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.

Options for a floor heating system

antonvk Member Posts: 2
I am very new to radiant floor heating, and I am trying to figure out what heating source options may work for me. I've contacted different contractors, and they give very different recommendations on the heater. I realize there is no perfect solution, and trying to understand pros and cons of various options.

I need to heat 2 rooms in my house. The rest of the house is heated fine by a gas furnace, but these 2 rooms are the farthest from the furnace, and their temperature is typically 3-4 degrees Fahrenheit below the temperature in all other rooms. These 2 rooms are bathroom and a walk-in closet, so I want their temperature to be a few degrees above the temperature in the rest of the house, which likely means that I need to install a separate heat source for them.

The 2 rooms are next to each other (share one wall), both have an attic above and an insulated crawl space below, high ceiling. The bathroom has 3 external walls and 2 large windows, the walk-in closet has 1 external wall and no windows. The bathroom floor space is 200 sq ft, and the walk-in closet floor space is additional 110 sq ft. Depending on the calculator I use, the heat loss from the bathroom is 9-12k BTU/h, and the heat loss from the walk-in closet is 3-6k BTU/h.

The crawl space underneath both rooms is accessible. The bathroom has a tile floor, and the walk-in closet has a carpet. I am not ready to replace the tile in the bathroom (so heater installation between the floor and the subfloor is not an option at this stage), but can replace the carpet in the walk-in closet.

I can heat the walk-in closet with a small portable heater, and the carpet there may limit the efficiency of a floor heating. Would be nice to heat both with the same system, but if I need to chose, the bathroom is a much higher priority.

I have a 40 gal water heater tank for the domestic hot water now, but I am going to replace it.

- Is installing a floor heating underneath the wooden subfloor between the joists is an option? I hear different opinions on it
- If it is between the joists, does it have to be hydronic, or can electric heating system be used there as well?
- If the carpet is a challenge for a floor heating system, would water heated baseboards be a good option to consider for the walk-in closet?
- I hear that a boiler is great, but even the smallest boiler would be too big to heat these 2 rooms (I don't have a boiler now)
- I hear that something called combi-core may be a solution, but some people say that it is very unreliable, and some say that it is very reliable. What is your experience?
- Is there an option to use a single on-demand tankless water heater for both the floor heating and the domestic hot water supply? What are its pros and cons?
- What other types of gas/electric heaters and mixers should I consider for this system? What are their pros and cons?


  • Jamie Hall
    Jamie Hall Member Posts: 18,966
    Lots to think about here -- and more than I can give you in one reply!
    Is in floor heating between joists feasible? Certainly. It's done all the time.
    That in floor heating can be hydronic (hot water) or electric. Electric is, in some ways, easier to control and install, but the cost of electric heating may be a real stopper, depending on your electric rates.
    Carpet is definitely a challenge for in floor heating; it's excellent insulation, which isn't quite what's wanted. I'd look at baseboards -- either hot water or, again, electric.
    You'd have to do a proper heat loss calculation to find out if even the smallest boiler would be too big. Without knowing any more about the space than given, I would guess that would be the case -- but without numbers...
    The problem with combis isn't reliability, it's that to provide the domestic hot water needed they are wildly oversized for the heating load, which is a problem.
    It is NOT an option to use a single on-demand hot water heater for both a heating load and domestic hot water. Never mix your heating water and you domestic hot water. People do do it from time to time, and get away with it, but there is a good likelihood of some real health problems (bacteria growth) unpredictably.

    Another excellent option, if you have LP or natural gas, is a small wall heater, direct vented (Rinnai makes some nice ones). But on the whole given what I've seen here, if your electric rates are reasonable I'd look at electric baseboards...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SuperTech
    SuperTech Member Posts: 1,792
    I would consider adding radiant floor heating or panel radiators to those rooms as well the rooms that are heated by the forced air furnace. Then you won't have too small of a load and you will have a heating system that provides greater comfort than forced air. Depending on how much your heating load is it may make a modulating condensing combi boiler an option.
  • hot_rod
    hot_rod Member Posts: 16,327
    You will really enjoy warm tile floors if you can make that happen, easily done from below with transfer plates. Possibly a large panel rad or heated towel bar in the closet, if you have wall space.
    Sounds like you have some heat in those spaces now, it would be tough to cover a 12K load with just that amount of floor space, figure mid 20 btu/sq ft output for a comfortable floor radiant. Rooms with a lot of glass and multiple exterior walls, not much floor space are a challenge.

    Radiant ceiling or walls are an option for the closet, might be easier than dealing with the carpet penalty on the floor. Warm floors are not so critical in closets.

    I have built a handful of small electric WH tanks for spot heating like this.

    Too bad this Laing unit has been discontinued, a pump and heater in a nice small unit.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jon_blaney
    Jon_blaney Member Posts: 273
    The ducts in these areas are probably undersized and/or your furnace is oversized. Look to increase airflow to these spaces. You may need longer furnace run times.
  • GroundUp
    GroundUp Member Posts: 1,406
    I had a similar issue in my own home with one bedroom being 50 degrees and the living room 60 with the rest of the house at 70 due to poor ductwork. Initially I had installed staple-up radiant below the subfloor using cheap aluminum transfer plates, insulated the joist bays heavily, and it more or less sucks trying to heat through thick carpet and pad. I am at a point now, that I am seriously considering adding kickspace heaters between the joists below and connecting them to the existing duct registers in the floor as the main duct is out of air by the time it reaches these areas anyway. Panel rads would be great, but they're hideous IMO. If I were willing to go through the work and spend the money, personally I'd do radiant walls if comfort is the end game. I'm too lazy to tear up the walls so that will not be happening in my house. As far as a heat source, the Combi-Cor is a pretty decent unit if you can find one. I've been told by every local supply house that they are discontinued but nobody else seems to have trouble finding them. I have had a few customer projects where I built my own combi from a standard WH tank and piped an external plate heat exchanger and circulator between the drain and relief ports on the side, using the other side for my space heating and eliminating the Legionella issue that comes with blending DHW and space heating. Personally I don't see the value in buying a very expensive boiler and buffer tank for such a small load when DHW tanks will do the job and you already have one.
  • hot_rod
    hot_rod Member Posts: 16,327
    There used to be a few manufacturers offering plate HX kits to connect to an existing DHW tank for small radiant loops, I think some Canadian brands are still offering them.

    I used a Solar Wand on a small bathroom job. It's a copper HX that drops into one of the top ports on the water heater. Intended for solar input.

    I just used it backwards as a means to pull a small amount of BTUs for a radiant bath floor.

    This version replaces the cold nipple, connect water to the side, the two upper tubes, 1/2 & 3/8 tube are the S&R. Good for maybe .75- 1 gpm, takes a medium head circulator..

    Butler Sun Solutions still offers them. Easy to build your own.
    I think he has a PV wand now also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 18,966
    That's a potentially very useful gadget. Thanks @hot_rod !
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • antonvk
    antonvk Member Posts: 2
    edited February 2020
    Thanks a lot, this is very helpful!

    If I understand the situation correctly:

    Walk-in closet
    - Water baseboard heaters is an option, but they require water of a different temperature from the heated floor
    - Heated walls is also an option
    - Heated floor is not really an option because of the thick carpet

    - Hydronic heating in the joist space underneath the wooden subfloor and tile floor is a good option
    - Do not mix space heating water with the domestic water (not worth taking the risk)

    - Does plumbing code usually allow having a completely separate heater (such as a small electric heater or an on-demand tankless one) for the floor heating, if this is only one room, and this is a secondary heating source (the primary one is forced air)? I hear that it is allowed to use a water heater for the domestic water supply or for both domestic water and space heating, but not for space heating alone
    - Between two options - (1) using a single heater with an external heat exchanger for the hydronic loop vs (2) using two separate heaters for the tap water and the hydronic loop, what are the pros and cons?
    - I hear comments that electric heater for heated floor is inferior to a gas fired one, because its recovery rate is too low. What are the drawbacks, especially if I need to heat only one room?
    - I understand that Taco X-Pump block can be a solution to the first option. Are there alternatives?
    - Are there alternatives to the Laing EHT (heater-circulator) unit for the second option?
    - Is it ok to install tubing underneath build-in items in the bathroom covering the entire area? I have a shower, jucuzzi, cabinets, built-in table, toilet. I may decide to relocate them in the future, and I don't want to redo the tubing (its in the crawl space). At the same time, I want to make sure there is no damage to the wood, jacuzzi and the subfloor. I realize this means I am heating a little bit of unnecessary space - this is ok. What is your experience?