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  1. #1
    Join Date
    Apr 2009
    Posts
    19

    Default Heat Pump "Emergency Heat" and Heating Cycle

    I have a question regarding 'emergency heat' with respect to HVAC systems. I had a new Unico system installed with an American Standard 15 SEER compressor and an electric duct heater. The duct heater has separate electric elements (three stages) We had a cold nght the other night, about 12 degrees, and woke up in the morning with the house a cool 54 degrees.

    After talking to the installer - he informed me that I need to switch the unit to 'emergency heat' in order to maintain the house internal temperature during very cold nights. They also informed me that the supplemental heat does NOT run when the heat pump is running and will not automatically switch to a second stage electrical heat when the heat pump can't keep up.

    I had thought that the heating cycle would be a multi range cycle:

    1) heat pump only
    2) heat pump & 1st stage electric
    3) heat pump & 2nd stage electric
    4) emergency heat (in the event of a heat pump failure of some sort)

    Is this correct or should the duct heater only run when manually switched 'on'? Also - what would be the lowest outside air temp that I can expect the heat pump to generate heat? I have heard as high as 28F to as low as 6F.

    Thanks

    Brian

  2. #2
    Join Date
    Aug 2007
    Location
    Maryland
    Posts
    1,604

    Default Re: Heat Pump "Emergency Heat" and Heating Cycle

    The auxilliary heat should be automatic when the heat pump can't keep up. This can be done by outside sensor or a 2 stage thermostat. It's been a while since I dealt with heat pumps so the technology may have changed some. Having to turn on the aux. heat manually is bogus.
    Besides, when the heat pump is in defrost mode the aux heat should come on.

  3. #3
    Join Date
    Aug 2008
    Location
    Midwest
    Posts
    179

    Default Re: Heat Pump "Emergency Heat" and Heating Cycle

    I am really concerned that a contractor would tell you that it is normal to get up in the middle of a cold night and flip the switch. I would call Unico and ask them for the sequence of operation, you did not list your Unico model number. Unico is a very nice system. Usually the first stage of heat is heat pump, if it can't maintain the temperature within about 2 degrees of setpoint, the thermostat brings in stage two. It energizes the electric heat relay which poweers the resistance heaters. These strips will have sequencers (timed switches) which are preset to energize individual elements based on time (for example at 20,40, and 60 seconds. This is to prevent large voltage drops which would happen if they all started at the same time. If you switch the stat to Emergency, or the pump goes into a defrost cycle, the strips will come on. The emergency position will also lock out the heat pump in most cases. Your system may be different, you may have 3 stages of heat, but I do not know.
    Last edited by ZZZ; 12-14-2010 at 10:58 PM.
    "Lead by Example"

  4. #4
    Join Date
    Dec 2007
    Location
    MN
    Posts
    455

    Default Re: Heat Pump "Emergency Heat" and Heating Cycle

    Quote Originally Posted by rangercap View Post
    I have a question regarding 'emergency heat' with respect to HVAC systems. I had a new Unico system installed with an American Standard 15 SEER compressor and an electric duct heater. The duct heater has separate electric elements (three stages) We had a cold nght the other night, about 12 degrees, and woke up in the morning with the house a cool 54 degrees.

    After talking to the installer - he informed me that I need to switch the unit to 'emergency heat' in order to maintain the house internal temperature during very cold nights. They also informed me that the supplemental heat does NOT run when the heat pump is running and will not automatically switch to a second stage electrical heat when the heat pump can't keep up.

    I had thought that the heating cycle would be a multi range cycle:

    1) heat pump only
    2) heat pump & 1st stage electric
    3) heat pump & 2nd stage electric
    4) emergency heat (in the event of a heat pump failure of some sort)

    Is this correct or should the duct heater only run when manually switched 'on'? Also - what would be the lowest outside air temp that I can expect the heat pump to generate heat? I have heard as high as 28F to as low as 6F.

    Thanks

    Brian
    I think you are missing some basic points regarding the operation of the heat pump. Heat pumps will have a minimum temperature range in which it will run. When the outside temperature gets to cold the heat pump won't run efficiently so it's adjusted to have a minimum setpoint. In areas where the outside temperatures go below that setpoint heatpumps will have an auxillary heat source. Many times that will be electric heat strips or some will use a gas,propane or oil furnace as the aux heat.
    Heat pump coils will also frost up which the unit will go into a defrost cycle. Durnig the defrost cycle your heating system may blow cool air while the unit defrosts.
    Usually heat pumps will be configured to switch to aux heat coils automatically rather having to maually switch to emergency.
    Maybe if you lived in Florida you might need to manually switch on the aux. coils since it's not likely the outside temperatures would dip to low for the heat pump to supply enough heat.

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