Carrier HP suction and coil thermistors swapped?

[carrier-397b1f]

Hi all,
My parents have a Carrier communicating heat pump (25VNA0) at their house. According to the service manual on page 10 (which I'm not allowed to link to yet, but is the first Google result for "25vna0 service manual"), there's an outdoor coil thermistor between the outdoor EXV and the outdoor coil, and there's a suction thermistor and pressure transducer near the compressor inlet. From my limited understanding of refrigeration, in heating mode, the temperature should be coldest (at saturation temperature) right after the expansion device, then the refrigerant should boil off and become superheated in the outdoor coil without much pressure drop, and the suction temperature should then be just a little below the ambient temperature. What I'm seeing on the Infinity communicating stat under "heat pump status" is that the outdoor coil temperature is a few degrees below ambient, the suction temperature is quite a bit lower, and the superheat is is the range of 0-5 degrees -- which seems like it's lower than it would be for AC, but not sure if it's normal for a heat pump. I also measured (with a cooking thermometer...) the air coming out of the outdoor unit as being colder than the outdoor coil temp, which doesn't make a lot of sense to me. For some numbers -- recently, at 43F ambient, it was showing 39F outdoor coil temperature, 31F suction temperature, and 100 PSI suction pressure, 0 degrees superheat.
I'm wondering whether my understanding is faulty, or if there might be something wrong with the system! For example -- could the thermistors just be swapped? I can't trace where the wires go because that would require opening the unit. I would expect this situation to result in some kind of fault code, but there aren't any.
Thanks for any guidance you can offer!

[comfortdoc]

It’s normal for heating operation. With the electronic controls they can run superheat lower than you would typically see.

[DOGBOY]

as comfortdoc said, all is normal. you really can't accurately check refrigerant charge in heating mode.

[carrier-397b1f]

Thanks for your responses! Lower superheat with electronic controls makes sense. But, I'm still not sure I understand why the temp after the EXV and before the coil would be higher than the temp after the coil, by a full 8 degrees -- it doesn't make physical sense to me, though I'm not an HVAC person so I could be missing something that's obvious to you
Also, I wasn't really concerned about checking the charge -- just wondering why the temps seem opposite to my mental model of how the system works.

[CircusEnvy]

Outdoor coil thermistor could be off slightly if the thermostat is reading 39 degrees, but the suction line temperature (assuming sensor is on the true suction line) is reading 31 degrees. The coil should definitely be colder than 39 degrees at a 41 degree ambient. Usually those sensors are located on a distributor tube in between the EEV and coil manifold.

[carrier-397b1f]

Hi CircusEnvy,
I guess that's the question -- is the suction line sensor even on the suction line, if it's almost always reading 0 superheat? Because that's the temp reading I'd expect if it were right after the EEV, which is where the outdoor coil sensor should be, not the suction sensor. It was 43 ambient, not 41, so maybe 39 degrees coil temp is less suspicious than you thought. And yes, on this unit, the sensor should be on a tube between the EEV and the coil, so I'd expect that to be the coldest reading..

[CircusEnvy]

Superheat is usually very low on systems with EEVs and I can’t say for certain what that Carrier should be running at that very moment.

The suction line sensor is supposed to be located at the evaporator outlet, which in heating mode, would be the common outlet tube on the outdoor coil (between the coil manifold and reversing valve). It might also be on the true suction tube (between reversing valve and compressor). Either place should be the same temperature.

The coil temperature is something different. That could either mean (1) the coil temperature sensor, which is used to determine the approximate temperature of the outdoor coil for defrost, and is usually located on one of the distributor tubes between the inlet manifold of the outdoor coil and the EEV. This would be the coldest part of the coil and can be anywhere from 10-20 degrees less than the outdoor air temperature - a lot of factors play into this, like what stage it’s operating, how long it’s been running, etc. (2) This could also be the saturation temperature of the coil, which would directly relate to whatever PSIG measurement the low side pressure transducer is reading (100 PSIG would be about 31 degrees for R410A). Whether it’s (1) or (2), they would both be about the same temperature in heating mode regardless. I don’t think it’s (2) because the thermostat is displaying 0 degrees superheat, so it’s comparing the transducer reading with the suction temperature reading.

The concern is the “coil temperature” is 39 degrees at a 43 degree ambient. Saturation temperature or actual distributor temperature should be colder than that. If you think about it, the coldest part is going to be after the EEV, which are the distributor tubes, and as that refrigerant boils off and absorbs heat through the coil, the temperature of the coil only increases before the outlet. So if the sensor is reading 39 degrees at what should be the coldest point, either the sensor is amiss or there’s an operational issue. It may not be enough deviation to throw an error code and usually the coil thermistor is only used to determine the coil temperature for defrost purposes, not operation. Not sure if this is true or not with Carrier.

[Makeitcold]

I often find those little coil temp sensors are swinging in the wind, having popped off from what assume must have been ice growth pushing it off

[carrier-397b1f]

CircusEnvy: Thanks for your response -- this is helpful. The manual says the suction line sensor is between the reversing valve and the accumulator (not that it should make a difference in heating mode!) so my understanding is that it reads suction temp in both heating and cooling (even though it has a TXV for cooling).

It also says the coil temperature sensor is between the EEV and the distributor tubes ("The sensor must be securely mounted to the tube connecting the EXV and distributor.") So, makes sense that it should be the coldest part of the system. MakeItCold -- that's what I originally thought (that it might have come off), since it reads close to ambient when the system is running. Might try to see if it's visible from the top of the unit.

