Heat Pump Backup Heat Outdoor Thermostat Diagnosis, Inspection, Repair Guide
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Troubleshooting backup heat problems on heat pump systems that provide both air conditioning and heating

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The articles at this website describe the basic components of an air conditioning system and then we discuss how to estimate the rated cooling capacity of an air conditioning system by examining various data tags and components. The limitations of visual inspection of A/C systems are described. We continue to add to and update this text as new details are provided.

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How to diagnose and fix the backup heat in a heat pump system that is not working is discussed at BACKUP HEAT for HEAT PUMPS.

Diagnosing 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

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."

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 has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
BACKUP HEAT: on heat pumps, types of backup heat; problems with backup heat; begin here if your heat pump is not providing enough heat or if your air conditioning system provides heat when it should be providing cooling.
COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units including noises and compressor hard-starting repairs
A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
Dehumidification Problems - Air conditioner cools but does not dehumidify
DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
Fire dampers, and Heating and Cooling Air Duct Controls such as manual and automatic duct dampers, zone dampers, and fire dampers are discussed and distinguished at DRAFT REGULATORS - barometric damper
LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system
- What to check first if there is no cool air or not enough cool air
- Compressor failure diagnosis: basic checks of the air conditioner compressor
- Ducts & Air Handler diagnosis: basic checks of the indoor air handler (blower), air ducts, and filter system
OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
ZONE DAMPER CONTROLS . discusses manual and automatic air duct zone controls

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AIR CONDITIONING & HEAT PUMP SYSTEMS

A/C - HEAT PUMP CONTROLS & SWITCHES

AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING

AIR FILTERS for HVAC SYSTEMS

AIR HANDLER / BLOWER UNITS
BACKUP HEAT
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
COMBUSTION GASES & PARTICLE HAZARDS

COMPRESSOR & CONDENSING COIL, A/C

CONDENSATE HANDLING, A/C
CONDENSATION or SWEATING PIPES, TANKS
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
COOLING LOAD REDUCTION by ROOF VENTS
COMBUSTION GASES & PARTICLE HAZARDS
CONDENSATE HANDLING, A/C
CONDENSATION or SWEATING PIPES, TANKS

DATA TAGS on AIR CONDITIONERS
DEFINITION of Heating & Cooling Terms

DEHUMIDIFICATION PROBLEMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE

DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DUCT SYSTEM & DUCT DEFECTS
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST CONTAMINATION FROM HVAC?

EDUCATION, HVAC SCHOOLS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
EVAPORATIVE COOLING SYSTEMS
EVAPORATOR COIL or COOLING COIL
EXPANSION VALVES, REFRIGERANT

FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FAN NOISES
FURNACES WARM AIR HEATING SYSTEMS

GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION & MEASUREMENT
GAUGE, REFRIGERATION PRESSURE TEST

HEAT LOSS (or GAIN) in buildings
HEAT PUMPS
  BACKUP HEAT for HEAT PUMPS
  HEAT PUMPS, GROUNDWATER
  HEAT PUMP STRATEGY - Indoors?
  HEAT PUMP Thermostats - Outdoors

HUMIDITY LEVEL TARGET

INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS

LEED GREEN BUILDING CERTIFICATION

LOST COOLING CAPACITY
  What to Check First
  A/C Flow Too Weak
  A/C Filter Problems
  A/C Compressor Problems
  A/C Off - Condensate Pan Switch
  A/C Cooling Coil Icing
  A/C Not Dehumidifying
  A/C Air Duct Problems
  Air Conditioner Won't Start
  Air Conditioner Refrigerant Problems
  Blower Fan No Start / No Stop
  Compressor Diagnosis: Diagnose & Repair
  Cooling Capacity of the Duct System
  Repair Guide Master List

MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH

NOISE AIR CONDITIONER / HEAT PUMP

OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements

PORTABLE ROOM AIR CONDITIONERS
PRESSURE READINGS, REFRIGERANT

REPAIR GUIDE, AIR CONDITIONERS / HEAT PUMPS
REPAIR & DIAGNOSTIC FAQs for A/C

REFRIGERANTS
  GAUGE, REFRIGERATION PRESSURE TEST
  REFRIGERANT CHARGING PROCEDURE
  REFRIGERANT DRIERS & FILTERS
  REFRIGERANT LEAK DETECTION
  REFRIGERANT LEAK REPAIR
  REFRIGERANT METERING DEVICES TEVs
  REFRIGERANT METERING CAPILLARY TUBES
  REFRIGERANT PIPING & DISTANCES
  REFRIGERANT PRESSURE READINGS
RETROFIT SIZING for A/C or HEAT PUMPS

SEER RATINGS & OTHER DEFINITIONS
SOLAR ENERGY SYSTEMS
SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS
SWAMP COOLERS
SYSTEM OPERATION
  OPERATING CONTROLS

THERMOSTATS, HEATING / COOLING
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

THERMOSTATIC EXPANSION VALVES

WATER COOLED AIR CONDITIONERS

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.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
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 5/16/07

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"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]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
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 ($69.00 U.S.).
Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation

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Diagnosing backup heat that turns on when it should not.

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

Heat Pump - Backup Heat Control Problem Identification

Heat Pump Backup Heat Control Problem Diagnosis

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair