Can an air-to-water heat pump replace a gas boiler to supply hydronic baseboard heaters?
Because of my GBA subscription (and I guess my enthusiasm for the topic) people sometimes ask me green building technology questions that are well beyond my limited knowledge. Recently a friend referred a family member to me with the question below. Hoping folks here might have answers. The family is in Climate Zone 4C.
Right now we are using a Burnham Series 2PV gas boiler to heat hot water that we push through hydronic baseboard heaters, which are divided into three zones (one for each floor of the house). According to the labels (attached photos) the burner has an efficiency of 81.8%, an input of 130,000 BTU/hr, a max heat of 250F, and max pressure of 50 PSI. Based on our gas bill, we use about 130 CCF of natural gas/month to heat our house in the coldest months (December – March).
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I am not an expert on these things, but I am in the same situation: gas boiler doing rads and domestic hot water. I am looking at a Sanden CO2 heat pump because of high water temperature output and very green refrigerant with a GWP of 1, since it is CO2 https://www.eco2waterheater.com
To the direct question, maybe. AWHP's generally produce lower temperature water, maybe 130F tops compared to the 170F or so that boilers typically produce. The output of a radiator is directly proportional to the temperature difference between the radiator and the air in the room it's in. If the room is at 70F, a radiator at 170F has a difference of 100F and a radiator at 130F has a difference of 60F, so it will produce 60% as much heat. However, radiators were very often oversized, so it may turn out that the radiators are in fact big enough even with reduced output.
But AWHP's are still kind of a fringe technology, I wouldn't recommend them if you're not a tinkerer. Does this climate require air conditioning? What I would recommend is instead installing an air-to-air heat pump sized to meet the cooling need. In climate zone 4 your heating load is probably higher than your cooling load -- although it may be close -- so a heat pump sized for cooling will probably be somewhat undersized. So if you size for cooling you have to figure out how to make up the difference in heating. Some options:
* Install a bigger air-to-air heat pump
* Use electric backup heat
* Keep the existing gas boiler
* Use an AWHP with the existing radiators
If you prefer the feel of the radiators you could make the AWHP the primary and the AAHP the backup for heating.
The short answer is yes but you’ll need someone skilled. Operating costs will change month to month and hour to hour so that’s a moving target.
For gas: 10 x $/therm / efficiency.
For heat pump: 293 x $/kwh / efficiency.
It’s extremely easy to just keep both and use the heat pump for the vast majority of the winter.
The first step is to a heat loss calculation for the building. Then do an economic balance graph comparing the cost to heat using gas, and cost for a heat pump. You then look at the number of days at a temperature. The controls to switch between gas and a heat pump are not that expensive, so you can keep the gas for when it is below 8F.
The good news is that the utilities are looking a mixing hydrogen with the natural gas. Nothing wrong with NG providing it is plumbed and vented correctly.
Baseboard heaters run about 50F (27C) hotter than in floor heating. This will put a big dent in your COP figures Theoretical COP = Temp of source/ (Temp of source - Temp of sink)
All temps are absolute. So in this case the sink is your reservoir of 140 degree heating water. If your outside air temp is 0 F So you get 255/( 333 - 255) or255 or 78 or a bit over 3.
By comparison for a radiant slab running with 90 F water you get 305 K instead of 333 which gives 255/(305-255) = 255/50 = a bit over 5.