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  1. #1

    Air-to-Water Heat Pump vs Oil-fired Boiler

    Hello

    I have a home in central MD with an oil-burning furnace heating system for the 1st floor of the house. Heat is delivered via hot water baseboard heaters located in all rooms. We do not have access to natural gas. Last year, I replaced all the original builder-grade windows with Energy Star rated, low-e, argon-gas filled Viewpoint windows.

    The furnace is pretty old (30+ years) and I'm going to need to upgrade it before it fails. I also have an underground storage tank I'm not fond of, and rising oil prices with unstable future costs have me thinking about other options besides oil as my fuel source.

    I do like the baseboard heaters and I like the efficiency of using water instead of forced air. I would prefer to keep this component of my system.

    I like the idea of geothermal but the upfront costs worry me.

    My question to the community is what do you think about an air-to-water heat pump system to provide heating and hot water through the existing system? Are there models out there that can achieve the water temp necessary for heating in winter? Would I need a supplementary system for a few months out of the year? What are the maintenance needs of a system like this? I like the idea of using air instead of oil and think I could capture some efficiency. My electric costs are pretty reasonable (8.5 cents/kWh supply). I'm currently burning about 1,000 gallons oil per year for heating and hot water.

    Trying to understand the pros and cons to make an educated decision. Also thinking about propane furnace instead of oil because of commodity costs and efficiency. Open to other heating technologies as well.

    I appreciate any feedback. Thanks!
    Brian

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I think multiaqua and Mitsubishi both make air to water system. The problem is they are limited to I think 130F water temps. 130F water has about 20% the heat capacity of the 180F water your current boiler can deliver. You will need to have a heat load calculaton performed and the EDR of all your radators calaculated to determine if you cna get by with a lower water temp after your effcieny imrpovements. SO while you might be able to get the capacity you need, you'd be short on delivered BTU's due to water temp.

    Further, you may need higher flow rates since the delta T will be much lower. You may not have piping adequate to handle that. Low temp systm work better with low mass low temp systems like radiant floor heat and wall panels.

    Do you have plans ot add AC as well? You mgiht be able ot supplement capaity by adding a small air handler sized for your cooling load. Then you can run chilled water to it in summer for excellent dehumidification and cooling.

    There are ground source units that can reach higher water temps of I think 150F. That might be a better option. They can supply domestic hot water as well with a desuperheater. Vertical wells are pretty common now and the state and fed tax credits cover a lot of the installation costs.


    Maybe another pro knows more abotu the Bosch units.

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Multiaqua is on 3.2 ton @ 17F with 115F leaving water temp. That's way too low for baseboard and even marginal for an air handler. Best for a radiant floor loops. But you could supplment with an electric boiler if you have capacity on your service.

    the Mitsubishi city multi can do 160F water on their 2 stage "boosted" heat exchanger. No sure what the efficeincies are. IF you wanted AC, you could add small fan coils mounted in the floor in each room that could heat or cool. The radiators would need to be isolated during cooling... since they would sweat if you don;t manage humidity. You could however, run chileld water t othe fan coils first, then 60F+ water to the radaitors for radiant cooling.

    You would want a heat exchanger or a really good strainer and particulaate seperator so you don;t send crud form the old rads to the heating system... unless you have copper slant fin baseboards now.

  4. #4
    Thanks for the feedback, I will look at the Mitsubishi - the house has an existing central AC for this zone with an air handler and existing ductwork to floor registers. It's pretty old also. Fortunately we live on a well-shaded lot so the consistent 90 degree+ summer days do not take quite so much a toll on the unit. Not so good for the grass, but oh well.



    Quote Originally Posted by motoguy128 View Post
    I think multiaqua and Mitsubishi both make air to water system. The problem is they are limited to I think 130F water temps. 130F water has about 20% the heat capacity of the 180F water your current boiler can deliver. You will need to have a heat load calculaton performed and the EDR of all your radators calaculated to determine if you cna get by with a lower water temp after your effcieny imrpovements. SO while you might be able to get the capacity you need, you'd be short on delivered BTU's due to water temp.

    Further, you may need higher flow rates since the delta T will be much lower. You may not have piping adequate to handle that. Low temp systm work better with low mass low temp systems like radiant floor heat and wall panels.

    Do you have plans ot add AC as well? You mgiht be able ot supplement capaity by adding a small air handler sized for your cooling load. Then you can run chilled water to it in summer for excellent dehumidification and cooling.

    There are ground source units that can reach higher water temps of I think 150F. That might be a better option. They can supply domestic hot water as well with a desuperheater. Vertical wells are pretty common now and the state and fed tax credits cover a lot of the installation costs.


    Maybe another pro knows more abotu the Bosch units.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The VRF systems also allow multple indoor units. So you could use a air handler unit to replace the existing AC and still make hot water with the second unit. For the other zones you could use smaller concealed units or again, use small concealed fan coils for chilled and hot water heating and cooling. They can operate up to I think 50 indoors units and you can connect multiple outdoor units so capacities range from 6-30 tons.

    Maybe a Mitsubishi VFR expecrt will come on here and give more info.

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