At what outdoor temp should unit go into defrost?

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Old 11-26-20, 05:15 AM
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At what outdoor temp should unit go into defrost?

I have a new Trane heat pump that was installed in Sept. I live in Charles Town, WV. System has propane gas furnace for backup heat. System does not seem like it has been operating correctly since installed. Tech has been out here 3 times now trying to figure out what is going on.

Problem has been that when unit goes into defrost it is dumping a lot of cold are into the house and temp will drop 5-7 degrees before it recovers which sometimes can take up to an hour.

Anyway, my question for now is about defrost. When the tech was out here the other day he was explaining what happens to the unit when it goes into defrost. When I asked him what actually tell the unit to go into defrost, I understood him to say that it was a temp. sensor and usually would go into defrost when temp goes below about 40. Is this correct?

The reason that I am asking is that when I got up this morning the inside temp was 69 and the outside temp was 56. Not long after I got up the outdoor unit fan when off, so I guess it went into defrost and the backup heat came on. Again the indoor temp went down 4 to 65 within 10 minutes.

With the outdoor temp at 56 why would the heat pump go into defrost and the system have to run back up heat? Seems like a heat pump should be able to do the heating when the outdoor temp is that warm.

Thoughts on this?
Thanks
 
  #2  
Old 11-26-20, 07:20 AM
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There are 2 methods used for defrost. Time and on demand. We would need the model number of your heat pump to determine which one you have.
It would seem as though your furnace isn’t starting during a defrost cycle. Can you confirm this? Space temp shouldn’t drop at all. The outdoor unit should automatically control your backup heat during defrost.
 
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Old 11-26-20, 09:01 AM
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roughneck77, Thanks, I will get back to you with these answers for sure. Might not be till tomorrow though. Happy Thanksgiving.
 
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Old 11-26-20, 09:27 AM
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It's a Trane. It's probably not going to use the timed method.
Defrost is rarely about the ambient air...... it's more about humidity. The higher the outside humidity the faster the ice buildup. In the Northeast the humidity level is currently high with this storm front.

You have a dual fuel system. Hopefully the thermostat is programmed as such. It sounds like they missed the heat demand wire from the heat pump to the furnace. They may have used a dual fuel kit and wired the defrost heat incorrectly.

Dual fuel systems present a little challenge as the furnace cannot run when the heat pump is in the heating mode. Defrost is system in cooling mode.
 

Last edited by PJmax; 11-26-20 at 09:45 AM. Reason: typo
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Old 11-27-20, 04:05 AM
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PJmax, what range of outdoor humidity are you saying? As an example of what it is when this is happening:

1.) this morning when I got up the indoor temp had gone down to 63 with thermostat set at 69. The outdoor temp was 47 and humidity was 44%

2.) another morning indoor temp dove to 61, thermostat was set at 69, outdoor temp was 39 and outdoor humidity was 37%
 
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Old 11-27-20, 12:11 PM
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roughneck77 asked about the model numbers. Here they are.

2 stage gas furnace with 4 ton XL16i heat pump

Gas Furnace model number S9X2C100
Heat Pump model number T4TWX6048

 
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Old 11-27-20, 08:02 PM
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Your system should be shutting down the heat pump and firing up the furnace when the temperature drops 3 or more degrees below the set point. This should be a fairly basic setup. It's called dual fuel.

I'm not sure why your tech is having a problem.

There is a sensor mounted directly to the coil in the outside condenser. This sensor detects the coil icing up which then starts a demand for defrost. At this time the furnace should immediately start running.
 
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Old 11-27-20, 08:58 PM
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That unit has demand defrost. A defrost cycle will not initiate until the system determines a defrost is needed.
One question you haven’t answered. Is the furnace starting during defrost?
 
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Old 11-28-20, 04:36 AM
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Correction to model number information

Heat Pump: 4TWX6048H1000BB
Furnace: S9X2C100U5PSBAA

 
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Old 11-29-20, 03:55 AM
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I get what you are saying about the 3 degree drop. But what I seem to be seeing is that the 3+ degree drop happens when the outside unit is in defrost and the back up heat / furnace comes on. Somewhere in that cycle from outside unit going into defrost and the furnace kicking on to maintain the indoor temp. That is when the temp will drop and rather quickly. Say 3-5 degrees in a matter of maybe 15 minutes.
 
