Rinnai tankless heater&hot water recirc
I asked a similar question in the past, and got good answers, now checking to see if there’s any new opinion
I’m building a new home (it’s framed now), 5000 sqft, 3.5 bath, plus kitchen and utility room with a sink (yeah, it’s a castle, not a home, I know 🙂
The original plan was to go with 2 50 gallon water heaters, and a hot water recirculation system. Alternate suggestion now is to use a Rinnai commercial unit (2532 WC), with a 2 speed recirculation pump: at low speed, it recirculates hot water, if incoming temperature drops, the pump speeds up, and fires the heater until the temperature in the system is restored.
First question: has anybody else done this? Comments?
My shower can, in a worst case scenario, use 12 gpm (multiple body sprays plus a showerhead). The Rinnai unit can do 2.4 to 8.5 gpm, depending on the temperature rise. In Seattle, in the winter, incoming water probably is 40F (nice round number). If the unit is set for 120F, rise is 80F, flow is 4.2. The shower will mix some cold water, but still way short of 12 gpm.
It was suggested to run the unit at the max temperature (180F), and use a thermostatic mixer, to mix incoming cold water with the hot one, and get extra flow. But at 140F rise, the flow is only 2.5 GPM, so even mixing additional water won’t get me far… I must be missing something…
Apart from using 2 units in parallel (in that case the cost is way too high), is there any way to make this work? Any idea?
(yes, I know that 100 gallons in the water heaters will give me no more than 15-20 minutes at full flow, after mixing at the showerhead, so it’s not a great solution either)
Thanks in advance, Rob
Replies
Why is the cost of 2 units "way to high"???
You are biuilding a 5000# ft. castle.
with a mega dollar shower
your ganna have a 750K house with and indequate HW system.
spend the extra 2/10ths of a % and get a second heater.
Mr T
I can't afford to be affordable anymore
Good point indeed. Due to the layout of the house, the space available to mount the unit externally and the venting problems, the 2 units will have to be mounted far away, complicating the installation, and raising the cost well above the cost of the extra unit (or so I'm told). So it's significantly more than just one unit, plus extra maintenance, 2 things that can break, etc.
Can be done, but the fallacy of the "extra 10 of a %" (as I'm learning very quickly) is that after you apply it enough times, you end up 25% over budget :-)
Going with 2 50 gallon water heaters can provide enough flow, but there's the recovery time to take into account.
Just trying to see if there's any other idea. Spending more is always an option... the other option is to go with one unit, think ahead about what it takes to add another one, and do it only after having used the system in real life and seeing if it's really inadequate or not.
Any input is appreciated...
How about a boiler and a storage tank?
Showing my ignorance: why would this be different from a traditional water heater? It seems to me that a traditional water heater is pretty much an integrated boiler and tank, in one insulated unit... hence easier to install and cheaper to buy due to the higher volume of production
Do you have more info (or a link) to this kind of setup? Manufacturers?
Thanks!
The difference is the power available from a heating boiler vs, that of a gas water heater. For instance, if you had a Weil-McLain Ultra PLUS-40 indirect water heater and a heating (water) boiler of say 90 mbh output, the 40 gallon IWH has a capacity of 200 gph @ 115 degF (first hour), 150 gph @140 degF.
A 100 gallon commercial gas water heater has 144 gph first hour delivery rate at 100 degF temp rise. A high output 50 gallon residential gas water heater will have a peak demand output of 90 gph, standard version, about 80 gph.
If you are heating your castle with a hot water boiler (BTW, highly recommended if you're not), an indirect fired water heater (set up on a priority zone) is by far the best choice. Add a bronze 1/40 hp circulator and recirc the DHW with an aquastat to control the pump, you will have an ultimate system. Practically unlimity hot water supply, instantly available ate every fixture.
In addition to the output capacity of IWH, the combustion efficiency of a std gas boiler is 80-85% AFUE, whereas a GWH is more along the line of 53-63% (though not AFUE).
Edited 2/2/2005 11:24 am ET by Timbo
I "think" that some of the high output water heaters speced for space heating are also rated around 80% AFUE.I was kind of thinking about a hybred hydroair/heat pump setup.But after checking our rates I find that resistive would be cheaper operating expenses.So I stop looking at the details.
Before investing in tankless water heaters, I suggest you read the first two of the links given below. These are written by unbiased sources and can be considered pretty objective. The last link is written my an employee of a tank-type water heater company (which also sells tankless heaters). While the third one can be considered biased, it has many of the same observations as the first two!
http://www.pmengineer.com/CDA/ArticleInformation/coverstory/BNPCoverStoryItem/0,2730,141364,00.html
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http://www.askthebuilder.com//cgi-bin/column?451
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http://www.stateind.com/new/Tankless_WhitePaper.pdf
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Bill
Here's my real-world experince with replacing an old cast iron boiler and 40 gal WH (both NG) with a Combination boiler/water heater (Baxi Luna 310Fi). We had to go this route because code now requires direct vent appliances in basement areas. Since this heater can only supply about 3 gal/min, I added a 30 gal storage tank to the wall-hung unit to buffer the hot water supply and allow more than one tap running.Gas usage for heating and DHW is down 15% from the previous set up. That alone certainly doesn't justify the $3500+ price tag, but to meet code it was our only option.
