This article discusses the diagnosis & repair of backup heat systems used on heat pumps - how to find out if your backup heat is working or partly working, or not working at all.

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 and fix the backup heat in a heat pump system that is not working

Diagnosing no backup heat working or not enough backup heat

If you are getting no backup heat at all, or the heat is inadequate, your system may not be switching on the backup heat at all, or it may be trying to switch on the backup heat but the backup heat is not working.

Backup heat controls not working: when outdoor temperatures drop below a set point (varies by geographic location) the heat pump system will switch from cooling mode to heating mode.

In heating mode a heat pump is running pretty much the same as it did as an air conditioner, except in reverse.

That is, all of the same controls and components are involved.

In cooling mode where a heat pump is installed, the indoor evaporator coil is cooled in order to cool air blown across it. This is normal heating mode for the heat pump and backup heat is not called-for.

In heating mode the heat pump warms the indoor coil so that it will warm air blown across it, transferring heat from outside to inside.

When temperatures outside become too low for the heat pump to efficiently extract heat from outdoor air, a temperature sensor in the outdoor unit turns on the backup heat system.

Some of the critical controls that have to work in order for the heat pump to heat at all in cool weather, or to switch to backup heat mode include:

The indoor thermostat

- see THERMOSTATS

the outdoor temperature sensor

- see HEAT PUMP THERMOSTAT, OUTDOOR

the reversing valve that switches direction of refrigerant flow

- see REVERSING VALVE on HEAT PUMPS

Electric Backup Heat Not Working:

If the backup heat for your heat pump system is provided by electric heaters

see STAGED ELECTRIC FURNACES

below for an outline of the diagnostic steps needed.

The temperature at which backup heat should come on varies by where you live. Perhaps 35 degF. would be common.

If debugging the thermostats and temperature sensor do not lead to a fix, then the problem may be with the backup heater itself.

Hot water heat or water to air heat not working: 

If your backup heat is an oil or fired water-to-air system you'll need to check the operation of the heating boiler.

See HEATING LOSS DIAGNOSIS-BOILERS

for a detailed diagnostic procedure. You will need to examine no-heat diagnosis first of the controls and fuel source and then of the boiler fuel type itself, oil, or gas.

Warm air backup heat not working: 

If your backup heat is an oil or fired warm air system you'll need to check the operation of the furnace.

See HEATING LOSS DIAGNOSIS-FURNACES

for a detailed diagnostic procedure. As above, you will need to examine no-heat diagnosis first of the controls and fuel source and then of the furnace fuel type itself, oil, or gas (propane or natural gas) or electric.

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.

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

at HEAT PUMP THERMOSTSAT, OUTDOOR

for a step by step guide to diagnosing the cause of this particular "heat output only, no cooling" problem at a heat pump. Diagnostic details provided by a thoughtful reader, Neal Renn.

Here is a photograph of the heat pump outdoor thermosatt. 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

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 or HEAT PUMP COMPONENTS which discusses and describes how to identify the indoor and outdoor components of an air conditioner or heat pump system.

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.

Staged Electric Furnaces Using Sequencers to Control Heat

If the backup heat source for your heat pump system is provided by an electric furnace, use the diagnostic details below when the backup heat is not working properly. This information is discussed further

at ELECTRIC HEAT.

For economy, as Carson Dunlop Associates' sketch shows, electric furnaces often use a gang of electric heating elements that are turned on in stages rather than all at once.

As temperatures fall and more heat is needed in the building, more heating stages turn on. By leaving heating stages turned off when not needed we reduce electrical consumption and energy cost.

On a staged electric heating furnace each heating stage typically provides about 5,000 watts (5KW) of heating energy.

The fan limit switch [image] that controls an electric furnace may have a built-in delay so that on a call for heat the blower fan won't turn on until the heating element(s) have warmed up. We discuss fan limit switches in more detail

at FAN LIMIT SWITCH

How to Diagnose & Repair Electric Heat that Has Stopped Working or is Not Hot Enough

What do we check if our electric heat is not working? There are a few basic things to check yourself. Other steps require an expert. Sketches courtesy of Carson Dunlop Associates, Toronto [ carsondunlop.com ].

