Hi, I am looking at putting in 1,800 sq ft of radiant in a post and frame building that will be a woodworking shop. I am considering using a Takagi tankless hot water heater as a heat source, anyone have experience with the efficency and effectiveness of these units. I am planning to maintain 55-60 degrees during winter months in Minneapolis with a shop that is R-19 walls, R-44 ceiling and 2″ foam beneath the slab. The tankless heaters seem to be a good heat source from what I can tell? I also will be using LP for the fuel source.
Thanks
Aaron
What?
The Takagi is perfect for a hydronic floor system and it was the heat source of choice for the high-performance/green builder I worked for back in Wyoming. None of our clients would trade the tankless for a boiler.
Heat is heat. An open zone is the only time heat is needed in the system and the Takagi takes the water back up to the proper temperature and no more. Why raise the temperature of the water to a point where it then needs to be diluted with cold water? The Takagi only kicks on when heat is needed and is easily adjusted for the ideal temperature without the need for mixing valves/etc.
The Takagi works and works well in high-end applications, in cold climates, in real houses. This isn't speculation, wishful thinking, reflections on an article, or he said/she said. Where the rubber meets the road the Takagi has been a proven winner.
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IdahoDon | | #13
PS Please do read the Takagi warranty and website--hydronic heating is listed as an application. http://www.takagi.com/index.asp
PPS The Takagi is efficient warming the return water because of the computerized gas modulation valving--unlike boilers that need to run full-on or full-off.
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RBean | | #14I've just went through three techies at Takagi...all good guys...but please see the attached file below...<!----><!----><!---->
It is never as simple as it seems....like 35 psi or 80 ft head loss through the 235,000 Btu/hr model at 9 USgpm... in comparison to 9 US gpm through a typical boiler of 3 ft or so. With electrical rates going thorugh the roof...anyone trying to convince me to install a circulator/motor that was 27 times bigger than needed - they’d be thrown out of my office.<!----><!---->
Let me repeat this is just one example - 9 US gpm at 80 ft of head and that doesn’t include head loss through the system...I've been through all my circulator catalogues and there isn't a combination of residential circulators on the market that can do that…even if there was - throw away the appliance and let friction heat the fluid ( said tongue in cheek). <!----><!---->
Application and equipment knowledge is everything...the tech info in mfg literature and manuals is just that - information...in the wrong hands it creates unhappy customers which happens every time someone innocently reads on a public forum that all you need to do is throw in a water heater not understanding the consequences. Then they tell two friends and they tell two friends and before you know it - the entire country is filled with less than brilliant ideas about how to do radiant…some companies actually make a living off of the unsuspecting knowing that the unhappy girl on the east coast never talks to the unhappy guy on the west coast...its a pride thing amongst humans – few ever want to admit they’ve done something less than intelligent...companies know that...some use it to market products and services.
A note: This isn’t targeted at the reputable guys at Takagi – if you talk with their senior guys they understand the limitations and applications - always have - unfortunately in a rush to do things the cheapest way rather than the best way - important engineering, information like 80 ft of head in the example above - gets ignored in the field by overly enthusiastic sales people…it is an epidemic across the country…it falls right in there with using bubble foil insulation under slabs.<!----><!---->
Heatloss drives flow rate - flow rate drives head loss which together drive the circulator selection. Fluid temperature is a function of tube spacing and floor R value and heat load.<!----><!---->
The manufacturer can tell you if their appliances will do the job….what they wont tell you is how to pick the big honking commercial pump that is needed to move the water though the unit plus your system. <!----><!---->
600 sf is not 1800 sf is not 5000 sf is not 50,000 sf- and you can't ask a piece of equipment do something it is not designed to do...You have to do the math – that’s why the word ‘designer’ is not a fashion statement within building construction – it’s a mathematical process…grade 10 math at that.<!----><!---->
RBean
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
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TJK | | #23Here is a tankless DHW heater/boiler that does both tasks and includes the circulator pump and 90% of the other external gadgets needed to make a hydronic system function:http://www.wallhungboilers.com/luna310fi.htmlThe notion that "mass" is needed in a boiler system is old thinking. That may have been true for cast iron honkers with on/off burner controls, but with modulating burners and a few smarts in the control circuit, the only mass needed is the water circulating in the pipes. It allows the system to respond quickly to sudden changes in demand (e.g. a deicing zone valve opens) without wasting energy overheating 300 pounds of iron.
