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Air to air heat pump truths

When we are out there talking to our clients about heat pump efficiencies it's important to recognize that the numbers can be misleading. A quick look at the AHRI ratings will tell you what your system is capable of producing at various temperatures, and the rated efficiency is clocked at a fixed temperature.

Sounds obvious enough, until you consider defrost cycles. During defrost cycles your system goes into air conditioning mode and electric heaters in your air handler come on to offset the cold air produced. This has to happen to get heat out to the outdoor coil to melt the frost that you will sometimes notice collecting there.

But defrost cycles don't happen on a fixed schedule.

Defrost cycles happen based on need, and that need is in more demand during times of high humidity. Believe it or not we have very high humidity in the winter in central pa. The system in my own home sometimes defrosts 10-12 times a day. I have a 10KW electric heater to offset the chill, and it's not uncommon for that to cost me $3.00 - $5.00 a day on cold humid days.**

People wonder why they still have high electric bills in the winter with high efficiency heat pumps and this is why. You system may be producing 70% of it's rated capacity below freezing, but it is defrosting more than you think and that is not accounted for in the ratings. It's one of the reason that the newer standard that came out this year reduce the ratings. They should!

What can you do about it?

Two things:

The most important thing is that you keep your air filter clean in the winter. I have said this ten thousand times and it seems like people still don't get it.

Airflow is the key to efficiency.

If your system can't deliver the air, it cant deliver the heat.

If your thermostat sees that it is taking too long to heat the house it will stage on the electric auxiliary heat.

Do not go out and buy an MERV 15 air filter and leave it in for 6 months. It will cost you hundreds of dollars in electricity. A good rule of thumb is no higher than MERV 10 and replace it every three months until November. Then change it every month until March.

Airflow, airflow, airflow.

The second thing you can do is stop with the temperature setbacks. Unless you are lowering the temperature at night for sleeping comfort, there is no economic justification for lowering the temperature while you are at work or otherwise out of the house for less than a few days.

Your thermostat will stage on the electric heaters when the thermostat sees a large demand that will take an unreasonable amount of time to satisfy.

If you take this advice, the solids in your house will stay the same temperature. Why does this matter? Air is easy to heat, but the solids are not easily heated with warm air. Let your system do a lot of light lifting throughout the day and keep the house the same temperature.

You can't stop aux heat cycles when defrost is necessary, but you can let your system breathe, and you can keep the load as steady as possible avoid aux heat staging.


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