Comments on: Why Are Heat Pumps So Dumb About Frost?

By: Ben

Ben — Thu, 02 Apr 2015 17:15:32 +0000

Does anybody have the p-h
Does anybody have the p-h diagram for the H2i Mitsubishi Heat Pump System? Or explain how it works?

By: wendy

wendy — Mon, 02 Mar 2015 20:43:10 +0000

thanx dave, bunches. i'll put
thanx dave, bunches. i'll put the timer back to 60min. it's been rainy and cold here for a week now and i have run the emergency heat twice at night only; the extra money spent is worth a good night's sleep for me anyway. i don't want to be calling an hvac co. at 3am because something failed during the night.

By: David Butler

David Butler — Mon, 02 Mar 2015 02:13:36 +0000

@Stan, do not run your system
@Stan, do not run your system in cooling mode in cold weather unless it has a low ambient kit, as this can damage the system. keep in mind that the outdoor fan is turned off in defrost mode. That makes a BIG difference.

By: David Butler

David Butler — Mon, 02 Mar 2015 01:23:16 +0000

@Wendy, your heat pump will
@Wendy, your heat pump will not go into defrost until the timer tells it to. Depending on the control design, there's typically a sensor that disables defrost above a certain outdoor temp, and another sensor that ends the defrost cycle. But cycles are always initiated by the timer.

If frost is building up on your outdoor coil, you should change the timer to 60 or 30 minutes, as may be necessary when outside temp hovers for long periods between 30 and 35 during wet weather. Running in EMergency mode is silly as it's only going to double or triple your heating bills.

By: David Butler

David Butler — Mon, 02 Mar 2015 01:14:38 +0000

@Bob & Wendy, I think
@Bob & Wendy, I think William was addressing a *potential* side effect of the after-market control described in the article.

Keep in mind that the colder it gets, the less likely there will be enough moisture in the air to form frost. So a pressure-based control that locks out defrost in the absence of frost will necessarily eliminate defrost during the coldest weather. That's a good thing from an efficiency standpoint but if oil migration away from the compressor is truly a problem, then such a control would need to have additional logic to protect against this.

@Mike, do you know anything about this?

It's well known that oil will migrate away from the compressor during cold weather during extended off periods, which is why most HP's have crankcase heaters, and why it's important to turn off the heat at the t'stat during an extended power outage and leave it off for several hours after power is restored so the crankcase heater can draw the oil back to the compressor sump.

However, I've not heard of oil migration being a problem during normal operation without defrost. I would be interested to know if anyone has published a paper on this.

By: wendy

wendy — Mon, 02 Mar 2015 00:45:15 +0000

hi stan; (the hot water
hi stan; (the hot water solution is todd's, not mine.) interesting thought on ac use; however, i read somewhere it's not recommended the ac portion of the unit be operated when outside temps are below 60's; and since i'm still under install warranty, i don't want to be burning anything up that will come back as my fault. anyhow, i can just short out the 2 pins that are near my timer selector which are located under the hp panel, but it's a pain to remove the cover when it's pouring rain out. this short will throw the system into defrost.

so, anyone have an answer to my original question? will the unit's defrost cycle OVERRIDE the timer if a defrost is actually needed?

By: Stan

Stan — Sun, 01 Mar 2015 23:03:04 +0000

what is wrong with putting
what is wrong with putting your HP on cool and turn the thermo down for an emergency defrost for a little bit until the defrost issue is fixed rather than hot water?

By: wendy

wendy — Sun, 01 Mar 2015 19:48:03 +0000

hi william; are you
hi william; are you commenting to wendy or todd?

wendy has a 4ton carrier unit.
will the unit's defrost cycle override the timer if a defrost is actually needed?

By: Bob

Bob — Sun, 01 Mar 2015 19:38:17 +0000

I don't see a compressor
I don't see a compressor running for 6 hours straight and it not being cold enough to go into defrost at least once during that time.

By: wendy

wendy — Sun, 01 Mar 2015 08:24:53 +0000

howdy; if i understand this
howdy; if i understand this article correctly my heatpump won't defrost until my timer tells it too, whether it needs it or not? correct? i ask because i presently have lots of frost on my heatpump and have no clue when the timer kicked in and when the defrost mode will actually occur. my timer is set at 90 min. i'm afraid to let the system run again for fear of damage if the wait for defrost is too long, so i 'shut the system down' and put the thermostat on aux heat strip only. is my heatpump designed to override the timer if a defrost is actually needed? did i do the right thing by shutting down?
thanx; wendy