Heat Pump outdoor unit defrost cycle:

This article discuss the reasons for, duration of, and control of the defrost cycle at the outdoor compressor/condenser/fan unit of a heat pump when in heating mode.

This article also discusses the diagnosis and repair of problems with the outdoor compressor / condenser fan and fan motor, including fans that won't run, fans that run at slow speed, and fans that won't stop running.

HVAC Compressor/Condenser Unit Defrost Cycle

Note: this heat pump defrost cycle Q&A were posted originally

at FAN, COMPRESSOR / CONDENSER UNIT - be sure to see the repair advice at that page.

In both of our outdoor compressor/condenser unit photos shown here, the snow cover itself is not a functional issue for the equipment, but the placement of the condenser unit just a few inches from the building wall may be an operating issue by restricting air flow around and through the unit.

Why and when a heat pump goes into a defrost cycle when in heating mode

It is normal for the outdoor fan to run while the outdoor unit compressor is running AND the system is in heating mode, not in defrost mode.

When your heat pump is in heat mode, the outdoor unit (compressor condenser) is extracting heat from (even chilly) outdoor air.

The outdoor unit fan runs, air moves across the outdoor coil, heat is absorbed from outdoor air into the coil and into the refrigerant inside the coil where ultimately that heat is delivered indoors through the indoor air handler.

Why does ice form on the outdoor coil?

In heating mode, as heat is extracted from the outdoor coil, its surface becomes colder than the outdoor air temperature and can drop below freezing. Then moisture in outdoor air freezes on the coil surface.

Why is a heat pump defrost cycle needed?

Ice on the outdoor coil blocks air flow across the coil; with no air flow, no heat can be extracted from the outdoor air and worse, the compressor motor itself could be damaged.

So periodically the outdoor unit will go through a defrost cycle. A control board in the outdoor unit is the control over when a defrost cycle occurs.

Just how often a defrost cycle will occur is determined either by a timer on the outdoor unit’s control board (when in heating mode) or by sensors that some heat pumps may use to detect the presence of ice on the coil.

Depending on your heat pump brand and model - thus the heat pump's design - the outdoor unit in heating mode will run a defrost cycle as often as once every 30-35 minutes OR (on some more sophisticated designs), the heat pump may only run a defrost cycle if sensors tell it that the outdoor coil is iced over. 

Heat Pump defrost cycle timer

Typically on the defrost cycle control circuit board, the installer can set a jumper to control the defrost cycle at 30, 60, or 90 minute intervals once the defrost temperature sensor on the outdoor unit senses that outdoor temperature has fallen close to 31°F.

The timer starts when two events have occurred:

1. The outdoor temperature has fallen to about 31°F as reported by a temperature sensor in the outdoor unit (OD unit)
2. The indoor room thermostat is calling for heat

The defrost cycle is initiated at the end of the defrost timer cycle.

The defrost cycle ends when the temperature sensor at the outdoor coil senses the temperature of the coil at about 75 °F or when the circuit board's timer overrides the sensor.

Other more sophisticated defrost control circuit boards monitor other operating details such as refrigerant pressure in order to avoid wasting the energy of a defrost cycle if it's not actually necessary. Fewer defrost cycles - eliminating unnecessary defrost cycles - means a more efficient heat pump system and some energy savings.

Factors that affect the frequency of a heat pump (in heating mode) outdoor defrost cycle include:

• The heating load - amount of heat being delivered indoors
• The compressor and control design (timer versus ice sensors)
• Outdoor temperature (closer to freezing, more frequent defrosting needed). At about 30°F you will usually begin to see ice forming on the outdoor coil or heat exchanger
• Outdoor humidity (more humidity, faster ice formation, more frequent defrost needed)
• On many heat pumps, a defrost cycle control board in the outdoor unit uses a timer and other information to initiate a defrost cycle.
  
