# Heat pump defrost



## samandlillie (Nov 3, 2011)

Would like to increase the speed of defrost cycle on my Fujitsu ductless heat pump.I was wondering if a small waterproof heater(infrared) placed directly in front of the outside unit would decrease the  5 min. defrost cycle to a couple of minutes?
Wayne in Nova Scotia


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## woodgeek (Nov 3, 2011)

I doubt it.  Compare the BTUs, your unit is prob pumping 15-25 kBTU/h directly into that ice, while your radiant output is 5000 BTU/h (max), with a radiant coupling eff of what ? 50% to the ice.  More importantly, the BTUs your HP is pushing cost you a LOT less than the electric coil on the radiant.

So, you are not going to save BTUs or loonies doing this....  There is a lot of money to be saved in not defrosting when unnecessary, but your unit is prob on the better engineered defrost controller end of things (compared to the cheap HPs in the southern US, and next to my house).

IF this is a comfort issue during defrost, you might look at redirecting the supply register so it doesn't blow on you.  It will heat just as well.


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## GaryGary (Nov 3, 2011)

Hi,
I'm just curious -- how often does it go through the defrost cycle?

Gary


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## karl (Nov 3, 2011)

GaryGary said:
			
		

> Hi,
> I'm just curious -- how often does it go through the defrost cycle?
> 
> Gary



It depends.  When the temperature outside falls to around freezing a timer in the heat pump turns on.  This timer is different for different heat pumps, but every 30, 60, 90 minutes is the norm.  They stay in defrost mode until the outside coil reaches close to 60 degrees.  Some of them adjust the time based on outside temps and temps coming from the heat pump itself.

Some heat pumps have demand defrost. Where they defrost only when necessary.  I'm not totally sure how this works, but I assume its measuring freon in and out of the heat pump and determining how well heat is moving as well as outside humidity.

A good amount of heat pump efficiency improvements have come from the defrost cycle.  Who wants to air condition their house for 5 minutes every hour so in the winter time.


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## BrotherBart (Nov 3, 2011)

Moving to the DIY room where more people will see the thread and post answers.


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## Corey (Nov 8, 2011)

Doubtful.  The unit most likely has an outer case which would block a lot of the IR.  Ice further reflects a lot of IR so you have two factors running against you.

Is this a new problem or an 'always been that way' ?

At any temp above freezing, the best thing is to run the heat pump in short bursts and let nature defrost the coils in between runs.  Below freezing, you have to run the pump in reverse to heat the outdoor coils.  I believe some units have the option to run axillary heat during this time to keep you from feeling like the air conditioner is running in the middle of winter.  But either way, at some low temperature point the COP drops and you're putting as much energy into defrosting coils as you're saving on heat pumping, so it's better just to turn on axillary heat.

The reason I ask if it's always been that way:  There are a lot of timers, valves, relays, heating/cooling stages, primary/secondary heat sources, etc which all have to work to make the most efficient use of the heat pump.  If any one of these things has failed, or been adjusted incorrectly, it can give poor performance, so suddenly seeing long defrost times on a unit which used to work would mean something else has changed/failed.


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## samandlillie (Nov 8, 2011)

Thanks everyone for the replies. My 2 year old Fujitsu goes into defrost a few times a day, the higher the humidity, the more defrost cycles. The most common occurance is in the am when a heavy frost starts to melt. In the past, the heat pump(ductless) would give heat when the temp. was as low as -20 deg. and the air has very little humidity so no defrosting occurs. During the last 2 years, instead of burning  150 bags of pellets($1000) I use an extra 300 KWH of elect. so I am really happy.Thanks again everyone.


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