Thread: Testing AC Systems

I tell buyers I can't run the AC system when the outside temperature is below 60 degrees as it can damage the unit. But what do you do when the temperature rises during the inspection from say 55 degrees at the beginning at the inspection to about 70 degrees by the end? Do you run the system or do want there to be a sustained period of 60+ degrees for 3 hours, 6 hours, 12 hours, etc prior to testing?

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Originally Posted by Nick Ostrowski

I tell buyers I can't run the AC system when the outside temperature is below 60 degrees as it can damage the unit. But what do you do when the temperature rises during the inspection from say 55 degrees at the beginning at the inspection to about 70 degrees by the end? Do you run the system or do want there to be a sustained period of 60+ degrees for 3 hours, 6 hours, 12 hours, etc prior to testing?

I turn it on and if it cools it cools! In my area it is common to have a30 to 40 degree swing in temps. Right now it is 47f and at my inspection in about an hour it will be up to around 65f and by the end of the inspection it should be in the 70's. I will run the AC and not even give it a thought.

If it is turned on, I'll test it. If turned off: 65 degrees for 48 hours. So I only turn them on in July and August.

In those borderline situation, sometimes I'll turn it on to make sure the compressor at least responds and check the unit for unusual noises and vibration. However, I don't run it for any length of time since it wouldn't really prove anything if the weather is that cool. I explain this all to my clients.

If it rises to over 60 during the inspection, I run the AC. It's my understanding that the prohibition below 60 is to keep the refrigerant from condensing inside the return line before it hits the compressor, which (being designed to compress a gas and a liquid cannot be compressed) it can tear up. So as soon as the air temp is over the magic 60 deg, that shouldn't be a problem. There is some safe margin about the 60 as well--an old HVAC guy told me I was being over-conservative and could run them down to about 45 or 50 before that happens. (Of course, he was the Realtor on that deal, but he's one I actually would trust to be pretty well right.)

The issue is not how long you run it nor what the exact temperature is when you run it.

The issue is liquid refrigerant migrating to the cold suction line and lying there waiting for an unsuspecting HI to turn it on and shell a perfectly good compressor. (liquid does not compress well).

Since only a small percentage of units are going to experience this phenomenon if you run them in much colder temperatures and the manufacturers build in the afore mentioned safety factor "I" don't worry about it too much and test them when the temperatures are warm enough when I am there. If it has been MUCH colder the night before and not expected to exceed the 60 degree mark, no testing for me.
It also makes a difference if the unit is in the shade or sun. If in doubt, I take a temperature of the cabinet of the exterior unit in a part not exposed to the sun's direct rays.
Bottom line is you want enough warmth on the refrigerant line and exterior unit to move the refrigerant to another part of the system or boil it off.

I go ahead and run them. We can go weeks where the temp drops below 65 at night, so if that was the case I'd never be able to test them, nor would the owner's be able to use them.

Back in the olden days I remember seeing a HVAC tech wrap a blanket around a condenser coil in what I believed to be an effort to simulate a warmer day.

Since way back then many system have been installed with a low ambient controller that permits compressor operation down to ~35°F (it varies from mfr to mfr).

To run a condensing unit below 60° is really not a big deal as long as you don't leave it running for hours on end. A low ambient controller just locks out the condenser fan and you can simulate that with say, hmmm, a blanket wrapped around the condenser coil?

Originally Posted by Rod Butler

Since way back then many system have been installed with a low ambient controller that permits compressor operation down to ~35°F (it varies from mfr to mfr).

To run a condensing unit below 60° is really not a big deal as long as you don't leave it running for hours on end. A low ambient controller just locks out the condenser fan and you can simulate that with say, hmmm, a blanket wrapped around the condenser coil?

I disagree, it is not how long you run the system but IF there is enough liquid present to slug the compressor on START UP. True, many of today's systems are capable of cold weather operation, trouble is you don't know which ones are or aren't. Better to be safe than shell a compressor.

Unless you can confirm a low ambient kit is installed it's best not to start below 65. The scroll compressors are less likely to be damaged but, do you want to take that chance?

To test the AC operation, you need 14 to 22 degrees temperature differential between the return and supply. If the temperature inside the home is cold like 60 degrees, you may not be able to obtain the 14 degrees. However, I run the heating plant in the AM to warm up the home prior to testing the AC. Then test the AC towards the end of the inspection (above 60 degrees). A reliable HVAC tech told me that most newer condensers have a heating coil to prevent damage by accidential operation during cold weather.

There are a lot of factors involved concerning what outdoor temps & indoor temp & humidity loads are needed for safe operation.

I've been involved in the air conditioning field since the mid-1970's.
In the early years some condensing units had suction line accumulators factory installed; heat pump of course have them; they let any liquid coming back toward the compressor to drop into the accumulator reservoir.

Some have crank case heaters to boil the liquid refrigerant from the compressor oil which reduces the possibility of slugging on a cold compressor startup.

Some have liquid line solenoids that shut off the flow of liquid into the evaporator on shut down, which helps reduce flood-back on-startup to the compressor.

Those with TXV refrigerant metering devices usually shut off the liquid flow to the evaporator on shut down, which helps some to protect the compressor.

