Insulating Liquid Line on Heat Pumps?

[SurreyStevo]

Last summer, I had installed a Keeprite heat pump at my parents house. About 20' of the lineset is exposed to the outdoors. I have never seen it out in the field, but shouldn't both lines should be insulated? The liquid line returning to the heat pump in the winter is around 85-90 deg F. It's precious heat is being lost to the outdoors. Therefore, a loss in efficientcy.

[cjpwalker]

No. The indoor coil acting as the condenser has already rejected as much heat as it can into the house. After the coil you have subcooled liquid heading toward the outdoor metering device in the heat pump. It will have to flash off anyway, so keeping it warm won't be any benefit at all.

[cboe]

got to have subcooling!

[mark beiser]

I'd insulate the LL where it runs through the attic, to help with cooling mode efficiency/capacity, but don't insulate it anywhere that the ambient temperature is lower than the liquid temperature.

In your case the "precious heat" you are "loosing" is actually bad heat!

I say that because immediately after the liquid refrigerant passes through the metering device, some portion has to boil(flash) off to cool the rest of the liquid refrigerant down the the saturated temperature of the outdoor coil. It is called flash gas, and 100% of it is wasted energy, robbing from the total capacity of the system.
Loosing some of that heat, before it gets to the metering device, actually helps the heating capacity of the system.

Having said that, the difference would be difficult to measure outside of a lab, or even in one.

[beenthere]

Don't have a chart for 3/8". but 1/2" copper pipe with the liquid being 99 degrees warmer then the outdoor air, will lose 47 BTUs per hour per foot. So on your 20 foot outside 940 BTUs an hour loss if the liquid line is 90° and the outdoor temp is -9.

[SurreyStevo]

That makes sense Mark Beiser. Colder liquid refrigerant would give a colder evap coil (heat pump) and possibly increase your outdoor ambient TD.

[mark beiser]

That makes sense Mark Beiser. Colder liquid refrigerant would give a colder evap coil (heat pump) and possibly increase your outdoor ambient TD.

Not really, you would just have slightly less flash gas after the metering device, so slightly less capacity loss.
In the heat mode we are talking about nearly unmeasurable differences.

On the other hand, heat gain in the liquid line in the cooling mode can be a serious issue.
In the cooling mode, with a long line set running through a hot attic, insulating the liquid line can make a very noticeable difference.