Defrost switch closed at 38F...normal?

[PhiliBob]

A couple of weeks ago I have been noticing the heat pump defrosting for a really short duration (about 30-45 seconds) with about 35-40 degrees outside. The symptom went away last week as the weather warmed up a bit. This week it's been cooler so the defrost has been coming on and actually having ice to melt off. I checked the switch yesterday and found that it was closed at 38F (unit was at ambient, hadn't run for hours). The install manual states that it should close at approx 30F. The switch is working, just seems it likely turns off at ~40F. Is this normal?

[BaldLoonie]

Usually they close around 30 and open 70-80 but with a little snappy like that, 8° off isn't that unusual.

Time to buy a unit with demand defrost

[PhiliBob]

I've been thinking about this all wrong. After looking at the logic diagram again and thinking about it I now see there is about a 30 degree hysteresis with the defrost thermostat (for whatever reason I thought there was another switch involved and the defrost switch turned on below 30 degrees). So anytime the liquid line drops to about 30 degrees the defrost switch will close and hold until it reaches 60+, which would be when the unit defrosts. Well now I can sleep easier knowing it is working.

I'll get one with demand defrost when the time comes, but this install is pretty new so it will be a while.

[SBKold]

I personally think all new units should have demand defrost.

Goodman especially builds these nice 2stage 18seer units....and then still uses the same defrost technology they did 20 years ago.

[fearlessfurnace]

A couple of weeks ago I have been noticing the heat pump defrosting for a really short duration (about 30-45 seconds) with about 35-40 degrees outside. The symptom went away last week as the weather warmed up a bit. This week it's been cooler so the defrost has been coming on and actually having ice to melt off. I checked the switch yesterday and found that it was closed at 38F (unit was at ambient, hadn't run for hours). The install manual states that it should close at approx 30F. The switch is working, just seems it likely turns off at ~40F. Is this normal?

yeah thats 30F line temp. the line is colder than the air (thats were the heat is coming from)

[abcHybrid]

A Demand Defrost controller unit that can be installed on most existing time based defrost heatpumps is now available. visit www.acbhybrid.com for details