Testing Furnace and A/C

When doing an inspection:
(1) After running the furnace and then shutting it off, how long should I wait before turning on the A/C
Or if I do it the other way:
(2) After running the A/C and then shutting it off, how long should I wait before turning on the furnace.

Immediately. There is no reason to delay checking the functions of each. Many inspectors seem to think there must be a wait time to avoid damage to the equipment. I never heard of that before I joined an inspector association. Wives tales. It isn’t made of Tiffany glass and is designed to withstand a lot more stress than you or I can put on it using normal operating controls. I hear guys say you can’t check the furnace during summer…sure you can…it is a FURNACE! Designed to get HOT as HELL!!!

I agree. Many thermostats have an “auto” setting that will keep the temperature set to a desired level using either heat or AC, whichever is necessary.

My understanding of the matter is that it’s only an issue when operating heat pumps. That is why many heat pump thermostats have a delay built in when switching from one to the other.

Heat pumps operated in heat mode with outdoor temperatures over 70 degrees may trip out on high head pressure safety controls to prevent damage or components. You may be paying for a service call if you do not know how to reset or cause daamge to system. Some manufacturers printed literature do not recommend operation in heat mode over 70 degrees outdoors.

35 years in HVAC business.

Appreciate the info. Could you post a link to this?

Hmmmm so does that mean if the outside ambient in the summer time is above 70 degrees one should not operate the unit in A/C mode:shock::shock:

Carrier Heat Pump manual says max 66°. See pg 2, step 4.

Thank you.

[FONT=Arial][size=1][size=4]Step 4
[/size][/size][/FONT][FONT=Arial]—[/FONT][FONT=Arial]Operating Ambients[/FONT]

[FONT=Times New Roman]The minimum outdoor operating ambient in cooling mode without additional accessories is 55 [/FONT][FONT=Times New Roman]°[/FONT][FONT=Times New Roman]F, [/FONT]
[FONT=Times New Roman]and the maximum outdoor operating [/FONT][FONT=Times New Roman]ambient in cooling mode is 125[/FONT][FONT=Times New Roman]°[/FONT][FONT=Times New Roman]F. [/FONT]
[FONT=Times New Roman]The maximum outdoor [/FONT][FONT=Times New Roman]operating ambient in heating mode is 66[/FONT][FONT=Times New Roman]°[/FONT][FONT=Times New Roman]F.[/FONT]

This makes absolutely no sense why the MFG would say something like this is beyond me. If one thinks about the refrigerant cycle and where the heat transfers takes place within the system in different modes with a 66 degree max outdoor ambient in the heat mode would represent the same theory as a indoor temp of 90 degrees ambient in the cooling mode and implying do not operate the system in the cooling mode

If you talk to Carrier, let us know what their response is, Charley.

Charley It is a get of out warranty Clause lol

The manufacturer recommends these limits to prevent extra wear on the compressor.
Some systems actually have outdoor ambient sensors sending a signal to the t-stat to prevent operation within temperature ranges that are not desired.

In A/C mode the compressor gets a nice cool vapor from the indoor coil (except for the first several seconds when the vapor line is warmer). Compressors like this cool vapor.

In heat pump heating mode, on a warm inspection day, the compressor is getting the warmer vapor from the outside coil that has ambient air forced across it. This “warmer than desired” vapor is what the manufacturer does not want getting compressed.

Since the heat mode on a heat pump actally invloves much more than just “A/C running backwards” it is a good idea to check it but only for a short period in warm weather. At some point it should not be run at all but that exact number is up to the inspector on that particular day.

Ok lets look at this realistically Take the heat pump in a heat mode the compressor discharges high pressure vapor through the reversing valve Into the larger of the two copper lines then travels indoor to the A-coil at above 100 degrees or there abouts. Indoor blower fan condensing the high pressure high temp vapor back to a liquid at roughly the indoor temp of the home then as a liquid travels back outside to the condensing unit and passes through the metering device on the exterior condensing unit which drops the temp to roughly 38 to 42 degrees within the exterior condenser coils. Outside ambient lets use 80 degrees is passed across the 42 degree exterior coil raising the temp of the freon to 80 degrees or less which would be the temp of the freon entering the suction side of the compressor through the reversing valve. 80 degree suction temp would not create damage to the compressor for the short term perhaps if operated for hours under that condition but for a short 10 minute test no harm would be caused.

I agree completely. It is a “test”, a brief test of the heating function. You ain’t trying to bake bread inside the house and there is absolutely no reason to let it run for very long. I thought everyone understood that. Like I said, I never heard of some of these tales until I joined a Home Inspector Association. Never heard of nor was I taught this in HVAC school. IN light of the man’s original question, I think the thread, like so many, has run off the track and taken a side road.

:shock:I want to see everyone in Arizona turn off their air conditioners when it hits 125°!:shock:

All that stuff from Carrier is about “operating” conditions. It doesn’t say anything about “testing conditions”. Operating conditions is a design criteria.

As they said:

This means the equipment is not designed the way it is to operate continuously below 50° without additional components installed to modify the operational design conditions of that piece of equipment.

As for switching back and forth in the heat pump mode; what happens during the defrost cycle? It goes between heat and cooling in a split second, twice every 45 min.

Please explain that drastic change in operating temperature of the components and why it doesn’t destroy the equipment after a single occurrence.

If this were true my mother in law and her friends would have either frozen to death or have to replace their heat pumps yearly.

Two Thoughts:

To be honest I know how I do it and could care less what anybody else does. Last summer I saw a real interesting thing happen.

A “couldn’t keep his hands off it buyer” kept playing with things. He was a real dumbo and the agent went along with it. I mentioned right up front, there was a cat litter box on the floor so even though I had not seen a cat / Treat the house like there is one. DO NOT leave doors open - IF you walk into the garage OR out onto the patio / SHUT the door behind you. Him and the agent kept coming in and out AND leaving doors wide open AND sure enough eventually a cat AND little minature something dog appeared and shot out an open door. By the time the inspection was over, they still had not rounded up fluffy the cat.

I had continually kept telling him to keep his hands in his pockets AND not play with stuff / He didn’t think the A/C was getting cool enough fast enough, so he kept fiddling with the T-Stat - turning it on & off. I kindly explained to him that when the A/C was running and turned off - IT needed to EQUALIZE the pressures before trying to start again. About the 5th time he did that I heard a loud WHOOSH and watched black oil start running out from under the condensor (28 year old unit).

When the seller and listing agent called that night wanting to know WHY the home inspector let the dog & cat out AND broke the A/C unit AND wanted to know HOW I was gonna pay … I politely explained that I had watched the realtor and buyer do the deed AND offered to write a letter if these slease bags did NOT offer to pay.

The A/C unit should NOT be IMMEDIATELY recycled. You got NO idea if the unit has a delay OR not / Nor do you know if its working.

Dan you and I both know the A/C needs to equalize before restarting but the thread was originally about switching from cool to heat or heat to cool which does not require a delay by the operator of the thermostat most newer units have a time delay built in.

There is also a device in the compressor which will turn it off in about 5 sec. if it can not start. This will go on till the pressure equalizes, and it will start.

There was something wrong with the compressor to start with. It just hadn’t happened yet.

Was it capacitor oil or did the terminal block blow out of the compressor?

Better then, than later I guess.

I consider this activity “Normal Operation” as the stupid fumbling client probably turns down the t-stat and then back up every day, just to see what the temperature is set at!..

The equipment should be able to take it.
If it can’t, there is a thing such as a time delay relay that should have been manufactured into the design.

Do we test garage door reversal?
That is not normal operation, but is it normal for it to reverse if the door is stuck against something.