  Some heat pumps use only a timer; others use a timer combined with a temperature sensor or thermostat that monitors temperature at the outdoor coil
• On some heat pumps, an air flow sensor detects air flow pressure change across the coil in order to detect icing or frosting of the coil (on some units)

What happens at the outdoor heat pump unit during the heat pump defrost cycle?

Usual design: a control circuit board in the outdoor compressor/condenser unit controls the defrost cycle timing and length.

The heat pump switches from "heating mode" back to "cooling mode" in order to warm the outdoor coil and thus melt frost or ice thereon.

The compressor motor will continue to run - compressing and thus warming refrigerant in the system.

In most heat pump designs, the outdoor fan will stop when a defrost cycle is initiated, and will remain off until the ice has melted away.

In some designs, both the compressor and the fan may stop and electric heaters may be used to perform the necessary defrost.

When does the defrost cycle stop (for a heat pump in heating mode)

The defrost cycle will typically stop after about ten minutes maximum, and often sooner, possibly after as little as one minute of defrost time, depending on weather conditions.

The defrost cycle will stop when temperature sensors at the outdoor unit detect that the coil’s temperature is high enough that no ice would be expected to remain on the unit.

That's a somewhat simplified description. More-sophisticated heat pump control designs add features to protect the compressor motor from damage.

For example, once the heat pump outdoor unit has completed a defrost cycle, if the outdoor unit fan has remained OFF, a timer may prevent the initiation of another defrost cycle even if the outdoor unit's defrost sensor (basically a thermostat) switch closes to try another defrost cycle.

Watch out: do not turn off your heat pump if it’s in the middle of a defrost cycle, or if you must turn it off, leave it off for at least 30 minutes to avoid damaging the compressor motor.

What happens at the indoor air handler during the heat pump defrost cycle?

During a heat pump in heating mode defrost cycle, the indoor air handler will normally stop blowing air into the occupied space - that's because the system is not receiving heat and the designer doesn't want to blow cool or cold air onto occupants. 

Or: the indoor unit will switch into backup heat mode and make use of the alternative heat source such as an electric, oil, or gas furnace.

Indoors at the air handler, or on some systems right at the thermostat, a colored or blinking indicator light may tell occupants that the delivery of heat has stopped for a few minutes to permit a defrost cycle.

Diagnostic note: just as we expect to see about a15-20 degree F temperature drop across the indoor coil when in cooling mode, we expect a similar temperature rise across the indoor heating coil when the heat pump is in heating mode.

If the indoor air through the air handler drops more than just one or two degrees F during the defrost cycle, then no backup heat is operating.

If the indoor air handler is blowing cold air on building occupants during defrost cycle, then the system is not operating properly. The problem could be a bad defrost controller outdoors.

Abnormal Outdoor Heat Pump Unit Fan-On Cycles

Outdoor Heat Pump Fan Runs When it Shouldn't

If your thermostat does not call for heat for an hour or more (perhaps because the indoor thermostat is satisfied), I would not expect the outdoor unit to require more than one additional defrost cycles and I'd not expect the outdoor compressor/condenser fan to continue to cycle.

So if that's happening, I suspect a temperature sensor or a condenser/unit control board error that's worth a service call.

If the outdoor fan runs when the compressor is OFF AND the system is NOT in DEFROST mode, then the repair may be simply to replace a bad fan relay.

Outdoor Heat Pump Unit Defrosts Too Frequently

If the outdoor unit is defrosting too frequently (at intervals of less than 30 minutes), something is wrong.

• If at the same outdoor temperature that frequent defrost cycle has occurred since original installation, the unit may be under-sized and unable to provide the amount of heat being asked of it.
• If the problem is a more recent one, check to confirm that air flow through the outdoor unit is not blocked by snow, brush, leaves, debris.
• If the indoor air handler air filters or blower fan are dirty then the indoor air flow and heat delivery are reduced and the heat pump may be over-taxed (as it can’t satisfy the thermostat).
• If the coil in the indoor air handler is dirty, that too blocks air flow, reduces heat delivery, and over-taxes the heat pump.

