Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Tankless Heater for Radiant

SeattleRadiantQs Member Posts: 3
edited September 2021 in Radiant Heating
I live in Seattle area in 2.5 (1, 0.75 and 0.75) bath home with 2 kitchen sinks and 2 laundry rooms. The main and upper floors are heated with ductless radiant that are working sufficiently. I am remodeling an existing 800 sq ft basement I would like to heat with radiant heat using warmboard for radiant heat piping. My plumber has recommended a tankless Navien NPE240A to be mounted on the exterior of the home. Is this a good unit for this job?


    HVACNUT Member Posts: 5,828
    edited September 2021
    Agree with @kcopp.
    That's not a boiler, it's a water heater. There's zero benefit, especially with low temp radiant.
    The space shouldn't need more than 20K BTU so maybe another ductless, or a small electric boiler with radiant if feasible. 
  • Derheatmeister
    Derheatmeister Member Posts: 1,528
    edited September 2021
    Never heard of "ductless radiant"...
    Regardless, Water heaters are not a good match for a space heating application
    We had to remove a couple systems of such design for the following reasons
    1. The inlet strainer/filter would clog up causing the waterheater to lockup (No heat calls).
    2. The waterheater was very hard to adjust for altitude.
    3. The waterheaters supply temperature did not match the outdoor temperature (Out door reset) which caused the home to overheat in milder weather (50F) and underheat in sub temperatures(-30 F )
    4. The anual fuel consumption was approx. 30% to 40% higher than a Modulating condensing boiler which will base its supply temperature on Outdoor reset.

    We have also seen waterheater setups using a tekmar outdoor reset control but this still does not make any sense and could be compared to trying to adapt a turbo onto a Yugo engine and then installing it into a Porsche. :s

    BTW..199.000 BTU/HR is way to much Boiler if you are just trying to heat a 800 sqft basement.
    Even if this waterheater is at its low output at will have approx 20.000 BTU/HR which is still Grossly oversized, This will cause the equipment to short cycle (Stop and Go) which is one of the main factors of premature system component failures (Gasvalve,fans,blowers,relays/Controls)

    It is better to just pay a little more to avoid the problems,have a better comfort level and save energy.
  • Wow! I'm overwhelmed by the speed and quality of responses. Sounds like the websites out there advocating tankless water heaters for radiant heat are inaccurate or outdate information. Glad I found this site with professional willing to share their information.

    Derheatmeister you are correct. The term radiant has gone through my mind so much recently I've attached the term to my electric ductless heat pump. The ceiling in the basement is already just under 7' so I'm trying to avoid any ducting in the area. Hence my focus on radiant heat for an area that will be used as an office or play area.

    Your information brings up another questions. Are boiler systems used for hot water supply as well or is the temperature from boilers too hot and require separate systems hot water supply?
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    For you last questions -- "it depends". On several things. A boiler -- pretty much any boiler -- can be controlled to produce very hot water. However, they can also be used to supply much lower temperature water, such as would be used by a radiant slab, simply by mixing some of the cool water returning from the slab with hot water from the boiler. However, for best efficiency, some boilers -- called "mod/cons" -- are constructed with materials which can handle much lower temperatures themselves on the fire side. These can be controlled by varying -- modulating -- the burner rate to match the heat demand, often by sensing the outdoor temperature -- and producing water which is just warm enough to supply the correct amount of heat for the structure. If properly sized and set up these can be remarkably efficient.

    Any boiler can be used for domestic hot water supply as well, and that often is a reasonable way to get hot water. What is done is to run hot water from the boiler through a separate storage tank with a heat exchanger in it to separate the boiler water from the domestic water. These are called "indirects". They too are quite efficient -- usually more so than a stand alone similar water heater.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,840
    An 800 sqft basement in a mild climate like Seattle is a very poor application for a radiant floor because the heat load of the basement will be extremely low, maybe like 8,000 btu/h for a handful of hours per year. This means that the temperature of the floor will be extremely low as well (like 72 degrees), basically cancelling out all benefits of a radiant floor. Not to mention that you'll spend a ton of money (~ 5 figures) on warmboard and an appropriate boiler (as described by the other responses). I'd price out electric baseboards for the basement or a ductless minisplit if you want AC down there. You'll get just as much benefit comfort wise for probably $10k-20k less.
  • Thank you Hot_water_fan. The main criteria for our design is to limit loss of ceiling clearance and try to avoid a cold floor since it'll be a play area for children. That is how we ended up on radiant heat over mini-split (ceiling clearance) or baseboard (cool floor).

    The recommendation from our interior designer was for electric radiant. I was discouraged by this because Seattle electric rates increase based on energy consumed and mini-split is already costing more than my old 80% efficiency gas furnace it replaced.

    Your summary clarifies electric radiant operations cost will not overrun install cost of gas radiant for decades. The finances for this small space seems to work out in favor of electric radiant.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,840
    @SeattleRadiantQs radiant floor is still problematic for comfort - the floor will be basically room temp. The warmboard technical documents show that a 75 degree floor will give you 10 btu/sqft. It's possible your heat loss on the coldest day of the year is below that number (basements have low heat losses compared to above ground floors), so you're looking at 72 or less for 80% of the winter. A rug would feel warmer. Take another look at minisplits - not the ones mounted high on the wall, but the ones mounted lower on the wall, like this one:
  • Pollymath
    Pollymath Member Posts: 9

    I was discouraged by this because Seattle electric rates increase based on energy consumed and mini-split is already costing more than my old 80% efficiency gas furnace it replaced.

    Wait, do you already have a MiniSplit elsewhere in the house?

    These are the questions I have concerning keeping my existing hydronic system or upgrading to MiniSplits as heat pumps. Electric prices are not going down and natural gas prices are staying relatively stable at the moment. While certainly heat pumps may be more efficient, and may reduce fossil fuel usage, there are many of us who still weigh the overall costs of those systems.

    More on-topic - one benefit to going with electric baseboard or electric radiant is relatively easy to add WarmBoard later on if you decide you like radiant a lot.