I am confused by your comments. You replaced an old inefficient boiler which heated your house with a modern appliance and you saved money. I don't see where that relates to the choice between one type of water heater vs. another for water heating alone. The efficiency of your old boiler was so low that I am not surprised that you saved a lot of money plus you spend a lot more to heat your house than you do to heat your water. In fact, the average homeowner with a gas water heater only spends about $200/yr to heat water. Hence, if you saved 15%, it would take a very long time to pay off any substantial investment.
My post was a reply to the comments and articles claiming that tankless heaters are similar in efficiency to the regular gas or electric units. I was simply pointing out that in my case, the tankless unit is more efficient, but not enough so to use the fuel savings as the sole justification. When the unit operated in DHW mode last Sping and Summer, the fuel bills were about 10% lower. I suppose if some manufacturers got off their cans and started using modulating valves and better burners, the tank units might draw even again.
Edited 2/3/2005 3:31 pm ET by TJK
I respectfully disagree. Modulating valves and burners have no practical place on a storage water heater. The difference you are seeing...the 10% savings...is probably the difference between an old (less efficient) storage water heater vs. a newer tankless water heater. If you look at the efficiency numbers of new water heaters today you will find that the best storage water heaters are more efficient than many of the tankless water heaters. It is true, the best of the tankless are slightly more efficicent than the best of the storage heaters. However, that difference is sufficiently small that the additional cost is often hard to justify.
Bill
I used specs from water heaters and boilers that I sell, so they may not be typical of the entire industry, but they are fairly representative of units used for domestic hot water.
Thanks everybody for the great input. This forum has been my lifesaver while building my house (and gets me a mix of hate and respects from the subcontractors: hate for asking all the questions, respect when I understand what they are talking about. I minimize the hate by having them go with what they are most comfortable with, once they understand my concerns)
Currently the house is planned to be heated with forced air (we don't like in-floor radiant heat, just a personal preference for our lifestyle, where we are out of the house for many hours a day, and we cycle temperature a lot). The house is 3 stories (one is a basement: that adds a lot of sqft, and helps offset the cost per sqft). The proposed system has a furnace (96% efficiency) for the basement and main floor, a smaller 96% furnace for the upper floor, with an option to install an air conditioner later on (if needed: Seattle is not that warm in the summer). As far as I understand, the air conditioning coils will be installed now, but not the compressor/heat pump and the rest of the system.
If I read your answer correctly, you are suggesting to use a water heater for everything, presumably using an heat pump? Can you elaborate a bit more, please?
Also, thanks for the links on the traditional-vs-on-demand. I'll digest them as soon as I have a moment
"If I read your answer correctly, you are suggesting to use a water heater for everything, presumably using an heat pump? Can you elaborate a bit more, please?"
From the standpoint of hotwater only, I was suggesting using a high efficiency boiler to heat the domestic hot water in what is known as an "indirect-fired" water heater. It uses the boiler water (which is in a closed system, never consumed) in a heat exchanger/tank device that has an internal thermostat. You set the water temperature, like on any other water heater.
I also have forced air, for some of the reasons that you listed and also for air conditioning. Dual use out of a single device makes sense. I also have a gas fired water heater and a hot water recirculation system.
I wish that I had a boiler, an indirect water heater (w/ recirc) and instead of gas-fired furnaces, air handlers with air conditioning coils forr cooling and hot water coils for heating.
I don't know where you live, but where subzero temperatures are normal in the winter, I would not recommend a heat pump. They work and work well. However, in areas like mine (northern Illinois) they are not very practical or common. When you have to call a service company out, you don't want to hear anything like "never seen one of these before...".
An air handler with a DX (air conditioning) coil and a hot water coil is less expensive and simpler than a funace with an add-on coil, cheaper to install (no gas line or vent/comb air pipes to run) and with the proper controls have a practically infinitely variable capacity where a furnce will at best have two stages of heat. With modern boilers, and particular with boilers like the Weil-McLain Ultra boilers, controls and outdoor resets and DHW priorities are built-in, and they have a 5 to 1 turndown ratio (that means that it can modulate anywhere from 20% power to 100% power), to boot, and they're 92+% efficient.
Got it. I also did some research after reading your answer, and I found the boilers you mentioned, with a water/water exchanger instead of a traditional water heater.