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constantin | | #24Allow me to quibble a bit...The notion that no buffer tanks are necessary at all is perhaps a bit optimistic. For example, many whole-house instant water heaters require a minimum flow of 0.5GPM to fire. This is going to be a problem when a hot water faucet is cracked part-way. A buffer tank would deal with that problem, which is why combi units like the smaller Vitodens incorporate one.The Baxi also only offers a turn-down ratio of 3-1 (35-105kBTU/hr), whereas most condensing boilers achieve 4-1 at minimum and some like the Trinity 400 from NTI can even modulate 16-1 (from 25-400kBTU/hr). I suppose you could incorporate a 0.5GPM recirculation system to ensure minimum flow at all times with an instant water heater and use the mass of the water in the pipes as you suggest. Lastly, the glitz of the Marathon site and all the promises they make leave me unimpressed. There is no data to bite into, just marketing. For example, there are no flow curves, published efficiencies, etc. Lest we forget, just about anything can save 53% over a traditional gas hot water heater. I'll be happy to take another look when Marathon offers more information online. Cheers!
Edited 9/29/2005 12:07 pm ET by Constantin
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TJK | | #25
We've been using the Baxi system for over a year and on average it saves us 15% over the iron boiler and 50-gal. NG tank heater it replaced. On the DHW loop we do have a 30 gal. electric heater installed as a storage tank to handle the multiple spigot contingency. A "buffer" tank in the hydronic loop is not necessary for most applications.I don't believe in the added cost or complexity of recirculation systems. IMO distant HW taps are best handled with small 2-4 gallon undersink electric heaters plumbed in the HW line.I agree the Marathon web site is lacking - perhaps they don't want to P-O their distributors by giving out too much info on a product designed for "professional installation". Here is the install manual (warning 1.3 MB download) if you are interested in more details.
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constantin | | #26
Thanks for the info. I will read it as soon as I have more time!As a seperate data point, Mike T. on the Wall replaced his 80% CI boiler with a Vitodens and saved 43.6% on a degree-day adjusted basis. WM Ultra owners have reported 35% reductions. Thus, modulating, condensing boilers certainly have a leg up on the CI competition.I agree that hydronic systems generally do not need buffer tanks the way DHW systems may. After all, filling a jacuzzi with water quickly or feeding just a little hot water to a faucet is a different challenge than keeping the huge mass of a home comfortable.I don't share your disdain for recirc systems though... I think there is a place in homes for them, particularly if the point of use option you describe means retrofitting lots of point-of-use water heaters. A properly-insulated and controlled recirc system consumes almost no energy and minimizes water consumption. For example, ours runs for 5-minutes on the hour in the morning and the evening, ensuring minimal water waste during the most obvious times of use in a given day.
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rich1 | | #27I'll quible too. At 35,000 btu minimum, I gaurentee that mosts hydronic systems will cause this boiler to short cycle. If the cycle is short enough, you won't be getting the full efficiency available. The mass is the water, most zones have very little mass.
I think most low mass boiler systems could use a buffer tank. In fact Siggy's software has a module just for buffer tanks.
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RBean | | #28
The first wall hung I worked with was back in the very early 80's - Paloma, then Hydrotherms version then the Bosch...following our venture into low mass wall units we bit on the bait tossed to us by a German fellow named Prell of a little known (to Americans) boiler manufacture called Viessmann…at one time we were the #1 rep in the country having sold a whopping three units...how’s that for circa dating myself…I always remind Harold and Ken at Viessman of this fact. Over the years, our companies have worked with Raypak, Laars, Buderous and few other stragglers including Baxi when it was still part of the Blue Circle Corporation eons ago...What I have learned with all these products is application is everything. A low mass or no mass boiler works great when married to a high mass systems...put a low mass single stage on/off gas fired unit on low mass heating system like a fan/coil and listen to the baby do the snap, crackle and pop. That’s not new or old thinking that’s just every day reality. One of the comments above made me grin when it alluded to boilers having on/off controls. The low mass Laars Mini Therm and Raypaks came with full modulating gas valves (thermostatic) and that was 20 plus some years ago...modulating gas valves are not exclusive or new to wall hung boilers or water heaters...every boiler manufacturer offers them – always have. What is interesting is how everyone (well most of them) of the old school manufacturers slammed the low mass units of decades gone by; but today many have had a change of heart…a lot has do with the ‘new’ (I loathe that) high mass radiant vs the old low mass baseboard…so its not so much that the boiler manufactures changed as it is the type of systems changed and the low mass units found their niche. (Next time any of you run into a cast iron sales guy from the 70’s or 80’s ask them if they ever thought they’d see wall hung aluminum boilers in North America…their eyeballs usually roll back into their heads).<!----><!---->
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Constantin is far more knowledgeable than I on the combustion efficiencies - I do know from owning and having owned several styles of hydronic systems that constant flow, variable temperature on a modulating flame regardless of mass does beautiful things from an efficiency and comfort perspective. Short cycling is the enemy and storage is a solution for many but not all applications. As long as there are a gazillion different homes and a gazillion different HVAC systems there has to be a gazillion different solutions…its just the way it is. All I’m wanting to communicate in this post is ‘once again’ that when all one has is a hammer everything becomes a nail…and consumers – like the people reading this post - get screwed by firms who use this one size fits all approach – and that’s just plain irresponsible. <!----><!---->
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As far as the combined operating cost (combustion and electrical), I agree again with Constantin and others and was privy to some early development of purchased consumption programs that Danfoss Denmark was working on for district energy. Consumers purchased units of energy on a program card just like a phone card and downloaded them into the building control system…haven’t heard anything about the program - it dates back 4 or 5 years…its just a matter of time. Will residential CHP make it…I would love to think so…but I see homes becoming more efficient first before seeing more high tech mechanical efficiency…even so one would have to make a hard case for individual CHP plants vs an efficient co-gen/ district energy systems feeding an network of efficient homes. As an aging boomer, the last thing I want is complication in my life…I want heat and power…not necessarily (and preferably not owning) the fancy equipment that provides it…hook me to a district energy system any day! Hope I live long enough to see what the younger generation decides…<!----><!---->
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p.s. Aaron...sorry for participating in the hijacking of your post.
