Using tankless H2O heater for garage radiant
Hey Guys-
I looked and searched, and i was SURE I’d find a thread that would answer my questions, but either I, or the search engine isn’t great at searching. Probably me, so feel free to just send me to the right thread!
A friend wants to finish off his barn/’garage so he can work on his race cars. He’s excited about a 2 post lift, but I’m thinking about the right wat to heat the space. It’s basically a 20 x 30 space, with 8′ ceilings on one side and 14′ on the other. R30 in the ceilings, R19 walls, with two 2 x 3 windows, two insulated garage doors, and one entry door. Now, I’m NO HVAC guy, so feel free to correct my numbers, but, my quick calcs (using the CT state as a location, 5500 degree days, 5 degree min temp) show I need 22K BTU. So lets say 25K.
Now, he won’t be using the shop every day, and doesn’t need it to be a balmy 70 all the time.
My gut says a simple radiant system with a ceiling fan over the high area would be more comfortable than either ceiling radiant units or a Modine Hot Dawg. Your thoughts welcome on that.
Since it seems like I really won’t need a large heating unit, I thought of hot water heaters. But, the typical cheap tank style worries me, and the stainless ones run in the $3K+ range. Ouch! So I am considering a tankless style. He’ll be using propane.
I’m wondering about the cycling on/off issue, mixing valves, etc.
I KNOW you guys have done this with your eyes closed. Thoughts?
Thanks!
Replies
What's the cost
of electricity per KwH?
A gas tankless wouldn't be my choice for space heating. It is designed to heat a volume of water very quickly (i.e. 175K Btu input). Using a tankless to heat an indirect tank, and using that tank to heat the floor would make some sense to me. More logical is using a small wall-hung gas boiler with its own internal vessel with a few gallons or more. There are plenty of those available and they generally have radiant controls built in.
If electricity is affordable, then something like one of the small (~10 Kw) Electro boilers would be worth considering. No chimney, no heat up the chimney.
Or.... a mini split heat pump. Much more efficient than burning fuel to generate BTUs.
The comfort part.
For that application I would certainly choose radiant in the floor. The operating temperature could be held pretty constant if that area is used for a daily garage. Could probably run the system at 55 and work with just a sweatshirt. I installed a woodburner in my shop/garage and could kick myself every time I go in there if only to grab a tool or some material. My wife could kick me everymorning (but she's got a seat heater for her morning drive to work.
As far as the means, you know the space and temp needs..........I would think you'll have no problem applying the applicable heat source.
How much does he want to spend, more or less?
I would look at adding a solar collector to passively keep the slab above a certain temp, then use a gas heater for the rest of the delta when needed.
I'd say something like a Munchkin MC50 contender boiler would be a good choice for heating water. High efficiency, but about 2k bucks. Plus all the radient hardware.
I'd just go for the hot dawg myself.
Too Low on Calcs
I'm no expert either, but using the garage room volume of 6600 cubic feet (IE 20 x 30 x 11 feet avergare height), a target temperature of 68F, and your coldest temperature of 0F, you will need nearly 3X the amount of BTUs you have calculated to keep the garage even modestly warm.
See this caluclator: http://www.herman-nelson.com/BTU_calculator.cfm?type=F
Not sure that even 2 80 gallon used heaters could keep up with the demand.
That said, and do not forget, you will need to make sure you also have at least 2" of foam insulation placed down to the footing on the inside or outside of the foundation, and/or a minimum of R-5 foam insulation placed under the radiant coils depend what your state energy code will require.
Another consideration is that the heaters themselves will need to be elevated to at least 18" above the finished floor to avoid any flame or electrical connections from igniting gasoline vapors.
If using tank type water heaters, they will need to be installed to avoid impact from vehicles...like installing bollards in front of them.
I'd certainly consider a heated slab, but your costs are going to be a lot more than you now figure imho.