Other causes of too-frequent defrost cycles or an outdoor unit stuck in defrost mode include:

• Failed outdoor temperature sensor or poor sensor contact with the refrigerant piping or temperature contact surface
• A failed defrost control board, or one that is mis-wired or a failed defrost control board contactor or relay or timer motor
• Damage to the outdoor coil, such as a large area of bent or smashed fins that block air flow or dirty, debris-blocked outdoor coil
• Low refrigerant - a refrigerant leak - drops the system temperature and increases the defrost cycle rate (until so much refrigerant is lost that the system simply stops working)
• A bad outdoor unit fan motor or fan relay or fan start/run capacitor: fan doesn't run or runs too slowly - the outdoor unit will keep defrost-cycling frequently
• Outdoor fan motor running backwards (possibly after a lightning strike)
• Low voltage causing slow outdoor fan operation

Outdoor Unit Does Not Defrost - No Defrost Cycles

This might be normal, for example if the unit is running in cooling mode, not heating mode, OR if the outdoor temperatures are high enough that no frost or ice is forming on the outdoor coil.

However if temperatures are below 35°F, we expect ice formation (or perhaps you see ice formation) and the unit should be self-defrosting.

If it is not, the temperature sensor could be bad or the defrost cycle control board could be bad.

Or the reversing valve may not be working.

Outdoor Heat Pump Fan Does Not Run when Heat Pump is Running

When the heat pump is operating to deliver either warm or cool air the outdoor compressor/condenser unit fan should be operating.

If the outdoor unit fan does not run when in heating mode, you will see abnormally low discharge temperatures at the outdoor unit, faster icing at the coil, and possible system damage as well as loss of heat delivery indoors.

(If the outdoor unit fan doesn't run when in cooling mode, the system will overheat, will not deliver liquid refrigerant to the expansion valve and cooling coil (indoors) and you will see loss of cool air delivery indoors.)

Also check this article: SHORT CYCLING AC COMPRESSOR

Outdoor Unit Compressor Stops when Defrost Cycle Starts

The outdoor unit compressor normally runs during the defrost cycle in order to compress refrigerant and thus raise the temperature of the outdoor coil.

If the compressor stops, the problem may be

• Defective fan, fan motor, fan start/run capacitor: fan not running
• Defective defrost timer
• Temperature sensor or thermostat defective, loose, not properly installed
• Air pressure differential switch failure (the air pressure sensor confirms that air is flowing through the outdoor coil)
• Defective defrost control circuit board or contactor/relay
• Thermostatic expansion valve (TEV) is jammed or stuck, possibly from debris, or has been improperly adjusted
• Refrigerant leak, low refrigerant

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2022-04-11 by Inspectapedia Com Moderator - Carrier heat pump system diagnostics

@LYNND,

Let's be sure we are not confusing the backup heat system, which in your case is probably electric, and which should only run in winter, with heater strips that might be used in the defrost cycle on the outdoor coil on a heat pump. So the indoor coil may be defrosted at a different time and using different heaters from The Outdoor Equipment.

Bottom line is it sounds as if you need service from a carrier trained service technician.

On 2022-04-09 by LYNND

@Inspectapedia Com Moderator,

Thank you for the quick reply. My spouse is about to inspect the wiring for himself rather than take the word of the tech that nothing appears to be wrong with it.

We have only had two companies out since 2017, the first assigned to us by the home warranty company. (That HVAC provider seemed unfamiliar with Carrier and refused to check any electrical issues even though our reason for calling them out was for the high electric bills after closing on the house in summer 2017.

That service provider claimed the refrigerant was about ~3% low but in the time it took to go in the house to get my checkbook they claimed to have recharged it. They also claimed that the refrigerant being slightly low was NOT an indication of a leak because it wasn't low enough.

Needless to say I didn't have that company out again because I don't know if they actually put any refrigerant in the system or not. It took me less than five minutes to go get my checkbook and they said it was done. I never saw for myself what they did.)