I didn't think of the air handler (I thought you'd use something similar to a heat pump to move heat from water to air, but the water out of a boiler is hot enough). I was not thinking of a traditional heat, as in humid climates like Seattle they can have problems on cold days, spending too much time on the de-icing cycle, and needing a booster system. I wish I lived close to a lake, as that seems to work much better (using the lake mass as geothermal source)
I'll check with my installer, but I have the impression that I'll get one of those "never done before" looks that scare me. I'd rather go with a slightly sub-optimal option that they know and understand well, than something they grudgingly install (and don't know how to troubleshoot if anything goes wrong). Given the 3 stories of my house (and the challenges in running the ducts in a way that works for the wife), we had to have 2 furnaces (one in the attic) and running extra hot water pipes around might be more complex than a gas line...
I wish I knew all of this before :-). This forum is great
As a follow-up to my posts, I thought that what I learned in the last 2 days could be helpful to others.
The suggested design of my system was to have a commercial Rinnai water heater, cranked up all the way to 180F, feeding into a thermostatic valve (set at 120F), in order to have >8 gpf flow (on a very cold day, it could not have been able to provide >8 gpf at 120F, but would still provide enough hot water on demand... and on most days would have the right flow)
The hot water recirculation system was supposed to have a 2 speed pump with thermostatic control, circulating at low speed (on a timer, like any other recirc systems), until the temperature in the recirc loop fell below a threshold, triggering the higher speed and having enough flow to start the on-demand Rinnai.
I was worried about the "sandwich effect", where the .6 second delay in the firing of the on-demand heater would cause a series of short bursts of cold water in the recirc loop. One of the Rinnai tech notes suggested adding a small insulated tank in the recirc loop (connected close to the heater), to help smooth out any cold water burst, and to add to the system's inertia (smoothing out temp changes). We decided to put the heater and the recirc system in an easy to access mechanical room, and add the small tank only if needed in real life.
Why am I using the past tense? Well, because it turns out that Rinnai will dramatically reduce the warranty every time an on-demand heater is used with a recirc system. I was told that the reason is because the heater is working sub-optimally (frequent short bursts), and more likely to develop problems over time. The suggested solution is to add a 2-5 gallon electric heater in the recirc loop with a low speed pump. unless a faucet is turned on, the electric heater provides hot water for the recirc loop. As soon as a faucet it turned on, the on demand heater takes over (it always feeds into the electric heater, but the hot water from the Rinnai is enough to avoid using electricity). It also helps when a faucet is only partially open and doesn't trigger the on-demand firing: hot water is provided by the electric heater.
In the end, the cost of a Rinnai system with the extra electric heater and the required extra plumbing (Rinnai location) just didn't make sense for us. I also think that any potential saving offered by the Rinnai would be offset by the electric heater (esp. when considering the extra initial cost)
We decided to go with 2 x 50 gallon water heaters, plumbed in parallel, with the option of turning one of them off when there are only 2 people in the house, and bringing the second one on line only when extra capacity is needed.
Is there any real downside to having a water heater sitting unused for long periods of time?
Thanks for all the help, Rob
Man, that all sounds so complicated.
Why not just get one big super-efficient water heater to meet all your needs. How many concurrent outlets so you really need to feed, and for how long?
Something like a Polaris (or equiv) puts out 190KBtu which should be plenty for all your hot water needs. 95% Thermal efficiency.
http://www.americanwaterheater.com/WHBrowser/gas/Polaris50.cfm
Then run a standard Grundfos UP recirc system to keep the pipes warm.
http://www.grundfos.com/web/HOMEus.NSF/Webopslag/37829454ADF8A525C1256C3D003D8AD8
Simplify your design and save energy.
Well, that's what we ended up doing: 2 high-efficiency water heaters, 50 gallons each. The sub was pushing a Rinnai system (I suspect margins are better) but, once realizing how complex all of it was, we reverted to a traditional system
We have a 10 gpm shower (when the body jets are on). Add a washing machine and a dishwasher, and just one 50 gallons heater can be marginal when there are guests visiting. To reduce waste (ok, I do realize that it's ridiculous to talk about reducing waste when talking about 10 gpm showerheads :-), we plan to have one heater on line all the time, bring the second one up only when needed
I'm just curious to know if leaving a water heater off for months at a time could damage it..
Rob
"I'm just curious to know if leaving a water heater off for months at a time could damage it.."
Still not convinced you need 2 water heaters @200KBtu each. Getting a single larger water heater would be cheaper, or you could go with a supplemental plain storage tank. The high efficiency HWHs are not cheap like home center specials. Standing losses are not significantly higher with a larger tank.