So none of you think my nutty theory about the wires being switched could be possible?
Since it was a few degrees colder today, some new numbers:
37F or 38F ambient, coil temperature 36F, suction temperature 25F, suction pressure 86 PSI. It's a variable speed unit, and it was running faster at this lower temp than it was when it was 43.

[CircusEnvy]

CircusEnvy: Thanks for your response -- this is helpful. The manual says the suction line sensor is between the reversing valve and the accumulator (not that it should make a difference in heating mode!) so my understanding is that it reads suction temp in both heating and cooling (even though it has a TXV for cooling).

It also says the coil temperature sensor is between the EEV and the distributor tubes ("The sensor must be securely mounted to the tube connecting the EXV and distributor.") So, makes sense that it should be the coldest part of the system. MakeItCold -- that's what I originally thought (that it might have come off), since it reads close to ambient when the system is running. Might try to see if it's visible from the top of the unit.

So none of you think my nutty theory about the wires being switched could be possible?
Since it was a few degrees colder today, some new numbers:
37F or 38F ambient, coil temperature 36F, suction temperature 25F, suction pressure 86 PSI. It's a variable speed unit, and it was running faster at this lower temp than it was when it was 43.

It’s unlikely they’re swapped. The coil thermistor generally has a metal clip that is only big enough for 3/8” copper. The suction line thermistor would have a larger clip to accommodate 3/4” copper. The only place where they could be swapped is on the board, but I think the plugs are different shapes to prevent that from happening. It really sounds like the coil thermistor is inaccurate or it’s not clipped on.

[Tjune54]

From reading your first post numbers and last post numbers, the only thing sticking out is the 0F superheat. The evaporator sensor, at the location you state will be the warmest part of the evaporator. The refrigerant has just passed through the EEV, it still has to pass the distributor and into the coil, where it will do it's work. Only a small part of the coil will add superheat and yours's does not seem to be doing that. You may need to get service out there to check the sensors and control board for the EEV.
There is a lot going on inside the refrigerant circuit, too much to explain on a forum.

[comfortdoc]

From reading your first post numbers and last post numbers, the only thing sticking out is the 0F superheat. The evaporator sensor, at the location you state will be the warmest part of the evaporator. The refrigerant has just passed through the EEV, it still has to pass the distributor and into the coil, where it will do it's work. Only a small part of the coil will add superheat and yours's does not seem to be doing that. You may need to get service out there to check the sensors and control board for the EEV.
There is a lot going on inside the refrigerant circuit, too much to explain on a forum.

Zero superheat is quite normal in the heating mode with this unit.

[carrier-397b1f]

Alright -- turns out my mother (this unit is at my parents' house) took a picture of the outdoor unit while it was being installed, which happens to have a clear shot of the board -- and I can see that the color-coded sensor wires are definitely going to the right plugs on the board. I also looked down through the top of the unit, and it turns out I can see the sensor wires from there, and the sensors are in the right place (and as someone mentioned earlier, they indeed have different sized clips.) So, sorry for wasting all your time with this silly theory!

I can't get a very good view of the outdoor coil thermistor, from outside the unit I can really only see the metal band and the wire coming off it, but that metal band does look firmly attached.

However, the suction line thermistor looks quite loose (picture attached). Should we avoid operating it until someone can come attach it better, or is it okay as long as it's not throwing any faults? As far as I can tell, it's operating normally.



Lastly, I still don't really understand why the part after the EEV wouldn't be the coldest part of the low pressure side, but I can let that go for a while, as long as the system's operating correctly

[carrier-397b1f]

Best shot I could get of the outdoor coil sensor (or at least I think that's what that is.) Sadly not showing the actual thermistor, just the clip.

[CircusEnvy]

Looks like everything is in the right place. Next step is to call a pro to test the coil sensor and make sure everything is working right. Thermistors commonly fall out of range just from age.

[carrier-397b1f]

CircusEnvy, thanks for taking a look! Did the suction temp sensor look loose to you or no? I suspect the sensors themselves are alright, since they all read the same as ambient when the system is off, and if I remember correctly that's been true over a fairly wide range of weather. We'll probably have someone out here soon enough for other reasons (some persistent new-furnace-smell, but that's for another post...) and I'll let them know my concerns beforehand, so they can take a look at the heat pump too.

[CircusEnvy]

Doesn’t look loose to me, but a tech has to get inside the unit anyway, wouldn’t be much to ask him to check.

[pecmsg]

CircusEnvy, thanks for taking a look! Did the suction temp sensor look loose to you or no? I suspect the sensors themselves are alright, since they all read the same as ambient when the system is off, and if I remember correctly that's been true over a fairly wide range of weather. We'll probably have someone out here soon enough for other reasons (some persistent new-furnace-smell, but that's for another post...) and I'll let them know my concerns beforehand, so they can take a look at the heat pump too.

You can suspect all you want....A tech needs to take reading(s). It could be off its been known to happen. If your that concerned call your service provider, tell them your concerns and see what they say.

[Makeitcold]

Agree with Pecmsg, if it's out of range it could lead to unnecessary defrost cycles. While not damaging, it will turn on the backup heat and will have to restart and run in its heating cycle around 15 minutes to achieve it's running efficiency again