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York heat pump/auxilliary element heat

Got a York system that had  a double mercury switch thermostat (4 mercury vials).  Does this indicate a 2 stage system? Customer wanted a set back thermostat.  Installed and system working.  By putting on emt switch I can get heat pump to cut out and heat only with the electric elements in the air handler.  But when things warm up a bit, is it possible to only heat with the heat pump and if so how? BTW, at what temp do you recommend the customer notuse the heat pump part of the system? Thanks, Ken


  • Empire_2
    Empire_2 Member Posts: 2,343
    MFG. w/ different control

    To answer you Q;, yes it is going to control stg,1 and 2.  Having said that, on a balmy day when HP is up and running stage 1 will control the HP and stg 2, if the temp continues to drop will initiate your supplemental back up heat ( Same as your Emergency heat). As temps continue to fall, at some point, say,....35* a control, temperature actuated relay (gen. located in condenser) will kill the HP circuit and automatically revert to your supplemental heat as stage #1.  This way the customer has nothing they have to do.  EMG heat switch is nothing more that a way for the customer to switch to the supplemental heat in extreme conditions if they feel there is a problem with the system.  Bear in mind that manufacturers do not all do the same control of this type of system.  So, it's nice to have a wiring diagram to see what they are trying to do, and when.  That is key.  In a perfect world everyone would do the same thing, but that's where an experienced tech. comes in.


    Mike T.
  • Spence
    Spence Member Posts: 316
    HP Operation

    If you have installed a fairly sophisticated programmable thermostat, you can set parameters for heat pump and electric heat operation by choosing options in the installers guide. However, I fail to understand why you would want to do this, as locking out either HP or back-up heat is normally reserved for fossil fuel combinations. The COP of electric heat is 1; a heat pump will always be more than 1 as long as the ambient is above -17 degrees. Run a heat loss, then plot that loss on a balance point chart. Look up the capacity of your HP at 47 and 17 degrees and plot that on the chart. This gives you exactly how much heat you are producing as compared to the heat loss at given ambients. I believe you will find that adding ODTs on the strip heaters to stage them based on your BP chart findings will make your client more comfortable and happier with their energy consumption.
  • unclejohn
    unclejohn Member Posts: 1,766

    Pretty much covered it. But the answer as to when to turn off the heat pump is never or, when its not working at all.