Seems residential home pre-sale inspections cannot be run for the A/C side if the outside air temp is below 60F

[NotMe]

Home pre-sale inspections cannot be run for the A/C side if the outside air temp is below 60F.

New to me and I'm curious why this might be so?

TIA

[recyclebinned]

One reason I can think of is that the evaporator might freeze and cut-off the air flow which would give inaccurate readings.

[The Daring Dufas]

At low outside temperatures an AC system can't build up enough head pressure to operate properly and get any meaningful performance measurements from. When I have a customer who owns a restaurant or other type of business where the AC may be run year-round, I install a head pressure control or condenser fan switch. The fan switch keeps the condenser fan off until the high side pressure gets high enough for the system to operate without causing damage to the compressor. The on off pressure switch is the simplest but crude for controlling head pressure which is why I prefer a variable speed fan control which provides a more consistent level of performance. The most inexpensive residential AC systems lack a factory installed head pressure control which is why it's not prudent to run them in cold weather. ^_^

TDD

[Ernie]

Why couldn't a thermostat that can sense condenser coil temperature be used to turn on the fan when the coils reach some pre-determined temperature?

[The Daring Dufas]

That's exactly what the fan speed modulators do in the links I provided. The ICM manufactured controls I use, come with a temperature sensor that's attached to the high pressure line on the condenser coil to operate the control. ^_^

TDD

[trader4]

Because it's not needed and just one more thing to fail and would then need to be serviced. As for the AC being damaged by operating it lower than 60F, personally I think it's BS. If it were, all the AC's out there would have a lockout to prevent them from starting below 60F. That and a warning to not do so. I'd like to see a warning from a major home AC manufacturer that says it's going to be damaged by operating below 60.

I would think it's more of an issue that you can't really measure the performance because it;s outside it's normal operating range. And also, most home inspectors are notorious for doing as little as possible. It would seem to me it could at least be turned on at 50 to see if it starts up and runs.

[Ernie]

You kept mentioning "head pressure control" which I interpreted as some sort of pressure-sensor (sensing line pressure) as a way to operate the fan.

[The Daring Dufas]

Some methods use a simple pressure switch to turn the fan on/off. Some use a pressure transducer which provides a variable signal to the control and the others use a temperature sensor to provide a variable signal to the fan speed control. ^_^

TDD

[Ernie]

I don't think the AC would dammaged by low-temperature operation, but clearly the efficiency of heat-transfer would be affected if the operation of the condenser fan resulted in insufficient line-pressure in the evaporator line.

This would explain why most residential AC units don't seem to cool a house very well when ambient outdoor temperatures reach "room temperature".

[trader4]

IDK what you're talking about there. My AC's have all performed fine, run a lot less, etc as the outside temp drops and approaches room temperature. There is more cooling, not less.

[The Daring Dufas]

Believe me, running an AC unit during periods of very low ambient outdoor temperatures CAN and WILL cause damage to the compressor unless steps are taken to control head pressure in the system. The refrigerant can migrate to the compressor crankcase causing liquid slugging of the compressor which is supposed to pump vapor not liquid. That why most systems have a crankcase heater to keep the compressor warm to prevent the majority of the refrigerant charge from winding up in the compressor. One of the things I do for a living is servicing and repairing refrigeration and air conditioning equipment. ^_^

TDD

[MurphyM]

The Daring Dufas wrote in news:kdhsci $io4$ [e-mail]:

Well then, I would hate to hire you because you don't know what you are writting about.

[The Daring Dufas]

I wouldn't want to have you as a customer. Your type of know it all would last about 2 seconds with me and my intolerance for morons. ^_^

TDD

[Stormin Mormon]

It can. However, what is needed is the higher pressure. The two are related, as the pressure temperature charts show. But, the pressure is the variable that's needed.

Christopher A. Young Learn more about Jesus

.

Why couldn't a thermostat that can sense condenser coil temperature be used to turn on the fan when the coils reach some pre-determined temperature?

[Stormin Mormon]

Writting? Like writting of habbeas corrpus?

Christopher A. Young Learn more about Jesus

.

Well then, I would hate to hire you because you don't know what you are writting about.

[bud--]

Along the same lines, but even more extreme, I have seen systems here (Minneapolis) for commercial kitchen refrigeration where the compressor is in the basement, condenser on the roof, and where there is an automatic valve to bypass the rooftop condenser when the head pressure is too low.

[bud--]

A common requirement for starting an outside compressor/condenser (at least when it is cold out) is to power the unit for a while before running the compressor to let the compressor crankcase heater warm up.

[trader4]

And that is like 59F? The claim was made that it can't be run below 60F outside temp.

The refrigerant

A residential AC doesn't. That's what was being discussed. And if it's such a threat to destruction, why don't they have a lockout that prevents it from running at 50F? Would seem there would be a lot of AC systems shot from say a kid or someone accidently turning it on under those conditions, no?

[trader4]

dhsci

Which typical residential AC systems don't have.

[The Daring Dufas]

I'm sorry to disappoint you but most AC condensing units installed in homes have crankcase heaters. Running a home AC system in cold weather does not cause it to explode immediately. Damage accumulates over time with liquid refrigerant instead of vapor being returned to the crankcase diluting the oil and causing it to foam which will result in a large amount of oil being picked up by the compressor and pumped into the evaporator. The more expensive home AC condensing units not only have a crankcase heater but will have a variable speed fan, high and low pressure safety switches to shut the unit down when the pressures are out of spec. Some units have a freeze sensor on the low pressure line to detect frosting of the line which indicates a problem with the operation of the unit and shuts the unit down. I don't recall any new AC unit I've installed in the past ten years that didn't have at least a crankcase heater. O_o

TDD