Reversed lineset

[Jim Katen]

Last week I saw an air conditioner that looked fine and worked fine. The temperature differential was about 19 degrees. However the suction line was tepid and the liquid line was cold. I've never encountered this before.

1999 Trane XE1000.

Anyone have any notion what was happening?

- Jim Katen

[Terence McCann]

Hi Jim,

What was the outdoor temperature? What was the indoor temperature? Any icing of the suction line at the condensing unit or by the evaporator? How many ton was the unit? I can advise on line set size if I know the tonnage.

Did you trace the lines out? It's damn near impossible to reverse the connections as the tech would really have to go out of their way to make up special fittings to reverse connections.

Assuming that the lines were correct: A cool liquid line and a warmer suction line indicates a low freon charge. If the outdoor temperature was very cool you will have a cooler than normal liquid line as well. With regards to the delta T across the evap, 19° is spot on however, was the filter very dirty? Was the evap fan running on low speed vs high? You can get a good delta T with poor air flow even though the charge may be low. Inadequate return air? Supply ducts closed?

[Jim Katen]

Originally posted by Terence McCann

Hi Jim,

What was the outdoor temperature? What was the indoor temperature? Any icing of the suction line at the condensing unit or by the evaporator? How many ton was the unit? I can advise on line set size if I know the tonnage.

ODT = 61

IDT = 72

No icing anywhere.

2-ton unit. Small house.

Differential = 72/53

Did you trace the lines out?

No.

It's damn near impossible to reverse the connections as the tech would really have to go out of their way to make up special fittings to reverse connections.

I know. That's why it seems so peculiar. The system is behaving as if the lines are reversed. But I'm not even sure that that'd be possible.

Assuming that the lines were correct: A cool liquid line and a warmer suction line indicates a low freon charge. If the outdoor temperature was very cool you will have a cooler than normal liquid line as well. With regards to the delta T across the evap, 19° is spot on however, was the filter very dirty? Was the evap fan running on low speed vs high? You can get a good delta T with poor air flow even though the charge may be low. Inadequate return air? Supply ducts closed?

The filter was new.

I didn't check the fan speed.

Return air was ok-ish. A central return, probably about 20x20.

The supply registers weren't closed, but they all had those stupid foam filters in them. Perhaps that slowed the air enough to mask a problem with the differential.

- Jim Katen, Oregon

[Terence McCann]

With an outdoor ambient of 61 and indoor of 71 there was little if any load on the machine. The liquid line had a ton of sub-cooling so a "cool to the touch" temperature is to be expected. At 60° outside the head pressure on the machine would be (assuming a 30° rise and the unit is R-22) 168#. Toss in 10° plus of sub-cooling and you have a liquid line temperature of 75° or so, cool to the touch. Normal liquid line temp is around 105° give or take. If it was a newer, high efficiency unit with an oversized condenser the the liquid line temperature will be even cooler.

The suction line being warmer than normal might be of concern however, if I were checking this unit, in these temps, I would tell my client that I just want to make sure it runs and that I can not properly test the unit in these temperatures.

[randynavarro]

Wow, OK. Air conditioning is definitely my weakest area. Was never 'formally' trained on how to inspect them and I probably only see 2-4 per year.

Jim, if ODT was only 61 deg. doesn't that risk damaging the compressor? Myth?

Any good resources for how to properly inspect an air conditiong system as part of a normal home inspection? I've got books on HVAC but they're way over my head and are published to train technicians.

[Jim Katen]

Originally posted by Terence McCann

With an outdoor ambient of 61 and indoor of 71 there was little if any load on the machine. The liquid line had a ton of sub-cooling so a "cool to the touch" temperature is to be expected. At 60° outside the head pressure on the machine would be (assuming a 30° rise and the unit is R-22) 168#. Toss in 10° plus of sub-cooling and you have a liquid line temperature of 75° or so, cool to the touch. Normal liquid line temp is around 105° give or take. If it was a newer, high efficiency unit with an oversized condenser the the liquid line temperature will be even cooler.

This liquid line wasn't cool, it was cold. I didn't measure its temperature, but it felt about the same temperature as a jug of milk in the fridge -- probably about 40 degrees.