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Old 11-29-20, 08:00 AM
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But is the furnace actually starting up and running? During defrost.
 
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Old 11-29-20, 10:47 AM
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The furnace should be able to generate enough heat to maintain inside heat during the defrost cycle.

I'm fairly certain that once a defrost cycle has started..... the thermostat cannot interfere with it. That means even if the temperature drops 3 degrees..... the furnace should still come on if the stat tells the heat pump to shut down. That should be confirmed by your tech.
 
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Old 11-29-20, 04:43 PM
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I'm not sure if the furnace is kicking in as soon as defrost starts or not but I think the tech felt like it was slow to start. I am very frustrated at this point. I have had tech out here 3 times now. The last tech that came out last week did the following, at least according to what he told me.

1. made 2nd stage heat come on sooner.
2. turned Smart optimization off
3. change to aggressive recovery or made it a more aggressive recovery. not sure which. My notes just say changed to aggressive recovery.

I was a mechanical / electrical troubleshooter in an automated industrial factory for 30 years. I understand that it can take time on some issues. I am not an HVAC guy but all those years of troubleshooting just make this stuff go through my head. But what they are doing doesn't seem to be addressing the issue. I don't think they are getting to the root cause. And I sure don't think that just because the HP goes into defrost it is normal for the temp in the house to drop 4-7 degrees in the matter of only a few minutes.

I am planning on getting an infared gun to check the temp of the air coming out of the vent when this is happening and see if that tells me anything.

I am continuing to take notes every day and I plan on calling them out again. I have already requested that they have a Trane rep. accompany the tech but they said it would be a few weeks before one would be available.. That is something that Trane told me that I should ask for.
 
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Old 12-04-20, 03:36 AM
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Finally have the Trane rep coming out with the local tech next week. Maybe we will get somewhere on this problem now. Will post again after they come out and let you know if they fixed it.
 
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Old 12-12-20, 08:30 AM
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Just curious - what thermostat are you using with your system?
 
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Old 12-27-20, 03:50 AM
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Update. Well guys, I think we have finally found what is going on with the system. The main complaint or symptom was that when the heat would come on, the temp in the house would quickly drop 5-10 degrees before taking a while to recover and increase to the thermostat setting.

After having the tech out to the house 4 times, he finally looked in the wall through the 14x14 return vent in the wall. What he found was that the wall cavity was being used at the return. But not only that, but that there were big openings to other wall cavitities and as I found out yesterday after cutting into the wall, there was one opening, about 8x10 that was open to where the bathtub is which in turn on the other end of the bathtub space opens to the attic. So yes, the unit was pulling air pretty much straight from the attic and mixing that with the air from the house.

My first indication of this prior to cutting into the wall was that I put a temperature probe in the wall cavitie, ie. the return to monitor what the temp in the return was doing when the heat came on. When the heat kicked on the return duct air temp would drop up to 10 degrees in a matter of 5 minutes. This confirmed the techs theory. That is when I started cutting into the wall to look for the culprit.

So long story short, I think I am on track to get the system working as it should. I am working on closing up the wall cavity leaks so that it is only sucking in air from the rooms of the house as it should. So far I have only sealed the opening mentioned above and last night the return air temp didn't drop when the heat came on.

Another thing that I have found is this. The upstairs and downstairs return vents are using this same wall cavity. And where they cut a hole in the OSB subfloor upstairs to draw air from the down stairs return vent is only 56% as big as the downstairs vent size. So my next step today is going to be cutting that hole bigger so that it is at least as big as the return size in sq. inches.

Well I just wanted to update you guys on what I/we have found. I am hopeful that once I have this all sealed up the unit will work as it should.
 
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Old 01-09-21, 05:33 AM
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PJmax,

I have found that the functional problem is not related to the defrost cycle. Main problem has been that the temp in house drops 4-6 degrees withing 15-20 min. after the furnace kicks on.

One of the main things that we have found, the tech and I, is that the return trunk, which is acturally a wall cavity was open to the attic and therefore the return was sucking in cold air from the attic when the furnace would come on.

I have since corrected that problem. Temp in house is still dropping when furnace kicks on..

My question to you is this,

Is it ok to start a new "Topic" to address the initial drop in the indoor temp during this time. It just seems like it would start a new conversation on this problem instead of people thinking the issue is with the defrost cycle.

Thanks
 
 

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