• Is the thermostat asking for heat? 
  
  Be sure you know where all of the thermostats for your electric heat are located.
  
  Some heaters such as flush-in-floor electric heaters may use a wall-mounted thermostat while other electric baseboard heaters may have individual thermostats on each baseboard unit. In a bathroom with electric radiant heat in the floor, Carson Dunlop suggests finding that hidden thermostat [image], perhaps inside the sink vanity.
• Is electrical power on to the heater? 
  
  Check the fuses or circuit breakers that supply each electric heater.
• If the electric furnace does not seem to be making heat,
  
   the electrician or heating technician will use an ammeter to test each of the heating elements to see if one or more of them is not working.
  
  She will also check first to confirm that electrical power is on to the unit. If one or more of the staged electric heaters in the furnace has failed, the furnace may make warm air, but not enough warm air in very cold conditions.
• Check the air temperature rise across the electric furnace.
  
  When the furnace has been operating for 20 minutes or longer, typically the temperature at the return plenum (the lower thermometer in the sketch) will show about 70 deg .F. and the supply plenum temperature will be between 120 deg f. and 125 deg f.
  
  If the supply plenum temperature is too high (over130 deg .f.) something's wrong and you should call a heating service technician.
• Check radiant heat ceiling panels
  
   for wire interruptions. If someone drove an nail into a radiant panel heat ceiling [image] or in the attic above they damaged a wire or heating panel, you may find that that room no longer has heat.
  
  We use a hand-held infra-red thermometer scanner to quickly check radiant heat floors and ceilings.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-03-30 - by Gale - Trane emergency heat comes on without being switched on.

My Trane emergency heat comes on without being switched on. The auxiliary heat works as normal when it reverses but sometimes it switches to emergency heat by itself, especially on a very cold night. Reversing valve, a switch, ?

On 2021-03-30 - by (mod) - My Trane emergency heat comes on without being switched on.

Gale

It's common for a heat pump system to be set such that backup heat will come on (in response to a call for heat at your indoor thermostat) automagically if an outdoor temperature sensor determines that outdoor temp is so low that the outdoor unit can't keep up with the call for heat.

Only IF that is NOT your case, would I think that this sounds like a problem with

- an outdoor temperature sensor

- a control board or relay in the outdoor unit

On 2021-03-23 by Pete - Bryant heat pump backup heat not working?

I need a little advice and hope that you can help me. I have a, 8 year old Bryant 213C heat pump & Bryant gas furnace.

This morning (at 35 degrees outdoors) I upped my thermostat from 64 to 69 degrees. The heat pump and furnace fan came on and ran for just a couple of minutes. Then, due to the call for 5 degrees of heat all at once, the Aux gas heat was initiated which makes sense. I heard the gas burner fire up and the vent pipe got hot.

My concern is this: I can hear that my heat pump is still running which of course concerns me as I learned that the 2 units (HP & gas furnace) should never run at the same time. I suppose it could be that the HP is only running in a different mode, such as defrost or possibly in some way that would not be harmful, but it worries. So, I immediately dialed back the thermostat to shut it down.

I don’t know how to confirm if this is a ‘dangerous’ situation or not. I don’t know if the heat pump is supposed to ‘run on’ in some otherwise safe mode temporarily. I am afraid that if I let it run on just to see if it eventually shuts down I might do damage in the meantime.

The fact is that it may have been doing this all along but since I installed a new thermostat I am being hypervigilant now. I am very confident that my thermostat wiring and setup is correct. I would feel better if I knew that the HP is supposed to behave like this under these conditions. Can you comment on this?

On 2021-03-23 - by (mod) -

@Pete, are we sure that the actual compressor/condenser was running at the heat pump outdoor unit? If so and if the backup heat was on, that'd be unusual and I would TURN OFF the equipment - at least the outdoor heat pump unit - and call your local repair technician for advice and repair. It could be a bad control relay or control board or, of course, something else.

