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re: heat pump screams occasionally
2 jan 2006
cbhvac  wrote:

>>>... its in the 50s right now, and my units running...outdoor coil temp 
>>>will be about 20f...its working fine.....
>> it wouldn't ice up at a 36 f coil temp.
>and it wont demand defrost at 36f either...
>it will go into a timed automatic defrost.

sounds like a silly waste of energy.

>>>>>depending on outdoor temps, humidity, and time of operation, you will 
>>>>>get a light layer at the least of frost on the coil.

>>>> that seems avoidable, eg put a temp sensor on the coldest part of the 
>>>> coil and turn off the compressor and keep the fan running if it's below
>>>> 36 f and the outdoor air is warmer.

>>>>>makers have overcome this by simply allowing the reversing valve to 
>>>>>open... and the unit takes heat from indoors and melts the frost... 
>>>> heating the outdoor coil with indoor heat sounds less efficient.
>>>how? your strips come on, or, they dont, depending on how its wired, and 
>>>the frost is gone in seconds.
>> ice is avoidable, but if your system were completely screwed up and
>> you wanted to melt 10 pounds pound of ice off a coil, would you rather
>> a) move 36 f outdoor air through the coil with a 2470 cfm 90 watt fan
>> for t hours, where (36-32)2470t = 10x144, so t = 0.145, ie 9 min, using
>> 13 watt-hours of electrical energy while removing no heat from the house, 
>> or b) run your heat pump with a cop of 3, stealing 10x144 = 1440 btu of
>> house heat and wasting another 10x144/3/3.412 = 141 wh of electrical energy?

in the calc above, melting ice with the fan takes 13 wh. the compressor
uses 141 wh and steals 1440 btu from the house, which takes 141 wh to
replace, so it uses 242 wh, ie 19 times more energy than fan-only.
>now...this is hvac sarcasm:

enough already.

>now, you keep using it air temp, or coil temp?

a coil temp.

>what would *i* rather do? id rather have it switch over to cool, drop the 
>fan, run 90f or higher refrigerant into the coil, watch the ice melt in 90 
>seconds, and get back to the business of heating.

that could be more fun to watch, but suppose you are paying the bill? :-)

>and you are not *wasting* anything.

that would waste 229 watt-hours of energy, compared to the fan alone.
>>>honestly...what basic part of this operation do you not fully understand?
>> gee... hvac arrogance? :-)
>might be  to a point. but then, even when i was just a tech, i had no 
>problems understanding why it worked the way it did.

the how seems easier than the why, which may have more to do with marketing
decisions, eg selling a lower cost system that uses more energy, and trying
to brainwash people into thinking its efficiency cannot be improved.

running the coil at 20 f on a 50 f day might extract latent heat from
the air and increase the coil's effective conductance and capacity, but
it looks like fan-only de-icing uses a lot less energy than compressor
de-icing, even if it requires more sensors or smarter software.


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