# Heat pump starts initially but will not come back on to maintain temps



## johnniebravo (Oct 4, 2011)

First post here and looking for some assistance with troubleshooting my heat pump. I have been living in my house for 2 years, I believe the system is original to the house and was installed in 1998. When I first moved into the house the heat pump worked fine, the system would heat and cool fine. In heat mode, if the temp dropped too much the aux heat would kick in. The system is a carrier weathermaker 8000TS heat pump system with a natural gas aux/emergency heat backup, the outdoor unit is a comfortmaker. The air conditioner works fine, no problems except with it freezing up twice this summer because 1) the indoor blower motor died and 2) after i replaced motor I had too many vents blocked off preventing sufficient airflow over the coil to prevent freezing. To elaborate on the previous cooling problem, I replaced the inside blower motor and capacitor as well as the outside compressor/fan capacitor. No more freeze ups once i corrected the mechanical failure and dumb move of blocking vents to boost airflow to upper floors. Lesson learned. 

The AC part of the system works fine, if i turn it on via the thermidistat (TSTATCCPRH01-B), the indoor blower starts as well as the outdoor compressor and fan. House cools fine.

Now on to my current problem that first appeared last winter. With the heat pump system switched to off (for an extended period) and then switching it to heat, the thermostat (set at 72, with an indicated indoor temp at 70 or less) will show the heat indicator on the lower right. The indoor blower will turn on and the outdoor compressor and fan will kick on. The low and high pressure lines will quickly warm and the system will heat very well. House heats up quick, Yay! Well, after the target temp is reached the system will turn off as it should. This is where the problem happens, after a few hours the house temp will slowly fall and the thermostat will kick on and show the heat indicator again in the lower right and the indoor fan will turn on BUT the outdoor compressor and fan will not turn on. The thermostat will continue to run the indoor fan and show the heat indicator but the outdoor compressor and fan will not turn on again. Usually this happens after i go to bed in the evening and the fan will run all night with the temperature slowly dropping all night. I usually wake up to a cold house and then switch the system manually to Emergency heat. The emergency heat works wonderfully! The house gets warm in a matter of minutes. Thing is the system used to switch to e-heat automatically, it doesn't do it anymore (another symptom?). Over the past few days the temps were in the 40s, which are plenty high for the heat pump to work efficiently (I live in the Washington DC area).

So my question is what would cause the heat pump to heat perfectly fine for the first cycle of heat when turned on from an off position but not allow the outdoor unit to come on when its re-called after temp drops? Second is why does the system no longer switch to aux/e-heat on its own? 

Just this morning i noticed something strange, the system was running as it normally does when i wake up....fan running with the heat indicated on the thermostat but outdoor unit not running and house temp dropping. Well, today is supposed to be 75degrees so i just turned it off for the day. WEll before turning it off the system showed house temp as 70 deg with a target of 72...it was attempting to get there. Approx 2-5 minutes after shutting the system completely off, i walked by the thermostat again and it said the internal temp was now 72 degrees. Is this a sign of a defective thermostat? it said it was 2 degrees cooler with the system running just ~3 minutes ago???

Additional details, again, i just replaced the indoor blower motor due to a failed front bearing, replaced capacitor. Indoor coil looks OK too. Outside, i hosed off and vacuumed the compressor unit, and changed out a weak capacitor. The contactor is aging but works perfectly fine, visually inspected it as it turned on and off. Air filter is clean and is replaced regularly. The contactor is getting 240v and the defrost board is also getting 24v.

My obvious observation is when the system is running in heat mode, not on the first start-up, the contactor is not getting a signal to activate the outside system. What could cause this condition? what would cause aux heat to no longer automatically switch on? Am I looking at a bad defrost board outside? With the indoor furnace working fine as well as all the fan controls I am inclined to say that part is working fine and the only electronics or parts are on the outside unit. I am convinced that the problems lies outside. To my knowledge, the outdoor unit has never frozen over. Last thing, the LED on the indoor board has not indicated any faults, its a solid red LED all the time.