Too Low on Calcs
Man,
I'm questioning the calculations you are using regarding radiant heat in the slab. While bringing up the temp in the room to say 68 might take more btu's, the comfort level in a shop situation where all the work is 6 ft off the floor can be acheived with less. My opinion is not done with any calculations, just the experience as a user of in the floor heat.
Not close
By your calculations, this garage would need, 70 MBH to heat it? 116 btu/sf?!? You are certailnly no expert.
I have to assume that have made a mistake and have no means or experience to determine what is reasonable. There are a few others here that constantly spout off about which they know nothing. Welcom to the group!
While "square footing" is no substitution for actual heat loss calculations, it does serve as a good "gut check". In the upper midwest, where dT's are in the 85 to 95 degree range, old poorly insulated, drafty buildings took 50 btu/sf to heat well. Now, tight, well insulated buildings can be heated for 15 btu/sf or less.
Accurate
Connecticut Energy Code requires this garage to be designed to be heated to a minimum 68F wintertime temperature if it is going to be heated at all.
The volume of this garage space is the approximate equivalent of a ranch style home with the approximate dimensions of 28 x 30 x 8.
75,000 BTU/hr is therefore a very reasonable and conservative amount of energy required to heat a house or garage space of this volume... based on a mean winter design temperature of 0 to 5F and given insulation as stated. Given the actual conditions, it could actually require more.
That said, this does not mean it will be necessary to expend 75,000BTU/hr nor that it is necessary to keep the space heated to 68F.
It merely means the heating system cannot be designed for anything less....and still meet the building and energy code requirements.
Sorry.
20x30x11, r19,r30, slab on grade with perimeter insulation, 2 double pane 2x3 windows, 2 garage doors 7x9 same r as the windows, 5f oat, 70f iat, needs about 16,000 btuh.
There is a point worth remembering in this. Though generally you won't heat a garage to "normal" indoor temps, and likely will use the area less in colder weather, many/most people would keep the heat turned down when not occupying the area, and thus would want enough extra BTUs to warm it fast when needed.
Not going to heat a slab fast. Ain't going to happen. If you need fast recovery, you need scorched air or fin tube. Trust me on this.
The difference between keeping the temp at 70f or 45f is 4470btuh.
Ct has a current delivered price of about 23.00 per thousand cubic feet of gas. Works out to about 2.50 a day at design temps or probably about 1.25 on average over the heating season to keep it at 70 vs 45.
Betcha milkbones he sets it at 65 and keeps it there.
Which is one good reason why I'd probably go for the warehouse heater.
Based on the dialogue you provided in this one response, you clearly really don't have much idea of what is going on. You certainly don't know how to say it and in this format, you might know the world, but if you can't express it, it's worth little. I've seen energy codes that are designed to accomodate 'minimally heated spaces' that don't require full comfort conditions if they are to be heated at all. I don't know about the IRECC, but I'd suspect it has similar provisions of other energy codes I've worked with.
I actually did a calc on the OP's garage ... 20 x 30 x 11 ... his dimensions and aproximate average roof height and his R-values with some reasonable assumptions about windows and doors.
Again, Your 75KBtuh is not even the same as the very reference/link you provided for us to look at ... you are 36% higher than the referenced link.
Yeah, actual conditions can exceed the official design temperature used ... duh.
You mean I have the option of not heating to e.g. 68 deg? Duh.
I must say, it appears that you understand some fundamentals, but you are misinformed on other aspects of heat loss and/or energy codes.
Sorry
is an apt description. There is no way, with the ASHRAE design conditions for CT that this space will require more than 1/4 of what you term as "reasonable and conservative". That is to heat the space to 75 degrees.
Please explain how you arrived at this grossly incorrect estimate, then I might be able correct your misunderstanding.
