Some of you may be curious how our Sanden CO2 combi heat pump system is working out.
The Sanden unit is an air-to-water heat pump that pulls energy from outdoor air to heat water. We’re using it to heat both the water supplying our radiant slab (to keep the place warm in winter) and to heat the water at our faucets. Hence the “combi” system.
Here’s a basic run-down of its major parts and what they do.
Cold water is run directly to the outdoor unit:
which uses CO2 to pull heat out of the air in a reverse refrigeration cycle and then heat the water. The warmed water is then fed into the insulated storage tank in the mechanical room:
If there’s a call for hot water from our laundry, a faucet, or the shower, the water from the tank directly supplies those fixtures.
If there’s a call for heat from the radiant slab (as determined by a thermostat), this little green machine kicks into gear:
This is a “Taco X-Pump Block”, a box containing two small pumps, a heat exchanger, and controls that enable ‘plug-and-play’ combi systems. When there’s a call for heat from the thermostat, the circulator pump moves hot water between the tank and the X-Block’s heat exchanger, while the other pump circulates the water through the hydronic system, also passing by the heat exchanger. This allows heat transfer to the closed loop hydronic system without mixing with the domestic hot water.
So how’s it working?
One of the reasons that CO2 works so well as a heat transfer medium for a domestic hot water system is that it loves a high “lift” – in other words, it can turn really cold water into really hot water. Amazingly, this heat pump can pull heat from air as cold as -15 F (-26 C brrr) and, according to its specification, it can heat water up to 175 F (79 C) (although I expect it would need warmer than -15 F air to get the water that hot). AND – it can do this extremely efficiently, producing up to 4.5 times as much energy as it takes to run the equipment. Compare that to, say, a natural gas boiler, which always produces less useful energy than it consumes.
We’ll have to do some more detailed monitoring to test the efficiency claims, but I can report on whether it’s keeping up with our hot water demands. The short answer is: with ease. Our 119 gallon tank (the biggest option) is serving both our home and our small rental suite and we have yet to run out of hot water.
The trick with a combi system is sharing the load between heating and domestic hot water demands. The big question (at least in my mind) is how to most efficiently provide hot water when we want it for a shower, while also providing sufficient heat to the radiant slab when it needs it.
175 F is pretty darned hot – WAY hotter than we’d ever want the water coming out of our taps. In our fiddling with the system controls, we adjusted the supply temperature as high as 160 F and we’ve since dialed it back to a more standard 140 F, which is plenty hot enough for the nearly-scorching shower lovers out there.
The reason we initially dialed up the supply temperature was to see if we could meet a call for heat from the slab and, at the same time, get really hot water at the faucets. It did seem this was possible in a highly insulated home like ours, based on our limited late winter testing, but it’s not a very efficient (or necessary) way to run the system. We would be heating and maintaining a tank full of water way hotter than we need it to be most of the time.
A couple of friends nearby have similar systems in their passive houses. They’re both using programmable thermostats to top up the heat in the slab overnight when there is no demand for domestic hot water, and then just let it coast during the day. This strategy makes great use of the thermal storage effect of the concrete slab. It takes a while for a slab to lose its heat, so once it’s up to temperature, it will stay there and take very little input to keep it there – especially in a Passive House. I could demonstrate this by doing some math, but, well, it’s a summer weekend and I just don’t feel like it.
I’m confident that the system will tick along nicely this winter using a programmable thermostat to prioritize slab heating overnight and domestic hot water use during the day, while keeping the supply temperature at 140 F. Stay tuned when the temperatures start to dip again and we get to see it all in action. I might even pull out some equations.
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