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nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12
Reviews:
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reply to pandora

Re: The addition saga continues

YES! You are wrong. The refrigerant can't do work without a pressure change. It can't change pressure without using energy.

Where does this magical heat for the defrost mode come from if it's not pulling it from inside?

You keep insinuating you have something special, and you don't.

»www.achrnews.com/articles/unders···peration
--
If someone refers to herself / himself as a "guru", they probably aren't.

pandora
Premium
join:2001-06-01
Outland
kudos:2
Reviews:
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First, I claim nothing special. My claim is for the units I have had experience with as this isn't my vocation. I don't have a lot of experience with a diverse number of heat pumps.

Second, the coils heat and defrost the unit. In defrost mode the coils become very hot fairly quickly. I don't understand why the fan turns off in defrost mode, but it does.

The compressor can heat or cool the coils based on the direction of the refrigerant through the system. I assume if a unit is frozen (in need of defrosting) the frost will cool the vapor without any need to visit the air handler.

I'll hazard a pure guess here, the defrost sensor may determine if the refrigerant is being cooled when in defrost mode (indicating a need for defrosting). I have seen the units enter defrost mode and exit within seconds. Why run refrigerant to "cool" the air handler, when frozen coils are sufficient?
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"


PSWired

join:2006-03-26
Annapolis, MD
reply to nunya
Maybe the heat contained in the refrigerant loop, along with the heat generated by the compressor itself during normal operation, is enough to defrost the coils in his unit? The mitsubishi ductless units I have at work don't even have heat strips and they run through a defrost cycle just fine. The indoor air blower is off during defrost.

harald

join:2010-10-22
Columbus, OH
kudos:2
reply to pandora
well, I do claim to be an expert. I was the engineer who handled the reversing valve product for Ranco, who manufactured over 90% of the reversing vales in the world.

There is one loop in a heat pump, and all the refrigerant passes through it. There is no capability to prevent the refrigerant from passing through the inside coil.

pandora
Premium
join:2001-06-01
Outland
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Reviews:
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said by harald:

well, I do claim to be an expert. I was the engineer who handled the reversing valve product for Ranco, who manufactured over 90% of the reversing vales in the world.

There is one loop in a heat pump, and all the refrigerant passes through it. There is no capability to prevent the refrigerant from passing through the inside coil.

Noting your past tense, and accepting your implicit assumption technology never changes, I'll defer to your expertise.
--
"If you put the federal government in charge of the Sahara Desert, in 5 years there'd be a shortage of sand." - Milton Friedman"


cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
reply to pandora
said by pandora:

Second, the coils heat and defrost the unit. In defrost mode the coils become very hot fairly quickly. I don't understand why the fan turns off in defrost mode, but it does.

The coils become hot the same way your indoor coils heat the house. When in defrost mode though, you want the coils to heat up to melt the frost/ice, instead of heating up the outside air in general. The outdoor fan turns off so that the coils heat faster and the heat isn't wasted warming the surrounding air.

The indoor fan may turn off initially in defrost mode in an attempt to defrost the coils without having to use previously conditioned air, emergency heat strips, and/or other fuel as a heat source. If there isn't a call for heat, but a call for defrost, it may run until the indoor coil reaches a temperature cutoff and the blower turns on. This is when some report that they feel cold air from their vents, as the air isn't heated as hot as when in heating mode. If there isn't an auxiliary source of heat like heat strips, there may be no choice but to blow cold air during the defrost period.

I assume if a unit is frozen (in need of defrosting) the frost will cool the vapor without any need to visit the air handler.

Yes the frost would cool the vapor, if the unit was in cooling mode. However in heating mode, refrigerant returning from the air handler is in a cooled, condensed form. It needs to be EXPANDED back into a relatively warm gas absorbing more heat. The outdoor coil acts as a heat sponge of sorts. If the coils are frosty or iced up, it act like an insulator preventing air from circulating around and the coils absorbing heat from the air.

I'll hazard a pure guess here, the defrost sensor may determine if the refrigerant is being cooled when in defrost mode (indicating a need for defrosting). I have seen the units enter defrost mode and exit within seconds. Why run refrigerant to "cool" the air handler, when frozen coils are sufficient?

You're last statement is backwards. It's the outside coils that are "cooled" when in heating mode.

If the unit enters and exits defrost mode in a few seconds, something is wrong. You're units for instance has a "SmartShift" feature where the compressor shuts off for a short period before and after mode changes, as well as a short cycle protection which is hard on the compressor shortening it's life.

robbin
Premium,MVM
join:2000-09-21
Leander, TX
kudos:1
reply to PSWired
said by PSWired:

Maybe the heat contained in the refrigerant loop, along with the heat generated by the compressor itself during normal operation, is enough to defrost the coils in his unit? The mitsubishi ductless units I have at work don't even have heat strips and they run through a defrost cycle just fine. The indoor air blower is off during defrost.

As a result of your post I found an interesting study on a Fujitsu running in defrost cycle. Fujitsu behavior is as you described for your Mitsubishi.

"During the reverse cycle itself, power usage is modest since the system is taking heat from a mild source and dumping it to a very cold sink (the iced-up coil in the outdoor unit). Since there is no electric resistance backup heat (as would be found in a standard residential heat pump), the overall effect of the reverse cycle is negligible. Also, there are no times where the supply air temperature dips below the return air temperature."

www.bpa.gov/.../BPA-Report_DHP-FujitsuBenchTest-July2009.pdf


Hagar

join:2004-10-31
Sunnyvale, CA
Reviews:
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1 recommendation

reply to pandora
Pandora,

You have some nice heatpumps COP 1.22 at -10 Fahrenheit, very nice.
Your heatpump does not blow cold air during a defrost cycle and does not have heat strip in the indoor air handler. Both are good things and what is expected of a modern heat pump.
I personally hate electric heat strips in the indoor air handler, never the optimal solution IMHO.

Now where does the heat needed for the defrost come from? There are only two sources available, the compressor and the warm house. A good heatpump would use both because you want the defrost time to as short as possible. All the time you are in defrost mode the pump cannot heat your house which is the purpose of the heatpump in the first place.
Language like ‘The heat pumps do not use additional current when defrosting’ is not as clear as no heat strips in the indoor air handle. ‘No running of cold refrigerant the air handler’ sounds like marketing speak and is at best misleading, technically speaking it is incorrect. Much better to say not blowing cold air indoors during a defrost cycle.

Most people who have not seen a modern heatpump in action will be surprised how well they work in cold weather.
Laws of thermodynamics have not changed and defrost cycles still reverse the refrigerant flow and take heat from the house.