This is how heat loss is figured. Worst case design condition heating loss includes transmission and infiltration, with no credit for solar heating
IF there are three walls at 30 x 10 with an overall R-value of 20 (this would account for some windows), the heat transmission is: q=UAdT=.05x300x75x3=3375 btuh, a ceileing at 600 sf and an overall R value of 30=.033x600x75=1485, say the side with doors has an R-vaue of 5 and is 10'h x 30'w; q= 4500 btuh. Account for 1 air change an hour ACH: q=Vx1/60xACHx1.085xdT=6000x1/60x1.085x75-7200 btuh. This totals 16,560 btu/hr to heat the space to 75 degrees in high wind and outside air temperature of 0 degrees. THIS is a very conservative number that assumes 4 exposed walls, oversized walls and a high infiltration rate. I have made the assumption that slab will be insulated underneath and at the perimeter with at least 1" rigid foam which will practically negate perimeter losses.
Your "very reasonable and conservative" estimate is nothing of the sort and is off by a FACTOR of 4.5. Again, not even close. I sincerely hope that you do not make a living victimizing people in any industry related to heating and cooling. I have to surmize, given your abilities with math and heat transfer, that your understanding of the Connecticut Energy Code is equally "unique".
I can't help you with the btu requirements, but I can share my system with you. I'm in a much more moderate climate: PNW. I put radiant into my new shop (30x40 ft.) and use an Eemax brand on-demand electric water heater. This is both thermostatically and flow controlled. I can set the maximum output temperature and it will modulate its own output to sustain that. (as long as it is working within its own design parameters) Control is very simple: a single Taco relay is activated by the wall t-stat, the relay activates the circ. pump, once flow is established the Eemax turns on. There is no mixing valve needed since the heater will maintain a set output temperature. The Eemax unit cost about $250. I'm not getting real fast response times going from a cold slab to warm but it will maintain the set temp once it gets there. We just had a four day power outage and the shop temp dropped from 63º to about 40º. It took a good 24 hours to get it back up to 63. I have R19 walls and R38 ceiling.
Are you installing perimeter and/or under slab insulation?
I use a tankless water heater
to heat my garage, via infloor tubing and some fin-tube convectors along one wall.
The heat loss numbers are in the right ball park, the heat source will be oversized no matter.
These are the good things about this approach:
It (Bosch AquaStar) is a lot cheaper than a boiler and is made for low incoming water temps
Mine was free or to be sold because I hated it as the main source of hot water for my house
Even the cheapest gas tankless units modulate to maintain a constant leaving water temp - modulation/turndown in this application is good.
Controls are easy - line voltage thermostat to start the circ pump, everything else is in the unit standard (this applies to any water heater based system)
Wall mounted unit takes up little space and the installation is very "clean"
There are also some drawbacks:
A tankless, power-vented unit is 3 times the cost of a suitable tank GWH, with gravity venting. IF an inexpensive initial solution is the decision point, use the tank-type. The heat source can always be upgraded should that be necessary.
With the unit on low fire to keep the supply temps down (bare concrete you will be looking at 100 degree supply water, give or take a few degrees), mine does not operate consistently, i.e. needs to be "re-started" from time to time.
These units are extremely inefficient. At design use, 55 degree EWT and 125 or so LWT, they claim 80% AFUE on the best day. In this app, EWT is 85, LWT is 105, AFUE drops down below the 70's.
Honestly, for a seldom/semi-occupied space, the HotDawg is the most practical option. IRT gas heaters would be my second or third choice tied with hot water IFH. 600 sf is not that large and when your busy turning wrenches on "your baby" as long as your not freezing, who cares...
Thanks for all the replies, guys! I hadn't gotten any emails informing me of posts, so that's why I haven't responded sooner. I'll check my settings, perhaps email notices got reset in one of the board changes.
So, More info:
I was thinking the set and forget temp would be about 55 or so in the deepest winter. I assume the floor would be nearly constantly warmer than that, and working on or near it would make the place "feel" warmer than it actually is. Also, I figure he can come home Friday at 6, crank the setting to 70, and by Sat AM, have it pretty darn comfy in there, and set it back Sun night.
