GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Radiant heat slab in a wood shop

MCooper | Posted in Mechanicals on

I have an 1800 sf woodworking shop built on a 6″ concrete slab, insulated underneath and around the perimeter with 2″ of foam. The building is well-insulated, well sealed and has few windows.

I’ve been using a Takagi Jr. to heat it for the past couple of years but it’s been no end of headaches. I only have propane and electricity to work with, by the way.

I’d like to figure out a better way to heat the water, probably a direct vent system so that the wood dust doesn’t create problems with the burner (just one part of the TK jr. issues). It seems that my options are either a better version of the current on-demand hot water heater/boiler, or a tank-style water heater.

Are there other options I should consider, and are either of the above ideas impractical?

Thank you in advance for your input.

Upstate NY, near Ithaca.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. davidmeiland | | #1

    What are your costs for propane and electricity?

  2. MCooper | | #2

    Hi David,

    Looks like I'm currently paying 2.25/gallon for propane and 0.12/kwh for electricity. (Unfortunately, solar is not an option here, given the site.)


  3. MCooper | | #3

    Hm, I see now that there is a pretty similar thread happening in response to a question by Neil B., that seems to be coming to the conclusion that a ductless mini-split is the best way to go, in spite of the wood dust and the existing plumbed slab.

  4. davidmeiland | | #4


    I'm not sure about the efficiency of your slab as distribution, so adjust inputs as needed.

  5. MCooper | | #5

    Thanks, David. I have no idea what the efficiency rating for the slab would be, but this is useful as a comparison nonetheless if I set both examples to baseboard heat.

    Any thoughts on using a tank water heater vs. a tankless for heating a slab?

  6. wjrobinson | | #6

    Tank units rarely have issues. Simple installs available from manufacturer. Dust free room might be nice with furnace filters separating utility space.

  7. MCooper | | #7

    The dust thing is a real issue. I've been assuming that I'll need to install some kind of direct vented unit that draws combustion air from the outside. It's really the only reason that electricity started to seem attractive.

    The tankless has the advantage of being able to be mounted on the wall and not taking up precious floor space, but if it's a matter of hundreds of dollars of savings, I could live with a tank. Are tank water heaters generally able to keep up with the demands of a heated slab? Might require a slower pump, I'd imagine.

  8. davidmeiland | | #8

    A tankless heater is generally going to be able to put out a lot more hot water than a tank, because that's specifically what it's designed to do. If you're going to use one for floor heating, it might be worth talking to the manufacturer. I have seen a lot of tank heaters used for floor heating, and they seem to work, assuming your building doesn't need more BTUs than they provide. You have to evaluate first cost, efficiency, expected maintenance, loss of floor space, etc.

    We have much cheaper electricity (and more expensive propane) so electric boilers are common. It's like owning an old VW Beetle, very simple and reliable.

  9. wjrobinson | | #9

    Thousands of tank water heaters are heating slabs. Tankless too. Tanks are simple. Search Chandler for his systems. Check out Web sites. Radiantec etc. They design, sell, all you need. Not sure why your present unit is not doing the job, explain.

  10. MCooper | | #10

    Well, that's a complicated answer. The short answer is that the TK Jr. worked alright at first, but now gives a couple of different error codes, fails to fire sometimes, and seems to use a lot of fuel. I've spent a long time on the phone on several occasions with the supplier (which happens to have been Radiantec) and have never really worked it out.

    I have looked at many websites, of course, and am here to hopefully get unbiased information and recommendations from someone other than the vendors of the various units.

    As for 'Chandler'; can you be slightly more specific? Is that a person/contributor here? A manufacturer? Googling just gets me to Chandler AZ and the plumbers there.

  11. davidmeiland | | #11

    Chandler is Michael Chandler, one of the experts here.

    As far as tankless for floor heat, I've checked out a couple of systems recently that used various brands, and they seemed to me to be short-cycling. That's probably a control issue, but would result in high fuel use.

  12. GBA Editor
    Martin Holladay | | #12

    Although several people have told you that it is common to use a tankless water heater for space heating, I have heard of many problems with such set-ups. Tank-style water heaters and boilers are more dependable ways to provide hydronic space heat.

    Ductless minisplits are also an option. I agree with your conclusion that, if dust has been a problem in a past, any combustion appliance should use ducted outdoor air for combustion.

  13. MCooper | | #13

    Thanks, Martin. That's the direction I'm going now. I've looked at the Radiantec website (where I got the original parts for the system) and they like the Polaris water heaters. They look pretty nice, but they're very expensive ($3000+), more than I'm going to spend. I figure that knowing the system capacity in gallons and the pump flow rate, I could determine if a given water heater is going to have the recovery rate to keep the water warm. That, plus the requirement that it be direct-vented and use outdoor air for combustion, will narrow the selection I think.

