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  1. #1
    Join Date
    Dec 2010
    Post Likes

    Question Need fan below balance point (HP with oil aux)

    I just had a new Trane 15i heat pump system installed. I still have my old oil fired boiler, and it works OK. They installed a heat exchanger for the hot water from the oil boiler just after the new Trane air handler.

    The installers claimed that my new heat pumps are so great, I should never need to use my old oil boiler. However, they showed me that I just had to manually change the system setting on my thermostat from "heat" to "em heat" if I ever wanted to use my oil boiler.

    I have the Trane 820 thermostat, which I believe is just a re-labelled Honeywell Pro 8320.

    I reading up on the thermostat, I found that it could be set up to automatically switch between my heat pump and my oil boiler based on outside temperature. I went there and added the outside temperature sense, and modified the thermostat's installer settings. For example, I set "0340" to "02" and "0350" to "35".

    I watched that evening as the outside temperature drop below 35deg, and yep, my heat pump compressor turned 'off' and my oil boiler turned 'on'. Great!

    However, after a few seconds, I realized that the fan had turned 'off' too, when the compressor turned 'off'.

    Not good. I need the fan to continue to run. BTW, the fan does run when I manually select "Em Heat".

    I went back and started to re-read all the docs I had. I then found this one paragraph in the thermostat's installation manual, in the section dealing with having the thermostat control fossil fuel aux heating.


    When the outdoor temperature is below the selected
    Balance Point Temperature (ISU 0350), only the Fossil
    Fuel (auxiliary heat) operates and the fan (G terminal)
    does not energize
    when the comfort control calls for heat.
    So, apparently, the fan (G terminal) not being energized is proper operation.

    Is there anyway around this? Is there some other installer setting that could get changed to allow the fan to run when I am below my set balance point? Fan control, "0180", looked promising, but it isn't offered.

  2. #2
    Join Date
    Sep 2006
    Post Likes

    As you may know the most common configuration of a heat pump is to have electric heating coils (strip heat) as the auxiliary and back up heat source for when the heat pump alone can not carry the load (then the strip heat cycles in and out to supplement the heat pump) or emergency heat for when the heat pump is not functional for whatever reason such as malfunction or ice accumulation.

    Thus, with this setup the heat pump continues to run and will do so at a higher level of efficiency than most other heating sources down well below 35*F.

    Unlike the most common dual fuel systems where a heat pump coil is added above the hot air furnace, you have your supplemental heat source above the heat pump coil. With the typical dual fuel it is not good for the heat pump to run with the furnace running at the same time. But in your case there is no detrimental effect of letting the heat pump run and have the hot water coil cycle in and out as needed to supplement the heat pump when it can not carry the full heating load.

    This would be the most economical way to run your system. If this is what you want your system to do then you should have your installers return and configure the system to run in this manner.

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