Reverse air conditioner freeze up problem

[jeff5may]

Originally Posted by 8307c4

Don't mean to burst any bubbles but when it comes to heat, those 1,500w space heaters that put out about 5,120 Btu's pretty much sum it up. That's the bottom line, in terms of btu's / kwh.

Now, if you want to reduce your heating bills?
Insulation.

A typical 5000 btu/h air conditioner uses somewhere around 500 watts when running the compressor. The btu transfer pretty much follows outdoor temperature with a cap-tube metered unit, due to the (under) size of the outdoor heat exchanger. With a txv or eev metered unit, heat transfer is much more consistent, and a 5000 btu unit will actually do more heat transfer when outdoor temps are not extreme.

Also, most run-of-the-mill resistance heaters have been downsized to 1250 watts to prevent the heaters from popping 15 amp fuses and circuit breakers. The units that use 1500 watts are now labeled that they require their own dedicated circuit.

[jeff5may]

Originally Posted by F357

That's another thing I don't understand about the commercial window heat pumps. They often are rated a few thousand BTU less in heating mode. Shouldn't it be the other way around?

The reason for this derating lies in the test conditions. In cooling mode, they are tested at a much higher outdoor temperature than in heating mode. At the same indoor and outdoor temperatures, the unit will indeed move more BTU's in heating mode.

As a general rule of thumb, pretty much any heat pump will have more capacity as suction pressure increases. Suction pressure is dependent on the evaporator temperature, so as the evaporator temperature increases, total system capacity follows. As evaporator temperature falls, total system capacity falls with it.

In cooling mode, the AHRI or ASHRAE tests the units for SEER between 67 and 100 degF outdoor, centered around 82 degF. The test indoor temps are between 67 and 80 degF. Given these test conditions, one can see that there is a 13 degree overlap where the outdoor temperature could very well be below the indoor temperature! Obviously, during these conditions, the unit is going to perform very well.

In heating mode, the AHRI tested units are tested for HSPF between 62 and 7 degF outdoor, centered around 45 degF. The indoor test temps are between 60 and 70 degF. Below freezing (32 degF) outdoor temps, the averaging curve drops off real fast. This is due to the fact that until recently, nearly all heat pumps were equipped with "low ambient" cutout thermostats that switch the system over to backup heat below a balance point somewhere near freezing. The AHRI also disregards defrosting losses below freezing under the assumption that nearly all the systems won't be operating in this "low temperature" region.

When calculating HSPF, there is only a 2 degF overlap where it may be warmer outside than inside when the heat pump is operating. For the rest of the test condition range, the system will be working against a temperature gradient to do its work. Regardless of overlap, with the evaporator temperature centered around 45 degF in the testing, it will need to work much harder to extract heat due to the lower evaporator temperature. The system just cannot move as much heat with a 45 degree evaporator as it can with a 60 degree evaporator.