"James Sweet" wrote in message
news:rPoJh.11757$mh7.4952@trnddc04...



What would be nice is a valve to short circuit the metering device and let
the hot vapor from the compressor flow straight into the evaporator coils.
That would defrost it in a matter of minutes. I've heard of a few very old
refrigerators that worked this way but nothing modern does it.



Admittedly not a refrigerator, but still a refrigeration system, heat pump
HVAC systems do exactly that... the valves in the system are arranged and
switched so that as the condenser on the outside unit collects frost and
eventually ices up (heat pump mode), the system reverses the valves and the
condenser is heated and the ice melts (defrost mode). When the system
determines that the ice has melted, it switches the valves back to heat mode.
In defrost mode, the system is essentially an air conditioner.



Yes I'm quite familiar with those having installed several of them. Most heat
pumps you'll find out there don't actually sense frost buildup but rather have
a simple interval timer which triggers a defrost cycle every 30, 60, or 90
minutes. A defrost termination thermostat senses the temperature of the line
out of the outdoor coil (which is the evaporator in heating mode) and returns
the the unit to heating mode when this trips. I modified my own unit with a
surplus vacuum operated sensor I found, it has a diaphragm which uses the
pressure drop from the fan across the coil when it ices up to trip a switch
which is then reset by a capillary tube and bulb in the location where the
original DT thermostat was located. In my climate this dramatically reduced
the number of energy wasting unnecessary defrost cycles, not sure how well it
would work elsewhere. It's a big boost to comfort as well, my backup heat is a
gas furnace so I can only run one or the other, a defrost cycle means an icy
blast of cold air which can drop the room temperature by several degrees. A/C
is very efficient when the condenser is down near freezing. Other methods have
been tried in the past but all I've seen until very recently are the timers.
Some very high end units today use a demand defrost system which monitors
trends in the evaporator temperature to determine when a defrost is necessary,
I have no experience with these so I can't say whether they work well or not.


Yeppers... Until a few years ago, I had an old GE Weathertron heat pump system.
Terribly inefficient by today's standards, but it got the job done. It had a
vacuum sensor in the outside unit that sensed a drop in air pressure inside the
coils, whereupon it switched into defrost mode. It ran the auxillary heat
strips while in defrost mode to keep from cooling the house while in defrost. A
capillary sensed the coil temperature and switched back into heat pump mode when
the coil temp got high enough. It worked quite well; only went into defrost
when necessary.

Scared me the first time I saw it defrosting... the big column of vapor steaming
up from the unit was awesome. I thought the thing was on fire!!

I had to replace the entire system when the compressor locked up after 25 or so
years of operation. Don't know how the Lennox system I bought as the
replacement determines that it needs to defrost... could be a timer. But it
seems to work well, even in sub-freezing temps and high Florida humidity.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer to the end, the faster it goes.