Any help is appreciated. I've searched a bit but didn't find any answers. If I've missed a previous thread, please point me in that direction and will try what is recommended there.


----------



## Done That (Apr 28, 2011)

So maybe start checking the Tstat outputs for 24V signals. For example, is there a Y1 compressor call at the outdoor unit when the thermostat has it's pump heat icon active? The defrost board only calls for back-up heat during a defrost...normal back-up/emergency heat is called for by the W1 output of the thermostat, or the E emergency terminal (sometimes these are jumpered together anyway).

Sometimes outdoor units that run and then stop are tripping on their internal protector..due to issues like low refrigerant charge. When the protector resets the units can run again. In summer it can take hours for a reset...but you had no summer cooling issues so this doesn't appear to be your issue.

I lean to someting inside / Tstat / wiring.


----------



## johnniebravo (Oct 4, 2011)

I am leaning towards something inside too especially since the thermostat no longer automatically switches over to aux/e-heat on its own anymore. Is there a chance I have a low charge that is sufficient for summer cooling but not good enough for effective heat during cold months?

By compressor call? Do you mean does the unit outside get a 24V signal at the circuit board? I am also going to assume Y1 = the Yellow labeled terminal.

The defrost board also seems to serve as the brain for the outdoor unit. Is this a correct assumption?

A few additional details, the heat pump model # is: CHP242AKA1

The board in the outside unit is: ICP Products Defrost Control Board 1069364


----------



## Doc Holliday (Mar 12, 2011)

I'd start off with the stat as well as in replacing it. You did say you had no cooling issues which would lead me to believe all is well although if you had a lineset problem such as a stuffed drier then that could lead to pressure switch problems which would cut off the outside unit.

We'd need to know if you have low voltage to the outside unit on each call (cycle) for heat.


----------



## johnniebravo (Oct 4, 2011)

Would low line voltage show up when the unit is operating fine on the first startup? Or would it only show when the unit is not coming on to maintain heat?

I tested line voltages at the outdoor board yesterday and found the following:

compressor and fan OFF measuring Volts AC
R C O Y W
27v .010 .017 .049 .002


----------



## Done That (Apr 28, 2011)

The control board is always fed 24V from the indoor transformer. 

The thermostat turns on a relay to send 24V to the Y terminal on that defrost board when there is a cooling or heating demand that requires the compressor to run. So you should be able to measure 24V demand signal when your thermostat is outputting a heat/cool demand...on Y at the board. The control may have a 3 or 5 minute short cycle delay timer, and then it should energize the contactor from the CC terminals (if I have that board right in my head).

Be carefull because the fan relay on that defrost control is breaking line voltage...so that board can bite you if you are not paying attention.

In this case I'm with Doc...I would put on a new Tstat and see if that takes care of it...worst case you have a nice new Tstat.

Otherwise bring in a pro and he'll have you straightened out in no time.


----------



## johnniebravo (Oct 4, 2011)

Just turned the heat on inside and quickly ran outside, the Y terminal registered 26.72 volts and the unit came on. It ran for approx 3 minutes then shut off. Temp inside had climbed in that short period by 2 degrees....which i thought was fast. Similar to this morning when I shut it off and it then jumped 2 degrees.

anyway, after the outside compressor shut off, the heat icon was still on at the thermostat and the line remained at 26.72V. 

The voltage fluctuates from 25 ish to 26 ish.


----------



## Master of Cold (Aug 7, 2011)

Look inside the compressor area. You will have 2 red or pink (the colors fade) that are probably laying on the hot gas line. Examine these wires and see if they have been damaged.