- if you don't have the manual for your unit here is a copy from Bryant - among its "legacy" documents

BRYANT 213C HEAT PUMP OWNERS MANUAL [PDF] (2019) Bryant - 7310 W. Morris Street • Indianapolis, IN 46231, USA, retrieved 2013/03/23 original source: https://www.shareddocs.com/hvac/docs/1009/Public/03/OG-ACR410GEN-03.pdf

On 2021-03-23 by Pete: not sure the heat pump is running

Thank you for your comments and for the link to the documentation! I am not sure if the heat pump is actually running the compressor or doing anything that might be considered 'dangerous' or harmful to the system.

I would doubt that the system would allow for this. It just seems odd that the heat pump would continue making any sound at all once the aux gas furnace has engaged. This is my dilemma.

On 2021-02-25 by Carol E - where is the backup heat reset button?

Lost power from ice storm 10 days ago. Since then I don't think back up heat for heat pump has been working properly. Is there a rest button?

On 2021-02-25 - by (mod) -

@Carol E,

I'm guessing that your backup heat is electric - if not let me know.

Often backup heaters are not working but that problem remains un-discovered until there is in fact a call for back-up heat, so the problem may not itself be storm related.

If I'm right (electric backup) check first that the circuit breakers for that system are not in the tripped position. (Try turning the electric backup heat breaker(s) to the fully OFF position, wait a moment or three, then to the ON position).

If the circuit breakers are NOT tripped off, then

Open the cover to the furnace blower compartment and look for a red RESET button on or near the blower (fan) motor. Try pushing that button in (hear it click and see it stay "down").

Then re-close the blower compartment door, taking care to maker sure that the door is itself fully in position and latched shut.

This care is important because there is often a safety interlock switch on the blower compartment door itself that will keep power to the blower OFF when it thinks the door is open. That's to keep you from getting a finger cut off should the blower fan start while your hands are nearby.

On 2021-01-30 by Mel

My aux/backup heat won't go above 74° on BOTH heat pumps. I am attempting to purchase a home with this problem. I am having an electrician do a whole home inspection as there are existing electrical concerns. Is it possible the heat pumps' problems are associated with a greater electrical/wiring issue? Thanks, I 'll hang up and listen now...

On 2021-01-27 by Indy

On a dual fuel Bryant. When thermostat is set at least 5° higher then current indoor temperature. Gas heat doesn't come on. Yet when you put it in test mode the gas heat comes on just fine. Also noticed no outdoor temp sensor, so no set lockout temp. ( Wondering if that has something to do with it.

On 2021-01-27 - by (mod) -

Indy

Is it the backup heat that won't turn on? If so could be a tripped breaker or safety control.

On 2020-07-17 by Cindy - Can you test the back up heat on a heat pump system when the outside temperature is 90 degrees ?

Can you test the back up heat on a heat pump system when the outside temperature is 90 degrees ? I am selling my house and the house inspector said the back up heat doesn't work. It's 90 degrees outside and my HVAC guy said the inspector shouldn't even be trying to run the back up heat. Thoughts ?

On 2020-07-20 - by (mod) -

I think your HVAC service technician is correct

On 2020-03-06 by Linda - Heat pump compressor is down, will emergency heat work?

Trying to post a question regarding heat pump.

Heat pump compressor is down, will emergency heat work?

On 2020-03-08 by (mod) -

Linda

I can't say if your particular backup heat will work or not, since sometimes a backup system that has not been tested in some time may itself be defective or inoperative.

But in general, properly-installed, the backup heating system for a heat pump system is specifically designed and intended to be capable of providing heat to the building if/when the heat pump cannot operate - such as at very low outdoor temperatures.

On 2020-03-05 by Keith - Can i remove the gas and the heat pump still work on heat?

I have a house with a dual fuel propane backup heat pump. Can i remove the gas and the heat pump still work on heat?

On 2020-03-05 - by (mod) -

Keith:

You say the heat pump is dual - fuel. What's the second fuel?

Perhaps you mean it's a heat pump, normally operated by electricity, with backup heat provided by a propane fueled gas furnace / heater.

If that's the case and if you remove the propane then you have no backup heat.