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Get three names from the Radiant Panel Association website and Dan Holohan’s, Heatinghelp.com including whomever you're working with now and establish what is important to you....specify outcomes and performance - be specific – in writing – download the 11 Point Checklist at http://www.healthyheating.com/home-heating-system-design.htm and let the best business win.
Edited 9/29/2005 3:20 pm ET by RBean
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rich1 | | #31
I agree about the combined operating cost. The NRC has a bulletin that says a 90+ furnace with variable speed fan may actually burn more gas than a standard 80+. The reason is that the fan motor adds heat to the airstream. But the cost is offset by the lower operating cost of the variable speed motor.
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RBean | | #33
Would you happen to have the link for the NRC bulletin? If it's not readily at your finger tips...not to worry....I’ll dig for it.<!----><!----><!---->
It's not surprising...the see-saw effect...a decrease in one change causes an increase in another. <!----><!---->
It is like the application of variable speed injection for mixing control. It is good use of technology and have used it on large applications but the majority of circs are grossly oversized for the injection application so the motor efficiency is very poor. The selection calculation is based on worst case scenario so the designer picks a pump that even in maximum load situations is oversized but has no choice because of the limited selection…the real pain is felt for 95% of the year when the system runs at less than worse case scenario...<!----><!---->
When an injection pump is oversized it creates a case where the warmer it gets the less efficient the system becomes...totally bassackward - but injection enthusiast never look at it this way...it’s that nail and hammer thing again. Injection works great when the smallest available circulator runs in the middle third of the pump curve for most of the heating season. That is where the word ‘design’ comes in…rather than pulling out the hammer…whack VEEE ALVAYS DOO IT DIS VEY…(said in the tone of Hogan’s Heros Sgt. Hans Schultz)<!----><!---->
If it’s done to reduce the cost of installation then the lower cost also prevents the system from being manually operated in a failure mode. If one installs manual bypass valves to allow manual operation in case the pump fails then it becomes more money than a mixing valve - so why not use the mixing valve or better yet a two-way valve injection system, which is less money than any other combination and can be manually operated? <!----><!---->
I like variable speed, have used it, teach it, and will use it again but sometimes we do things for the sake of technology without looking at how and where the last domino falls.
Now I'm off to see the wizards at NRC about that bulletin...<!----><!---->
Edited 10/1/2005 6:36 am ET by RBean
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rich1 | | #34
I'll have to dig for it, it was quoted in a SaskPower bulletin.
When I was at The Little Red Schoolhouse, one of the instructors, asked some of his engineer friends how they choose pumps. Often, by that stage of the design, the budget is shot so,"big job, big pump. Little job, little pump."
We recently did a retrofit for DND. 2 new Laars boilers, 3 stages each, tekmar controls,pumps on the boilers, P/S, real nice system, except for one detail. The primary loop was 40' of 4". The primary pump was a 6"monster. I don't know the gpm of the pump, but I'll bet a case of Sleemans Honey Brown, that it was WAY oversized.
One of these years, I'll get to one of your classes.
Cheers.
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RBean | | #35
Thanks rich1,<!----><!----><!----><!----><!----><!---->
Haven't found it yet but will keep digging.<!----><!----><!----><!---->
Great story rich1...in the absence of logic ... go big!