The suction line being warmer than normal might be of concern however, if I were checking this unit, in these temps, I would tell my client that I just want to make sure it runs and that I can not properly test the unit in these temperatures.

Well, maybe. The thing is, I test AC units at similar ODTs all the time. Usually, the suction lines are cold. This one was an anomaly.

- Jim Katen, Oregon

[Jim Katen]

Originally posted by randynavarro

Wow, OK. Air conditioning is definitely my weakest area. Was never 'formally' trained on how to inspect them and I probably only see 2-4 per year.

Jim, if ODT was only 61 deg. doesn't that risk damaging the compressor? Myth?

No, not a myth. In the old days, and particularly on cheap equipment, there was a danger of liquid refrigerant being in the compressor at the moment of start up. If this happened with a piston compressor, it could self-destruct as the pistons tried to "compress" the liquid (slugging). Better units had crankcase heaters to prevent this. Some had outdoor sensors that locked out the units when it was cold to prevent the possibility of damage (Tempstar & Nordyne, for instance.)

Nowadays, most everything I see uses a scroll compressor. These compressors aren't damaged by the presence of liquid refrigerant. It just sort of squirts between the scrolls harmlessly.

In general, it's still not a good idea to test the AC system at low outdoor temperatures and the manufacturer's instructions will generally warn against it.

The issue isn't so much that harm is likely, but that there's not a lot of value to be gained by running it at that temperature.

Any good resources for how to properly inspect an air conditiong system as part of a normal home inspection? I've got books on HVAC but they're way over my head and are published to train technicians.

Start a new thread on this subject and I'll be happy to contribute later. Right now, I'm recovering from a cruddy cold and I have an 8am inspection of an 1880 house. In these parts, that's considered an old house. (Stop laughing, Kibbel.)

I need to get to sleep.

- Jim Katen, Oregon

[Terence McCann]

Jim,

Did you check the liquid line before or after the liquid line dryer? The reason I ask is that it is possible for the liquid line dryer to plug up. This usually happens after a compressor change-out due to a burn. The liquid line dryer now starts to act like a TXV of sorts. Moisture in the system will also give this affect. What's weird with moisture is that the system will start and run OK for a while but then you will start to see things, like your seeing, as the moisture start to freeze and restrict certain areas. In any case it is hard (if not impossible) to troubleshoot with out gauges and temperature probes which, are outside the scope.

[Jim Katen]

Originally posted by Terence McCann

Jim,

Did you check the liquid line before or after the liquid line dryer? The reason I ask is that it is possible for the liquid line dryer to plug up. This usually happens after a compressor change-out due to a burn. The liquid line dryer now starts to act like a TXV of sorts. Moisture in the system will also give this affect. What's weird with moisture is that the system will start and run OK for a while but then you will start to see things, like your seeing, as the moisture start to freeze and restrict certain areas. In any case it is hard (if not impossible) to troubleshoot with out gauges and temperature probes which, are outside the scope.

There wasn't a dryer on this installation.

Still, that's an interesting theory, I never knew that those things could plug up.

- Jim Katen, Oregon

[Bain]

Those dryers are also notorious for leakage, especially when they aren't replaced along with an exterior condensor. I have a leak detector made by TIF, and it's amazing how often I find leaks in the the soldered joints on either side of the dryer. And while we're at it, slide the filter out of most any heat pump and shine your flashlight up at the coil. More often than not it looks as if it's wearing a fur coat due to all of the gunk trapped between the aluminum fins. Talk about slowing down the air flow over the coil and screwing up the delta . . .

What Terry said above is true. There are so many variables--including properly sized ducts, clean blowers, etc--it's difficult to really know what's going on with a system unless you go way beyond the scope.

Jim, the phenomenon you described is fairly common around here in early spring or late fall when the indoor ambient is higher than the outdoor ambient. I've run the scenario by some talented HVAC guys, and it's pretty much like Terry described vis a vis a small load. The seemingly screwy differential is caused--according to those I've talked to--by the normal removal of humidity when it isn't too awfully cool outside.

John