Other service calls were associated with the prior owner but appeared to involve repeatedly replacing the thermostat. We only know that because there were so many receipts and tickets for installation.

We have had the same owner/operator tech out for all the calls involving the fuses (same guy who did our HVAC inspection). Prior to buying the house we also had a dedicated electrical inspection but none of these issues presented themselves up front.

It has been a learning curve to say the least. We see the meter come out and the owner-operator test for proper voltages but to our knowledge he hasn't used any specialty tools intended to isolate low voltage situations.

We can't continue to randomly pay to replace parts (with the markup) so it seemed fairly harmless to try replacing the board since the defrost boards aren't all that expensive. (We can always send it back if the tech actually rules it out when he shows up later today to revisit the recurrence of blown fuses.)

Overall, what do you think about my though process on the defrost board? If it were improperly going into defrost regardless of weather could that explain why the drop on the electric bill after re-running the wiring to the STAT has only fallen by about $100 from ~$900 during the summer.

Everything I have read on heat pumps suggests that the higher electrical cost should take place in winter, when the supplemental heat strips kick in, not summer — so the fact that our bills are highest in summer inferred (to me) that the heat strips were running. But I, too, was unclear on why the tech we had out — can't go any higher than him because he is the business owner — was not able to identify the wiring problem sooner.

Is it conceivable that a faulty defrost board would run up the electric bill by calling for defrost even in hot weather?

Thank you for suggestion the names of the tools. That was a big question — was our tech even showing up with the correct tools (other than an OHM meter) to properly isolate a short?

On 2022-04-09 by Inspectapedia Com Moderator

@LYNND,

What a remarkable, long, and painful Diagnostics Saga for your Carrier heat pump system.

You raise a number of diagnostic questions that are perfectly reasonable but I don't hear any more story that anyone has checked them. Even the question of the heat strips remains unclear from the narrative.

Give me the number of people who been to your site over such a long time I don't think we can be smarter just based on a text description.

A couple of remarque's might help get going on sorting this out.

It's common for a service tech to simply try replacing a simple part with the hope that that will fix the problem rather than going through a lengthy diagnostic routine, partly because it's the least expensive first thing to try. But certainly you don't want to keep replacing parts and certainly not replacing fuses nor over fusing equipment.

I would expect someone to use some test equipment like a DMM or vom to look at the current draw when the system is running.

Talk to the service manager and review the case history briefly and ask that she send an experienced senior service technician to sort out this problem. And ask the tech to review the repair history.

On 2022-04-09 by LYNND

This question pertains to a Carrier heat pump, 25HCD348A00310.

We have had a long ordeal of inexplicable problems and sky-high electric bills despite the fact that the unit was under five years old when we moved in.

During a housing inspection — summer of 2017 — the unit was found to be operating on fan but not blowing cold air. HVAC tech rearranged location in breaker panel and removed some clogged filter material a prior owner had placed behind the intake and the system began operating normally.

Upon moving in a couple months later, we inherited a box of paperwork from the previous (elderly) owner suggesting the homeowner had been charged repeatedly for service calls involving swapping out thermostats beginning within a year of the installation.

In 2018, our first full summer in our 1400sq home, our electric bills went through the roof despite the fact that the house is insulated and has newer windows (double pane). After much Google searching I suspected the heat strips were running with the AC but it took TWO YEARS of paying almost a grand a month in summer before the HVAC tech took a closer look at the thermostat wiring. (We got the sense that it was not possible to readily tell that the heat strips were operating in cooling mode. In any case this was the same owner-operator that had performed our pre-sale HVAC inspection.)

The first time we had the tech out on a service call was winter 2018 — before we had reason to suspect our electric bills would be so high the following summer. Tech found that the unit (indoor section) was fitted with a higher-rated fuse (we speculate this was a work-around given to the prior owner to keep it from blowing).