A lot also depends on the amount of heatload both sensible & latent that is absorbed by the evaporator coil to vaporize the liquid refrigerant flowing into the evaporator coil. Too low an indoor heatload can result in liquid flooding back to the compressor...

What you don't want is for the evaporator coil to flood liquid into the suction line.

Therefore, it is a combination of factors that determine whether it is safe to run an A/C unit; many if not most heat pumps have most of the safety factors built in.

Very little to gain running them when it's borderline... many $$$ to lose.

I think of it this way - You run it on a marginal day and it doesn't work properly/well/at all so you write it up. HVAC contractor comes out and blames you for running it on a cold day. Whether or not he's right you're stuck defending yourself and possibly paying for an A/C unit. Why put yourself in that position?

I play the numbers. I inspect +/- 300 houses a year. If I run all the A/C compressors on the 50-60 degree days I'd probably kill one every few years (or be blamed for doing so).... sorry, it's not in the budget.

I operate every Air conditioner. In cold weather (below 50) it is just long enough to verify that the compressor and fan turn on. If there is snow on the ground I would probably not turn it on but I have never inspected a home in those conditions.

Since the manufacturer does absolutely nothing to prevent a homeowner from operating the appliance under those same temperatures, I have to wonder how real the problem really is. After all, there is no safety switch, no temperature cut-off, nothing.

This issue is typically overblown, or exaggerated, and I suspect there are homeowners (or renters) in states where the temps swing wildly overnight then heat up fast in the afternoon, and they just turn the system on whenever they please.

Originally Posted by Dom D'Agostino

Since the manufacturer does absolutely nothing to prevent a homeowner from operating the appliance under those same temperatures, I have to wonder how real the problem really is. After all, there is no safety switch, no temperature cut-off, nothing.

This issue is typically overblown, or exaggerated, and I suspect there are homeowners (or renters) in states where the temps swing wildly overnight then heat up fast in the afternoon, and they just turn the system on whenever they please.

Oh, - in this instance the problem is real. Many compressor's are damaged because they don't have sufficient protections installed on the unit. Many reciprocating compressor's have had their valves damaged or blown-out because of liquid refrigerant slugging...

In states where temps swing wildly over night it is important to have crank case heaters installed on the compressor to keep the liquid refrigerant boiled out of the compressor's oil; so the compressor does not liquid & oil slug on startup. Compressors can only compress vapor, not liquid refrigerant...It takes the compressor shell & its oil much longer to warm up even after outdoor temps reach more acceptable levels.

They ought to have liquid line solenoids that shut off the flow of liquid into the evaporator on shutdowns. A positive shut off TXV refrigerant metering device can do that & also control liquid flooding under low evaporator heatload conditions.

No, in this particular case, this is an area where those threats can be real; additionally some levels of damage can occur without the user being aware of when or how it happened, & wouldn't be noticed until the hot weather occurs.

Sorry Darrell, but a disconnect exists between the equipment, the factory, and the end user.

The installed base of actual users (not inspectors, and not HVAC repair techs) doesn't suggest that the temperature at which an A/C system is energized causes real problems.

Given that the expense of warranty claims is astronomical, I would expect a statement, or warning, concerning the allowable operating temperature of HVAC systems be loudly proclaimed and visible; along with a disclaimer that such unapproved conditions would render the warranty invalid.

Your mileage may vary.

You're right Dom, a critical disconnect does exist.

It is strange that I don't recall reading that mfg'ers exempt those situations from their warranties; maybe it's too difficult to prove how it happened.

Some mfg'ers do warn against startup slugging & liquid flood-back under those causal conditions...

You certainly make excellent points, & I do not understand why they would not put more emphasis on those conditional situations, when since the mid-1970's I have witnessed a fair number of compressors with bad valves that had to be replaced, in all probability the valves were damaged trying to compress liquid refrigerant.

Improper refrigerant piping can also be a real contributing factor to liquid flood-back & I've never heard of it being cited as an excuse to void a warranty. Though many do explain how it needs to be piped...

Originally Posted by Dom D'Agostino

Since the manufacturer does absolutely nothing to prevent a homeowner from operating the appliance under those same temperatures, I have to wonder how real the problem really is. After all, there is no safety switch, no temperature cut-off, nothing.

This issue is typically overblown, or exaggerated, and I suspect there are homeowners (or renters) in states where the temps swing wildly overnight then heat up fast in the afternoon, and they just turn the system on whenever they please.

Good call, I agree. Temps as low as 50 F are not a problem. But paranoia reins .

So, I guess my question is this: Running an a/c unit on borderline days when it is cool outside tells you what about an a/c unit? Is it actually functioning properly-you won't know because it is too cool outside and test is negated-all it tells you is that it started up, nothing more. Thre real test of an a/c unit comes when the outside temps are such that you can run a unit long enough for it to actually start balancing out the inside temps, reduce humidity levels and make some sort of determination as to whether or the thing will cool properly when it actually gets hot outside.

