HVAC questions

[Fred Maiman]

My new-to-me house has a central air system that isn't working very well. When I purchased the house I was told the temperature drop over the coil was about half of what it should be (about 8 degrees instead of 15). I was told that it was probably low on refrigerant.

I called an HVAC person out to take a look, they made sure the filter was clean, and that the coils on the compressor outside were clean. They checked the temperature drop and agreed that it might be low of refrigerant.

He hooked his manifold to the unit, and added two pounds. At that point, he said there was too much pressure on the pressure side. Actually, this is what the invoice says:

08/27/13: A/C NOT COOLING DUE TO LOW REFRIGERANT CHARGE- CLEANED OUTDOOR COIL &
CHANGED (OWNER PROVIDED) FILTER WHILE WAITING FOR EVAP COIL TO THAW. ADDED
PURON TO SYSTEM AS NEEDED. UPON ADDING PURON, THE HIGH SIDE PRESSURE SPIKED,
AND LOW SIDE DROPED INDICATING A BAD TXV
S - SERVICE CALL 1.00 45.00 45.00


I do not believe the coil was actually frozen. While the unit has been running constantly, I'm getting water out the drain. My coil froze at my previous residence and it stopped dripping altogether.

I also took pictures of his manifold gauges. Unfortunately, I could not read the actual settings, but the blue (left side) was at approx. 10 o'clock, and the red (right side) was at about two o'clock. I did notice he could make the needles go up/down by adjusting the valves on the manifold, though.

I'm including a picture of the manifold, and of the tag from the unit. He said it would likely cost hundreds of dollars (as much as $800 to $900) to evacuate the system, replace the value, and then pump the system back up to the correct level. He said the unit may be overcharged by the two pounds he put in there, that it may not have needed more coolant, and that we should switch it off until it can be fixed.

I did find another thread at another forum where a member was given almost this exact same diagnosis, and had the repair done (it only cost him $550), but it didn't help, and there was no follow-up after that.

Any ideas on how to make sure this is the right fix? Will continuing to run the unit hurt it?

[Dan Hintz]

Taking a guess based upon the gauges here, that looks like 30F/100psi (blue, low pressure) and 140F/550psi (red, high pressure) for R-410a/puron.

[Duane Meadows]

If you doubt the diagnosis, I'd get a second opinion. Another service charge is cheaper than the alternatives! Or you could ask other woodworking forums!

[Bill ThompsonNM]

If you doubt the diagnosis, I'd get a second opinion. Another service charge is cheaper than the alternatives! Or you could ask other woodworking forums!

Yes, definitely get a second or third opinion, and additional estimates. I'd look for local smaller hvac firms, the kind the owner comes out to look at the problem even if he has a few employees to do installs, etc. and ask, if this doesn't fix it what else could the problem be?

[Lee Schierer]

Did he check the air flow through the coils in the main duct. Inadequate air flow will lead to a frozen coil. Also a dirty evaporator coil will give poor performance. Trace the coolant lines all the way to the evaporator and insure that there are no kinks in the lines. Did you ask what a TXV is? Is this also a heat pump used heating the home? If the switch over valve is bad you won't get proper flow through the system. The unit is only 5 years old, so I wouldn't expect the compressor to be bad, but it does happen.

[Rod Sheridan]

TXV is thermal expansion valve.

It meters the refrigerant into the evaporator coil, based upon the coil temperature. If it doesn't open enough you'll have low capacity.

Regards, Rod.

[George Bokros]

I see it is a Trane system. I would get a second opinion from a Trane service technician. It can be costly to evacuate a system and replace that valve and recharge the system.

George

[Mark Paavola]

There is a fix. Find someone who can diagnosis a TXV system. You need to know at the minimum what the subcooling is. For a proper operating system the subcooling will be around 10 - 12 degrees. Just pressure readings is not enough to diagnosis a bad TXV. Have someone else check it and find the subcooling of the system. If it is high have them remove the TXV bulb from the suction line and warm it up in their hand. This should cause a change in pressure and temp drop across the coil. the TXV is designed to maintain a specific superheat in the system.

[Joe Tilson]

Check to see if there is an old filter in the indoor air handler. This has been a problem for many people. They change the filters in the house vents, but don't know they have one in the air handler.

[Joe Tilson]

By the way, if the tech added refrigerant to your system, you have a leak somewhere that needs to be repaired. These are sealed systems and refrigerant should never have to be added to them. Get a second opinion for your on safety. Protect your wallet!

[Dick Strauss]

Am I missing something here? Shouldn't a decent AC tech be able to evacuate the puron from the system, weigh the original refrigerant, and add back what is needed to bring it up to the 5lbs 11oz (as noted by the manufacturer) rather than guessing by adding 2 lbs? Your guy may have added too much refrigerant which can be worse than not having enough.

My AC tech did all of that and added enough refrigerant so that it met the factory spec for about $100 including the service call. I was told that all systems leak some and that every 10-20 years you might want to have it checked even if there is no issue.

[Joe Tilson]

These systems are sealed at the factory, if your tech says all systems leak, find another tech. They are just setting you up for repeated service calls. I worked with HVAC for twelve years and know for a fact they are sealed systems.

[Dick Strauss]

These systems are sealed at the factory, if your tech says all systems leak, find another tech. They are just setting you up for repeated service calls. I worked with HVAC for twelve years and know for a fact they are sealed systems.

Are there mechanical connections (access ports to test high/low pressures) or other copper connections anywhere in the AC system? Every connection is a possible source of leakage no matter how small. Do o-rings and seals dry out and degrade over time? Even soldered copper connections can fatigue from vibration over time. I don't believe there is such a thing as a permanently sealed system. A system might hold a good vacuum for 15 minutes but not for roughly 16,000,000 minutes (30 years).


My info came from an AC company owner that has been in the business for 30+ years and still does service calls himself. Most HVAC techs want you to have your system serviced at least once a year. I don't agree that this helps his business if he tells me I shouldn't have to worry about a recharge again for 10-20 years. The AC unit will probably be ready for replacement by that time.

[Duane Meadows]

Do they fail? Of course they do. That doesn't mean the leak(and yes there is one if refrigerant needs to be added, unless it was under charged to begin with) should not be found and repaired first. The fact that the "tech" added refrigerant, doesn't mean the system needed it either! It may just be overcharged now. I would definately get another opinion.

[Lee Schierer]

These systems are sealed at the factory, if your tech says all systems leak, find another tech. They are just setting you up for repeated service calls. I worked with HVAC for twelve years and know for a fact they are sealed systems.

When you install a HVAC system in the field you have to run copper lines between the evaporator and the condenser. Unless you buy prefitted lines of the right length you have to evacuate and pressurize the system in the field. which means you could have too little or too much refrigerant and even leaks where the connections are made, not to mention leaks due to corrosion of the piping or damage caused by the owner.