Anyways, would they be run in series, or in parallel with some sort of shutoff valve arrangement when not in use? If in series, the unused tank would be flushed with new water constantly, so you would be unlikely to breed harmful organisms. An unused tank with stagnant water would be at risk.
Just me and thinking again.. Gets me in trouble..
What agout a water heater on each side of the house feeding the 1/2?
I think strange sometimes but thinks this could work. Easier to run the power than water!
Csnow: I need a direct vent unit, and that limits the choices. I have been told that it's actually cheaper to use 2 x 50 gallon tanks than one 75 or 100 gallon. Mostly due to supply and demand (50 gallons are common and easier to get). not sure if it's true, but I know that I will end up paying more for one bigger unit, and that's all it matters :-)
The tanks are connected in parallel (if in series, methinks that after a night standing unused, the water in the unused tank would be cold, and the other heater will have to warm it up again... at that point it's probably more energy efficient to keep both of them on all the time). I need to see if it's possible to flush the unused tank (it will be off for potentially months at a time. I was thinking about damaging the heater, not about being a breeding ground for scary stuff... good point
Bsaver: not sure about the real temp of incoming water, as I never measured it. But 40F or 50F makes a relatively small difference when you are trying to raise it to 180F. The point is: an on demand water heater has the highest flow only when the temp raise is relatively small. It's downhill from there. And, believe it or not, humidity is usually not a problem in Seattle. With forced air heating, in the winter we have an humidifier going almost all the time in our current home (we have a leaky house, which doesn't help). The summer is really dry, too (it usually almost never rains from July to September). We will have an "AC ready" system, but I'm pretty sure that with tile roof and light-color stucco we should not need AC (or, if we do, only for a few days/year, and we can bear it)
Will George: I thought about having 2 heaters in 2 places. But it wouldn't buy me much (I need to route gas piping instead of PEX, and Pex is cheaper), not to mention that I still need to run cold water to the heater and the faucets. And the problem is that the house is 3 stories (and relatively compact), so I'll have to put a water heater in a closet on the top floor, with concerns about weight, space loss, etc). And the vertical run is less than 20'
"Csnow: I need a direct vent unit, and that limits the choices. I have been told that it's actually cheaper to use 2 x 50 gallon tanks than one 75 or 100 gallon. Mostly due to supply and demand (50 gallons are common and easier to get). not sure if it's true, but I know that I will end up paying more for one bigger unit, and that's all it matters :-)
The tanks are connected in parallel (if in series, methinks that after a night standing unused, the water in the unused tank would be cold, and the other heater will have to warm it up again... at that point it's probably more energy efficient to keep both of them on all the time). I need to see if it's possible to flush the unused tank (it will be off for potentially months at a time. I was thinking about damaging the heater, not about being a breeding ground for scary stuff... good point"
In series is safer and easier. Legionella and all that...
Here's what you could do. Buy just one really nice 50 Ga gas water heater. Then buy one relatively cheap 40 GA electric water heater. Plumb them in series, with the electric HWH 1st. On the rare occasion you need more capacity, fire up the electric HWH. This would save you some $1700 just on equipment.
Or perhaps better, you could install a plain tank in series (after HWH in this case). With the recirc setup properly, this would provide more HW storage, probably negating the need for a 2nd HWH. Many folks actually use electric HWHs for this sort of thing instead of plain tanks because they are often cheaper.
Both are really good ideas. Somehow I didn't think about having the "cold heater" first (duh...)
I'll discuss both options with my installer and see what the costs are...
Thanks!
"I have customers with big bathtubs that have an insulated storage tank in unison with a water heater. A circ. pump moves the water between both. they say it works great."
It's worth noting that a plain storage tank can also be located remotely. Not an unusual setup for larger buildings.
You could get a tankless setup and a storage tank. Check out http://www.takagi.com
A storage tank doesn't heat the water, so the life expectancy is much greater than a standard water heater.
The other advantage of a storage tank is that the tank is not vented at the top like a typical gas water heater, so there's a lot less standby loss. Same advantage applies to electric tanks.
Yes, I see... I found the "concept design" on the Takagi webpage, and it makes sense. A holding tank would also smooth out any temp fluctuations when the water is turned on and off frequently (dishwashing), and works well with the recirc. It's one extra pump, but might be worth it. What I really like, is that I can easily plumb for it now, and put in the tank only if it turns out that I need it in real life. The small extra cost is a cheap insurance...
Thanks for the help, and please keep sharing ideas :-)
I have customers with big bathtubs that have an insulated storage tank in unison with a water heater. A circ. pump moves the water between both. they say it works great.
Are you sure about that 40 degree temp for incoming water? That seems really low to me. I figure 50 here in Ohio, and that is plenty conservative.
Also, on the issue of AC, don't forget about the dehumidifying aspect of it, not just the cooling issue. I would think in Seattle humidity is more of a problem than cooling.