I'm evaluating how to best heat a pool \ hottub room 24' x 24' with 11' ceiling and 4' ICF crawlspace. About half will contain a swim spa and hot tub recessed into the crawlspace, and the other half will be decking over the crawlspace.

The room is sealed off from the rest of the house. I would like it to be fairly warm in winter (80F, Minnesota) but exact temp control not needed and if it gets colder sometimes in the dead of winter that's OK. There will be 50% glazing on West and South sides. Summertime temp control even less important due to operable windows, fans, plus not used as much.

I plan on having an air handler around the perimeter which can provide some heating and cooling\dehumidification via heat pump ERV. The hot tub and swim spa also give off some of their own radiant heat. However for the main heat source I am considering superheating the crawlspace floor (the open area, not the area that has the swim spa and hot tub on top) with radiant tubing which will be about 4' lower than the proposed wood decking. The wood decking will have gaps which would allow hot air to rise between them, and I'm hoping the decking itself will absorb some of the heat and feel warm.

One alternative is to install the tubing in the suspended floor (concrete, thermalboard?) but I'm liking the option for a simple wood deck as that will allow water to fall through to the crawlspace and dry\drain there, easier access for maintenance, and because the decking will transition nicely to the same decking on the exterior patio.

Is superheating a crawlspace in this manner acceptable? I know it's not ideal, and need to get a manual-J, but wondering if anyone has done this or has experience with a suspended system like this. Thanks

I don’t think having 4 feet separation of the PEX tub and floor will work very well, if at all. I think you would be better off placing the PEX tube in the floor, either in concrete or something like Warmboard. However, I don’t know if Warmboard is really suitable for a humid and even wet location. So I am thinking a concrete or tile HR radiant floor would likely be best...if you really must have a HR floor.

Yes, you need a good Manual J that accounts for all the other heat sources (e.g., heat tub, etc). I am familiar with ERV/HRVs and heat pump cooling/heating systems, but not a heat pump ERV? In any event, good dehumidification would be critical for this application.

You are trying to heat a pool/hot tub room by proximity? Using the floor as radiator to heat another floor? Seems like not the most efficient method. Because you want to heat the air with the floor why not just use a "fan coil" to heat the air. You could use the warm water going to the pool and put it through a fan coil first. Still not great though.

I don't think a ERV is going to provide dehumidification at all in the summer....winter maybe, summer no. You will need a way to dehumidify. I am not on the minisplit band wagon but sounds like a good application.

You will need a way to cover and uncover pool easy to help keep it covered most of the time. Pools loss heat mostly through evaporation which ruins the room because of the excess moisture....

Also minimize air movement because that also increases evaporation.

OK thanks for the feedback. Since I'm doing radiant in the rest of the house, I was trying to apply it here too even though I knew it wasn't a good fit.

So either move the tubes to the main floor, or use a fan coil. The fan coil could use the same heat source as pool and hot tub, which will be hot water storage with exchanger in the crawlspace. I am also considering an off-peak HPWH for that which would provide cooling in the summer (if I decide to keep the windows shut). Natural gas is not available.

The pool and hottub (two different systems) will each be covered by a lightweight thermal blanket to minimize evaporation and reduce heat loss. Also the swim spa is not very big -- not a typical "pool" size -- just enought to swim in (SwimEx Triton), so the amount of evaporation and humidty shouldn't be that great anyway. Probably less overall humidity than the bathrooms generate.

I agree that minimizing air movement is a good thing, but would also like frost-free and dew-free windows, so I was planning to route the ERV output ducting there, but with a fan coil it probably makes sense to use that fan instead and duct elsewhere for the ERV.

On the "heat pump" ERV -- I should have said CERV (Build Equinox) on a separate zone. It is basically a smart ERV with a heat pump so it can provide some minimal heating\cooling\dehumid during air exchanges. It also knows the exterior temp so can trigger air exchanges to provide some "free" cooling\heating when exterior temp is within that band.

The CERV is an interesting concept. Hope it works. Would love to hear the results.

Is this room attached to the house?

>> Is this room attached to the house?
Yes through a hallway \ catwalk and with a separator door (self-closing). Note the build is not yet started; expected to start this Fall or next Spring.

We have a pool in our basement that will be open to the rest of the house once we are complete. I feel linking the he whole house as one through a 300 sq ft stair well will dissipate any moisture through the house and reduce a chance of any problems. I think it is good that you have the option of connecting the rooms if you want for both heating and cooling and moisture control.

We plan on keeping it covered almost all the time and will be a salt water pool to reduce any chance of chlorine smell. House is in the final build stages right now and pool is not finished. Pool is around 9000 gallons and have calculated evaporation over surface. It will add 7.13 quarts to the air at 50% humidity and 77 degrees water temp. Air movement increases evaporation a lot so being inside we will minimize that when the pool is uncovered. Our Geo unit dehumidifies at just about the same rate so not planning for a dehumidifier at this time. One can be easily added later if needed. Right now I have a very hard time keeping the humidity in the house it has been in the ~20-25% range all winter.

I can share my calculations if you would like.

Having an open pool like that would be great. But everything I've learned is have a separate room for humidity, humidity, humidity and some concern about damage from residual chemicals.

IMO your plan will probably work fine, especially if you don't have a hottub. There's always compromises. If it doesn't initially work to goals, add a humidistat that triggers your ERV or separate exhaust fan, or add a sealed automatic cover, or just drain it and plant a tree

Your pool holds 3x as much water as my swim spa, and probably at least 2x the surface area. My hottub will likely give off more evaporation than the swim spa, depending on usage.

Codes and safety are another concern. When I was a kid, I probably would have jumped from your staircase into the pool... Anyways I want my swim spa and hottub recessed completely into the floor, so that makes it easy to fall (or crawl) into, so that IMO requires a separate room and self-closing door. But then again, the house is 100' from a lake so not exactly safe to let toddlers run free in general.

I'm doing everything reasonable to address humidty:
- Separate pool room with its own sealed HVAC zone (but not separate system)
- CERV with ability to remove some humidity and perform additional air exchanges based on humidity
- Thermal blankets
- Planned high air temperature (ideally a couple degrees above pool temp to reduce evaporation)
- Air routed around perimeter windows (not over the pool, which also helps to keep condensation off windows)
- Triple-pane non-wood windows (lower condensation)
- HVAC exhaust above hottub
- ICF construction and greenboard (no rotting walls or drywall)
- Possibility of HPWH in crawlspace which will reduce humidity in summer
- High ceilings (11') so more cubic feet to dissipate
- Ability to open windows and turn off HVAC (in summer)

There is a old saying....The solution to pollution is dilution....hence if you have something bad you spread it out and then it is not so bad....works in the gulf, should work in your house with humidity.

We have fiberglass windows and going one step farther than green board, using durarock.

Hot air can hold more water so heating it kind of makes sense but when it hit a cold surface it condenses and that is a bad thing.

The pool will be covered most of the time and is raised out of the ground about 24 inches so you can sit on the ground and interact with the kids. There is no way to jump into pool from stairs, we did joke with the kids in the beginning of adding a trap door to drop unsuspecting friends into the pool form the first floor.