HVAC for a swim spa underground
I’m the client, building a PHIUS Passive House in Seattle, with an architect and builder who have done Passive certified projects before. However, we are doing something they have never combined before – passive and a pool (swim spa).
We are doing a 16′ Hydropool Executive Trainer 16EX for my wife in the new house. This is one of those pools you swim against the current against, and is about the size of two hot tubs.
It will be in a 225sqft room in the basement. It will be outside the passive envelope, with a tightly sealed and insulated door and walls between pool and house. The all walls will be insulated with: 1 wall facing a bathroom, 1 wall facing the garage, 1 wall partially exposed to the outside down an embankment with a small window, and 1 wall buried completely. Slab will be insulated, and the roof, with patio above it, will be insulated as well. Contractor is going to seal the heck out of the room.
Pool will likely be kept at 82°F, with a light cover, and used for 0-60 minutes a day. We hope to heat it with a SANCO2 water heater, if we can get the pool company to install the bypass loop.
I generally understand that if we keep the room 2°F above the water temp, evaporation will be greatly reduced.
HVAC contractor (who I’ll be following up on some more questions about later) proposed a Honeywell TruDry 120-pint dehumidifier and MSZ-FS09NA heat pump. It both seems overkill, and doesn’t address fresh air.
Pondering options:
Belt and suspenders: Heat pump, HRV, dehumidifier
Crazy Combo: Minotair V12 Magic Box (I have an email out to them)
Trust in HRV: HRV + Minisplit
Dehumidifiers are technically heaters: Dehumidifier with radiant heat panel, need HRV?
What’s our best option for this small, sealed, warm, damp, room?
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I would be very cautious about adapting off-the-shelf dehumidification and heat pump equipment for a pool environment: pool chemicals have the potential to make for a corrosive atmosphere, potentially damaging coils and equipment. There are "off the shelf" pool dehumidifiers/conditioners (look up DesertAire, Dectron, PoolPak) engineered for these conditions. As a first cut, you might want to reach out to the manufacturers of equipment you're selecting, and ask them whether pool use is acceptable or not.
Second, in addition to the mechanicals, the enclosure is at much higher risk of wintertime condensation with the extreme interior dewpoints (air moisture levels) of pools. As background, this is BSC’s monograph on how to build pool enclosures—i.e., all exterior insulation.
1. BSI-055: In the Deep End
https://www.buildingscience.com/documents/insights/bsi-055-in-the-deep-end
Some further information on a presentation I gave on this topic.
Building Science Experts' Session | DAY 2: November 8, 2018 | Westford, MA
https://www.buildingscience.com/sites/default/files/pools_special_use.pdf
Pools_Special_Use.pdf
Kohta Ueno
In terms of fresh air: the standard recommendation is to install an exhaust system that will *depressurize* the pool space. This both contains potential pool chemicals/odors from the rest of the house, and more importantly, air leakage will tend to flow from outside-to-pool, thus containing pool moisture. The opposite, of pressurizing the pool, pushes pool moisture out through the shell, potentially resulting in a spectacular enclosure failure:
Indoor Pool Building
https://www.buildingscience.com/project/indoor-pool-building
But lastly: a pool that spends 90%+ of its time covered makes the room much safer from these condensation risks... but does not eliminate those risks.
I stumbled upon this interesting thread about HVAC for an underground swim spa. I was just wondering how things turned out for you. It's such a cool project combining passive design with a pool!
We are in the middle of building this, still figuring out a few details.
But we settled on a Broan AI series HRV, as they can be specifically calibrated to run at slight negative pressure to stop moist infiltration.
Then we are going to have a dehumidifier, basically a crawl space model that is easy to replace when it fails.
We don't think we need much heat, since the pool will bleed heat, but we will have hydronic radiator.
Contractor is working through all the waterproofing details, I don't have those at hand to share right now.
Hi, How goes your project. We are in the middle of doing a similar project in Portland. I've had no luck finding a local contractor that knows what is needed. Does the space stay warm enough without a heater? How was the room sealed - specifically the ceiling? We are placing in basement and have all concrete services professionally sealed but still have the ceiling to address.
Thank you