On 2020-01-21 by J'net - heat pump can't switch to backup

Have dual heat unit. Came on for one cycle this morning, now nothing. Breaker is ok. Thermostat set at heat, but small print comes up that says aux heat. Nothing happens. Have e-heat for when power is out, but that's not working either. Any suggestions?

On 2020-01-21 - by (mod) -

J

It sounds as if your heat pump is telling you that it's trying to switch to backup heat and that your backup heat isn't working. Is your backup heat electric? Have you checked the power is on to that system?

 

On 2018-11-05 - by (mod) - -not getting backup heat

Re-posting:

I had a new furnace and Guardian heat pump installed over the summer. The a/c worked perfectly during the summer. I turned on the heat pump and it will run about five minutes, heats great, and then will still run with fan on but no heat. There is a Nest thermostat. The installer doesn't know what is causing the problem.

I would appreciate any suggestions you would have. - anon by private email

MOD Reply:

The good news, Anon, is that the installer is obligated to fix the new system and get it working properly. They can't simply give up.

If I were the tech I'd start by eliminating the thermostat from the equation by removing its wires at the heat pump end, jumping the thermostat heat terminals to call for heat, and see what happens.

On 2018-10-16 by Joe Hangge - heat pump won't switch to backup heat

Why won't the heat pump switch over to aux heat in really cold weather (<35 degrees)? the heat pump works in warmer weather (>35 degrees), and will keep the house heated to the set temp, but it won't switch over to aux heat (gas heat) in cold weather.

When the weather is really cold (<35 degrees), the thermostat says "aux heat source" (or something like that) when the heat pump can't support the heating needs in the cold weather, as it's trying to use the aux heat source (gas), but the furnace does not kick on with gas heat. (fyi, this is all in "heat" mode)

when i switch the thermostat to em heat (emergency heat), the furnace turns on with gas heat and will heat the house to the necessary temp.

On 2018-10-16 - by (mod) -

Joe

When a heat pump won't switch over to aux heating when it should I suspect

- a failed temperature sensor outdoors

- a failed control board in the indoor unit or outdoor unit

- the backup heat itself is not working; for example on occasion the service tech will find that electric heaters are burnt-out

Start by confirming that power is on to the backup heating system and if it's gas, that there is gas supply

 

On 2016-03-02 by Anonymous

Can I turn off the electric coils on my heat pump so only the pump works to heat my house

On 2016-03-02 by (mod)

If you turn off the backup heat system the heat pump will work to heat your house until outdoor temperature is so low that the heat pump can no longer extract enough heat - then it will try to switch to backup heat and you'll have no heat.

Question: my backup heat is not working on my heat pump

(July 6, 2014) Anonymous said:

My mitsubishi mr.slim MUH-18RV have heating kit which were not working i bypass the kit and try to stat the compressor seperatly but when i try to start it try to start sounds and become heated , but could not start , before starting the compressor voltage are 212 VAC but when i start compressor voltages downs to 130 volts and all wires become hot . what is the problem is this due to bypass the compressor kit , or its motor is short ? please help me regards kalim ullah

(Dec 22, 2015) Pete said:
On my GE wall mounted heat/ac unit the fan blows but I get no heat or AC. What could be wrong?

Reply:

Kalim it sounds like a seized compressor motor.

Pete:

IF you are in a warm climate I suspect your compressor unit is not working. If you are in a cold climate and your heat pump unit is not producing heat it may be that your backup heat (perhaps electric) is not working. Check for a tripped breaker to start.

Question: do I need a backup heat system with my heat pump?

(May 28, 2014) Jane said:
Thinking about purchasing heat pump ..do I need back up

May 28, 2014) Anonymous said:
Thinking about purchasing air heat pump is it compulsory to have a back up heat source

Reply:

Anon,

The requirement for backup heat for a heat pump system depends on where you live and what cold weather temperatures you can anticipate. If you are in an area where the outdoor temperatures never fall so low that your particular heat pump system will be unable to heat the home comfortably then yes you need backup heat.

Question: Air handler won't shut off after heat pump shuts off

(Dec 15, 2014) carl said:
air handler won't shut off after heat pump shuts off. must trip breaker & reset breaker.only dose this when out side air is above 40 degrees.