----------



## Marty S. (Oct 31, 2009)

johnniebravo said:


> Just turned the heat on inside and quickly ran outside, the Y terminal registered 26.72 volts and the unit came on. It ran for approx 3 minutes then shut off. Temp inside had climbed in that short period by 2 degrees....which i thought was fast. Similar to this morning when I shut it off and it then jumped 2 degrees.
> 
> anyway, after the outside compressor shut off, the heat icon was still on at the thermostat and the line remained at 26.72V.
> 
> The voltage fluctuates from 25 ish to 26 ish.


Since you still had 24 volts at Y outside and the compressor was not running then that narrows it down to the heat pump. There's the defrost board, a high pressure switch and maybe a discharge sensor. Narrow it down by checking to see if the sensors are open or closed when the system faults. If they check out fine then it's the defrost board. If the sensors are open then there's likely still an air flow issue inside and the sensors are doing their job protecting the compressor.


----------



## johnniebravo (Oct 4, 2011)

Master of Cold said:


> Look inside the compressor area. You will have 2 red or pink (the colors fade) that are probably laying on the hot gas line. Examine these wires and see if they have been damaged.


I looked in the compressor area and did not notice red lines, there are yellow lines wrapped around the smaller copper tube (high pressure line??). It's dark out now so I will have to hold out until tomorrow afternoon.


----------



## johnniebravo (Oct 4, 2011)

Marty S. said:


> Since you still had 24 volts at Y outside and the compressor was not running then that narrows it down to the heat pump. There's the defrost board, a high pressure switch and maybe a discharge sensor. Narrow it down by checking to see if the sensors are open or closed when the system faults. If they check out fine then it's the defrost board. If the sensors are open then there's likely still an air flow issue inside and the sensors are doing their job protecting the compressor.


I would like to check the voltage to the board after the thermostat has called for heat for an extended period and the temp is falling...like when I wake up in the morning. Problem is the temps are back up to 75ish degrees now. So, not sure if I will have the opportunity any time soon.

Regarding the sensors being open or closed, I have no idea of how to do that. I assume with my multimeter?

And with the air flow, all vents are open with the exception of the sole vent in the basement. It is very high pressure considering how close it is to the indoor unit.

Should I hold off on a thermostat until i can test the rest?


----------



## jfrotten (Aug 14, 2011)

It sounds like a tstat to me... the tricky part in diagnosing is hopeing that the original installer wired with the traditional colors (probably). Take the tstat completely off of the wall and go get a wire nut. Nut the red, yellow and green wires together. This is the heat pump in heat mode and the air handler. If your unit runs and heats normally, add the white wire. This should be your secondary heat, gas or electric. The heat pump should continue to run but your secondary should kick on. Connecting these wires at different times is all your thermostat really does... if you can do it manually and everything works, then your thermostat is either not wired/set up correctly or it's bad.


----------



## johnniebravo (Oct 4, 2011)

jfrotten said:


> It sounds like a tstat to me... the tricky part in diagnosing is hopeing that the original installer wired with the traditional colors (probably). Take the tstat completely off of the wall and go get a wire nut. Nut the red, yellow and green wires together. This is the heat pump in heat mode and the air handler. If your unit runs and heats normally, add the white wire. This should be your secondary heat, gas or electric. The heat pump should continue to run but your secondary should kick on. Connecting these wires at different times is all your thermostat really does... if you can do it manually and everything works, then your thermostat is either not wired/set up correctly or it's bad.


Jfrotten,

Thanks for the reply. The thing to consider here is that if i turn the system on after it has been off, it will heat the house up to the desired temp very quickly. After a few hours when it needs to come back on, it won't, but the indoor blower fan does come on along with the heat icon the tstat. Could I be dealing with an intermittently functional tstat? It seems to me that the only way to tell is to wait till it acts up and then see if the voltage is present at Y.