We can see it now…xspurt thumbing to Page 49 of the unpublished but infamous hydronics DUH-signers manual...a 4" line = use… hmmm...lets see hmmm - a 1.2 safety factor = 6" circ, UPS courier guy looking over xspurts shoulder, soundsboutrightbuddy…then next day xspurt fishes for page 65 of said unpublished manual .... says here - If you’re a radiant DIY’r you need 7/8 PEX ... why?(Sales guy thinking to himself - that's all we stock) but says outloud - it worked on the last 1800 sf job and will work on any DIY project (even bigger than 1800 sf) so long as it involves cash burning a hole in the DIY’r pocket - funny how those suppliers in far off ever never lands are there at ones ankles to pick up the hot dimes when they fall....dime timing is everything as they say counting on the innocents of the DIY’r - they'll never know the difference 7/8ths or 8/7ths who cares - PEXSHmEX - its just retirement money - there is never an end to the brilliance...go big or go home says the DUH-signers manual. ...ok I'm done ranting.<!----><!---->
Good beer…will buy ya one without the wager next time we meet.<!----><!----><!----><!---->
Next seminars out west are in <!----><!----><!----><!----><!----><!---->Calgary<!----><!----><!----><!---->, end of November.<!----><!----><!----><!---->
Would be great to have your participation.<!----><!----><!----><!---->
By the way here's an interesting link from our Alberta Safety Codes Council on certification requirements for designing hydronic systems...it's actually an update which lists the approved certification bodies and few other minor details...i.e.: thou shall not pretend to be a designer without suitable white collared shirt and pocket protector…glasses and hush puppies optional.<!----><!----><!----><!---->
http://www.healthyheating.com/Conference%20Calls/Bulletins/Hydronics_Standata.htm<!----><!----><!----><!---->
Stay well.<!----><!----><!----><!---->
RB
Edited 10/1/2005 2:04 pm ET by RBean
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constantin | | #30
TJK,Thanks to your kind posting of the BAXI Luna Installation instructions, I was able to look at some of the specifications for the unit. Allow me some observations.The thermal efficiency of the unit is not the greatest at 83% when its firing at maximum output. The condensing boilers I have referred to reach steady-state efficiencies above 94%... Now, I imagine that the heat exchanger works better under lower firing condition, but it still won't approach the surface area and heat flux of a Vitodens, Ultra, etc.I am also not a great fan of flat plate heat exhangers to heat DHW. If you have well water or a high-hardness water from the city, these are much more likely to seize up with calcium carbonate than heat exchangers that have a larger surface area and that are hence less likely to need very high supply temperatures to provide the hot water you want. ... the lower the needed supply temperature, the lower the rate of calcium carbonate formation.Also note the minimum flow rate of 0.5 gallons per minute to activate the Baxi. I imagine this has to do with their minimum firing rate of 35kBTU/hr (though I am too lazy to do the math just now). If, as Robert points out, you have a low-mass heating system, this may or may not work too well. If this is legal in your jurisdiction, it might work well enough for a slab or similar high-mass system or as a hot water source for an indirect. However, I would sleep better at night with a slightly-higher priced system such as a HTP Munchkin or NTI Trinity that are sold at a very competitive price point, that have been shown to have a higher modulation range and thermal efficiency than the BAXI, and that have widespread code approval.
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Tim | | #37"It is never as simple as it seems....like head loss through the 235,000 Btu/hr model at 9 USgpm... "
9 GPM through 235 mbh works out to a 42 degree delta T (at 80%). Actual use of a tankless water heat as an infloor heat source would be more along the lines of a 10 to 15 degree DT, as you well know. In a realistic application, the flow would be approximately 2.5 gpm and the pressure drop through the heat source would subsequently be around 2.7 ft of head.
You are right, however, "application and equipment knowledge is everything...".
I'm not arguing for or against the use of a tankless water heater as a space heating source. However, if you are to make a competent argument in favor of your opinion, at least use realistic numbers.
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RBean | | #38
Right you are Tim, where the challenges lie for a competent argument also lay in the data of the attached graphic. If you locate the load for a 2.5 gpm at a 20 deg F rise it works out to a nominal 25,000 Btu/hr…so is the consumer being sold a 235,000 Btu/hr model when the load is 1/10 of that to keep the head loss to a reasonable value? Another way to look at it is at 5 USgpm, with a typical 100 deg F supply temperature and 80 deg F return temperature, the pressure drop is a nominal 11 ft of head for that model…still very high…so ask is a 235,000 Btu/hr model for a 50,000 Btu/hr load at 11 ft of head without accounting for system head losses something one would recommend to a client?
