Testing Furnace and A/C

[John Allingham]

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.

[Doug Edwards]

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!!!

[Joe Funderburk, CMI]

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.

[John Overholser]

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.

[rramsey1]

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.

[Larry D. Kage]

Originally Posted by rramsey1

Some manufacturers printed literature do not recommend operation in heat mode over 70 degrees outdoors.

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

[Charley L. Bottger]

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

[Michael R. Boyett]

Originally Posted by lkage

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

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

[Larry D. Kage]

Originally Posted by mboyett

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

Thank you.


Step 4

—Operating Ambients

The minimum outdoor operating ambient in cooling mode without additional accessories is 55 °F,
and the maximum outdoor operating ambient in cooling mode is 125°F.
The maximum outdoor operating ambient in heating mode is 66°F.

[Charley L. Bottger]

Originally Posted by lkage

Thank you.


Step 4

—Operating Ambients

The minimum outdoor operating ambient in cooling mode without additional accessories is 55 °F,
and the maximum outdoor operating ambient in cooling mode is 125°F.
The maximum outdoor operating ambient in heating mode is 66°F.

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

[Larry D. Kage]

Originally Posted by cbottger

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.

[Wayne B. Wilson]

Charley It is a get of out warranty Clause lol

[Bruce A. King]

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.

[Charley L. Bottger]

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.

[Doug Edwards]

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.