Air-to-Water Heat-Pump Water Heater for Radiant Floor
I currently have a gas steam boiler and am considering removing it and switching to radiant floor heat for my first floor; but instead of a boiler, using a 83 or 119 gallon SanCO2 (Sanden/ECO2) air to water heat pump. Although this is considered a DHW unit, the 150 F water temp is plenty for radiant heat. The only potential issue that I foresee is that the recover time for the unit wouldn’t be able to keep up with the radiant and domestic hot water use.
I already have an infinity series carrier mini split on first floor. Second floor would get a new ducted heat pump for bedrooms. The goal is to finish basement so I would do about 400 sq/ft of radiant floor there and the 1st floor is another 900 sq/ft or so.
Climate zone 5. 1930 Colonial with densepacked walls, 16″ cellulose in attic, air sealed etc.
What am I missing? Any advice or criticisms are appreciated.
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The Sanden option is possible, but would be crazy expensive and risky. Double check the warranty, but I don't think your climate zone would qualify (if it did, the warranty for combi systems is a fraction of DHW). If the power went out during the winter, the potable water circulated outside would be at risk of freezing and bursting. Then there's the installer familiarity - do you want your system to be the first an installer has seen or the 1000th? Ditto for emergency maintenance. All for 1300 sqft of a well insulated house with floors that a Sanden can only bring up to about 3 degrees over room temperature (8000 btu/h / (1300 sqft heated * 2 btu/sq/(surface temp - air temp)).
With assumed access to the first floor joists, a floor ducted heat pump with electric radiant floor where it really matters would be much more reliable, probably 50%+ cheaper and more efficient too.
Thanks for responding Paul. What would be the difference be between another air to water heat pump in this scenario that is designed for heating? I like the Sanden due to the environmentally friendly refrigerant and positive reviews (for dhw at least.) I don't think the potable water actually circulates to the exterior either. You are certainly correct with the installation being uncommon. I do have a 1 ton mini split that services the first floor as well which would be taking on a significant portion of the heating load. I figure the unit is 6k plus another 4k in pex and materials. This wouldn't be much more than installing a natural gas combination unit. I don't want to run ductwork in the basement due to headroom concerns. That is a big driver in taking out the steam pipes and doing radiant to begin with. I appreciate any other feedback.
There has been a couple of studies that looked at the Senco unit for combi heat. When used like that, the efficiency numbers for the unit was suprisingly low (bellow COP of 2 if I remember correctly).
The issue with these used for space heat is the return water temperature is too hot. CO2 heat pumps need a very large delta T to work well (this is why you need a big tank to allow for stratification). It takes pretty careful design and a lot of tweaking to get there, I doubt it is possible in a retrofit type of situation.
Even if you do manage to design a system that works, the cost of the parts is so high that there is no ROI vs a simple resistance floor heat.
If you want to take it on as a science project, my recommendation would be to use a glycol loop to the Sanden unit with a reverse indirect (domestic hot water runs through the heat exchanger coil not the tank) as a buffer tank and for domestic hot water. Make sure the tank is large, design your radiant for very low return water temperature and be very careful with flow rates to maintain stratification in the tank.
It would be great that there was a single box out there that could do exactly what you are looking for (hot water plus a bit of radiant heat), unfortunately we are not there yet.
It has pretty similar downsides to an air to water heat pump (complexity, rarity, cost). It’s very limited in heat output which is an additional limit. I’m pretty sure it requires heat tape for the outdoor tubing, so there’s the freeze risk, another downside. Also no warranty. I think you’re spending significantly more than $10k here on materials and installation will not be cheap. You’re not gaining warm floors. Hydronics is expensive for new houses, and scales down terribly, especially for retrofits. Likewise, a combi gas boiler would also be a terrible choice for an 8k btu load.
What about solar + minisplit or solar + resistance? The solar needn’t be onsite either. Same environmental benefit without taking on a five figure risk.