Appropriate Heat Pump Pressure Readings

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Old 02-20-11, 01:45 AM
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Appropriate Heat Pump Pressure Readings

I understand and respect the rules about describing the technique for using refrigerant on the forum. I am NOT interested in messing around with the refrigerant in my new Carrier, Infinity heat pump.

That being said, I have many years experience with automotive AC and was involved in research and development of new lubricants during the change over from R12 to R-134A before I retired from a major oil company automotive laboratory back in 1993.

Last Spring, I had a new Carrier, Infinity installed and I am very happy with my choice. It does everything that Carrier says it will and so far with no maintenace.

I intend to have the first annual service on the unit within the next month or so and I suspect the technician will check the system pressures. When I had my previous heatpump serviced, there was sometimes disagreement about the appropriate pressure readings. On one occasion, the technician told me the system refrigerant charge was low. Six months later a different technician (same company) said the pressures were too high.

I know that ambiant temperature has much to do with determining the proper pressure and I have used pressure readings on automotive systems to help me determine if a system was charged properly. However, there is a rather wide low to high band that is considered normal pressure in any given system. I never relied on just pressure and always use outlet temperature and ambient temperature at the condenser to help adjust the refrigerant level.

I felt much more comfortable when I evacuated the system and weighed in the designated amount of refrigerant. When I did that, the pressures were always right on target and the outlet temperature was usually at it's lowest designated temperature.

My question concerning whole house AC is.... because the tubing that carries the refrigerant from the condenser to the coil can be different lengths, the technician does not really know the exact capacity of the system, so he has to rely on pressure as the main measurement to adjust the refirgerant level. I guess the temperature of the air in the plenum could be measured but I never noticed the technician placing a thermometer in the duct over the coil in the air handler when he was measuring the pressures. He usually just read the high and low pressures to determine if the system was properly charged. We never discussed where the pressure was located within the allowable pressure band.

When I have the system checked out in the next month or so, I would like to be able to discuss the pressure readings intelligently with the technician. I would appreciate input about what a technician looks for (other then the pressure reading was somewhere between the allowable high and low limits) that helps him determine that the system is accurately charged.

I am NOT asking anyone to violate the forum rules about the technique used to charge the system. I am only trying to acquire a better understanding of what goes though a technician's mind that helps him determine if the system is optimally charged (not just close) when he is reading the pressures. Thanks for any and all input. Bill
 

Last edited by worahm; 02-20-11 at 02:10 AM.
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Old 02-20-11, 10:28 AM
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Your unit displays the desired subcool on the outdoor unit nomenclature if it is a single speed or inside the panel if it is a 2 speed Infinity. (the model numbers can be viewed at your thermostat)
 
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Old 02-24-11, 01:23 PM
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Thank you. I appreciate your response. It is exactly the information I was looking for.. It will be interesting to compare your technique to the my technician when he checks the pressure in my system. Bill
 
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Old 02-27-11, 05:48 AM
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New System

Sinse this syaytem is new I would imagine you still have all the original paper work and books that came with it.

Look through the installation manual for the outdoor unit. You should find what the outdoor unit was charged with at the factory. Say, 10# for the outdoor unit 15 or 25 feet of line set and a particular indoor unit.

There should also be a table that provides information of how much refrigerant to add or subtract when additional lineset is added or removed.

There should also be another table indicating addition or subtraction of refrigerant charge when using different airhandlers or coils that can be matched to your outdoor unit.

So, with this information and a tape measure you should come up with the approximate charge needed for the system when your Tech installed it in the beginning.

Further, there should also be perfomance charts. One for heating and one for cooling. At a given outdoor ambient, indoor wetbulb and dry bulb, the suction and discharge pressures should intersect on a graph at specific temp readings along a curve on the graph.

Also, as Houston said, there should be a desired subcooling reading at the outdoor unit. But, this is usually what is desired in cooling vice heating. It's not called a "Reversing Valve" for nothing. Which is why the heating "Check" chart is just for checking the charge.

To really determine if the system has the correct charge, have the Tech come in the summer when the system has a good cooling load and the system can run for some time without satisfying the thermostat.
 
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Old 03-01-11, 04:45 PM
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Thanks Jarredsdad. That is exactly the information I was after. Now that you mention it, the installer did explain the compressor was pre-charged to accomodate the matching coil and a typical line set

I intend to have the system serviced early in April. A new line set was installed when the new unit was installed last April and the system has been running flawlessly since it was first installed.

It will be interesting to see if the technician monitors outside and coil downstream tempertures when he checks the pressures.

Today, I was suprized the unit started running when the inside temperture was 2 degrees lower then the AC set point. Then I realized the unit will run to control humidity independent of temperture, which was exactly what it was doing.

Thanks again for the information. Bill
 
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Old 03-02-11, 07:08 PM
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I just wanted to add. IMO I dont think any normal variation in rfrigerant line length is going to make a hill of beans. consider the suction line is all gas and the diameter of the liquid line is narrow, so really how much could it hold with that volume and pressure (with added length considered).. I have spent weeks tweaking mine in all different weather conditions, considering sun shades and even some kind of device that your water line runs through thus removing energy and preheating for the hot water heater.... I even went to the trouble of upgrading the suction line to 7/8" from the existing 3/4 (and thats an understated nightmare that i got lucky on first try). I am sure your high eff unit states 7/8 line too. You will most likely find the carrier dealer did not upgrade it.. In the end I wind up lucky to get 18 degrees temp change same as anyone else. So I got a NET ZERO benefit as far as I am concerned....

Also I understand there is some type of "Shot glass" transparent bubble type device you can place in the suction line for perfect tweek. But I am not sure if that,or how it applies to a two speed unit...
 
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Old 03-03-11, 09:50 AM
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Like said the true way to check proper charge would be super heat/sub cooling. That being said your system is R-410 and the oil is very hygroscopic, meaning it absorbs moisture very easily. If your tech is not on top of things he can easily introduce non-condensables into your refrigerant circuit through his hoses. In my opinion I would avoid pressure checks if at all possible unless there's an issue.
 
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Old 03-04-11, 02:07 PM
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So, and to continue not to focus on charging issues. Can I focus further on potential run problems that may be associated. you brought up an interesting point. I was trying to understand what you meant and how a tech could even let foreign in to a positive pressure situation, and then is dawned on me that this could happen if the charging lines were not bled prior to the final lock with the jug. So it really makes me wonder how many times I did that?!?!!? I did tweak with it a TON.
So now I am wondering, if my performace was sluggish prior to the recent failure, and forgetting the current problem assuming unrelated, Just how sensitive is it? HOw many improper tank attachments would it take to compromise correct operation? And would just one application of a line drying filter most likely get it all?
 
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Old 03-05-11, 06:21 PM
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<Quote> If your tech is not on top of things he can easily introduce non-condensables into your refrigerant circuit through his hoses.

When I was working on automotive AC systems back in the old days, I generally (not always) purged the gauge lines with new R12 before introducing it into the system. I did not bother to purge the lines when just checking pressures.

If all the tech does is check the pressures, and assuming he understands how the valves work, I am not certain I understand how non condensables can be introduced into the system via the hoses on the gage set. What am I missing? Bill
 
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