Seeking Tips on Getting the Most from a Heat Pump

[kenelias]

I replaced our A-C/propane-heat roof-top unit (it died) with a heat pump about two years ago. Our goal was to move to something sustainable, and we have solar panels for electricity, but not enough for the heatpump. Even if a lot of the electricity provided by Edison is produced with a carbon footprint now, that might not be the case in the future.

I'm not an engineer, but I read somewhere that the colder it is outside the less advantage there is to a heat pump. And our experience is that when it's close to freezing outside, the heat pump struggles and is quite noisy, the unit spends a lot of time in defrost-mode, etc.

Our thinking is, perhaps heat pumps are more appropriate for a more temperate climate than where we live (San Joaquin Valley in Ca., elevation about 800 ft.). However, we have it now. So, how do we get the best from it?

For example, if it's 40F. outside at 10pm and it will be 30F at the 6a following, maybe we ought to crank it up at night so the house will stay warmer through the night. Or, perhaps we should be using some plain-old resistance-electric heaters in smaller areas of the house when it's near freezing outside.

If anyone can direct me to some technical data on this matter, I'd really appreciate it. Somehow I have not been successful in getting the information from the manufacturer of our unit. But this kind of information must be available in this modern web age.. I just don't know where to look.

Sincerely,

Ken Elias
Three Rivers, CA

[spimps]

This http://www.arthurhewett.com/id26.htm#cop might help with the basic temps.
Efficiency will fall slightly as the temp drops but will still be far more efficient in energy terms than using direct electric heating,should easily cope with temps down to 5F/-15c.

[dougheret0]

Based on the BINS for Travis AFB, the San Joaquin should be an ideal area for heat pumps, similar to my location in North Florida, but drier. Heat pumps are rated in heating mode at two temperatures: high (47F) and low (17F). A typical COP of a moderately efficient air cooled heat pump at 47F would be about 3.5, and at 17F about 2.4. So even at 17F, which is about as cold as it gets in your area (according to the Air Force BIN data), the heat pump is way more efficient than electric strips and about the same (in terms of “source” energy or carbon footprint) as a gas furnace. It is actually more efficient than a gas or oil furnace at higher outdoor temperatures. The ratings include a factor to account for defrost with electric heat strips.

The problem is the “quality” of the heat provided by a heat pump. Where a fossil furnace regularly delivers air at the register at 110 – 120F, a heat pump delivers air at temperatures as low as 85F on cold heating days, up to maybe 95F on warmer heating days. When 85 degree air is delivered to a space, it gives a sensation of cool drafts to occupants, remembering that the heating air is being cooled to whatever the room set point is, say 72F. This causes occupants to want to turn up the thermostat.

Also, the outdoor unit is cooling outdoor air, so ice can form on the condenser, requiring defrost. When the unit goes in to defrost, it reverts to cooling mode and starts to deliver cold air to the room, which is why electric heat strips are needed to temper the supply air during defrost. The same electric strips will also energize when the space temperature falls three degrees or so below the thermostat set point, although this feature can be suppressed. If it is not suppressed, then the electric heat will activate in the morning during warm up if the heat pump is set back (the thermostat set point is reduced) at night or when occupants are at work.

During winter in my climate, which has a similar temperature profile to yours, I set the indoor temperature at 68F. I disable the “warm-up” activation of electric heat when the outdoor temperature is above 40F. We wear sweaters around the house on cold days, because of the draft effect. A heat pump is capable of warm up without the heat strips, it just takes longer. When we leave for a few days, I set the thermostat at 60F. I've noticed that on our return, the heat pump will warm up the house at about one degree per hour without the heat strips.

[casstrig]

In colder climates to increase efficiency and reduce the draft effect by eliminating the defrost cycle ground coils are used with ground temperatures at one meter never getting below 70 degrees Fahrenheit.

[kenelias]

Sincere thanks for this wonderful trove of engineering information, and leads!

We deliberately left out the resistance-heat strips from our installation thinking that we wanted it to be the maximum green solution it could be.

And actually, the cool drafts have not caused me as much consternation as the sound of the poor thing when the evaporator (condenser during a/c) gets frozen. It sounds like a truck trying to drive up a steep hill in high gear, except it's on our roof! D'oh! For that issue I was considering remounting the unit buy surrounding it with an extra layer of angle iron and installing numerous neoprene isolation blocks which I would get from W.W. Grainger or the like.

I'm very grateful this forum is available, and again, sincere thanks to those who posted links, and particularly thanks to dougheret0 for the crash course on the engineering..

Sincerely,

Ken Elias
Three Rivers, CA