On 20 Dec 2005 20:13:07 -0800, "Stretch" wrote:

Heating Capacity of a heat pump drops as it gets colder outside. The
heat load load on the house goes up at the same time. Around 35
degrees oiutside the heat pump's heat output drops below the heat load
of the house. This is called the "balance point". Below this outdoor
temperature the heat pump will run constantly.

Only if there is no back-up heating unit. In real systems, below this
outdoor temperature the heat pump and resistance heating will both
turn on, but when you reach the setpoint temperature, the heat pump
will turn off.


Note that at -20 degrees outside the COP (Coefficient Of Performance)
Of a heat pump will be about 1.5. The COP of strip heat is ALWAYS 1.0!
Therefore, switching to emergency heat will increase your electric
bill, as with both heat strips and heat pump running your average COP
will be around 1.25 at -20 degrees outdoors. Not great, but better
than heat strips alone.


Does this calculation of the COP include the defrost cycles? Or are
you saying (below) that the cost of the defrost cycle is negligble
even at -20 def F?

Also since swithcing to emergency heat drops
all the heat from the heat pump mout of the system, you have just lost
some of your capacity and the system may not be able to keep up.

I believe units installed in northern climates are sized so the
back-up furnace can handle the entire heat load.

The outdoor coil freezes because the outdoor coil gets colder than the
outdoor air in order to extract heat from it. If the outdoor coil
temperature is below the dew point of the outdoor air, frost will form
on the outdoor coil. Leaving the system in emergency heat will shut
the compressor and outdoor fan off, so the outdoor coil will be at the
same temperature as the outdoor air. NO frost will form on the outdoor
coil unless it is from horizontally falling snow.

Yes, I thought this is what I was saying. If you switch to emergency
heat, no ice forms on the outside coil.

When the heat pump goes into defrost, the strip heat comes on inside to
temper the air, the reversing valve switches to cool mode so the
outdoor coil will get hot enough to melt the ice and the outdoor fan
will shut off so most of the heat in the outdoor coil goes to melting
the ice (latent heat of fusion at 144 BTUs per pound) and very little
goes directly into the outside air.

That heat that goes into melting frost ends up in the outside air.

In practice, what I see in very cold temps is that the defrost cycle
is *very* frequent. It's hard to believe that I'm actually getting any
benefit from the heat pump.

The fan running on the outdoor unit CAUSES the frost to form in the
first place, not prevent it form forming. If the outdoor fan was not
running, VERY LITTLE heat would be extracted from the outdoor air.

Yes. What I was trying to say is that when you switch to emergency
heat, the heat pump turns off and so the outside fan stops. If you
stay like that for a couple of weeks during a cold spell, and there is
lots of snow or freezing rain, etc., you may end up with a crust on
top of the unit that you have to manually remove. The fan alone (when
you turn the unit back to heat-pump mode) cannot blow away the snow
and ice. So that's a disavantage of switching to emergency heating
mode. If you leave it in dual mode, you may be getting zero or
negative savings out of the heat pump, but at least the blowing
outside fan keeps the snow and ice from building up on the outside
unit.

Terry