----------



## jfrotten (Aug 14, 2011)

It's definitely the thermostat. The question is, is it intermittenly defective (unlikely) or set up incorrectly. Either way, if you do your test and find no voltage at y, the it could only be the thermostat. If you do your test and find voltage at y, then there is an intermitten fault in the outside unit. I think that your installer set up on your thermostat is set to use your heat pump as your aux instead of your primary. It's either set that way or wired that way. It's almost easier to replace the thermostat then it is to explain how to ensure it is set up and wired correctly. I think that everything is functioning the way that it is set to function. Can you post a picture of the wires hooked into your thermostat back plate? One that shows both the wire colors and the letter labels of the terminals...


----------



## jfrotten (Aug 14, 2011)

johnniebravo said:


> anyway, after the outside compressor shut off, the heat icon was still on at the thermostat and the line remained at 26.72V.
> 
> The voltage fluctuates from 25 ish to 26 ish.


where did you measure this voltage? The red wire?


----------



## johnniebravo (Oct 4, 2011)

jfrotten said:


> where did you measure this voltage? The red wire?


No, i measured at the Y terminal which has a yellow wire connected. 

The red wire was measured yesterday and it has a constant 24V to it.


----------



## jfrotten (Aug 14, 2011)

Okay, so I've been posting from my phone. I just got home and logged in from my computer. I looked up your thermostat model and it looks like in order to adjust the installer settings you're going to need a carrier technician. I didn't realize that you had a freakin computer running your system. I can't find instructions for how to enter the installer set up mode. I'm thinking that your heat pump is set to run as the aux in heat mode. When the temp in the house is 2 or more degrees below the target, the primary AND aux heat kick in. Once your temp is within 2 degrees of the target, the aux kicks off and the primary cycles on and off to maintain the temp. I think that your heat pump is set as aux heat and whatever is set as your primary is not morking. That would explain why your heat pump will run when you first turn your heat on after its been off for a while And why your heat pump would not run when the thermostat is maintaining temp. Next time you get ready to turn your heat on after it's been off for a while, turn it on like you normally do. Make sure the heat pump has kicked on. Then, turn the temperature setting to only 1 degree less than whatever the temp in the house is. I bet the heat pump kicks off. The opposite should also be true. After your heat has run and got you up to temp the first time, when it kicks on the second time and the heat pump is not running but the fan is, try turning the temperature setting way up( like 5 degrees hotter than it is in the house). I bet the heat pump kicks on then. This would indicate that the heat pump is either wired or set as the aux instead of the primary for your heat. I hope I didn't get too confusing with all of that.


----------



## Marty S. (Oct 31, 2009)

It can't run the heat pump in heating and the gas furnace at the same time or the heat pump will trip the high pressure switch.


----------



## Master of Cold (Aug 7, 2011)

I don't think its gas..


----------



## johnniebravo (Oct 4, 2011)

jfrotten said:


> Okay, so I've been posting from my phone. I just got home and logged in from my computer. I looked up your thermostat model and it looks like in order to adjust the installer settings you're going to need a carrier technician. I didn't realize that you had a freakin computer running your system. I can't find instructions for how to enter the installer set up mode. I'm thinking that your heat pump is set to run as the aux in heat mode. When the temp in the house is 2 or more degrees below the target, the primary AND aux heat kick in. Once your temp is within 2 degrees of the target, the aux kicks off and the primary cycles on and off to maintain the temp. I think that your heat pump is set as aux heat and whatever is set as your primary is not morking. That would explain why your heat pump will run when you first turn your heat on after its been off for a while And why your heat pump would not run when the thermostat is maintaining temp. Next time you get ready to turn your heat on after it's been off for a while, turn it on like you normally do. Make sure the heat pump has kicked on. Then, turn the temperature setting to only 1 degree less than whatever the temp in the house is. I bet the heat pump kicks off. The opposite should also be true. After your heat has run and got you up to temp the first time, when it kicks on the second time and the heat pump is not running but the fan is, try turning the temperature setting way up( like 5 degrees hotter than it is in the house). I bet the heat pump kicks on then. This would indicate that the heat pump is either wired or set as the aux instead of the primary for your heat. I hope I didn't get too confusing with all of that.



