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jeff5may · 03-05-16, 01:05 AM

The air source units frost up the most when temps are in the high 20's and low 30's. It all depends on the dewpoint outdoors. Below about 25, the majority of the moisture is frozen out of the air, so there is not as much available to frost up the outdoor heat exchanger.

The old school units defrost on a time vs temperature scheme: when it's below about 40, they defrost every 30, 60, 90,or 120 minutes and don't switch back to heating until the heat exchanger warms to above 50 or so. If there's not much frost on the coil, they might only defrost a minute or two. The colder it gets, the longer it takes for the coil to warm up, and the more heating capacity you lose every defrost cycle. The newer units use more sensors, both on the coil(s) and in the open air. They can sense if the outdoor unit needs defrosting and when. They vary so much more stuff (fan speeds, compressor speed, flow rate of refrigerant, etc.) to do their job than the old school units that it is difficult to compare them with each other in side by side tests.

The ground source systems take energy efficiency to a whole new level. When it is -10 outside, they straight up whoop an air source unit if the ground loop is designed correctly. Having a 40 degree heat source to pull from, they don't take a penalty in power or capacity. If the wind starts howling, they don't mind at all. The unit might run a little longer, but that's because the wind is robbing more heat from the house.

Combining a GSHP with a low-temperature radiant source can save you a whole lot of KWH to get the same BTU of heating capacity if the system is designed correctly. When combined with domestic hot water heating, the savings can be very substantial. A straight electric water heater has a performance level of 1. This translates to about 4 EER. An old school air source unit might have an EER of 10 or 12, a newer one maybe 20. An old school GSHP might start out at 20 EER, and the newer variable-speed "water furnaces" are pushing 40 EER. There have been measurements made by members of this forum where their GSHP systems averaged between 25 and 30 EER all season long (COP values of 4 to 5) and saved them the cost of their systems in electric bills in just a few years. Of course, a lot of them did a large portion of the labor involved on their own.

03-05-16, 01:05 AM	#18
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	The air source units frost up the most when temps are in the high 20's and low 30's. It all depends on the dewpoint outdoors. Below about 25, the majority of the moisture is frozen out of the air, so there is not as much available to frost up the outdoor heat exchanger. The old school units defrost on a time vs temperature scheme: when it's below about 40, they defrost every 30, 60, 90,or 120 minutes and don't switch back to heating until the heat exchanger warms to above 50 or so. If there's not much frost on the coil, they might only defrost a minute or two. The colder it gets, the longer it takes for the coil to warm up, and the more heating capacity you lose every defrost cycle. The newer units use more sensors, both on the coil(s) and in the open air. They can sense if the outdoor unit needs defrosting and when. They vary so much more stuff (fan speeds, compressor speed, flow rate of refrigerant, etc.) to do their job than the old school units that it is difficult to compare them with each other in side by side tests. The ground source systems take energy efficiency to a whole new level. When it is -10 outside, they straight up whoop an air source unit if the ground loop is designed correctly. Having a 40 degree heat source to pull from, they don't take a penalty in power or capacity. If the wind starts howling, they don't mind at all. The unit might run a little longer, but that's because the wind is robbing more heat from the house. Combining a GSHP with a low-temperature radiant source can save you a whole lot of KWH to get the same BTU of heating capacity if the system is designed correctly. When combined with domestic hot water heating, the savings can be very substantial. A straight electric water heater has a performance level of 1. This translates to about 4 EER. An old school air source unit might have an EER of 10 or 12, a newer one maybe 20. An old school GSHP might start out at 20 EER, and the newer variable-speed "water furnaces" are pushing 40 EER. There have been measurements made by members of this forum where their GSHP systems averaged between 25 and 30 EER all season long (COP values of 4 to 5) and saved them the cost of their systems in electric bills in just a few years. Of course, a lot of them did a large portion of the labor involved on their own.