Comments on: Why Does My Heat Pump Frost Over?

By: Allison Bailes

Allison Bailes — Fri, 21 Feb 2014 01:44:27 +0000

David B.:
David B.: I'll give you a hint. Mike MacFarland's intelligent defrost system measures pressure as well as using the built-in timer and temperature sensor.

Adam G.: Thanks! I've covered some of those topics already and will have an article about Mike MacFarland's intelligent defrost system out soon, next week I hope.

Curt K.: Yeah, I usually say 'outdoor unit' when I'm talking about heat pumps, too, but for some reason used condensing unit here and clarified with the footnote. Also, see my response to David Butler above for a hint about Mike MacFarland's intelligent defrost system.

Jehan: Thanks!

John S.: Yes, it's bugged me, too. I probably should've used 'outdoor unit' instead. In fact, I think I'll go change it now.

By: John Semmelhack

John Semmelhack — Thu, 20 Feb 2014 22:14:12 +0000

Thanks for the footnote about
Thanks for the footnote about "condensing units", Allison. That word bugs me every time when the topic is heat pumps.

By: Curt Kinder

Curt Kinder — Thu, 20 Feb 2014 02:51:28 +0000

I'm looking forward to the
I'm looking forward to the article about smarter defrost controls.

I prefer to refer to the box outside as the "outdoor unit" or even "compressor section" to avoid the inaccuracy of "condensing unit" during heating mode.

Around the time Atlanta had a 2" snowfall blizzard, Jacksonville experienced a rare condition - 35 degrees combined with rain (i.e. 100% RH. Defrost controls were taxed as never before.

By: Adam Gloss

Adam Gloss — Thu, 20 Feb 2014 01:24:28 +0000

Nice job! This was a great
Nice job! This was a great way to simplify a complex subject for the lay-person. (Although my jaw did drop when I saw the amount of ice on that heat pump). Heck you could do a whole series from this: The importance of proper heat pump maintenance, equipment and line-set sizing, installation and charging, issues due to improper airflow, freezing of indoor coils, etc.

Sorry - didn't mean to get excited and create more work for you ;)

By: David Butler

David Butler — Thu, 20 Feb 2014 01:12:03 +0000

I recall having to pour hot
I recall having to pour hot water on my first heat pump (circa 1979). That unit was a dog compared to today's heat pumps.

Typically, defrost initiation is based on a timer (30, 45, 90 minutes, set depending on local climate). The key for intelligent defrost control is the ability to stay in defrost mode long enough to remove most of the build-up, while ending defrost cycles quickly when there's not enough moisture to form frost. This is typically done with a temperature sensor on the liquid line.

In my dry (Arizona) climate, the defrost only operates about 20 seconds (except those rare occasions when we actually have some moisture in the air), so there's little impact on efficiency. In fact, I have my strips disabled and never notice the momentary loss of heat in the supply air stream.

Here's a picture of a different problem. Freezing rain accumulated overnight on top when the unit didn't cycle on in the hours after setback. Plus I think the gutters may have been clogged.