I realize, and so does he, that it won't be a 'fast response" system, and it doesn't need to be.
The slab will be all new, and it will have a couple inches of foam under, plus permimiter insulation. (My understanding is that the major heat losses occur at the perimeter of the slab). We'll be sure to insulate that area deep.
My thinking was that the normal tankless HWH would have sufficient BTU capacity. My calcs assume the insulation as stated, and a design temp of 5. It rarely gets that cold here, and frankly, if it's 0 out, he won't mind 65 inside! The walls are getting a hybrid closed cell/batt system, so things should be pretty tight.
I'm looking at spending a grand or so on the heat unit. less is better, more is probably a deal breaker. However if he buys this year, he gets a 30% tax credit. The incremental cost of radiant is the fittings and tubes and labor. The Modines run about $800 or so, IIRC, so that's nearly a wash. Getting the tax credit helps make the radiant more attractive.
Also, it's a lot quieter than the modines, and far more comfortable. (I have a Modine in my own shop and LOVE being able to work like a human in shirt sleeves, but don't like the noise and airflow.)
You guys seem split on the BTU requirements! Interesting! (Unexpected too)
Glad to hear somebody else is using a tankless as a source. The one worry I have is the possibility of constant or near constant use. What kind of costs have you seen in your use? I'm trying to guess what my cycle times would be. A typical winter day here is say 35 degrees as a high, 22 as a low. To maintain 55 that's about a 30 degree rise. The garage will NOT see daily opening/closing. So, is there a way to see what my % of load would be? If the thing will be burning fuel 22 hours a day, we're in trouble.
One note ... heat loss is linear w/ delta T. So if you're setpoint is say 65 and the design temp is 5 degF and say your load at those conditions are 25KBtuh, .... the load at say 35 deg is exactly half ... or 12.5KBtu. You're hinting at the down side of the tankless ... it kicks on a lot. Not sure if that is an issue. But as the other poster said ... a LOT more expensive than a tank water heater.
I had a guy tell me my tank style heater would kick on every time I needed heat in my slab ... which isn't true. If you maintain e.g. 110 deg in the tank, you only need part of that stored heat during mild weather to keep the slab idling at say 85 deg (or whatever is typical which I'm not sure of).
I have not
separated the cost for the garage heat from the rest of my heating costs, so I don't have an answer for you there.
It was 22 degrees this morning with high winds at my house and my unit did not fire during the 30 minutes I was in my garage. The inside air temperature is 51 to 52 degrees. I have a 20 x 30 garage with two 9x8 overheaed all wood doors, two double hung windows, R-13 in the walls and R-25 in the ceiling.
I have a 4000 gallon swimming pool, 16 ft diameter, that is a huge heat sink (I house my koi in the garage from Dec thru Mar in the pool) in my garage that moderates temperature swings.
Your space will require less than 20,000 btu/hr to keep at 65 degrees if the doors are closed and the OAT stays obove 0. The average GWH is rated at 40 MBH input. My tankess is rated at 175 MBH input. At 5:1 turnown and 70% efficiency estimation, the lowest output I can acheive is 24,500 btuh.
Depending on the slab thickness (4 to 6" right) and the underslab and perimeter insulation (1" of rigid foam will be very adequate) in CT, I would expect the average "burn time" to be on the order of 3 to 4 hours a day. Though response is "slow", even with my pool, I can raise the air temp from 50 to 55 in 40 minutes with a nice warm slab underfoot, should I need or want a warmer space.
garage heating
I followed this thread with some kind of amusment
You have a garage where you would want to work on your "baby" in your sparetime and not working in freezing conditions.
It appears to me that the time during the whole year when the temp is becoming uncomfortable is relatively short.
My finish carpenter had a garage (ca 20x20), insulated walls, cold floor. When he needed to work in it during times when the temp was below 12C (53F) he used 2 electric "ceramic" heaters (1500W each), cost 40$ each. He did not work in the garage on a constant basis, only a few days/mth but about 8 hrs during that time.