What happens in the real world is someone does a heat loss, picks a water heater rated for the same or bigger number, hangs it on the wall, or throws it on the floor goes to the local plumbing supply house and say according to the manufacturer, I need a high head pump...so the counter guy goes the heating manager (if there is one) who asks what size of heater the guy has and he’s told 235,000 Btu/hr so the guy either says instinctively that it must be 23 USgpm because that’s what the rule of thumbs says…not knowing that the manufacturer has a limit of 9 USgpm or he tells the counter guy to give the customer the largest high head residential circulator on the shelf and says this otta do it…and then the system doesn’t work as witnessed by the contractors that have to fix these applications. <!----><!----><!---->
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I appreciate your comments and value your opinion; we recognize that design has to include all of these issues to make good decisions…unfortunately; I don’t see that happening on a large scale particularly when it comes to selling to unsuspecting consumers.
When the complaints stop, then I'll change my position.
RBean
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
Edited 10/3/2005 2:52 pm ET by RBean
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Tim | | #39
"..so ask is a 235,000 Btu/hr model for a 50,000 Btu/hr load at 11 ft of head without accounting for system head losses something one would recommend to a client?"
NO. I would not recommend use of a domestic water heater for use as source of heat for comfort applications. I design hydronic heating systems around boilers. I have "helped" people in the past make do with inappropriate equipment, ie. using water heaters for RFH. I no longer do that.
I don't believe these units can produce anywhere close to that level of heat output with warm/hot water going in. These units will produce a 80 degree temp rise with EWT of 55 deg at 5 gpm, but with EWT at 85 or 95, that is not the case. The units are modulated on discharge temperature and have high limit. I have a discarded Bosch Aquastar that I use for heat in my garage. I set the outlet temperature to 95 degrees, let the line voltage t-stat start the circulator as necessary, and it works very well for my garage. But frankly I don't care if its 45 or 55 in there, so long as its not 10 deg. As the slab temperature comes up, the unit modulates down. I don't know what problems or complaints others have, but mine works quite well. However, as I stated above, what I find an acceptable experiment in my garage is not what I would consider as a professionally acceptable design or equipment selection.
Aaron,
You've gotten good advice from Constanin and RBean and Plumbbill.
To clarify a point you understood, but maybe some lurkers didn't: Many tankless heater have a constant heating rate. If your RFH can't absorb that much heat that quickly, there will be a problem. The best of which would be rapid cycling which isn't good for your heater, your pump or your efficiency. But it could be no heat or too much heat, depending on how you try to force mismatched equipment together.
A VERY rough heat loss calc, assuming -40F design temp and decent, modern, but not supertight construction, and the insulation you noted; gives 50,000 BTU/hour (at -40F). If you want freeze protection at the temp, you need to maintain at least 40F inside and that gives 43,000 BTU/hour minimum.
I used a direct-vent hot water heater in my own garage for RFH, but it is 728 sf of slab plus 500 sf of loft and very tight (except the damn garage doors, of course). 40,000 BTU/hour gross gives about 32,000 delivered and it has no problem keeping up at -20F or lower. I like HWHs because they are cheap ($200), easy to install and cheap and easy to replace. I like direct-vent HWHs ($400) because they help keep the structure tight (I have one in my house as well) and greatly reduce the risk of CO poisoning from backdrafting. Also, since the combustion is sealed off from the room air, you can place the HWH on the floor, instead of 18" up and keep the flame away from the gasoline/propane/paint/solvent fumes that can occur in a garage/shop.
If it were my shop, I'd be willing to do a slightly risky design and use a 40,000 BTU/hour direct-vent HWH (but I'd prefer a 50,000 BTU/hour unit if available). Then, when it running on a 0F day, check the circ pump run time (or propane consumption) and calculate its duty cycle. From there, calc the coldest day it can keep up with. If it isn't enough, throw an eletric heater (cheap, cheap) or two on a thermostat set to 40F. You'd only need a few days of that back-up heater each year, at most.
I'd never do that for a client - plumbers never get callbacks for oversizing systems. Sure, it cost too much and cycles too often, but the only thing the HO notices is insufficient. But for myself or a tinker, I'm willing to cut it closer and twiddle a bit with it later.
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BillHartmann | | #16" Many tankless heater have a constant heating rate. Many tankless heater have a constant heating rate. "I have never heard that. In fact I was under the impression that to fucntion properly as domestic WH that a tankless one did have to modulation the heat.You might have source water of 40 to 70 degrees and flow rates of .5 to 3 gpm (I am kind of guessing at those nubmers, but there is a range of flow rates).
