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  1. #1
    Join Date
    Apr 2012
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    Parkesburg Pa
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    Add heat pump to existing oil furnace

    I'm buying a new house that has a Bryant oil furnace with conventional air conditioning. I only casually looked at it, but tomorrow afternoon I'll be making a much more detailed inspection of the unit as well as trying to fire it up. The indoor unit looks beautiful, but the condenser looks 20 years old. The house was built in 93 so it's very likely the system is original. The house is a bank owned property so there is no history with the house.

    I was thinking of adding a heat pump to the system and running dual fuel to save costs. Right now the unit will cost about $32 per million BTU output where as a heat pump will be about $18. Based on degree days if I switch over at 40 degrees to oil I'll have an average cost of about $20 per million BTU. If I can find electric for cheaper than Peco I'll do even better.

    The ductwork is fiberboard and they used an uncased coil and used the fiberboard plenum to house the coil. I was going to cut the fiberboard up high enough and slide in a cased coil above the furnace. I normally work on heat pumps with electric backup or natural gas furnaces. This will be my first oil/HP hybrid. I'm looking for some pointers so I get the best efficiency. For one I saw that there can be an issue with the temperature rise when in defrost when running the oil furnace for heat. I saw that I could run a temperature limit switch or possible space the coil further away from the unit. Is the problem the temperature load on the coil when it's in A/C mode or is it the combined temp when it exits defrost and begins producing heat?

    What thermostats would you recommend? I'd like a 7 day programmable with an OAT sensor and a lockout so I can prevent the oil from coming on when the temperature is above say 40 degrees outside. Will that cause the unit to send cold air out the ducts if it goes into defrost mode? Or would the oil kick in then? Can/should I put an electric coil in for use with defrost or is that getting too complicated?

    Another option would be to simply rip out the oil furnace and go with all electric. The cost per million BTU would be slightly higher at around an estimated $22 based on the DOE calculator but it's not that far off and it's certainly cheaper than the oil costs. Lastly, since I have propane for cooking I could convert to a high efficiency propane furnace with HP and split the cost difference and be at about $21/million BTU.

    i'll have specs, pictures and more information tomorrow afternoon when I do my inspection and I'll post them here.

    EDIT: I can add that it has a Beckett burner with an outside combustion air setup.

    Until then, Thanks
    -=David=-

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    Dual fuel makes sense. We had a heat pump with oil backup in our office for years. Old mechanical DF kit but now with a Vision Pro or the like, pretty simple. Probably would want to add a plenum thermostat to be sure the oil has cooled down before the pump comes on if backup is allowed. If set so no backup, not an issue.

  3. #3
    Join Date
    Apr 2012
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    Parkesburg Pa
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    Thread Starter
    Would the plenum thermostat be integrated with the vision pro or more mechanical like a NC limit switch that opens the connection to the HP above a certain temp?

    Also, would that be placed above or below the coil? I'd assume below, correct?
    -=David=-

  4. #4
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    When doing the DOE calculations are you considering cost of the backup only? How does oil/propane/electric compare just for the backup? How much would the backup run over the course of a typical winter? What fuel source are you using for the water heater?

  5. #5
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by cobra2411 View Post
    Would the plenum thermostat be integrated with the vision pro or more mechanical like a NC limit switch that opens the connection to the HP above a certain temp?

    Also, would that be placed above or below the coil? I'd assume below, correct?
    Bellow would be correct.
    With the older style barrel heat exchangers it is best to ad a transition section on top of the furnace to raise the A coil up to provide adequate airflow distribution across the entire coil.

  6. #6
    Join Date
    Apr 2012
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    Parkesburg Pa
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    Thread Starter
    Ceilings are 9' or so, so that shouldn't be a problem. How much of a transition should I add?
    -=David=-

  7. #7
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    Apr 2012
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    Parkesburg Pa
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    Thread Starter
    Quote Originally Posted by 54regcab View Post
    When doing the DOE calculations are you considering cost of the backup only? How does oil/propane/electric compare just for the backup? How much would the backup run over the course of a typical winter? What fuel source are you using for the water heater?
    I looked at the heating degree days with 65 degrees as a baseline and then again with 40 degrees as a baseline and created a ratio that I used to calculate the costs. So assuming I use heat only below 65 and I switch to backup at 40 degrees then the ratio should be valid at least when comparing per million BTU's. I have 4412 degree days with a base of 65 degrees and 615 degree days with a base of 40. So if my math is correct I'll be below 40 approximately 14% of the time. So 14 * 32 (oil cost) + 86 * 18 (HP) / 100 = ~20.

    Here are my estimates that I calculated using Philly airport as my location. Everything is per million BTU. I used my current Peco electric rates but I'm going to be using PPL as that's who services the location and I don't have their rates handy. I doubt it will change much.

    $32.53 - Oil, 82% efficient
    $43.11 - Electric heat (resistance 100% efficient)
    $18.39 - Heat Pump, HSPF 8.0 (HP Only)
    $22.63 - Heat Pump with electric coils, adjusted HSPF 6.5 (KPHL)
    $20.37 - Heat Pump 8.0 + Oil 82%

    Hot water is via electric.
    -=David=-

  8. #8
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    Since you already have an oil heater in good condition and are replacing the AC condenser anyway, the Heat Pump + Oil sounds like the most logical option.

    When doing a comparison figure only when below 40f (why such a high temp for switchover?) for fuel costs. Resistance/Propane/Oil.
    14% may be the total TIME below 40f, but the total ENERGY used below 40f is a different story.

    What are propane costs vs. electric for the water heater replacement? Do you plan on keeping all 3 sources of fuel in the house?

  9. #9
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    Apr 2012
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    Parkesburg Pa
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    Thread Starter
    Quote Originally Posted by 54regcab View Post
    Since you already have an oil heater in good condition and are replacing the AC condenser anyway, the Heat Pump + Oil sounds like the most logical option.

    When doing a comparison figure only when below 40f (why such a high temp for switchover?) for fuel costs. Resistance/Propane/Oil.
    14% may be the total TIME below 40f, but the total ENERGY used below 40f is a different story.

    What are propane costs vs. electric for the water heater replacement? Do you plan on keeping all 3 sources of fuel in the house?
    Yes, I plan on keeping all three right now. I want propane for cooking - no questions... I haven't looked at the cost difference between electric/oil/propane/HP hot water - yet... There's an electric tank there that works.

    I've tried to compare apples to apples so that's why I went with the million BTU costs. Sure when it's below 40f I'm going to be at $32/mBTU. But that's only about 14% of the time. The rest of the time I'll be at $18/mBTU. So by the end of the year I would expect my total fuel costs to average $20/mBTU. How many actual BTU's I'll use is a different story.

    The furnace is a Tempstar Lincoln Series model number OHB5-F059-85-3 / BHF-5 with a Beckett AFII 85 burner I believe. It could be the 150 but I think it's the 85. The furnace has the 0.50 nozzle installed for an output of 73,500 BTU. My Manual J suggest a heat lost of around 57k BTU, so that seems inline. It's got a side vent kit on it also.
    -=David=-

  10. #10
    Join Date
    Jan 2004
    Location
    PA
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    You can probably set your heat pump lock out temp to 30 maybe even 25. Heat pump COP would still be well above 1.5. If you use a thermostat like the IAQ. You can set the furnace lock out at 35, and the heat pump lock out at 30 with a 2 degree droop. This will use the heat pump first between those temps, and if the indoor temp drops 2 degrees, it will switch over to the furnace.
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  11. #11
    Join Date
    Feb 2008
    Location
    harwinton, CT
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    Is the ducting system zoned? If so I like the EWC bm3000 panel... Even if there is no zones they are nice control panels because you can use any heat/cool stat and the panel has the inputs for outdoor air sensor and supply air sensor and has options for controlling the staging.

  12. #12
    Join Date
    Nov 2013
    Location
    Quebec, Canada
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    for the defrost, I generally install a plenum nc stat, 70-90* between the combustion chamber and the coil. This is only to inhibit the furnace during defrost demands, limiting the potential output and preventing pressure issues.

    to date, I have yet to have a call caused by high pressure due to a furnace starting after a call for first stage heat is terminated. This is assuming a proper dual fuel board or lockout relay system in used to prevent simultaneous operation. The heat pump will not throw a high pressure code if there is no demand to Y1.

    as for height above the combustion chamber, when possible I try to maintain a minimum of 12"(spec sheets normally require a minimum of 6" for non metallic evap pans.).

  13. #13
    Join Date
    Apr 2012
    Location
    Millsboro, DE
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    I'd be quite surprised if the fibrous plenum isn't prohibited by Code, Manufacturer's instructions and/or codes referenced therein: It is "combustible" by definition.

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