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  1. #1

    Home heating conversion - hot water oil boiler to air to water heatpump

    Hello,

    I am playing with the idea of converting/modifing my current heating system which is a NY thermal oil fired hot water boiler with baseboard radiators over to something more efficient. I was told that there are some air to water heatpumps now on the market that I may be able to use but I am not sure if they will work.

    My home is 10 years old and approx 3600 sq ft incuding basement. It has good quality windows and doors. It is insulated ok with 2x6 walls, attic with blown in R40. Basement with batting inside 2x4 walls framed inside concrete.

    I would like to find some way to keep my oil boiler but incorporate it with a air to water heatpump. Is this something possible or am I into a whole new system. If it is possible what is the basic concept. I am a controls/instrumentation tech so I am familiar with this type of stuff but on a industrial level but I also do minor manitenance with heating systems such as mine but I am not familiar with all the new systems.

    Any help will be appreciated.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,308
    Most air to water heat pumps won't heat the water hot enough to use with baseboard.

    Do you have central A/C. If so, an air to air heat pump with a hydro coil as aux heat could be done.
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  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Most all heat pump whether air or water cooled are most efficient with supply temps (whether air or water) under about 110F.... 100F is even better.

    Could you add a air to water unit for mild weather, then use your oil boiler for a 2nd stage? Possibly. But I think if you want AC or already have AC a conventional heat pump will have a better return on investment.

    You might also look at Geothermal. Vertical wells requre very little space and their proces have come down. Waterfurance makes a water to water unit that can deliver up to 150F water. But depending on how well insualted your house is and how much baseboard you have, that might still not be enough. BTU output drops by about 1/2 going from 180F to 140F and 1/2 again going to 115F.

    Look at it this way, if in cold weather at design conditions (between 0-10F for most of the northeast) your existing boiler is only running about 1/4 -1/3 of the time when maintaining a comfortable temprature, then you might be able to use 115F water.


    Again, the lowest cost overall, migh be to install a conventional heat pump and minimal ductwork, then use the baseboards for auxillary heat with a thermostat that measure outdoor temprature and add a hot water coil for reheat during defrost. Then run your boiler with return temps as cool as the mfg specs allow.
    Last edited by motoguy128; 02-13-2013 at 08:56 AM.

  4. #4
    Quote Originally Posted by motoguy128 View Post
    Most all heat pump whether air or water cooled are most efficient with supply temps (whether air or water) under about 110F.... 100F is even better.

    Could you add a air to water unit for mild weather, then use your oil boiler for a 2nd stage? Possibly. But I think if you want AC or already have AC a conventional heat pump will have a better return on investment.

    You migth also look at Geothermal. Waterfurance makes a water to water unit that can deliver up to 150F water. But depending on how well insualted your house is and how much baseboard you have, that might still not be enough. BTU output drops by about 1/2 going form 180F to 140F and 1/2 again going to 115F.
    I dont have AC however I do have an air exchanger with ducting ran to every space. Could this unit be replaced somehow with an air to air heatpump and use that ducting. the ducting is not the metal type but the flexible stuff. It is insulated and ran in the attic with blown in insulation all around. Just curious.

    So from the sounds of it using the existing oil boiler in conjunction with an air to water HP is not going to work, I had my doubts anyway. I guess I could do this but I would have to replace all the rads with low temp units but then I would not be able to run my boiler with them since the max water temp most of these LT rads is 120F. True?

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Possibly. Depends on the duct sizing. IT's probably a little small even for jsut a 1.5 or 2 ton heat pump. If it's accesible, it might be easy to upsize it as well. Its' not hard to make a register slightly larger. TO balanec the airflow to the heat loss or gain of the different rooms, you'll liekly need to unpsize some of the ducts anyway. You can keep the air exchanger in service and tie to to the air handler.

    If we're talking a a <1600sqft home in a northern climate, a 2 ton heat pump would probably supply all your heating down to 30-35F depending on window and insulation and supply at least 13/-1/2 the heat down to design conditions and still be cheaper to operate than the oil boiler. But again, to prevent drafty cold air, you'll want to add at least a small hydornic coil ot temper the heat pump air in colder temps and during defrost. Although a hte price of oil, you might just use a small stright electric coil for that purpose too.

    The nice thing about a boiler, is that if you lose power, you can run it and the circulation pump on a very small protable generator or even a battery pack-up. It probably only needs arund 300 Watts at most.

  6. #6
    Quote Originally Posted by motoguy128 View Post
    Possibly. Depends on the duct sizing. IT's probably a little small even for jsut a 1.5 or 2 ton heat pump. If it's accesible, it might be easy to upsize it as well. Its' not hard to make a register slightly larger. TO balanec the airflow to the heat loss or gain of the different rooms, you'll liekly need to unpsize some of the ducts anyway. You can keep the air exchanger in service and tie to to the air handler.

    If we're talking a a <1600sqft home in a northern climate, a 2 ton heat pump would probably supply all your heating down to 30-35F depending on window and insulation and supply at least 13/-1/2 the heat down to design conditions and still be cheaper to operate than the oil boiler. But again, to prevent drafty cold air, you'll want to add at least a small hydornic coil ot temper the heat pump air in colder temps and during defrost. Although a hte price of oil, you might just use a small stright electric coil for that purpose too.

    The nice thing about a boiler, is that if you lose power, you can run it and the circulation pump on a very small protable generator or even a battery pack-up. It probably only needs arund 300 Watts at most.
    If I were to go with all new ducting and registers, how small can the main trunk be made. The reason I ask is that my bacement is developed with dropped ceiling and I have about 3 1/2" between the t-bar and the floor joists, width shouldn't be an issue. I would hate to have to take all this down.

  7. #7
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
    Posts
    377
    You can run any reasonable temp to Low Temp baseboard; they have ratings up to 200* like standard base.

    A simple and inexpensive add on to your present boiler would be outdoor reset. If properly adjusted, you should see at least a 15% savings in fuel. Don't set the bottom of the water temp curve below 130* or flue gas condensation may occur.

    Look at the Tekmar 256 for starters:

    http://www.tekmarcontrols.com/produc...stems/256.html
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  8. #8
    Quote Originally Posted by bobboan View Post
    You can run any reasonable temp to Low Temp baseboard; they have ratings up to 200* like standard base.

    A simple and inexpensive add on to your present boiler would be outdoor reset. If properly adjusted, you should see at least a 15% savings in fuel. Don't set the bottom of the water temp curve below 130* or flue gas condensation may occur.

    Look at the Tekmar 256 for starters:
    http://www.tekmarcontrols.com/produc...stems/256.html
    Sounds like an intersting controller however I am looking for more of a life long solution. Thanks for mentioning it though, didn't know they existed.

  9. #9
    Guys, am I better off just keeping my current boiler and doing the ductless system (mini split).

    If so what are the pro's and cons.

    I think I would need about 7 inside air handlers to have heat in every room (living room, 3 bedrooms, kitchen, hall, & basement recroom). Is there a reliable system out there that can accomodate this many inside units or would I have to install 2 separate systems. Typically what does these systems cost. As mentioned above my house is a 10 year old 3600 sq ft ranch style with basement.

  10. #10
    Join Date
    Oct 2009
    Location
    God's country - Shenandoah Valley, VA
    Posts
    377
    There are multi zone units as well as one on one. You probably don't need a unit in every room. We do mini splits all the time and have never had a dissatisfied customer.

    Use the heat pump function until it can't keep up and then let the boiler kick in.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

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