Heat pump control (C) D Friedman N RennHeat Pump Backup Heat Outdoor Thermostat or Control Relay Diagnosis, Inspection, Repair Guide
     


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Heat pump back-up heat thermostat diagnosis & repair guide: here we discuss how to troubleshoot problems with thermostats on heat pump systems that use back-up heat, including problems with backup heat that does not turn on or off when it should.

This article series answers most questions about central air conditioning & heat pump system troubleshooting, inspection, and repairs. We describe how to inspect residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects.

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Troubleshooting backup heat control relay or thermostat problems on heat pump systems

Article Contents

How to diagnose & repair backup heat that turns on when it should not.

  • Heat Pump Provides Heat when in Cooling Mode: if your heat pump is heating when it should be cooling there may be a simple problem with a thermostat, thermostat setting, or with a temperature sensor or control inside or outside the building. "Only gets heat when in cooling mode."

The following diagnostic tips were provided by a thoughtful reader, Neal Renn who describes the problem of a heat pump that insists on turning on backup heat when it is not needed. That is, during the cooling season, the heat pump insists on providing warm air rather than cool air to the building.

The family woke up to an 85 degree house even though the weather remained in cooling season. The occupants found that the heat pump system was running in heat mode. (A Goodman™ 5 ton heat pump and Goodman indoor air handler with propane backup heat.

  1. The indoor thermostat was checked to be sure it was set to cooling mode. (Thermostats might be set to "heat", "cool", "Auto", or "OFF" depending on the model.)
  2. Backup heat on: The occupants observed that the propane heater was running even though their indoor thermostat was set for cooling and outdoor temperatures and indoor temperatures were high enough that cooling was required.
  3. Examine the dual fuel relay and terminal block.
  4. Examine the outside compressor condenser unit - check the outside thermostat (GE Morrision). If the outdoor thermostat is not responding properly the system defaults to heat.

Detailed Case of a Heat Pump that Put out Heat when Cooling was Required

The following diagnostic details were provided by a thoughtful reader, Neal Renn.

On my heat pump system, the thermostat is set for Normally Closed when the temperature is above the temperature setting. When the temperature drops, the contacts cycle. This is to pull in [turn on] the heat pump's backup heating system. When the thermostat lost it freon charge, the thermostat cycled and diverted to the emergency heat position. When the thermostat calls for emergency heat, it routes the request to the outdoor heat pump compressor to the white, heat control wire - always.

This is for my system. The replacement thermostat used different terminal designations but worked the same way. The whole heat pump outdoor thermostat repair kit was $44 and was available for same day delivery.

Heat Pump - Backup Heat Control Problem Identification

Indoor heat request at the indoor thermostat only gets heat from the backup heat system not from the heat pump: when you request heat at the indoor thermostat, if you get heat from the backup heating system only, something is wrong with the outdoor compressor or with the heat pump controls. Normally when the indoor thermostat calls for heat, the heat pump will be used to provide heating until until the request is more than 2
degrees above ambient air at the thermostat, or when the outside ambient air temperature is below the outside
thermostat setting. The outdoor thermostat will usually be found in the outdoor compressor/condenser unit.

Indoor cooling request at the indoor thermostat only gets warm air, coming from the heat pump's backup heater, and it's uncontrolled, that is, the system will heat forever.

Heat Pump Backup Heat Control Problem Diagnosis & Repair Steps

The first clue was that the heat came on while cooling (air conditioning) was requested on a heat pump system. If weather is intermittently cool enough that you might be calling for heat, this problem may remain hidden until warm weather makes you sure that your indoor thermostat should only be calling for cooling.

Here are a couple of photos of the offending part. It is housed in the control box corner of the outside heat pump compressor/condenser unit. This heat pump outdoor unit has a control board, a contactor, a start capacitor and the outdoor thermostat which is used to determine when backup heat is needed. Photographs courtesy of Neal Renn show a Goodman Manufacturing Corp. outdoor thermostat # OT18-60A (below left) and a back view of the control showing additional part numbers #B13708-66 (below right).

Heat pump outdoor thermostat control (C) D Friedman N Renn Heat pump control (C) D Friedman N Renn

When I first started looking at this, we thought the worst - a failed outdoor compressor unit.

First try the indoor heat pump thermostat: the indoor wall thermostat was checked for proper settings (calling for cooling), but even though indoor temperatures were well above the indoor thermostat's cooling temperature set point, the heat pump would not operate.

Next check the outdoor heat pump controls: Warning: removing the covers to access equipment controls can expose you to potentially fatal shock hazards. Do not attempt these steps unless you are properly trained. The diagnostician, (the owner in this case) pulled the cover from outside heat pump unit controls and pushed the contactor switch
in manually. This caused normal compressor action - it began running.

If the heat pump or air conditioning compressor runs in response to this step when it would not turn on in response to the thermostat it is natural to next suspect a problem with the heat pump controls, not the compressor unit itself. (There are exceptions such as a hard starting compressor that might sometimes start easily but have trouble starting against a head pressure when it was just turned off moments before.)

Investigating the heat pump controls further: being sure that the " heat only" problem was a control problem - either in the wall thermostat (indoors) or in some other control the owner tried swapping in another thermostat (in this case the old, original mercury unit). The result was the same symptoms, un-changed.

Checking the heat and backup heat controls: That's when the owner started digging into the dual fuel kit and its wiring.

Diagnosing a gas-charged outdoor heat pump thermostat sensor problem: This heat pump unit was installed originally in 1993. The old outdoor thermostat was a B13708-66. That thermostat was later superceded by a Goodman™ B13708-67. Mr. Renn wrote: the more I think about the uncontrolled heat issue when the thermostat loses it's charge, I'll bet Goodman reversed the contacts in the replacement control so that it will simply not turn on the heat pump compressor and you get NO heat on a thermostat failure. I have not confirmed this - but from a safety standpoint, it would make sense. We left our house unattended for two months in the summer while the family traveled. If this failure would have happened unattended, we would have had a burned up furnace, minimum, house most likely.

Replacing the outdoor heat pump thermostat involved a "bolt-on" replacement part, connecting wires according to the manufacturer's instructions, and testing the unit again.

What caused the failure of the outdoor heat pump thermostat? a plastic nut holding the supply line to the inlet valve. The nut was cut with a sharp edged relief at the base of the thread - which defined a nice - "break here" mark. Immediately the owner replaced all supply nuts with brass (only had 2 that were not). He observed that another of these plastic connecting nuts appeared ready to fail like the first. He also finally installed the pressure regulator for the house and cut the line
pressure back to about 75 psi. We were only about 90 before. "Stressed plastic will fail."

For photographs of other air conditioning and heat pump parts, and for an explanation of where these air conditioning components are physically located, see A/C COMPONENTS which discusses  Indoor A/C Components and Outdoor A/C Components

Since the failure of the heat pump's backup heat to turn on, loss of heating capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioning or heat pump system, after reviewing the lost backup heat diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below.

To return to our air conditioning, heat pump, and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.

If your air conditioning or heat pump system won't start see

Continue reading at LOST COOLING CAPACITY or select a topic from the More Reading links shown below.

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