Replies
Well I don't recommend a "tankless" for this application tankless water heaters use less energy than tank types cause they run in short intervals not long sustained periods. For heating application a reservoir of hot water is best, unlike domestic applications in heating your trying to get the heat out of the water so if it works like planned your tankless will constantly fire to make up for loss . If that makes sense to you.
Plumbbill,
Thanks, that does make sense. In short any time the radiant system calls for heat the "Tankless" has to run. How about standard domestic H2O heaters? don't they have to make up the same amount of heat to transfer to the slab?
well not really. Lets put some #'s on it if your source "tankless heater" puts out 120 degrees & your system takes out 50 degrees then the water going back to the source is 70 degrees so if the system is not firing then you have 70 degrees going back to the system. If you have a reservoir then you have 70 degrees mixing with the reservoir water so the out put 120 mixed with 70---make sense--- If your going to use a domestic water heater the best way to do this is to turn up the heater & use a mixing valve cause radiant floor systems run at lower temps than baseboard systems so then your only pulling from both your system & the reservoir. hope this helps
So what do you feel is the best heat source for a radiant system?
IMO, you're best off with a boiler that is meant to run a radiant system. There are many choices out there, with the T50 from HTP probably being the best choice for your application. You could couple it with an indirect to make your hot water as well (they can modulate up to 80kBTU/hr output just for hot water).
Hi Constantin,
Once again you're bang on with your advice.
Not sure if you have this issue, Radiant Panel Report, A Square Peg in A Round Hole
July Issue Excerpt:
"For those who still want to make those tankless water heaters, not tobe confused with tankless boilers, a part of their radiant heatingsystem, beware. The pitfalls haven't changed much in twenty years. Readthe manufacturer's recommendations and the warranty carefully and decideif you really want to carry all the risk."
August Reply to Editor:
"A Square Peg in A round Hole". We were burnt by this type of heaterlong ago and have forced ways to make them work. The cost of a pump togenerate sufficient flow makes the supposed savings over a boilerdisappear. If you plumb them according the manufacturer'srecommendations you will also suffer. Primarily, we get calls to try andsalvage jobs, where people and contractors believed the claims of themanufacturers but did not under- stand the design criteria anddifficulties."
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I thought it would find good company in your knowledge vault.
Cheers,
RB
RBean
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
Edited 9/26/2005 8:33 pm ET by RBean
Edited 9/26/2005 8:33 pm ET by RBean
Edited 9/26/2005 8:38 pm ET by RBean
Robert,
Many thanks for the kind words. I've meant to reply earlier, but some issues back at the house have kept me very, very busy. Like you, and many others, I believe that tankless water heaters have their place. Specifically, they do a great job of heating water in homes that are used intermittantly and where the flow is high enough to allow safe operation. Allow me to present a couple more points to the boosters of tankless systems.
For one, I believe it is only a matter of time before the utilities in urban areas get smart and start imposing maximum demand charges, like they already do for commercial accounts. They're already arming themselves for that day in our area by switching to electronic time-of-day meters that reflect not only your total consumption but when and how much. In areas like Boston that suffer from chronic issues with old, undersized gas and electrical distribution systems, it is only a question of time until utilities will also impose demand charges for residential customers.
Traditional gas and electrical water heaters are the darlings of the distributors of energy for a number of reasons. For one, their input ratings are relatively low. Furthermore, they recover over a number of hours, smoothing out demand. Lastly, gas water heaters are not terribly efficient at retaining heat or putting it into the tank, which is great if you're selling energy.
If everyone were to switch to electrical or gas on-demand systems, the distribution networks would see a huge increase in the variability of demand. This is a nightmare for distributors of energy because neither the gas or the electrical networks can usually sustain much of either. My guess would be a huge spike in the morning and the evening for gas, for example. So, think of your water heater tank not only as a buffer for the thermal needs of the home but also for the distribution system that feeds it with heat.
Naturally, this isn't an issue for those on LP tanks... such users only care about total demand.
Using a traditional water heater for RFH is playing with your life, IMO. I have read too many reports from home owners wondering why their gas water heater hooked up to RFH systems is rusting out from the inside. The answer is simple: A gas water heater is built to be cold maybe twice a day. However, in a RFH system it can be cold all day, condense ad nauseum, and degrade very, very quickly. The potential of CO poisoning lurks just around the corner under said circumstances.
So, you save some money up front in terms of equipment cost by installing a plain-vanilla gas water heater to supply your "small" RFH zone - but to make the whole thing safe, you'd have to spend a lot more money. IMO, your money is better spent installing an appliance rated, tested, etc. for the application that you intend to use it for.
