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

>>> wrote:
>>>> stretch  wrote:
>>>>
>>>>>the reason the fan alone is not used to thaw the coil is it takes too
>>>>>long, especially if the outdoor temperature is below 32 degrees f.
>>>>
>>>> that would take a long time to thaw :-)
>>>>
>>>> but why run the compressor vs heat strips, with air below 32 f?

note "air below 32 f"." 
 
>>>> and why not turn off the compressor before the coil ices up?
>>
>>>the coils going to normally operate close to, and far below, freezing.
>>
>> "far below" sounds like strip heater time.
>
>why? 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.

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

you won't, with a 36 f coil temp.

>> 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.
>
>gee....think thats what an outdoor thermostat is for?

gee... hvac sarcasm?

>>>makers have overcome this by simply allowing the reversing valve to open...
>>>and the unit takes heat from indoors and melts the frost or ice off...
>>
>> 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?

>honestly...what basic part of this operation do you not fully understand?

gee... hvac arrogance? :-)

nick




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