I take it the hotrod operation is not a business but he will work on it maybe a few hours during the evening or on the weekend.
When the outside temp is above 55F he does not need to heat the garage at all. And cars do not need to be heated . We lived for over 20 years in the cold Canadian prairies, yes, we had a garage that was insulated but it was always unheated, the cars always started. When I really had to work in the garage I left the door to the house interior open and was wearing a sweater and warm socks
To me, this was a more economical solution rather than spending thousands on a heating system.
I remember a guy in Calgary with whom I was "wrenching" in his garage. He had a turntable in the floor to move his car in any direction, his tools were assorted on green velvet the garage was nicer than someone's livingroom but even he never saw a need for a heated garage
Form follows function
:=)
No this is a non permit job.
In thinking about the cycling, I recall that thermostats have settings that can be used to control cycling. Alowing the temperature to run over a few degrees, then run under a couple before triggering might keep the unit from running on and off constantly.
Using a tankless does seem to me to have, in this case, certain advantages. One of course is the decreased cost compared to a conventional boiler. But the major one is that it's designed for low output temps. I can see setting the output in the 100 range. With a boiler I'd need mixing valves. And i imagine the return will be in the 50 range, so it should be well within it's comfortable range of operation..
Better Check Again
You had better check again.
Connecticut Energy Code says differently.
http://www.energycodes.gov/states/state_info.php?stateAB=CT
Permits Required
Permits will almost assuredly be required to heat this space.
And with that comes compliance with the typical state mandated minimum energy code requirements.
For Connecticut, that means compliance with either the 2006 International Residential Code or the 2006 International Energy Conservation Code.
That also means designing to a continuous conditioned area minimum heated interior space winter temperature of 68F when the thermal envelope is insulated to at least R38 for ceilings, R-19 walls for walls and R-10 for foundation walls in slab on grade construction.
This means the design load would remain at about 75,000 BTU/hr to maintain a 68F temperature based on the volume of the room.
http://www.herman-nelson.com/BTU_calculator.cfm?type=F
Just because on can easily be comfortable working at 55F or lower does not mean you can design a conditioned building to be built to only provide a maximum 55F or lower. You can't.
All due respect, but I think you are waaaay off base with your heat calcs and design criteria on this one, and if you want a code compliant structure for Connecticut, you need guidance from your local code professionals, not strangers on the internet.
You need to be talking with your local mechancial code and energy code inspectors for this one.
Your design as proposed does not meet code required minimums anywhere the IRC or IECC is used.
I beg to differ
"This means the design load would remain at about 75,000 BTU/hr to maintain a 68F temperature based on the volume of the room.
http://www.herman-nelson.com/BTU_calculator.cfm?type=F
Just because on can easily be comfortable working at 55F or lower does not mean you can design a conditioned building to be built to only provide a maximum 55F or lower. You can't."
Your reference 1st of all is lame. You give them a volume and they give you a heat loss??!! Give me a break. 2nd, your own calculator says about 55KBtuh, not 75 for this man's situation.
As for designing for a different setpoint temp inside, that may be debateable. A shop may be able to be designed for a lower comfort level. It's not like a normally occupied space ... and it may indeed be an option.
Local code professionals are not the ones to 'give advice' about heat load calcs, necessarily. In fact many code professionals don't give advice period (and often for good reason). Personally, if he wants to make sure, you are right ... he should seek the guidance of e.g. a professional mech. engineer who can do the calcs and do them right and not rely on us strangers on the internet. After all, we only have a smattering of details provided by the OP. But given the details he provided, I estimated a heat loss at 70 deg setpoint (WAY high for a radiant system IMO) of around 32KBTu. As I indicated earlier, I calc'd a load on an 1,800 sqft house built back in the 80s at 35KBtu in a cold climate,
Your comments are a bit strong and maybe YOU should consult a professional yourself.