I understand what you are saying, I will test it at my first opportunity. 

If I switch to my e-heat setting, the gas furnace does kick in. At the same time, the aux heat icon lights up at the bottom of the thermostat. Wouldn't that indicate that the gas furnace was wired as the aux?



jfrotten said:


> After your heat has run and got you up to temp the first time, when it kicks on the second time and the heat pump is not running but the fan is, try turning the temperature setting way up ( like 5 degrees hotter than it is in the house).


Well usually after a long night of cold, the temp showing on the tstat is like 62-65 when the target set for the heat is 72. Are you suggesting to set the target to 77 versus the already set 72 that its already not maintaining? or to just turn the temp up and see if the heat pump responds?

As far as programming for my current tstat, taken from another thread, this link shows how to enter setup mode:
http://www.docs.hvacpartners.com/idc...tstat-23si.pdf


Thanks for the large amount of feedback so far! I greatly appreciate it. I will have to make sure I pay back the advice over in the automotive forum since that is more of my area of expertise.


----------



## Master of Cold (Aug 7, 2011)

What control is being used to lock out the heat pump during gas furnace operation?


----------



## johnniebravo (Oct 4, 2011)

Master of Cold said:


> What control is being used to lock out the heat pump during gas furnace operation?


my thermostat has the option to operate in heat, eheat, cool, and auto modes. If i select heat, the heat pump comes on and aux heat (i.e. gas furnace) used to kick in when needed.

If i use e-heat, its only using the gas furnace. heat pump will not come on.

obviously cool is cool.

I have never operated the system in auto mode.


----------



## Master of Cold (Aug 7, 2011)

Next time the heat pump wont start check for voltage on the yellow wire, if you have it, check the wire that feeds the contactor. If you don't have voltage there then time for a new board. Im a little suspicious of the 24 volts at the condenser, when you have 27 at the thermostat.


----------



## jfrotten (Aug 14, 2011)

johnniebravo said:


> If I switch to my e-heat setting, the gas furnace does kick in. At the same time, the aux heat icon lights up at the bottom of the thermostat. Wouldn't that indicate that the gas furnace was wired as the aux?
> 
> Well usually after a long night of cold, the temp showing on the tstat is like 62-65 when the target set for the heat is 72. Are you suggesting to set the target to 77 versus the already set 72 that its already not maintaining? or to just turn the temp up and see if the heat pump responds?


Just because the gas is your emergency doen't mean that it is also your aux... it should be but that depends on the installer settings in your thermostat. As for the second part, what I meant was when your fan is running but your heat pump isn't, try turning the setting up far enough to trigger the aux heat (usually 3-5 degrees higher than the current temp) to see if the heat pump kicks in. 
New idea... follow those instructions you posted the link to and write down all of the settings that are currently set. Don't change anything yet. If everything looks good from there the next thing we'll do is the test that it talks about in step 5. Run through the installer setup and write post what all of the current settings are.


----------



## Patrick Eubanks (Oct 6, 2011)

*voltage reading*

When the Thermostat is calling for heat and the condenser outside is not running, go outside and get a measurement of AC voltage between the common and Y terminal coming into the board from the thermostat wires from the inside unit. If it is showing 24 volts or higher, its in the outside unit. If its not showing 24 volts then its inside.


----------



## johnniebravo (Oct 4, 2011)

Master of Cold said:


> Look inside the compressor area. You will have 2 red or pink (the colors fade) that are probably laying on the hot gas line. Examine these wires and see if they have been damaged.


checked the two yellow wires wrapped around the hot line and they are not damaged. No red lines in the compressor area.

Another note, this outside unit is not the original compressor. Most all of the parts in the unit including the fan and compressor are dated mid 2000. House was built between 96-98 and sold in 1998.


----------



## Master of Cold (Aug 7, 2011)

If this were mine..I would take the yellow wire off of the defrost board, and connect the contactor directly. See what happens.