Reply:

Carl that sounds like a relay or control board problem. OR the fan switch is set to ON, or the thermostat is never satisfied.

Check first that the MAN ON switch is not set on your thermostat or in the air handler - those can tell the fan to run continuously.

Question: Amana heat pump aux light stays on

(Dec 26, 2014) Anonymous said:
I have an AMANA package unit that is just out of warranty (of course) and the AUX light comes on (and stays on) even when I lower the heat setting. It is not freezing weather her in Eastern NC and it is kept set to 68-69 degrees.

The unit is not 'iced up'. The 'aux light' stays on even when I lower the setting. What might be the problem? Is it most likely that the 'reversing valve' is stuck?

Reply:

I'd check first for a failed outdoor temperature sensor.

See HEAT PUMP THERMOSTAT, OUTDOOR

Question: Thermostat goes into AUX mode and system blows cold air

(Jan 7, 2015) tom Geiger said:
Occasionally when heat pump is operating, thermostat goes into "aux" mode and then blows cold air, presently 32 degrees outside temp. Electric heat sequencers replaced.

Reply:

Still, Tom, if you're not getting warm air out of the supply registers when the aux heat or backup heat is "on" then I suspect it's not really on. There may be a control or wiring problem if the heaters themselves are intact.

Question: aux heat kicks in too soon

(Jan 24, 2015) Anonymous said:
I have an electric heat pump for AC and Heating of the home. There is also an auxillary heat that is supposed to come on when temperatures get very frigid. My heat comes on normally with the outdoor compressor activating.

After several minutes the auxiliary heat kicks in even when temps are moderate 30 degrees and above. Thermostat is working correctly, wondering what component in the compressor unit is controlling the activation of the auxiliary heat. Electricity use is way to high this way.

Reply:

Anon

Your HVAC service tech will probably check first the thermostat settings and wiring and then the control board and relays for the heat pump.

See HEAT PUMP THERMOSTAT, OUTDOOR

Question: equipment failure at certain temperatures may relate to wiring or controls

(Feb 17, 2015) Anon said:
Knocked the ice off the top of the electric heat pump's outside unit and now it won't run. We then poured hot water over it to clear away any ice chips

Still won't run. It was operating before we knocked the ice off. What do we do now? We have only aux heat with frigid temps next few days.

(Feb 18, 2015) mike stasko said:
when really cold outside 0 degree electric heat wont work. When temp goes up to 10 degrees everything works fine.When 0 degrees and things don't work turn thermostat to emergency heat outside unit shuts down and indoor blower runs but no electric heat. when temp gets to 10 degrees everything normal.Three service companies checked out this problem and said no problem.

Reply:

Mike I'm surprised the service companies found no problem since you indicate that the electric heat doesn't work when it's quite cold.

Try hiring an experienced licensed electrician to find the failing control or bad connection.

(Feb 19, 2015) mike said:
Dan thanks for your response. It just doesn't make sense to me that outside temp would effect emergency heat from the thermostat. looking at wiring diagram from the unit e lug should go right to the air handler in parallel is white wire to outside unit with an optional outdoor thermostat which I'm not sure is in the wiring.

Now with the wire note #3 that wire is removed and could effect w2. What am I missing?

Reply:

Mike

Certainly heat pumps include an outside air temperature that controls the unit's switch to backup heat, right?

Question: propane backup heat for heat pump

(May 18, 2015) kim said:
I have an air source heat pump with propane back up. The temps are in the 40s but air source heat pump doesn't come on ,the propane furnace does. Why isn't the heat pump engaging?

Reply:

Kim take a look at the diagnostics at LOST COOLING CAPACITY in the ARTICLE INDEX given at Continue Reading .

We don't know enough about your system to diagnose it's failiure to cool. Causes range from an improper thermostat setting to failures in the equipment itself.

...

Continue reading at HEAT PUMPS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• DIAGNOSTIC GUIDE A/C or HEAT PUMP
• DIAGNOSTIC DETAILS A/C or HEAT PUMP
• LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system

Suggested citation for this web page

BACKUP HEAT for HEAT PUMPS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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