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  1. #1
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520

    Comparison of Fuel Costs

    If the energy consumed by the blower is included in the HSPF ratings for a heat pump, then should the power consumed by the blower and inducer be factored into the calculations for total energy costs of a furnace vs. a heat pump?

    For example if I removed 50 watts from the furnace for the inducer and approx 30% of the energy for the blower (since it's heat is recovered with a COP of 1 while my equivalent COP for the furnace is 2.3, but it runs about 1/2 as long) so about another 50 Watts, so 100 Watts total, my balance point shifts by 5 degrees.

    Should I factor in the inducer and part of the blower power in the furnace operation since it's included in the heat pump operation? In my case, that 5 degree shift has a significant impact on how much the heat pump will be utilized.

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Quote Originally Posted by motoguy128 View Post
    If the energy consumed by the blower is included in the HSPF ratings for a heat pump, then should the power consumed by the blower and inducer be factored into the calculations for total energy costs of a furnace vs. a heat pump?

    For example if I removed 50 watts from the furnace for the inducer and approx 30% of the energy for the blower (since it's heat is recovered with a COP of 1 while my equivalent COP for the furnace is 2.3, but it runs about 1/2 as long) so about another 50 Watts, so 100 Watts total, my balance point shifts by 5 degrees.

    Should I factor in the inducer and part of the blower power in the furnace operation since it's included in the heat pump operation? In my case, that 5 degree shift has a significant impact on how much the heat pump will be utilized.
    For the sake of the audience, perhaps we should distinguish between "economic balance point" and "thermal balance point". Thermal balance point is when the heat pump is producing the same amount of heat the structure is losing to the outdoor cold air. Economic balance point is when does it become more feasible to run the back-up source of heat by itself vs. combined with the heat pump (locking out the heat pump when outdoor temperatures are very low).

    In your case it looks like you're talking about a dual fuel system, so there is no combining of heat sources typically, since the indoor coil is above the furnace heat exchanger and can't be used while the furnace is in operation. It then becomes a matter of determining the point of changeover from heat pump alone to furnace alone. I would think local fuel and electric costs would figure, along with personal comfort...if the heat pump is running like crazy to just hold your setpoint, but you're feeling drafty and cool, lock it out and turn on the furnace. The heat pump gives you the edge during cool weather, the furnace during cold to very cold weather.

    And offhand, I'd say yes, the inducer and blower motors should figure into energy consumption of the furnace, not just cubic feet of gas used.

    In the end, you may find you need to strike a happy medium between economic balance point and a third one...comfort balance point.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I just wanted to avoid running my heat pump when it's less economical than the furnace. But accurately determining that balance point isn't easy since you have to factor in defrost. Ideally I'd have differnt balance points for morning vs. afternoon due to higher RH in the morning, when defrost is more likely to occur.

    Of course the Visionpro IAQ gives you several different options for how to set balance points.

    I'll probably go ahead and lower the HP lockout temp from 30F to 25F which is pretty close to my thermal balance point, but leave the aux lockout temp where it is at 35F (using option 2 on function 0345) where my the COP of the HP has a clear advantage. from 25F-35F both the furnace and HP and fairly close depending on the outdoor humidity levels, and indoor temp.

    I actually find the HP more comfortable most of the time than the furnace because the increased air movement and longer run times create more even temperatures and better circulate warmer air at the ceiling. I also run the furnace at the higher of the 3 blower speed options for the same reason.

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