Heat pump testing-a/c mode - InspectionNews

daniel nantell · #1 (**permalink**) 10-30-2009, 08:05 PM

I know during cold weather with a conventional gas furnace and outside condensing unit, you do not test the a/c unit during temp . below 60 degrees as it will cause the migration of oil out of the compressor. Does a Heat pump work the same way, not advisable to run the A/c mode in colder weather and how do you check the electric back up furnace is temp is not cold enought to cause the Electric back up furnace to kick on. I don't see many heat pumps in my area but need to know. thanks

Jim Luttrall · #2 (**permalink**) 10-30-2009, 08:17 PM

Not a problem with the a/c in cold weather, in fact the unit is switched to "cooling" without the outdoor fan running for the defrost cycle. Heat pumps are more robust than their cooling only cousins. You don't want to run it in the heating mode when outdoor temps are too WARM to keep from overheating/high head on the compressor. The electric heaters are typically controlled by the indoor stat, not outdoor ambient temperature. Set the indoor stat one or two degrees higher than indoor temperature to call for first stage heating (the heat pump) and confirm proper operation. Then raise the setting several degrees to call for back-up resistance heat and confirm operation. Then move the system switch to "emergency heat" setting to shutdown the outdoor unit and confirm you are still getting heat (although less than with the outdoor unit contributing) and that the outdoor unit did in fact shut off. You may have a different control system, especially if you have a multi-fuel set-up, but the explanation above fits the vast majority in our milder climate. You may also see an exterior thermostat designed to keep the heat pump from running in low ambient situations.

Jim Luttrall · #3 (**permalink**) 10-30-2009, 08:22 PM

Quote:

Originally Posted by daniel nantell

I know during cold weather with a conventional gas furnace and outside condensing unit, you do not test the a/c unit during temp . below 60 degrees as it will cause the migration of oil out of the compressor. Does a Heat pump work the same way, not advisable to run the A/c mode in colder weather and how do you check the electric back up furnace is temp is not cold enought to cause the Electric back up furnace to kick on. I don't see many heat pumps in my area but need to know. thanks

By the way, the reason for not running the A/C in cold weather is NOT oil migration. The reason is liquid slugging. Compressors are designed to compress refrigerant VAPOR, not LIQUID. Liquid refrigerant will migrate to the compressor during cold weather and settle there and in the suction line and will be immediately sucked into the compressor suction port when energized and tend to damage valves, rods, and pistons.

Ted Menelly · #4 (**permalink**) 10-30-2009, 09:25 PM

I have operated AC in the temps in the upper 30s, 40s, 50s, 60s for as long as there has been AC condensers. I guess I am the lucky one that nothing has ever happened because they were my own that I was operating.

Never had problem one.

Jim Luttrall · #5 (**permalink**) 10-30-2009, 10:26 PM

Ted, back in the olden days when you and I were comin' up, they put crankcase heaters on the units to prevent damage but now those are "options" on most residential equipment. I run equipment colder than I ought (did I just say that out loud?) but only with what little knowledge I have from assessing the equipment and weather history. I have been around when compressors are slugging liquid and it is not something you want to repeat. It makes me want to hit the deck for fear of something coming apart! It is NOT a good sound!

Scott Patterson · #6 (**permalink**) 10-31-2009, 08:52 AM

If the unit has a Scroll compressor you can run the A/C at any temp and it won't be damaged.

Ted Menelly · #7 (**permalink**) 10-31-2009, 12:48 PM

Quote:

Originally Posted by Jim Luttrall

Ted, back in the olden days when you and I were comin' up, they put crankcase heaters on the units to prevent damage but now those are "options" on most residential equipment. I run equipment colder than I ought (did I just say that out loud?) but only with what little knowledge I have from assessing the equipment and weather history. I have been around when compressors are slugging liquid and it is not something you want to repeat. It makes me want to hit the deck for fear of something coming apart! It is NOT a good sound!

I have no idea what you are talking about. I am only 28

Jim Luttrall · #8 (**permalink**) 10-31-2009, 03:06 PM

28 and holding?

Nolan Kienitz · #9 (**permalink**) 10-31-2009, 09:06 PM

Quote:

Originally Posted by Jim Luttrall

28 and holding?

Sounds very good!

My Mom was "39 and holding" to the day she went Home a few years ago at the wonderful age of 92!

Miss both her and my Dad to be able to sit and talk over things.