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Air-to-Water Heat Pump for Heating, Cooling, and Hot Water

BrianRJH | Posted in GBA Pro Help on

My gas boiler is 25 years old, and still working fine but  it will die some day. I am also trying to get off of fossil fuels.  Existing cast iron radiators  are oversized at water temps as low as 140*F. Please see attachment below for how I arrived at that.  I live in Maryland, stick built 1925 1 1/2 story house.  The basement is Not in the conditioned zone. I have now almost completely insulated the basement ceiling.  Conditioned area is 1275 Sq Ft.  Outdoor design temp 15*F, indoor 68*F,   and 95/75 during the summer. HDD is about 2853.   Moderatly humid, 40 grains difference. Winter ACH 1 per hour; summer .5 per hour.  3 occupants, plus dog.  1 pellet stove in Living room, tight.
Total heating load 25,399 BTU/h  and cooling load 18,314 BTU/h.
And here I just found what must be an error , under Energy Cost Analysis manual J show 4500 HDD, and only 1250 CDD. shows 2853 HDD for South Atlantic,  and 1964 for cooling.  I ran the  Manual J my self.

My room by room calc. shows my lossiest room at 4942 heat loss at design temp and my radiator emits 7560 at 140* water. All my existing room are like that.  I know Air/water heat pumps only give out 130-140* water, but it seems I should be fine with that.   The idea comes from John  Siegenthaler at HPAC.

A look at air-to-water heat pump systems

This involves use of a buffer tank, with a coil for DHW, but I still don’t know how that would work in the summer, and a fan coil unit for forced air cooling.

I have had energy audit s done, and I am trying to finish up on those recommendations, and it seems I will need a mechanical engineer to design the system.
Is there any reason why this could not work?

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  1. walta100 | | #1

    As long as you do not mind spending more than twice as much money to design and install the system, twice as much to service the equipment and 15% more to operate the equipment keeping the hydronic system will work great.


  2. kieran973 | | #2


    Did this ever work out? I have extremely oversized cast iron radiators in some of my bedrooms as well, so I've been checking out air to water heat pumps (Arctic, LG, Chiltrix, etc). Using the chart that you supplied in your attachment (thank you, btw), it looks like the water temp of my radiators would only need to be at 105F or so to meet the necessary heating loads in the bedrooms.

  3. BrianRJH | | #3

    Heating can work with old radiators, there are articles here that address that. But then you may need to resolve AC in the summer months.
    So I went for Chiltrix CX34-2 , which I installed myself, mostly, with some help from an electrician, and a plumber. I also incorporated the basement into the conditioned zone, mainly by using Insofast 2" foam insulation. I am more than 3/4 of the way done with that, I ran into a structural foundation issue, that I am nearly done completing work on ( That is a really tough job cutting into and structurally pinning a block wall, from 1925, needless to say, these "masons" didn't kneel at the alter of Standard Block Layout, but it's not as if I can go dig up their graves and yell at them).
    The Chiltrix is working ok now ( the highest honor mechanical equipment can rate, in my opinion) although it has taken quite a bit of troubleshooting and massaging to get it to that state.
    But yeah, I ditched the radiators, and got Fan Coil Units, and DHW 60 gallon tank. No buffer tank.

  4. BrianRJH | | #4

    One more thing about the fan coil units, for heating, they both radiate heat, much less than cast iron, but it is constant, and of course they blow the heat out as well.

  5. charlie_sullivan | | #5

    Thanks for the report of your conversion. That's great to hear and nice that it worked out that you could do a lot yourself.

  6. kieran973 | | #6

    Thanks for all the info, Brian. I checked out the Chiltrix CX34 (and the Insofast). I know what you mean about the basement masonry - I have a fieldstone foundation that just looks like it was built by children with randomly selected rocks. The Chiltrix looks awesome, but I don't really have the skills to install it or troubleshoot it myself. I think what I may do is leave the radiators in for now, put some air-to-air heat pumps on the first floor (where we just have baseboard radiators) and see if any simpler air-to-water units come on the market in the next few years. I was hoping there was an air-to-water heat pump that was about as difficult to install and maintain as a heat pump water heater, but the elegance of the air-to-water units seems like a major maintenance issue, especially if you don't know what you're doing (which I don't)....

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