Replaced fuse and everything was back up and running. However, a day later the fuse and the transformer blew. Tech replaced those parts and tested continuity, claimed everything read correctly. He did not find any issue with the wiring, either, so he replaced fuse again.

Ran okay until winter 2019 when the same situation occurred This time he replaced another part involved in the thermostat call for the unit to switch on (there was again snow on the ground and apparently it was sticking). Replaced this part (sorry don't recall the name) and installed another fuse. Ran until winter 2021 when it "died" again on the coldest night of winter (snow again on ground). Tech then tested the blower/fan/relay and found it to be operational. Replaced the fuse and it began running again.

When he went to put the panel back on the outdoor unit, it shorted again. Tech reported the wiring looked okay (no breaks) but cut the ends and reconnected just to be sure the connections were tight. (Speculated that vibration from the outdoor unit may have caused the wiring to short.)

Questioned the tech as to the repeated bills associated with paying to replace fuses every year but he essentially shrugged his shoulders. This time, however, he left two spare fuses behind. Got another six weeks of use from the system without issue before the system died again, this time in 90 degree Spring weather. (All previous instances were in very cold weather.)

Spouse popped in one of the two spare fuses the tech left behind but after running normally for about 20 seconds the fuse tripped again. No sign of icing on the outdoor unit. No visible blockages.

As an aside: Tech was asked by myself on his prior call ~6 weeks ago if he checked refrigerant levels and admitted he did not because checking them would allow some of the refrigerant to escape. Said he put his tool on the line and it was sufficiently cold that he didn't think that was necessary. Was that method of verifying that there is enough refrigerant circulating adequate or should he be doing more?

I reminded the tech of a conversation I had had with him the year before when I related that I had come across a report online — which unfortunately I lost track of — in which it was suggested that Carrier had an issue with some of their control boards causing intermittent low voltage issues.

Asked him to check with Carrier to see if the board itself needs to be replaced — but he did not follow through on that request prior to the fuse tripping again the other day. We're back to Square One and concerned that the tech isn't doing anything more than metering for 24volts. Might there be another way of identifying a short (a tool he should have or that we should inquire about)?

We are at wits-end with this. Spouse works in IT and can swap out boards on his own with no issue but the question is which board?

Last night I pulled the trigger on an aftermarket defrost control board on the guess that it might be operating improperly because our electric bills are still excessive in the summer (~$700) despite having the supplemental heat strips rewired.

My thinking is that an intermittent fault with the defrost board could be calling for it to defrost even in warm weather, which could conceivably explain why our electric bills in the summer did not drop as much as expected after replacing the thermostat wiring to address the heat strip wiring problem. Am I on to something? If so, did I order the correct part # HK32EA007 (found on Amazon)?

Lastly: At one point about a year ago our HVAC tech suggested we replace our thermostat but we found it difficult to believe that would be necessary. We didn't like the one that was installed when we moved in so the one he was suggesting we replace was not even a year old (Emerson UP310). Our STAT replacement in 2018 came after a long-line of consecutive STAT replacements by the prior owner.

We are supposedly dealing with a master tech (business owner), yet my impression is that that rather than go through the entire troubleshooting necessary to diagnose a low voltage short, the HVAC techs who have come to our address current and former have simply cut corners by putting in replacement fuses and/or by recommending the customer replace the STAT. As a result, I tend to be skeptical that the thermostat is the culprit.

Should I replace the thermostat anyhow?

Ultimately our fear is that if these fuses keep tripping a more expensive component will be damaged. Is that a valid concern?

Any suggestions on where to go from here would be IMMENSELY appreciated. Thank you.

On 2022-03-22 by Inspectapedia Com Moderator - outdoor fan should stop during defrost mode

@Gregg Nemec,

Details on the condenser unit (outdoor unit) defrost cycle are above on this page;

Indeed on a heat pump unit in heating mode as frost may form on the outside coil, the unit will periodically switch to "cooling" mode to defrost the coil (cooling mode will cause the coil to warm up). In this mode the outdoor fan should stop - at least on all the units we've examined/read-about/etc.