Robert also raises some good points about the very high pressure drop across the heat exchangers found in most instant water heaters. Even in their intended application, this can be marginal in many localities due to insufficient water supply pressure. For example, on my street we have a full-flow curb-side pressure of 30 psi. Boy, it would be fun to try and make a high-head HX work under those circumstances, particularly if you have an apartment at the top of a 3-story house!
Lastly, the electrical costs of various appliances are not factored yet into their overall ratings on this side of the pond, AFAIK. Thus, the AFUE between the Viessmann Vitodens and the WM Ultra is about the same - yet, according to the ACEEE, the electrical consumption of the WM Ultra is 8x higher than that of the Vitodens. I hope that in due time the total energy consumption of an appliance will be considered. Otherwise, the waste heat from pumps and other parasitic losses will continue to unduely increase the ratings of the appliances they're used on.
Constantin: What do you do for a living? You have an unusually expansive viewpoint. I forwarded your demand-pricing thoughts on natural gas to my FIL who did his PhD research and subsequent consulting work in that area regarding telephone and electric rates.
"Traditional gas and electrical water heaters are the darlings of the distributors of energy"
Yes, and more so in some countries. Some locations in Europe, electric HWHs are radio-controlled. Since it only has to come on once a day (if you take really quick showers), the utitility is only promising to turn it on a few hours SOMETIME each day. giving them tremendous flexibility in balancing peak loads.
"Gas water heater hooked up to RFH systems is rusting out from the inside."
I'm not so sure that should discourage someone from doing RFH with a HWH, only that it needs to be considered. My house needs about 17,000 BTU/hour at -40F (very well insulated and super tight). So my HWH (which does both RFH and DHW) has a pretty low duty cycle AND can maintain temperature. I run it a little hot (140 instead of 110-120F and temper that down for RFH and for DHW) to keep from ever being in a condensing mode. And I don't see any rust.
I DID get some rust formation during construction when the insulation and vapor barrier weren't in place and the HWH couldn't keep up. But I vacuuming all that out and haven't gotten any more since the house was closed in.
And doesn't that rust come from the water-combustion gas HX? It did in mine. So a leak would manifest as a water leak, not as a CO leak. And, if my inner exhuast duct leaked, it would only leak into the incoming combustion air, not to the room air.
I think a HWH sees more rust from a super-long shower - the kind that results in cold water at the taps - than from RFH demands below its heat rate. The super long shower definitely results in a cold tank and acidic condensation on the HX. RFH use maintains 120-140 in the HWH tank and avoid those condensing conditions. (unless you try to skip the tempering valve and turn the HWH down towards "vacation" settings. That is an inappropriate "savings" of $60.
"Lastly, the electrical costs of various appliances are not factored yet into their overall ratings"
Yeah, that bugs me too. Not only the electrical consumption, but the much higher price of a kWh compared to 3,413 BTUs. Are there truly any 93% efficient HWHs out there? I think not.David Thomas Overlooking Cook Inlet in Kenai, Alaska
David,Many thanks for your kind words also. I'm a consultant in the Medical Device Industry, though I used to work at Arthur D. Little. Among other things, the cost models I built were used as the basis for the Washing Machine, Water Heater, and Residential AC energy efficiency rulemakings. Yes, I was one of the pinatas in the middle with the DoE folk, being batted by the industry folk and the environmentalists...I also helped develop some appliances, among them an outdoor gas water heater. That was a long, hot summer... I've also lived in France where people literally set their dishwashers, washing machines, etc. to run during the nighttime, when nuclear energy is looking for something to do. The house I lived in was filled with an electric water heater in each bathroom on 24-hour timers also... Our local (MA) electrical utility has announced not only a 23% rate hike, they're also now offering time-of-day plans for consumers. I guess those upgraded meters are going to be put to use by whoever can shape their electrical demand to be primarily by night. For example, you could follow the NM utility model, build a high-mass home, insulate it well, and only cool it by night. That's the beauty of buffers, be they thermal, etc.As for the health of your water heater, your carefully constructed home pretty much may ensure that the heating load is low relative to total output. Furthermore, you take the time to monitor the condition of the inside of the heater... few people do, even if they know how. Have you pulled/inspected the flue baffle plate? If so, how does it look? ...and in most homes (which won't be nearly as well-constructed as yours), a thermostatic valve is inadequate to prevent flue gas condensation. It may do so during the shoulder months, but it does not protect the boiler from prolonged exposure to cold returning water in the colder months. However, how to convince HO's to invest about as much into the return temperature protection as the water heater they just bought?As for CO production, the issue is in the HX. For example, if you get significant corrosion due to flue gas condensation, then it's only a matter of time before you get flame impingement and hence the generation of nasties begins. Now, as long as you still have a strong draft, you'll be OK because the CO will end up poluting the outsdoors, not killing you. It's just the days on which the draft diverter doesn't work due to ambient wind conditions, etc. that you may fill up your home with CO...When I helped develop that outdoor gas water heater, we hit it from every direction with tons of wind. Getting a water heater to work outdoors in all wind conditions without the benefit of a forced draft is tricky, tricky business. It's all too easy to have a delta-P develop between the intake and the exhaust, which then leads to bad combustion, impingement, etc.Thus, I laugh a bit when I see all the different ways in which people install indoor water heaters in outdoor locations... and wonder how often the pilot flame is blown out, etc. Best of luck!