----------



## johnniebravo (Oct 4, 2011)

Master of Cold said:


> If this were mine..I would take the yellow wire off of the defrost board, and connect the contactor directly. See what happens.


I don't have the wiring diagram with me at the moment, but is this yellow wire set from the high pressure limit switch?

Could all this be caused by a low charge that only manifests itself in heat mode?


----------



## Master of Cold (Aug 7, 2011)

Sorry..the yellow thermostat wire that attaches to the board..
Low charge could cause the compressor to overheat, and shut down.


----------



## Patrick Eubanks (Oct 6, 2011)

*dual fuel*

It sounds as if you have a dual fuel configuration. If so Im betting the outdoor temperature sensor tells the unit when to switch from Heat Pump to furnace. Is this correct? If so then check the wiring for the outdoor temp sensor. If its not shielded cable then that could be your problem. You may also want to change yout outdoor temp sensor I ran into this problem about a year ago on one of my dual fuel units. We had no way to get back to the sensor wire so I ended up installing a honeywell Prestige IAQ thermostat with the wireless outdoor sensor. Problem solved and a happy customer again.


----------



## johnniebravo (Oct 4, 2011)

I do have a dual fuel configuration, and I do have an outdoor sensor on the side of the outside unit. I will trace and check the wiring. If there a way to check the sensor (ohms??)?


----------



## Master of Cold (Aug 7, 2011)

There are too many hypotheticals going on here. Let's pick one problem area and start there.
You found 26v on the y terminal, and the condenser wasn't running. That's a malfunctioned defrost board. That will have to be replaced. You can wire the unit to test this theory if you wish, but you will not have the defrost option. Although the weather forecast for your area looks like you wont need any defrosting this weekend. 

The thermostat jumping temperature after a short run duration could be a faulty stat. Or you could have air from the hole in the wall behind the stat. Couple that with the outdoor temp not reaching lock-out temp, and you have a cold house.

But I would start at the condenser and go from there.


----------



## johnniebravo (Oct 4, 2011)

Agreed with the multiple theories, though I do appreciate all of the suggestions. I turned the heat on for the evening since the temp will drop overnight. Hopefully when I get up tomorrow the fan will be running. If it is I will check the voltages at the board and go from there.


----------



## johnniebravo (Oct 4, 2011)

Got up early this morning and sure enough the temp had dropped and the system had run all night. Headed outside to remove the panel but first saw that one of the components in the compressor area was frozen. I have a picture but don't have a place to host it.

I proceeded to check the voltage of the yellow wire at the defrost board and it was registering a steady 26.70v (at the Y wire). 

I then checked for voltage to the contactor from the two blue wires that feed into the side of it (to the coil??) from the defrost board and they didn't have power.

Lastly, I checked the voltage from the board to the two yellow wires (i still don't know the accurate name of this sensor/switch) and it was getting 26.50v. I also checked the ohms and it was OL or 0.000. Don't know if that means anything.

Link to pictures on Flicker 

http://www.flickr.com/photos/johnniebravo2011/


----------



## Marty S. (Oct 31, 2009)

Y energized at board,pressure switch closed and no low voltage at contactor means you need a new defrost control board. Get that replaced along with the defrost sensor.


----------



## Master of Cold (Aug 7, 2011)

Yes, time for a board..


----------



## johnniebravo (Oct 4, 2011)

OK, tomorrow I will place an order for the following:

ICM304 Defrost Board
DT1172766 _ICP _Defrost Thermostat

and while I am in there I will also put in a new contactor.

And before the cold sets in I will have a Pro come out and check my refrigerant levels.

So I can better understand what's going on, why is the one particular part of the system freezing rather than the whole outdoor unit? My assumption the board is not telling the system to defrost??


----------



## Master of Cold (Aug 7, 2011)

The part that is freezing looks like the txv(im on a mobile phone right now, I can't enlarge it).
The outlet of the txv assembly is the coldest part of the system.


