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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.
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.
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.)
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.
Examine the dual fuel relay and terminal block.
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).
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."
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.
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Neal Renn who described diagnosing the problem of a heat pump that "only gets backup heat and no cooling" to describe the problem of a heat pump that insists on turning on backup heat when it is not needed.
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The Home Reference Book, a reference & inspection report product for building owners & inspectors. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Thanks also to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website,
SJM Inspection Service LLC, serves the entire state of CT, sjminspect.com 203-543-0447 or 203-877-4774
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.