I have the same issue with heating units here, when do you stop testing it? When the attic temps are over 100* (when the air handler is in the attic), when the interior temps are 80*+, and if you do turn on the heating system,' just to see if it fires up", what did it actually tell you-not much. Turning on an a/c or heat just to see' "if it comes on" tells you what? It came on, it tells you nothing of the actual functionality of the system.

For us to properly test any component in the home means- testing it under normal operating conditions. What lunatic homeowner will turn on their a/c unit when it is 45* outside-virtually no one. Conversly, what homeowner is going to fire up their heating unit when it is 90* outside-no one. Why should we do it? We have standards to follow to keep our liability down to a manageable level. My level of care dictates that I have to follow some sort of guideline as when to operate any system inside someone's home. If you don't follow some sort of written guideline you will open yourself up to perhaps 'unacceptable levels of liability'.

But, as with most things in life, it's a choice. Make these decisions at your own peril.

I do know this though. If a HI came to my home and turned on the a/c when it was 45* outside and the standard is around 60*...it had better work properly when the time comes for it to actually be operated and if it doesn't..I am calling him with the bill for repair or replacement.

My take.

The problem is that there isn't a unified standard. Most condensing unit instruction manuals that list the minimum vary a bit.
Carrier states 55 in one or two of it's models guide, but doesn't list it all in several others.

So I agree, as a operating protocol, pick a "standard" from a verifiable source and stick to it.

Dom.

Around here, Excel (our main power supplier) will not run an A/C unit at temps below 65. It is a generally accepted rule (at least around here) that you can damage the compressor if you operate it below 65 degrees. So, it seems to me that you open yourself to big trouble if you turn on a unit when the outside temp is below 65 and damage occurs to the unit.

Now, having said that, if it is 60 degrees and the compressor is in the sun, I often run it.

I think the take away from this discussion isn't whether operating the A/C below 65 or 60 is bad, but rather, what could be your liability if you test something like the A/C under parameters that are not industry acknowledged or acceptable?

Originally Posted by Lon Henderson

I think the take away from this discussion isn't whether operating the A/C below 65 or 60 is bad, but rather, what could be your liability if you test something like the A/C under parameters that are not industry acknowledged or acceptable?

To me that’s the core issue right there. If I turn it on under questionable circumstances, and later it doesn’t work correctly; it’s easy for the (current or new) homeowner to get a HVAC person to say as the HI I caused the damage. True or not. Even if it was damaged long before I got there, how would I ever prove it? And I have had a claim over just that when I fired one up at 12°C (54ºF) and it didn’t work. Current homeowner claimed I damaged it right then and there. When reality was that it probably hadn't worked for years. But my position was undefensable.

I put a clause in the report explaining why I didn’t inspect it (due to temp); and just move on to the next item. I’d much rather be in a position of explaining why I chose not to risk damaging it than why I took the chance.

In Ohio we have 7 to 8 months in a typical weather year when we don't expect the temps to rise to a level where we feel it safe to test a cooling system. The test is not an on/off to see if things fire up, the test we perform is a very basic Delta-T test. We use 12 to 20 degrees as a standard acceptable differential. If the outside temps are at or above 60 F. at the time of the inspection and the over night temps did not dip below 45 +/- we normally test. If the outside temps are in the 50's to 60 during the time of the inspection and....the condenser unit is or has very recently had sun "on it" we typically perform the test.

We have very good relationships with several HVAC techs / companies and all of them have concurred that if they were in our position, this guideline would suffice.

Something else we tend to look for since relocations are fairly common in the area is to follow the guide of relocation company inspection protocols when it comes to testing cooling systems and heat pumps in general. Below 65 F: we don't test per Relo instructions.

We've only ever been called on the carpet once or twice in 20 years and with the backing of the major HVAC players in the area we have a pretty good level of comfort.

A comment in the report stating why you did not test is as importasnt here as it is for any other system you restrict.

We use 12 to 20 degrees as a standard acceptable differential.

That temp-differential could be way-off & not at all acceptable depending on airflow CFM & the humidity levels indoors, which can greatly vary the heatload through the evaporator coil, whereby the indoor temp system split may not be at all 'an acceptable' performance indicator.

You need a quick humidity & reasonable airflow CFM check to begin to ballpark a system's performance.
E.G., A 12-split with just 'a very-low' indoor humidity would be unacceptable, add the usual low airflow & it's a totally unacceptable indoor temp-differential. Generally speaking to be acceptable it should have +23 to even a +25-F temp-differential with those extreme indoor conditions.

Even a low cost anemometer can be helpful for revealing systems with unacceptably CFM airflow; far too many A/C techs neglect checking that all important airflow!

The indoor relative humidity will have the biggest effect on the indoor temp differential. The higher the humidity the greater the latent heatload & the lower the temp-split-differential will be.

It also depends on whether the airflow is low, as that will raise the indoor temp-differential-split, & lower the outdoor condenser temp-split.

Condenser temp-spits also vary as to the SEER Rating, higher SEER Ratings have lower splits at varying conditions.

The old 6-SEER condensers used to have a very high temp-differential.

You could also check the temp-differential off the outdoor condenser; the temp-split will be higher when the indoor humidity is high.

The mfg'ers charts can be used to see if the system is performing near its rated performance data.