wrote in message
...
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.
So is building 500HP toys, but I do it anyway..
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.
Damn...take ALL the fun out of it will ya?
Now, you keep using 36F...is it air temp, or coil temp?
A coil temp.
Clairification noted.
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? :-)
I am.
I took a really nice, high dollar ThermoPride oil burner out and put in my
heat pump when I bought this old place.
Im about to upgrade again, and the one I use is only a few years old.
Not for energy savings, but for the fact that the model that I have, has
been dropped by York, and its not real smart to be a dealer and have
obsolete equipment.
I AM however, going to try a higher SEER model this time, altho I have some
serious issues with running a 19SEER unit in this climate.
And you are not *wasting* anything.
That would waste 229 watt-hours of energy, compared to the fan alone.
So...you are honestly suggesting that the makers of heat pumps drop the
defrost mode, just run that 1/2 HP fan to defrost the
coils.....ohhhhhhkay...
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.
Right now, no ones brainwashing anyone. Units efficiency ratings are
climbing yearly. So is the cost. I suppose you could tell GM and Chrysler
and the rest that if their cars and trucks would have a sensor in them that
would allow the engine to shut off while coasting and start back up again
when demand was there, they could add a few more MPG points to each
sticker...
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.
Some units, even the ones I sell, have an outdoor air temp sensor, a line
sensor, and the software to know if its going to need a demand defrost, or a
timed defrost, or neither.
So, given that, the possibility exists that at some point during the cycle
of the unit, a defrost would be close to being needed, however the
thermostat would be satisfied in teh home, the unit would shut down, and the
defrost would occur due to outdoor ambients being above freezing anyway, so
that would save even more eh? Its already there. Running the fan motor
alone, in an attempt to thaw a coil is pointless, particularly if the coil
will not pass air due to being a sold block of ice....and that happens. You
have to find a way to insure that the unit will thaw, no matter what,
quickly, to allow the unit to get back to a frost free condition, where its
making more heat than it will when the coils frosted.
Taking THAT into consideration, the units run time would be shorter still,
the need for the fan to continue to try to thaw the coil is eleiminated
together, and IF the unit DID need to go into a defrost cycle, at this
point, lets consider that the strips DONT come on, you eliminate that load,
the fan stops, you eliminate that load, the compressor uses NO more power
than it was before, it runs for only a few moments in defrost, and the units
output when it goes back into heat mode is increased, therefore, you are
gaining more than you used.
Dont forget the fact that overall run time, NOT those few moments in defrost
mode are not being taken into consideration with your figures. In other
words, you seem to forget that you get more with a heat pump than you pay
for.
Nick