Your link, IMO is virtually worthless as a calculating tool beyond a rough calc and even then, it appears that it makes some huge assumptions that the user is not privy to. So to the casual user ... e.g. the OP, he could end up buying a system that is WAY oversized for his needs ... even more so if he feels compelled to go to the next larger available unit and e.g. install a 60KBtuh unit to meet the projected load.
Back Down Mr. Manhattan
You need to stop using that BTU calculator you are linking. t's a totally useless calculation for sizing a heating system. You are basically just making up a number that has *no relevance whatsoever* to a heat load calculation. It doesn't even allow you to specify an R-value for anything!!!
I'm going to say this as plainly as I possibly can: You have no idea whatsoever how to calculate a heatload. You may know some codes, great. But you have NO IDEA whatsoever what the numbers you are linking are saying. the pile of people telling you this are right. You are wrong. It's time to accept this, learn from it, and move on.
For reference, and to back up the other posters: I have designed hundreds of heated garages and well into the thousands of home heating systems for buildings from the north coast of alaska to southern texas and coast to coast. You want to know how many times I have seen a BTU/sq ft requirement over 50 BTUs a square foot? I can count them on one hand. Never in garages. Especially not with spray foam. Not usually even in basically unheated houses in new england! That's the realm of greenhouses and sunrooms that are all glass.
If you needed 50 BTUs/sq ft to heat a space, his feet would melt into the floor. Typical heat load for a garage, a leaky one, is 20-25 BTUs/sq ft at a 70 degree room temp.
Regarding this garage:
If this fellow is ok with maintaining a set temperature, I would *without hesitation* recommend a tank water heater, though many codes will prevent that unless the heater is doing domestic as well. use a heat exchanger.
If it needs to recover more quickly from time to time, I would recommend 6" o.c. pipe, relatively short runs, and a tankless that HAS A WARRANTY WHEN USED AS A HEATER. Not many meet that criteria. This would allow heat up time for the slab to be reduced.
otherwise, electric boiler or a cheap mod/con.
Primary Tubing
I will throw in my two cents...Go ahead and put in the tubing and zone valves. I just did the same thing in a 2100 sq foot basement. Even if you change your mind later, you can NEVER go back and do it.
I used 7/8" tuning from the radiant floor heat company in Vermont. Good comapny and I am not soliciting business for them, just worth the call, and they know the codes as well in the northeast.
1/2 would have been enough.
never do this
http://www.pmmag.com/Articles/Column/31ed5e1f0bfc7010VgnVCM100000f932a8c0____
there is never a good reason to use gigantic pipe like that. You are far better off using more, smaller pipe at a tighter on center.
Or use smaller pipe at the same on center and save even more money, if you have a heat load since the larger pipe has practically no effect on heat output at all. all it does is allow a higher volume of water flow which has a minimal impact on heat output: worth thinking about in some cases, but nothing like some of the claims some less than fully educated radiant salespeople make.
I personally consider using large pipe in non-commercial spaces as a sure sign of a hack designer or an overeager salesman.
Thanks for the new responses guys.
Yes, the 75K BTU recommendation freaked me out. I reran my calcs and came up at less than a third of that.
This job is for a friend of mine. He currently works in his 'barn' in the cold with a salmander type kerosene heater. He cranks it to about 50 or 60, shuts it off, and then turns it on when he's cold and the fumes have disipated.
I find it odd that I'm being told that my suggestion that he rid himself of that health hazard and install a more legit system is an absolute no no. But it's ok for him to breath kerosene soot. LOL. And that he must do his work at 70 degrees.
Annnnyway, he has decided that no matter what, we will insulate the new slab and run the tubes. Knowing him, he's going to go ahead with the 'next step" once he sees those pipes. He's now considering adding some windows for light, and adding some more (15 x 15 x5) heated space.
Appeciate the calcs and advice you guys have provided. I feel much better now about this than I did when I read the "Strident and skeptical" posts.