But give the brand and model and let's confirm that with the manufacturer of your specific unit.

On 2022-03-22 by Gregg Nemec

My outside unit fan keeps running in defrost. This is a new 3-ton system. I am pretty sure that when it calls for defrost the outside fan should not be running.

On 2022-01-18 by Inspectapedia Com Moderator

@James,

Thank you for helping out. Working together makes us smarter.

On 2022-01-18 by James

@SEM, if it’s a heat pump it will run constantly with the temps that low. I have mine set to turn on Aux heat if it gets more than 2 degrees below the thermostat set temp. It’s probably to cold for the heat pump to keep up with the thermostat set temp.

You can set it to run simultaneously with the indoor heat on most systems. I would measure the heat coming out of the vents with just the heat pump running and see if it’s putting out heat. The slow down is probably to defrost but you would need someone to look at it to be sure.

On 2021-12-23 by Inspectapedia Com Moderator - nonstop running may be time for service

@SEM,

Best to shut down the unit entirely; you'll have to runon backup heat - assuming we're discussing a heat pump.

It's possible that the defroster on your outdoor unit is not working - or of course there could be some other cause as we haven't a shred of information about your installation.

I'd turn it off and call for service.

On 2021-12-23 by SEM

Outside unit continues to run all day, periodically slowing for about 5 minutes, then clicking back to normal. It has been running for almost 48 hours straight. Temps here today are going to be around -8 to -10° C. How can I stop this?

On 2021-11-30 by Inspectapedia Com Moderator

@Marty,

I don't know the jumpers on that control board, though doubtless they're detailed in the Carrier manual.

But I'm confused. Given that air temperature is lower after a defrost cycle, isn't that telling us that the defrost cycle is both normal and needed? Why would we want to turn it off, blocking our coil with ice?

On 2021-11-29 by Marty

I have a Carrier heat pump with hk32ea001 defrost board in Venice, FL. After a defrost cycle the air duct temperature is 10 degree F cooler than before and remains there for a long time.

I suspect the reversing valve may be leaking across. I would like to experiment with deleting the defrost cycle. Can I do this by not selecting a defrost interval on the board ie disconnecting the time select jumper wire?

On 2020-11-06 by danjoefriedman (mod) - power outage may have caused control board damage

Philip

It's possible that a power surge has damaged the control board

On 2020-11-03 by Philip

So had power outage that lasted about 40 hours. After power back on I turned heat back on. Now the outdoor unit sounds and acts like going into very quick 3-5second defrost cycles.

It will do this about 10 times quickly back to back then start running normal. I get the air brake sound and fan outside will stop during entire session of cycle. Even kind of sounds like air brake sound at indoor blower unit.

On 2018-12-19 by danjoefriedman (mod) - smoke indicates possible fire risk and need for repair

Ms. Dill

IF you see smoke at the outdoor unit that sounds as if there is an electrical failure and possibly even a fire risk at your outside compressor/condenser unit.

Leave power to the unit OFF and call for repair.

IF on the other hand the unit is simply going through a defrost cycle that might keep the compressor from running for a time.

On 2018-12-19 by Ms. Dill

The fan on my outside heat pump wont run for a few minutes. Smoke or humidity is coming from unit when this is happening. The indoor heat unit is running but coils are cold as if in A/C mode. After a few minutes the fan outside comes on and all is well inside and out. Is this normal?

On 2018-11-05 by (mod) - outside fan may run even when no heat is blowing inside

Aaron; 

It is normal for the outdoor fan to run while the outdoor unit compressor is running AND the system is in heating mode, not in defrost mode.

On 2018-11-05 by Aaron

Heater on heat pump works. But, the outside fan turns on several times per hour even when no heat is blowing inside. Is this normal?

...

Continue reading at FAN, COMPRESSOR / CONDENSER UNIT - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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