----------



## JJboy (Oct 12, 2010)

tvx restriction, power head should be stuck


----------



## johnniebravo (Oct 4, 2011)

JJboy said:


> tvx restriction, power head should be stuck


Will a new board and the sensor fix this?


----------



## Master of Cold (Aug 7, 2011)

If your heating performance is good, don't worry about the txv.
The only other check would be to check the voltage at the contactor. I know you checked the blue wires at the contactor coils, but those are on the common side. I wold check the other side of the contactor.


----------



## johnniebravo (Oct 4, 2011)

There are only contacts on one side of the contactor, the other side doesn't have leads similar to those that connect to the coil.

And yes, the system heats very well when that particular part is not frozen.


----------



## JJboy (Oct 12, 2010)

Master of Cold said:


> If your heating performance is good, don't worry about the txv.
> The only other check would be to check the voltage at the contactor. I know you checked the blue wires at the contactor coils, but those are on the common side. I wold check the other side of the contactor.


Maybe the check valve inside of txv is good in heating, but not the metering device in cooling......I'm no sure:huh: or the check valve is inside of the filter?????


----------



## Master of Cold (Aug 7, 2011)

He has good heat when it runs. He is just running too long. The coil gets too cold and the txv shuts down. The little refrigerant that comes through is freezing at the orifice.


----------



## johnniebravo (Oct 4, 2011)

With recent weather conditions I don't have much of an update. I replaced the defrost board (ICM304C), I don't know the part number for the defrost thermostat so i have not changed it yet. All the parts online look nothing like the one my unit uses. 

After a bit of thinking about the logic behind this system, if in fact the board is bad, it would explain why the system no longer auto switches to aux heat. If a defrost cycle is never initiated it never gets the signal to switch.

As soon as the weather allows, I will be back with feedback.


----------



## Master of Cold (Aug 7, 2011)

You can use just about any board on any unit, as long as you have the proper freeze thermostat.


----------



## johnniebravo (Oct 4, 2011)

Thats the dilemma, the existing defrost thermostat looks nothing like the ones offered online. The ones i have seen look like they clip on to a part of the coiling while the one i have has an enclosure with a small nut holding it on. I suppose i could change it out to a different type but i am unsure where to place it. In the photos i posted earlier you can see my defrost stat.


----------



## Master of Cold (Aug 7, 2011)

Place it next to the old one.


----------



## johnniebravo (Oct 4, 2011)

How critical is it to match up the thermostat exactly? The number i pulled off the thermostat comes up on parts sites but it looks different from the one i have. 

I was able to see, on the thermostat, 37TU31. I could see there was a HH carrier/bryant number but couldn't read it.

Is there anything that would prevent me from moving to an ICP defrost thermostat?
http://www.hvacpartsoutlet.com/Defrost-Thermostat-ICP.aspx


Lastly, I removed the stat and placed it in a bowl of ice water for 5 minutes and it tested ok using my multimeter. So, I don't believe i NEED one at the moment but i may get one just in case as a spare.


----------



## johnniebravo (Oct 4, 2011)

Had the opportunity to run the heat pump last night. Turned it on at about 7:30pm and it ran fine and shut off when target temp was reached. About 90 minutes later it came back on as it was supposed to do. I though everything was ok. Woke up at about 3am this morning and it was back to its old ways, inside blower fan running and thermostat calling for heat but pump not running outside. Got up at 8am to re-check the outside unit and it was frosted over just like before (same as pics above). 

While it was frozen the pump was still calling for heat (y, 26v), red was still at 26v, and defrost thermostat was beeping using ohms with tone on multimeter. I measured voltage between the contactor coil posts and it was at zero on both sets (they are side by side). I pressed down on the contactor and the compressor did run when it was down and immediately stopped when i let up; i did this twice. Well after pressing the contactor for a second it looked as if the ice started to melt off of the frozen part. Not sure. 

