How to integrate an air to water heat pump into an single air to air hrv/erv system in a multiplex
I am in the process of designing a 6 unit multiplex in Madison WI climate zone 6. Units are 3-1144 sqft 3 bedrooms, 3-924 sqft 2 bedrooms and 1-924 sqft heated garage. For simplicity of field application, I am utilizing dual stud R30+ walls without sheathing and a corresponding ventilated roof system of R70. I would like a critique of the building systems. Specifically of my planned Cooling, heating and especially of the planned ventilation systems.
I have built this unit before utilizing standard 2×6 sheeted wall construction and r21 batt, 12,000 btu through the wall AC units, and a NG fired single unified high temp boiler system with high temp wall emitters, which also heated DHW. I had a unified air to air heat exchanger supplying all units that had supplemental electric heating of incoming winter air. The DHW and heating system was a huge win as far as operating costs go, but the AC struggled with temps in the 90’s. The apartments were cooled but the units ran continuously. The air to air heat exchanger was expensive to run during the winter due to heating winter air temps. The air to air exchanger was located in the truss space and vented through the roof.
This time around I am doing things differently because the units are now being built as condos. Water will be metered individually.
1) Planned heating will be individual NG fired combination heating/dhw boilers. This time however I am planning to do only LOW temp heating via either infloor hydronic or low temp wall emitters and would like some input as to which to use.
2) Planned AC this time is via single-zone mini-splits of 24k btu size maybe even 18K, after I finish the calculations for this superinsulated structure.
3) Planned Ventilation is my big problem. Individual ventilators are problematic, so I am planning a single HRV/ERV unit. However, I want the heat transfer efficentcy to be high enough not to require heating of incoming air. I will place the hrv/erv unit inside the heated garage this time venting to the east side of the building to limit wind effect issues of our blustery winter northern winds.
Other than the Equinox CERV heat pump units available on the market, is there any other air to air heat pump ventilation units on the market? If hot is there any other air to water heat pump I could tie into the larger HRV/ERV so that I could turn that heated air energy back into heated water to preheat the 40 degree well water or use it to heat the garage or even one or two of the units since space heating temps will be below the 105 degrees for infloor heat? I realize this is not a normal methodology but I sure hate wasting the hrv/erv discharge.
Thanks, Mark
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Replies
Normal HRV or ERVs warm the incoming air enough that you don't need to supply additional heat. If you want to, you can put in a water to air heat exchanger, on the ventilation supply air, after it comes out of the ERV. That will supply some of the space heating and won't decrease the amazing heat recovery performance of the units.
Trying to use the heat remaining in the exhaust isn't really worthwhile. You have already removed most of the heat with the heat exchanger. There's not enough left to do much useful, and it's not much higher temperature than the outside air. So if you want to run a heat pump, run it from outside air.
If you like heat recovery opportunities and hate letting it go to waste, get a drain water heat exchanger. That can be a very cost effective particularly with cold ground water.
Thanks, Charlie I will look into the waste water recovery.
You may to look at the Messana ATU which is hydronic based unit with a HRV. That should be able to dehumidify supply air in the summer and pre heat in the winter.
I'm working on a smaller place right now and went with a single ducted cold climate mini split for both heat and cooling and an ERV instead of a bathroom fan for fresh air for each unit.
The cost of going gas was not worth it plus would already would have needed an AC unit, the additional cost for a heat pump was not that much.
I don't think eight units is not large enough to have anything centralized, there is no cost savings over individual units plus now you need to pay for operating the central unit.
In most of these multiplexes, units only have a few outside walls, in a building that is well sealed an insulated, the cost of heating with a heat pump would probably be less than paying for gas and the gas connection fee. I'm in the land of cheap gas and expensive hydro and heat pump works out only a bit more over a year.
With modern ERV/HRV you don't need pre-heat for the intake, frost control is part of most units. Make sure to go with a unit that does recirculating defrost, exhaust only defrost tends to use a lot of heat in very cold climate. In cold weather the fresh air feed will be a bit colder, simplest is to duct it to the air handler to do the conditioning and distribution.
Akos, Thanks for the reply and I finally came to the same conclusion. Invest in the superinsulated building and windows use an ERV with mini split heat pumps and eliminate the gas.