    Any thoughts as far as choosing between a boiler and a tank water heater? Or recommendations of models to look at for either?

  14. GBA Editor
    Martin Holladay | | #14

    If you are trying to save money, you should shop around for the least expensive sealed combustion water heater with a BTUH rating that meets your needs.

    The fact is, I'm not a fan of in-floor hydronic heating systems. The components are expensive for what you get. If your alternative is an inefficient propane water heater, a ductless minisplit starts to look attractive.

  15. Expert Member
    Dana Dorsett | | #15

    The cheapest way to avoid dust issue for the TK-Jr is to buy the direct-vent conversion kit and draw combustion air from outdoors (as should be done on ANY tankless installation, IMHO.)

    Properly set up without over-pumping or over-cycling the tankless, and at output temperatures & delta-Ts where it modulates, it should be able to deliver reasonably good performance for at least a decade, but some of the "one size fits all" Takagi based slab heating systems are extremely abusive to the water heater. If you don't already have one, install one of the wired remote controls on TK-Jr, which reports output & return water temps, as well as flow rates. Flow on the tankless loop should be set to no more than 4gpm (1-3 gpm is kinder), with a delta-T on between entering & output temp somewhere between 25-85F. On many (but not all) systems this may require a primary/secondary configuration with a separate pump determining the radiation flows (which may be higher volume, lower delta-T.) If you know the actual heat load of the place (which could be determined by fuel use against heating degree-days) it's possible to optimize both the tankless and radiation flows so that the unit spends most of it's life modulating rather than cycling, which is the only way to get the total ignition cycles per year numbers and burner firing rates down to something reasonable. If it's always blasting away at 75%+ of max-fire with 10 cycles/hour the odds of it lasting a decade are pretty remote.

    To get the cycle rates down to the same as a domestic hot water application you need to get it down to about one burn per hour, which should be possible with high thermal mass radiation like a 6" concrete slab. If you set up the delta-T and flow on the tankless to match the anticipated peak load and control the system with a pretty good PID algorithm thermostat set for a constant room temp, the thing SHOULD do pretty well. If the flow x delta-T on the tankless is more than 2x the peak load to the building it's abusing the tankless for no good reason. (For napkin-calculation, 2gpm is about 1000lbs of water per hour, BTU/hour is then delta-T x lbs/hr. So if your peak load is 40,000BTU/hr and your flow is 2gpm, you should be running with a delta-T of 40F at the water temp required to deliver the heat out of the slab, which has to be calculated separately based on the radiant design, or determined impirically by where it fails to keep up. Higher delta-Ts are much preferable to higher flows for achieving the heat delivery- these things are designed to handle 85F delta-Ts with some margin in hot water heating applications (35F water in, 120F water out.) The worst thing you could ever do to it is set up for 8gpm flow and a delta-T of 10F to deliver the same 40,000BTU/hr.

    But I'm sure you did all those calculations when you set it up, right? :-)

    At $2.25 and ~82% efficiency you get about 75,000BTU/gallon out of the TK-Jr, for $2.25, which is $30/MMBTU. With 12 cent electricity and 3412 BTU/kwh an electric boiler would deliver the heat at $35/MMBTU. With a properly sized mini-split and 12 cent electricity you'd be paying $8-16/MMBTU (local climate and season dependent), plus maintenance, which may be significant if you're going through a filter per week due to heavy dust accumulation.

  16. wjrobinson | | #16

    Matt, I deal with systems like yours now for decades. My post 6 is the answer. Buy local. Build a dust free utility closet. Standard tank units are bullet and idiot proof. Just installed an so Smith 50 that cost $1000 to customer. PVC vent and intake. 4 hours done.

    Burn some wood or pellets if you need less expensive fuel. Turn tstat down some. Raise your prices.

  17. Expert Member
    Dana Dorsett | | #17

    Also- just to be clear, all good heating solutions begin with a calculation of the load at the 99% outside design temperature for the area, which is 0F for Ithaca NY:

    With high-mass radiation and high R-values, few windows you could even "cheat" and design to a higher temperature, since it could go many hours before the instantaneous load shortfall of the system turned in to a comfort issue.

    But since you have a heating fuel history on the place, you can MEASURE the heat load based on how much fuel the TK-JR goes through per heating degree-day. That will tell you the surface temp needed for the slab to meet that load, and the tubing layout/design would then tell you the peak water temp needed to deliver that amount of heat, etc.

    Your true heat load number would be critical for sizing a ductless mini-split solution as well.

  18. MCooper | | #18

    Wow, thanks everyone. I've got some math to do, clearly. Can't sit down today, got a town board meeting tonight, working all day tomorrow... looks like Tuesday evening is my time to digest all this.

    Very helpful, all of this. I'll reply in greater detail as soon as I can.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |