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  1. #1
    Join Date
    Aug 2012
    Location
    Vancouver Island, British Columbia
    Posts
    16

    Thumbs up A solution to defrosting with an Oil furnace

    Where I live there are a lot of houses with oil furnaces, so granted some of our customers don't see the necessity in putting in an electric or gas air handler when they already have a 'perfectly' fine oil furnace*cough*. So we tend to do alot of heat pump add ons to these oil furnaces and throw a coil in the return or supply obviously depending on what is more accessible with the existing ductwork. moral of the story is new electric furnaces turn on the auxillary to keep sending warm air through the ductwork while the heat pump is in defrost. but with these old oil furnaces when defrost comes on of course cold air comes out of the registers and the oil furnace does not come on to supplement the heatpump while it is doing its defrost. of course it doesnt cool there house down due to how long defrost runs but why would someone want cold air coming out of the registers when its below 32 ambient.

    it tends to be a big complaint in the winter time and I hate having to say 'sorry defrost is an essential part to a heatpump being used in winter and there is not much we can do about that'

    Does anybody know of any relays or controls that we could use that would turn the oil furnace on to offset the inside coil being used as an evaporator for those few minutes.

    all help and criticism is appreciatedhttp://hvac-talk.com/vbb/images/smilies/2cents.gif

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,384
    The defrost board in the HP will trigger heat during defrost. I'd put in a limit above the heat exchanger so the burner can't put too much heat in the coil on a long defrost. We had dual fuel oil heat for years in our office.

  3. #3
    Join Date
    Jul 2005
    Location
    Boston/Cape Cod
    Posts
    66
    I would think any GP relay to TT of primary will turn the burner on, the real problem would be cycling the blower if board controls blower. If older style fan/limit, would be easier, and could even be added onto unit easy enough.

  4. #4
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    6,914
    We have a lot of satisfied customers with that setup, do it just like Baldloonie said.

  5. #5
    Join Date
    Jul 2007
    Posts
    12
    Or switch to oil at 50 degrees with an outdoor stat and no need for a defrost.

  6. #6
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    6,914
    Quote Originally Posted by jhef83 View Post
    Or switch to oil at 50 degrees with an outdoor stat and no need for a defrost.
    And defeat the purpose of having a heat pump.

  7. #7
    Join Date
    Dec 2007
    Posts
    4,574
    we do this set up all the time and only one customer has requested we disable the oil in defrost.

    we use a HW 8000 series stat, outdoor sensor, proper stat programming and a plenum switch in the evap coil plate- basically like Baldloonie said.

    blower control is STILL on from call for heat pump in this set up.
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  8. #8
    Join Date
    Mar 2005
    Posts
    56
    furnace bonnet temp (before evap) should be limited to 88F max. This will prevent overloading the indoor coil, which is now in cooling mode, with too much heat. it also should keep discharge air no higher than 68-70F which is desirable during defrost, so you dont satisfy the heating call while in mid-defrost.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,755
    Quote Originally Posted by BaldLoonie View Post
    The defrost board in the HP will trigger heat during defrost. I'd put in a limit above the heat exchanger so the burner can't put too much heat in the coil on a long defrost. We had dual fuel oil heat for years in our office.
    Bonnet sensor should be used on any oil furnace with a heat pump, but they are really needed on over sized oil furnaces.


    OP, don't put the evap in the return, it will rot out the furnaces heat exchanger prematurely.
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  10. #10
    Join Date
    Aug 2012
    Location
    Vancouver Island, British Columbia
    Posts
    16
    Quote Originally Posted by BaldLoonie View Post
    The defrost board in the HP will trigger heat during defrost. I'd put in a limit above the heat exchanger so the burner can't put too much heat in the coil on a long defrost. We had dual fuel oil heat for years in our office.
    That's an excellent idea for a heatpump with a coil in the return I will definately pass that on to my boss and we can pick away at it.

    But when there is a coil in the supply we use an isolation relay so that the furnace and heatpump will never run at the same time so it will not overheat the coil. Wondering if there is something that we could use in cooperationwith the isolation relay?

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,755
    Quote Originally Posted by VanIsleRefrig View Post
    That's an excellent idea for a heatpump with a coil in the return I will definately pass that on to my boss and we can pick away at it.

    But when there is a coil in the supply we use an isolation relay so that the furnace and heatpump will never run at the same time so it will not overheat the coil. Wondering if there is something that we could use in cooperationwith the isolation relay?
    Manufacturers tell you not to put the indoor coil in the return.

    Use a dual fuel thermostat with an outdoor sensor, no extra relays required then. And your customers will get what they are paying for then. Economy, and comfort.
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  12. #12
    Join Date
    Dec 2010
    Location
    Virginia
    Posts
    353
    I always wondered why people worry so much about how much heat is being blown over the evap coil during defrost in dual fuel applications, I dont see why it matters what the air temp is going over an evaporator in ac mode?

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,755
    Quote Originally Posted by millertime77 View Post
    I always wondered why people worry so much about how much heat is being blown over the evap coil during defrost in dual fuel applications, I dont see why it matters what the air temp is going over an evaporator in ac mode?
    Its more important when it switches back to heat mode. over sized oil furnace tend to cause a high head and trip the HPS on some units. As far as when its in defrost, its not a real problem, but best if not left to rise to more then 100F.
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