Radiant slab in garage: boiler vs. tankless vs. tank
Hi all, first post. I have been doing a lot of research (on this site and others), talking with manufacture’s, etc. about my setup. Nothing too complicated, I have a new 36×30 detached garage. Walls are 2×4, 22′ of the 36′ is attic space with 2×8 joists, the remaining 14′ is cathedral 2×12 for a lift someday.
I have calculated my heat load (albeit 4-5 months ago), and if I recall it is in the range of 30-35K BTU/hr. This might actually be a little on the high side (but sticking with it as this is a garage with overhead doors, etc.), but I did put into the software as much accurate info as I could for insulation, windows, doors, ceiling, slab insulation, etc.
The 6″ slab floor is enveloped with 2″ XPS on bottom and sides. I have 4 loops (single zone) of 1/2″ oxygen barrier PEX around 240′ per loop, terminated at a stainless mainfold with 3/4″ fittings to/from heatsource. Everything came from BlueRidgeCompany.
I know guys are getting away with tankless hot water heaters. I actually have a very good friend that has his up and running, and it seems to be working great. I understand that boilers are purpose-built for this application, but at 4x the cost I am seriously considering going this route as well. Takagi’s are rated for radiant (albeit a lowered warranty period). I am looking at the Jr2 & K4. K4 has more max BTU (unneeded/overkill) but it also can modulate to a lower BTU (more interesting to me).
I understand the head loss associated with tankless, and that a beefier circ pump is required. These particular units have a min head loss of around 10ft, which is the *max* that the Taco 007 can handle. So, immediately that pump is out.
More to the point, what I am struggling with is that these heaters modulate both the water flow as well as the burner. I think boilers only modulate the burner (assuming you have a boiler that can modulate to begin with). I don’t know this for sure about boiler operation, but it makes sense. I want to be able to keep a fairly consistent flow rate, but worry that this won’t be possible with the tankless.
The other is on the high end, the BTU output is way, way overkill for my space. I understand this, these things were designed to bring 50 degree water up to 120 degrees instantly. That requires a lot of heat. For a radiant floor with a delta T of say 20-30 degrees, is it possible for me to dial a tankless down so it is not short cycling and causing large temp swings?
Barring the head loss, some basic questions — can the flow modulation be disabled so that it can be consistent (i.e more like a boiler)? Can the water temp output set point be set low enough so that the unit runs at a lower BTU longer, instead of high BTU shorter?
I didn’t mention tank style hot water heaters, but I have sorta thrown them out due to the space requirements (although I do have an attic…). There would be no head loss, and the BTU is more in-line with what I really need. But, longevity is a concern.
Thanks!
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Replies
T.G.,
I don't know if you've already read them -- but here are links to two relevant articles:
Using a Tankless Water Heater for Space Heat
How to Heat a Garage
Martin, thank you. I cannot look at the first article. It appears that it needs a subscription. The second confirms further that a tankless will do the trick.
Maybe I am overthinking this? (would not be the first time!)
I have started looking at a primary/secondary plumbing config. A few advantages, but some disadvantages (two pumps instead of one, little more complicated setup, larger footprint on the wall).
It is a garage, so 100% comfort is not the goal. I just want the mechanicals to be reliable.
T.G.,
If you "want the mechanicals to be reliable," it may be worth investing in a one-month subscription to GBA so that you can read my article, "Using a Tankless Water Heater for Space Heat."
A one-month subscription to GBA costs $14.95 -- about as much as two ball valves for your new heating system. You can also get a trial subscription for free. Here is a link to a page with the details: GBA subscription options.
Martin, yes I was just looking into this. Thank you.
I read both articles. The first, combi-systems, are far more complex than what I need to deal with. Some of my concerns were echoed in that article as well. Neither of these really address my questions/design specifically related to system design/configuration. Thanks, I'll keep digging.
T.G.,
Among your questions are the following:
1. For a radiant floor with a delta T of say 20-30 degrees, is it possible for me to dial a tankless down so it is not short cycling and causing large temp swings?
2. Barring the head loss, can the flow modulation be disabled so that it can be consistent (i.e more like a boiler)?
3. Can the water temp output set point be set low enough so that the unit runs at a lower BTU longer, instead of high BTU shorter?
I'm not sure whether these modifications can be made -- but I have heard enough stories about the difficulties of using a tankless water heater for space heating to suggest that if you need modifications on this level, you have probably chosen the wrong appliance.
Martin, yes I agree. I asked the questions more towards -- "Has anyone done this successfully/reliably". I would rather not be the first. I am investigating the primary/secondary approach. This appears to solve some of my concerns. But, again still agree that this will require more "fussing around" than with the appropriately designed appliance. Thanks again.
I don't remember details clearly enough to provide a link that might help, but I've heard of others managing temperature swings with a small buffer tank (typically an unpowered electric water heater). Of course, once you add in the space and cost of a 10 or 20 gallon buffer tank, it becomes even harder to justify not simply using a 30 gallon water heater.
Do not use the tankless water heater . It will require extensive maintenance and when it does fail , and it will , you will be w/o warranty assistance from the manufacturer . Look into the Phoenix light Duty water heater from HTP . It is not shown as a heating appliance but is shown used for water / air coils . The reason it is not shown as a space heating application is simply because the base unit was licensed some time ago to Warmboard who uses this as a space heating appliance . Conflict of interest . Look up Warmsource , look up the Phoenix light duty to confirm these things . Mass for your radiant system , modulating and condensing , low head requirements , so MUCH LESS pumping power consumed .
Use the right stuff first and relax while not having to worry about when it will break down .
Tim, thanks. Yah, there is a lot on the net about this stuff. However, I don't find info on 5-10 year reliability reports with a tankless setup for my situation. I know they will "work", but for how long?
Richard, thanks for the info. That looks like a Cadillac (or if you prefer Lexus...) water heater, wow. At that price I would probably just do a wall hung boiler (a real boiler). I am not sure I can justify the cost or space requirements.
You've posted this question on another forum, and hopefully you've already read my considered response there. Your real heat load is less than half that stated, and a tankless hot water heater is a dreadfully poor choice for a slab heater when the real heat load is under 15,000 BTU/hr. The TK-4's min-fire output is about 10,000 BTU/hr which is clearly above your average load, and there's really no stopping it from cycling on/off- it's modulation range has limits.
So what's the "real boiler" wall hung solution going to look like? Even the lowest modulating mod-cons only go down to ~7500 BTU/hr out which is also probably higher than your average winter load, and surely higher than your full heating seasonal average. Since it would only be modulating on the coldest of nights, and essentially never during the day, there's no real benefit to a modulating gas-burner that never modulates.
Richard's recommendation of using the Phoenix Light Duty is a good one, since it's inherently self-buffered, much simpler and as a stainless steel tank should last longer than any tankless in a space heating application, and would perform at LEAST as well as a mod-con. And it would save on pumping power use. If you want to cheap out, you could do the same thing using the smallest Vertex, but with the glass lined tank it would probably crap out on you in about the same amount of time as a tankless HW heater.
On that other forum I also pointed out that a 2-ton Chilltrix air source heat pump/chiller has sufficient capacity at your +5F-ish 99% outside desgin temp and 100F-ish water temp requirements, nearly ideally sized for the load. A heat pump with a COP of 3 would be cheaper heat than condensing gas, and would have a lower net carbon emissions output than condensing gas at your state's Clean Power Plan target.
Whether heat pump or gas fired, if you paid a competent experience person to actually design it for you it'll pay back in both higher system efficiency and saving unnecessary or inappropriate hardware purchases.
At your low electricity costs I'd be inclined to mothball the hydronic floor and heat the place with a Fujitsu 12RLS3H or 15RLS3H mini-split, which would also be able to air-condition the place.
http://www.fujitsugeneral.com/PDF_06/Submittals/12RLS3HSubmittal.pdf
http://www.fujitsugeneral.com/PDF_06/Submittals/15RLS3HSubmittal.pdf
With a mostly-DIY installation (pay a qualified tech to do the line purge, refrigerant fill and commissioning tests), you'd be able to pull it off for about $2500.