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This website 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. 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.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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.

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

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 SYSTEMS.

If your air conditioning 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
• 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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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING SYSTEMS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNITS
BACKUP HEAT
CLEANING & Legionella BACTERIA
COMPRESSOR & CONDENSING COIL
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
ENERGY SAVINGS in BUILDINGS
EVAPORATIVE COOLING SYSTEMS
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
HUMIDITY LEVEL TARGET
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
  What to check first
  Compressor failure diagnosis
  Duct & Air Handler diagnosis
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
  REFRIGERANT LEAK DETECTION
  REFRIGERANT LEAK REPAIR
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
  OPERATING CONTROLS
THERMOSTATS
  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
CRITICAL DEFECTS
Air Conditioning "How To" Books

• 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.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, 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 design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
• 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.).
• ...

More Information on Building Diagnostic Inspections and Repairs

• 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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Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Building Inspection, Problem Diagnosis, Forensic Investigation & Testing, Repair Consulting

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