I then went into the house to see if i switched over to AC if the compressor would start. I switched to AC, changed the temp, and after 3-5 minutes, the compressor and fan kick on. I let it run for a minute, turned it off and switched back to heat. After the short 3-5 minute delay the heat pump kicked back on and is heating as requested. EDIT: The heat lasted only a few minutes and the TVX area frosted over again and shut down the pump.

Also, when i switched over from heat to cool inside, i could hear what sounded like the reversing valve doing something.

It appears that the system is not defrosting just that small part of the outside coiling and the ice is causing it to shut down. Could the defrost thermostat be in an improper place causing this symptom? Or could the defrost thermostat be broken but appear to be ok?


----------



## Marty S. (Oct 31, 2009)

The defrost sensor is just a switch that closes around 28 degrees and starts the timer on the defrost board. A pressure switch lock out would show 24V at Y but keep it from going to the contactor.
None of that explains why the thermostat or duel fuel control are not bringing on the aux heat when the heat pump can't keep up but it works on E-heat. Sounds like a fun one to work on in person but over the internet, not so much.


----------



## johnniebravo (Oct 4, 2011)

Marty S. said:


> The defrost sensor is just a switch that closes around 28 degrees and starts the timer on the defrost board. A pressure switch lock out would show 24V at Y but keep it from going to the contactor.
> None of that explains why the thermostat or duel fuel control are not bringing on the aux heat when the heat pump can't keep up but it works on E-heat. Sounds like a fun one to work on in person but over the internet, not so much.


Maybe it is a pressure switch lockout, it seems that as soon as the frost appears the system shuts off and the coil connectors do not show 24v. Seems like I am running out of options with a new board and a working defrost thermostat.


----------



## johnniebravo (Oct 4, 2011)

Had a local company send a tech out to look at the system. I explained to him all the checks I had already done and all of the maintenance items/replacement parts I had already added (capacitors, defrost board, contactor, indoor blower motor). He diagnosed the system as having a bad TXV valve at the outside unit. He said it was not opening and after the unit ran for 20ish minutes it would develop low pressure and the sensor would shut everything down until the pressure equalized. As it was explained my system has a TXV both outside and inside (cannot see the inside one) and the outside TXV is not used in cooling which is why the problem only manifests in heat mode. 

I also spoke with another competing tech and he had the same diagnosis. Actually, over the phone he described my problem so well that if I didn't know better I would have thought he had been to my house when it was acting up. AND, get this, he explained that i needed to weigh the cost of a repair versus the savings....is it cheaper to just run gas versus spending a large sum of money that i may only recover after 5-6 years of running the heat pump. Overall I was very pleased with his perspective and advice. He is an independent contractor. 

A big thanks to everyone here that gave ANY advice. I have a much much greater understanding of how my HVAC system works now. I pretty much had it narrowed down to the problem he diagnosed based on what you all suggested. I haven't scheduled the repair yet, but I will. The price is a bit steep at $800ish, but as I understand it is what the going rate is here to get the job done right (labor, TXV, drier, and system evac and recharge). If I had not replaced my own capacitors, contactors, and indoor blower motor I probably would have paid much more and probably would have been advised to get a new system. 

Also, i asked about how it was wired and he mentioned that my e-heat would not kick in until outside temps reached about 40 degrees or less. 

Just out of curiosity, I checked into the local HVACR program at my community college and it would only take 3 semesters/50ish credits to complete their degree program and get certified (shorter because i would transfer credits for all classes not HVACR related from my other degrees). I might look into it, my MGIB would pay for the classes and I am certain it would all pay for itself somewhere down line just repairing my own equipment. 

I will post an update with more details for closure and searching once the repair is done and i have an itemized receipet.


----------



## johnniebravo (Oct 4, 2011)

Had the system repaired last week, the txv was the problem. Evening else was ok, system is working great now....the best since I have lived in this house. Thanks for all the suggestions and help.


----------