Developing a new type of HWH? Let me tell you what I want to see mass-marketed.The marriage of a weedwhacker engine, a tanked HWH, and a alternator/generator. Even at retail those parts would add up to only $400.Designed, tested, approved, and produced as a local electrical source, like a solar panel can reduce your use of the grid. 30% efficient with a small IC engine but most of the other 70% of the BTUs goes into the water tank for DHW and/or space heating.Would run on natural gas (which is already plumbed to one's house) and could be viewed as giving you either 1) almost free electricity or 2) competetively priced electrons and free waste heat.We waste so many BTUs at central power plants. Produced locally, it could be utilized.David Thomas Overlooking Cook Inlet in Kenai, Alaska
David,
I co-developed that hot water heater some years ago. It was called the Weatherpro and sold by American Water Group via Lowes and other outlets. I guess the idea was that south of the mason-dixon line and in CA, etc. there are a lot of indoor water heaters being installed in garages, heater huts and other things. The Weatherpro would have allowed builders to avoid a number of installation costs, but AWHG never figured out how to sell them, and later took them off the market. I do have a picture of me standing next to one in a Lowes in Shreveport, LA taken by a startled employee...
Coming back to your suggestion, the concept is already in production by a number of manufacturers. Honda collaborated with Climate-Energy to develop a Micro-CHP as you've described it. I agree that these units have a lot of potential in terms of reducing the BTU losses at the point of generation, though their CHPs only cover a portion of a households needs.
Another manufacturer in this vein is Marathon engines. I happen to like their product better from a technical point of view because it doesn't have to be married to an ECR furnace or boiler like the Climate-Energy unit. With a higher output of up to 4.7kW of electrical and 12.5kW of heat output, it would also cover more of the needs found in homes. This is the unit I would investigate further...
Thanks for all the info, here is the setup that i am looking at for the Takagi in my RFH application (~1,800 sq ft).
http://www.radiantcompany.com/heatsources/Takagischematics.shtml
Let me know what you thinkg.
Aaron
Hey love the information that you & Mr Smith in Alaska have provided.
Being a 4th generation commercial union plumber in Seattle, I like to learn new things that I don't get to see a lot of doing high rises & massive Boeing structures.
I have only one complaint it's not a "hotwater heater" just a water heater cause if the water is already hot why would we need to heat it.
Water temp goes from 120 to 140. Over and over. Sounds like a hot water heater to me. lol.
Constantin & rbean have it right on when calling for a boiler for heating applications. Another link for good advice on this is a plumbing & heating trade magazine I get. Here is the link http://www.reevesjournal.com
I'm unclear on your "radiant" - is this tubing embedded in the slab? ...overhead panels that radiate heat?
I have seen a radiant heating system that routes the exhaust from a gas flame through pipes overhead backed by a reflector. This feels good (like sunlight) even when the air temperature is low. Check out the park building (the old pickle factory) on Nicollet Island in downtown Minneapolis for an example. (I'm also in Minneapolis - and I'm getting chilly...)
Hydronic heat - Infloor radiant.
Okay - that helps. I'm figuring on building a "garage" (with much shop space) next year and I've been curious about those overhead gas-fired heaters. I'll keep in mind the hydronic floor heating - imagine crawling under a warm, dry car on a warm, dry floor...in January.
Aaronb all the advice you have gotten from this forum is sound advice. But if you want to get down to the nitty gritty and actually design a system that will meet your needs I recommend you calling the guys at this website
I built an addition on my house and installed an elevated concrete slab with hydronic heating and my wife loves it. The guys at radianttec designed the system, calculated the heat loads, sized the water heater (small additon 600 sf) and made recommendations for controls. I built a very bare bones system but it works great. These guys can assist with a very simple or multizone type system. The thing to do is call them now while you are in the design phase not after you have poured 4" of concrete on a single 600' loop of 1/2" pex and wonder why you have cold spots. They designed the system for free and I felt their products were reasonably priced compared to all the other companies out there doing similar things.
Good luck,
Clark