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-   -   Temperature at Refrigerant line too low? (http://www.diychatroom.com/f17/temperature-refrigerant-line-too-low-92263/)

crakarjax 01-14-2011 10:59 AM

Temperature at Refrigerant line too low?
 
I have had some work done to my HVAC system, which consists of two Goodman air handlers and two 13 seer goodman heat pumps. There were apparently leaks in one of the systems' refrigerant circuits and those were repaired, but I am seeing some behavior that worries me and that the HVAC techs haven't been able to give me a good explanation for.

The temperatures, when read with an IR thermometer, at the outlets of the two heat pumps are 87 and 113 degrees, typically. It seems to me that since these units are identical, they should be putting out refrigerant that is heated to the same temperature! The only differeing factor that I can think of is that the circuit is physically longer to the upstairs zone since the handler is in my attic.

I contacted the HVAC service company about this, and they came out to check it out. They told me that I need a new PCB because my heat pump was stuck in defrost mode! Well, that's not the issue that I called about, nor is it an issue that I have before or since witnessed. I pressed them on this and they claim that there are "too many factors" that could cause the discrepancy in heating ability between the units?!?! The way I see it is this:

Refrigerant coming in at X degrees -> black box heating magic -> refrigerant out at Y degrees

The return lines are approximately the same temperature going into both units, so I don't see how there could be a 20 degree discrepancy between the two!


As a disclaimer, I should note that I have little experience working with HVAC systems (so far!), but I'm hoping that you resident experts can point me in the right direction.

Thanks in advance,

Russell

kb3ca 01-14-2011 12:39 PM

The run times are different since they are serving different areas. Maybe one is about to go into defrost and the other one just came out of defrost, for example. HVAC company can check the refrigerant charges and operating temperatures. I would not expect to see two heat pumps operating exactly the same at any given moment in time.

fitter1 01-14-2011 01:02 PM

Agreed with post#2. Also using an infra red thermometer can be a little tricky if not being used per instructions. They do not give an accurate reading on copper tubing or any shinny metalic metal. Use a thermocouple type meter and secure tightly to pipe and insulate. This will give a better reading for you.

crakarjax 01-14-2011 01:07 PM

Points taken, but I allowed the units to get up to a stable operating temperature before taking the readings. I'm assuming the units will heat the refrigerant gradually to X degrees and level off at a maximum temperature, and that's the temp I was trying to capture.

I was pretty careful about the temp. measurements too, moving the thermometer around to get a feel for the actual temperature and noting the highest temp recorded, as well as taking the temps at the same reference points on both units.

I guess my question boils down to this:

Assuming that I can accurately measure the temp. and the units have been running under the same load for the same amount of time, should the temperatures be the same (or similar at least)? I wouldn't be worried if it were a few degrees, but 20 degrees is a large difference IMO!

fitter1 01-14-2011 03:10 PM

Different air flows will give you different T.D's,which would reflect in different operating pressures. Also maybe a 2nd stage heat is on in one or the other. These are only a couple of things to look for as there are a LOT of factors involved and to check.

AllanJ 01-14-2011 06:28 PM

I take it that you are measuring the temperature near the heat exchanger as opposed to at the floor register(s).

It is possible that the heat pump producing less heat is a little short on refrigerant.

For heating, the refrigerant arrives at the indoor heat exchanger in gaseous form via a fat pipe. At this time its temperature is irrelevant. A compressor turns the refrigerant into liquid (simply by compressing it). Being liquified by compression makes it becomes hot. After giving up most of its heat to the air inside the exchanger, the refrigerant goes back to the outdoor unit via a thin pipe.

If there is a smaller quantity of refrigerant in the loop overall or if the outdoor temperature is lower, less heat will be produced at the indoor exchanger. This is because the quantity of gaseous refrigerang arriving at the indoor exchanger is smaller. The amount of heat more or less corresponds to the volume of liquid refrigerent coming out of the compressor. In milder weather, more of the liquid refrigerant will evaporate (re-boil or re-sizzle) at the outdoor exchanger and therefore more gaseous refrigerant comes back to the indoor excchanger compared with during frigid weather. If the outdoor exchanger freezes up and/or the fan out there fails, then cold outside air won't flush away the colder air in the exchanger, said air made still colder by the evaporation of the refrigerant back into a fat pipe, then less evaporation will take place, and therefore less gaseous refrigerant will head back to the inside exchanger.

It is possible for the compression to be done outside to reduce noise inside and then a thin pipe would take liquid refrigerant into the house as well as a thin pipe bringing the liquid refrigerant back outside. Here there is the great disadvantage that heat will be lost along the way.

beenthere 01-14-2011 07:38 PM

You need to check the temps at the air handlers, not at the registers. And you need to check the return air temp also. The unit in the attic, also has its ducts in the cold attic, so the register temps will be lower.

3 legged dog 01-14-2011 07:40 PM

Has it ever been the same supply temp, or is this something new?

AllanJ 01-14-2011 08:35 PM

Also check the heat exchangers and their air filters for excessive dust and dirt.

Marty S. 01-14-2011 10:36 PM

If you're talking about air temps then the 113 degree one has a heat bank on when the other does not. Assuming a typical 68-70 degree return air you can not have a 45 degree temp rise with just the heat pump. Ten to twenty seven would be the normal temp rise range for a 2.5 ton, depending on outdoor temp.

If you're talking liquid line temps then one or both units have a problem.

crakarjax 01-14-2011 10:50 PM

So, I guess I wasn't clear about what temperatures I was reading... I wish those were air temps! I am reading the temperatures not at the registers, but at the liquid lines on the compressors outside.

The attic heat exchanger was just replaced (had a leak) 3 days ago, so I'm sure it's clean. The air filter is also clean.

Marty, what kinds of problems might they have? Wrong amount of refrigerant is obvious, but I have had the hvac guys out here 3 times now and I hope they have been checking it when they come out.

Are there troubleshooting steps I can take on my own? I have a manifold gauge that I use to recharge my car A/C, so I might be able to read pressures with it. If it helps, I have a 2-ton goodman system from 2007, the compressor model is GSH130241A I believe.

crakarjax 01-14-2011 11:00 PM

Quote:

Originally Posted by 3 legged dog (Post 570219)
Has it ever been the same supply temp, or is this something new?

I have never checked the temps before so I cannot say. I will note that when the hvac tech first recharged my system, the air coming from my vents was warmer than it had ever been, and the air is now room temperature at best.

Marty S. 01-14-2011 11:05 PM

Pressure readings of both units along with outdoor temp at the time would be very helpfull. Return air temps, supply air temps and liquid line temps should also be taken.

I don't do automotive so not sure if your manifold set will work or not. Thought they had a different sized fitting then we use.

crakarjax 01-14-2011 11:10 PM

Quote:

Originally Posted by Marty S. (Post 570383)
Pressure readings of both units along with outdoor temp at the time would be very helpfull. Return air temps, supply air temps and liquid line temps should also be taken.

I don't do automotive so not sure if your manifold set will work or not. Thought they had a different sized fitting then we use.

I think you might be right. I may look for a proper gauge set tomorrow.

What do you mean when you say "return air" and "supply air"? Is that air going into the handler and air coming out of the registers?

Marty S. 01-14-2011 11:15 PM

Air coming into the air handler and air leaving. Registers can give you false information since there's heat loss in the duct so close to the air handler is best.


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