No defrost in heating mode

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Old 12-26-12, 08:05 AM
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No defrost in heating mode

Let me just say thanks for any help anyone is able to offer here before I begin!

This past summer I had a new compressor & thermostat installed and now the exterior coil is freezing up and not defrosting. I have checked the reversing valve solenoid and have continuity across the leads. I have also checked the reversing valve and can here the plunger move inside the valve when power to the solenoid is applied and removed. In my opinion, I think that the tech has installed the thermostat wrong or the thermostat just isnt compatable and that is why my unit is no longer defrosting in heating mode when the exterior coils freeze up.

I dont know how to tell if my tstat is compatible with my unit or how the tstat should even be wired. I don't know how the old tstat was wired and it was the original tstat that had come with the unit. I need help verifying that my tstat is wired correctly to the unit.

Thermostat is either a TopTech TT-P-411 or TT-P-421
(It doesnt say on the tstat itself, all I have is the manual)

Unit is an American Standard WCC024F100BB with a Heat Pump.


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Last edited by BigOch; 12-26-12 at 10:41 AM.
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  #2  
Old 12-26-12, 12:54 PM
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Defrost cycles are governed by the defrost board inside the unit, not the thermostat.

There could be a problem with the defrost board or defrost t-stat inside the outdoor unit. Your defrost board may have test pins which when shorted, initiate a defrost cycle.
 
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Old 12-26-12, 01:03 PM
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Is the defrost mode not essentially just the ac running ? After realizing the exterior coil had frozen up again last night, I turned the AC on for a few minutes and boom, no more ice on the exterior coil. I just dont understand how if it works one way why it doesnt the other, unless of course I am misunderstanding how it works.

I will take a few shots of the boards on the inside of the unit and upload them, maybe you or someone could educate me on the testing points if there are any and what I should see when testing.
 
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Old 12-26-12, 03:35 PM
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Defrost = a/c on, aux heat on, outdoor fan off

During a normal call for cooling, the t-stat internally makes the connection between terminals R(power), Y (compressor/fan), O(reversing valve), and G(indoor fan).

During a call for heat, the t-stat energizes terminals Y/G; when there's a call for aux heat, W2 gets energized as well.

When O isn't energized, most* defrost control boards monitor the temperature of the coil and initiate a defrost cycle every 60-90 minutes (it's adjustable) if it's below freezing; each defrost ends when the coil exceeds a certain temperature or after a certain amount of time has elapsed - whichever comes first.

*Some control boards don't work based on time.

So, a faulty defrost board or defrost thermostat can prevent the coil from defrosting even if the system works fine in cooling mode.

Try to locate the service manual online - it should tell you how to test the defrost control.
 
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Old 12-27-12, 09:49 AM
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Ive searched and searched online to no avail for anything to do with this unit. Maybe I am not using the right key words but nothing comes up in google with the model and or serial number search.

I have seen the defrost tstat on the coil line that runs back to the defrost board where it has labels for 60 90 ect as far as time incriments for checking I suppose? Ive not been able however to find a tutorial or guide on how to test the board itself.

I am gonna take the cover off again today and just give it a good lock over to see if I can see any test points but unlike newer units theres no test button way easily labeled way to force defrost from the board to test it.

When I look again today I will take some photos and upload them here in an hour or two.

Thanks again for your help!
 
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Old 12-27-12, 11:09 AM
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No problem.

If I remember correctly, the defrost t-stat closes when the coil is below freezing.

With the coil frozen, check it for continuity.

I found the spec sheet but not the service manual - http://www.mgasociados.com/trane/wcc10.pdf
 
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Old 12-27-12, 11:43 AM
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Heres a photo of the control board, not my actual board but the kind thats in my unit no less. I am assuming that jumping across the two polls just above the letters TST will force defrost and the outside fan should turn off as long as I have it in HEATING mode ?

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Old 12-27-12, 11:56 AM
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Ok so I tried shorting the two pins above the TST to force defrost. The outdoor fan kicked off just for a few seconds before almost instantly spinning back up. I tried both shorting it out for just a few seconds as well as keeping it shorted for a longer amount of time, or atleast until the outdoor fan started spinning back up. Both cases had the same results as stated above.

I had left to go an look at the board before I saw your continuity comment. The outside coil was slightly frozen over when I attempted to force the unit into defrost mode. Does the defrost board bypass the tstat when using the TST jumper ? If so, can I assume the board is bad since the outdoor fan almost immediately kicked back on ?
 
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Old 12-27-12, 01:02 PM
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I'm not exactly sure how your board responds; I'm pretty sure a defrost cycle won't occur even with the test pins shorted unless the defrost t-stat is closed. The pins probably have to be shorted at least until the defrost cycle begins.

This link might be helpful: Heat pump controls demystified | ZenHVAC

Given that the system works fine in cooling mode, either...

- There's a bad connection
- The defrost t-stat is bad
- The defrost board is bad
 
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Old 12-27-12, 01:17 PM
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Well a local parts house was able to order me both the control board as well as the tstat, so I will swap them both out tomorrow on the unit. They are calling for it to be in the 20's this weekend so thankfully they can get me the parts tomorrow morning!

I will report back to let ya know if thay for sure solved my problem. Both together werent more than $100. Thanks again for all your help, without people like you who help on the boards I would be stuck with paying a few hundred dollars to Bentlys lol.
 
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Old 12-28-12, 06:31 AM
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Muggle,


Last night, just for kicks, I tried defrost again via the TST points after reading the link you provided and watching a few youtube videos. It appears I was just shorting the test points for too long. Once I so to say perfected forcing my unit into defrost it would work perform with no issues. Twice last night I shorted the pins once the coil got a little icey and it defrosted great. This is making me lean more towards just the tstat. If its just the tstat it'll save me 85 bucks or so and so I wanted to get your opinion.
 
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Old 12-28-12, 06:43 AM
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When the coil is frosted up, check the t-stat for continuity with a multi-meter.

When you short the test pins, how long did the defrost cycle last? As soon as the stat opens, as far as I know the defrost cycle should terminate.

If the stat was stuck open, chances are shorting the test pins wouldn't have worked.
 
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Old 12-28-12, 07:11 AM
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Defrost lasted several minutes. I currently have the system off as I am at work and anticipated it just freezing up while running through the day. I will run the test again like instructed in the link you provided and measure continuity acorss the tstat before and during defrost.
 
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Old 12-28-12, 09:13 AM
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It's possible that it's defrosting properly, but not frequently enough for your area.

If it seems to be working okay, try reducing the defrost interval.
 
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Old 12-28-12, 12:43 PM
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Alright so ...

Checked continuity across tstat .. nothing. Ran heat without outdoor fan, slowly the meter started beeping then a solid beep, so I assume the tstat is working.

I have 50/70/90 options an so last night I actually moved it to 50 and it still managed to freeze over.

It is defrosting fine, but only when forced to defrost. The system itself isnt innitiating the defrost cycle however.

I went ahead an bought the board and tstat since the tstat was only 10 bucks and I had stripped the wires back on the line to measure it before. All in all it was $103 an change.

Gonna toss in the new parts once I get home from work today. I was really hoping to not have to replace the board but it seems like maybe thats the issue. Relay or IC not responding to the tstat or something ? I dunno. Either way its getting a new defrost board an tstat today!
 
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Old 12-28-12, 04:31 PM
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Help!




Huge uh oh! Was replacing the defrost board when I got home. One of the wires were on there pretty tight an I was giving it a firm pull when it came off and the board jerked an two or three other wires came off without me knowing from where.

The color wires are the orange that looks like it runs to the indoor thermostat, the brown that runs to the internal tstat in the unit on the coil line, and the two black wires that run to the reversing valve solenoid.
 
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Old 12-28-12, 05:02 PM
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Hang in there....we'll get you up and running.

Are there any part numbers on that defrost board ?
 
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Old 12-28-12, 05:09 PM
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Sorry about the other thread. Wasn't sure how active the forums may or may not be in the late evening. Guess I am just a little frantic knowing its gonna be so cold tonight lol. I really appreciate all the help I have gotten here!
 
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Old 12-28-12, 05:28 PM
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Oh and yes the part number for the defrost board is cnt1642 there is a picture of it further down in the post.
 
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Old 12-28-12, 05:34 PM
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Check out the link I left you. LOTS of great info on your board as well as wiring diagram. See if this helps. If not.....let us know.

http://www.hvac.amickracing.com/Heat...34-4101-07.pdf
 
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Old 12-28-12, 05:57 PM
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Check the link I posted earlier - it has a schematic for your model.

Switch to emergency heat mode for the time being if that's an option.
 
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Old 12-28-12, 06:48 PM
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Mug that's what I was doing, just breaking it down was hard for me though.

PJ I did find a wire diagram in there for my board. I think I have everything hooked up correctly now.

Ive got it turned on and nothing blew up so thats definitely a positive lol.

I cant even tell you how grateful I am for you two helping me!
 
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Old 12-30-12, 08:20 AM
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Status update ...

New board in and it still froze over. Ran a force defrost and then installed new thermostat and it hasn't had any problems freezing since, so I am guessing that even though the stat was working right maybe it was out of range ?


I have a curiosity question however. Currently the reported temp for my area is 25 degrees. I just got out of bed and the inside temp is 67 while the thermostat has been set to 72 since last night. The inside temp before going to bed was 72 as well.

The air coming out of the vents is a cool / warm mixture. I know the unit is a heat pump and from what I read they don't function well under like 50 degrees but should it struggle this much ? A 5 degrees inside temp difference seems like a lot.

My thermostat reads aux heat mode, is that just the emergency heat working in conjunction with the heat pump? If so, shouldn't it be heating better than it is ?
 
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Old 12-30-12, 08:48 AM
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If it is calling for aux heat strip heaters and you have low temps out of your vents then there is a problem with your aux heat. It could be how the stat is wired or an actual problem inside the HP
 
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Old 12-30-12, 03:36 PM
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The air coming out of the vents is a cool / warm mixture. I know the unit is a heat pump and from what I read they don't function well under like 50 degrees but should it struggle this much ? A 5 degrees inside temp difference seems like a lot.
Heating capacity matches cooling capacity at around 45-50F.

As the outdoor temperature drops, the amount of heat your house needs increases, yet heating capacity drops off. When your heatpump can no longer maintain the setpoint on its own, the backup heating elements should cycle on and off as needed to bridge the gap between heat loss and heatpump capacity.

A properly operating heatpump should be able to keep up without supplemental heat above 30-40F.

The supply air should be 20+ F warmer than the return air, which is still well below body temperature. (hence the luke-warm air)

Heatpumps are far more energy efficient than electric heaters, so it makes sense to keep them running even when they require some supplemental heat.

In your case, your aux heat isn't coming on. The fact that the heat strips come on in emergency heat mode (right?) but not normal mode leads me to believe that there's a problem in the control circuit.

If you get hot air (not luke warm) with R, G, and W2 jumpered together (t-stat base), the t-stat might be bad or not configured correctly.

In defrost mode, does the system blow very cold air?
 
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Old 12-30-12, 06:09 PM
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In emergency heat it blows hot air.

I jumped R, G, and W2 together at the inside tstat, its more cool than warm air.

In defrost mode its not "very cold" but its cool air none the less. About the same temp as when I jumped the wires on the inside tstat.

I may have a thermometer I can get tomorrow to measure the temps at the return and supplies to be more accurate, for now I am just going off how it feels to the skin though.
 
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Old 12-31-12, 05:58 AM
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I jumped R, G, and W2 together at the inside tstat, its more cool than warm air.
Does the blower speed seem lower in emergency heat mode?

Not having very cold air (like 55F) come out during the defrost cycle indicates that at least part of the heat kit is working.

The same goes for having luke warm air with R, G, and W2 jumpered.

It's possible that there's a relay or outdoor t-stat which keeps part of the aux heat off most of the time. (even with that, it should still keep up with part of the heat kit off)

This is something that you might have to call a tech for.
 
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Old 12-31-12, 07:51 AM
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Is it possible that I didnt hook the defrost control board back up correctly and that has something to do with this? I have printed the wire diagram out and plan to recheck everything this afternoon just to be sure.
 
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Old 12-31-12, 09:31 AM
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It's hard to know over the internet. The only option is to check everything related to the electric heat - low voltage control circuit, heat sequencers/contactors, the heat strips themselves, etc.

Did it it work properly right before you changed the board?
 
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Old 12-31-12, 09:38 AM
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Do you by any chance set back t-stat?

Is it possible that the system was just bringing the temp back up when you got up?

Heatpumps are best left at a constant temperature; the use of aux heat to recover wipes out any savings.
 
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Old 12-31-12, 10:43 AM
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Its a programmable tstat but I have it set to hold 72 degrees all the way through the program.

I dont know if it was working before or not honestly.
 
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Old 01-02-13, 09:38 PM
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When I use emergency heat, do I need to turn the fan on or should it run by itself? I've been crazy busy and not able to look at the unit yet an so I'm using emergency heat but when I move the thermostat to eheat all air coming from the vents stop immediately and nothing ever comes out. Do I have to change the fan from auto to on with eheat?
 
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Old 01-03-13, 10:05 AM
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I thought the emergency heat worked before.

The fan should come on automatically on most units these days; if it only works with the fan switch set to on, the t-stat needs to be configured to call for fan when there's a call for emergency heat. Check the manual - either there's a fuel switch (set it to E) or a setting in the menu which needs to be changed.

Having the heat strips operate without the fan can cause damage.

----------------------
Running the system in emergency heat mode can be very expensive, so I suggest calling a tech out to see what's going on. Over a single season, the extra cost of using 100% resistance heat can far exceed that of a service call.

Continuous heatpump operation with the heat strips running a little bit (using around 2.4kw for a 2 ton to provide 4-7+ kwh of heat depending on weather) beats resistance heat. (10-20kw to provide 10-20kw worth of heat)

Emergency heat is only for emergencies or extreme cold.
 
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Old 01-03-13, 01:40 PM
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The emergency heat did work before but the fan issue had been present, I just hadnt thought to ask until now. I completely retraced and rewired the thermostat to defrost control board today. Verified all wire runs are good.

In the process of all this, while testing continuity from tstat bank to the defrost board I noticed the thermostat itself actually has some switches on the back on the board, hidden from plain sight.

After looking at them, you have to manually set it to be used for either oil/gas units or heat pump units. These swithes were not configured when the new tstat was installed by the tech this past summer when the compressor and tstat were replaced. Apparently, he just slapped it on there no big deal and since it was using the air it didnt matter much at the time.

Tonight will be the ultimate test to see whether or not that resolves the issue with the aux heat but I have faith lol. I will post tomorrow to let you know how tonight goes with temps.

Again, thanks so much for your help. I now know more about hvac than I ever have
 
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Old 01-07-13, 12:21 PM
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Note 2: Jumper is required to use Auxiliary Heat for both Second Stage and Emergency Heat on units without separate Emergency Heat and Auxiliary Heat terminals.

The switch setting on the back of the thermostat fixed my indoor fan not being on when I turned emergency heat on. The jumper wire fixed the aux heat not working by jumping E/W1 & W2.

The only thing I have left to do is set the point at which the aux heat kicks in. Currently its 3 degrees. I dont know what the default is in general but I would like for it to be less than 3 degrees. With the tstat set to 72, at night the aux doesnt kick in until 69 degress.
 
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Old 01-08-13, 02:44 AM
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I gained alot from this as well, thanks everyone for your patience and perservierence
 
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