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Community and Q&A

Heat Pumps for Hot Water Systems

deanberg | Posted in Mechanicals on

I have a house built in 1888 with hot water cast iron radiators – fired with an oil boiler. We love the radiant nature of the radiators and would like to find a renewable way to heat the water.
I just came across the Daikin air-to-hot water heat pump. Is this a viable option? Can we really replace our boiler with this heat pump? Are there other heat pump (air or ground) options out there?

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  1. rhl_ | | #1

    The daikain system is off the market — but yes. However todays “single compressor” heat pumps only allow <= 120F water.

    The cast iron needs >= 140F to overcome its thermal mass.

    I installed Jaga low temp rads and floor heating with plates and I run my hydronic system at <= 120F all season long.

    You can do air to water cooling with the Jaga Briza fan coils too.

    I’m hopefully installing the SpacePak air to water soon.

    1. mikeferro | | #4

      Where did you source the Jaga rads and fan coils from? I've spec'd a SpacePak SIS-060A4 for my new house in Maine and am planning to use their thin wall fan coil units, but am open to other options if they're better or most cost effective.

  2. Expert Member
    NICK KEENAN | | #2

    Theoretically the answer is yes but there may be practical issues. Let me throw out two:

    1. Traditional boilers usually produced water at around 180F. Heat pumps are typically limited to about 120F. The heat output of a radiator is determined by the difference in temperature between the radiator and the room. Traditional radiator systems were designed to produce water at 180F and return at 160F, so the average water temperature was 170F, which is 100F difference if room temperature is 70F. With a heat pump you might get the return water at 110F, average temperature of 115F, so a difference of 45F. So the radiators can only produce 45% as much heat. That might be a deal-killer, or it might not. Your existing radiators may be oversized, the old guys didn't have computers to size heating systems like we do today, they just put in the biggest radiator that would fit. Or you might be able to improve the insulation of the house enough that the reduced output is sufficient. Or you might be able to strategically add some radiation to make up for the reduced output.

    2. Old heating systems tend to be dirty on the inside. Old boilers had big 2" piping and they didn't worry about debris. Heat pumps tend to be sensitive to debris, they have filters on the water. Again this may be a problem and it may not.

    There is a subsidiary of Google called Dandelion Energy that is dedicated to replacing oil burners with geothermal heat pumps. Presumably they have some smart people and they seem to think it's possible. Although I haven't read much about them lately and a lot of the early reviews were mixed.

  3. paul_wiedefeld | | #3

    This is 100% doable.
    However, you'll be limited to lower water temperatures. This may or may not be a problem based on the amount of existing radiation and your heat loss. 120 might be enough, might be more than enough or might be too low. You can keep the oil boiler for backup, as the highest water temperatures are needed only on the coldest days. This could decrease your oil use by a significant margin. Another option would be electric resistance backup or using a R134A water to water heat pump pre-heated by an air-to-water heat pump. You could also add more radiation, which would output more heat for the same temperature water. If you have existing ductwork, a central heat pump could function as backup.
    Since you have existing oil usage, start here to determine your heat loss:

    You'll be limited to lesser known manufacturers on the air-to-water side. Hopefully that changes in the US soon. Geothermal could also work, but would be much more expensive.

    Don't sweat the dirty old system, you can use a heat exchanger to protect the heat pump if needed.

    Here's a case study:

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