From: philip@mentorg.com (Philip Peake)
Subject: Re: How to cut back my heat bill?
Date: Tue, 24 Nov 1992 01:12:09 GMT
Distribution: na
Organization: Mentor Graphics
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In article <FSF.92Nov23144756@hpic006.mentorg.com>, fsf@hpic006.mentorg.com (Rick Farnbach) writes:
|> There's been lots of talk about using resistive heating rather than (or in
|> addition to) the heat pump when the house is really cold, but I just don't
|> understand why you'd even want this.
|> 
|> The efficiency of the heat pump is inversely related to the temperature
|> differential between the cold source (outside) and the hot source (inside).
|> Thus, your heat pump will save you the most energy when it is warmer outside,
|> or when it is cooler inside.  When you first turn the heat pump on after the
|> house has cooled, it should be enormously more efficient at moving heat into
|> the house than when it is running in temperature maintenance mode.  In
|> temperature maintenance mode it is trying to move heat into a hotter
|> environment, which is harder to do.
|> 
|> Why do heat pump manufacturers put in these self defeating thermostats that
|> turn on resistive heat sources during the interval when the heat pump would be
|> at its best?
|> 
|> It seems you really should use the resisistive heating only to get those last
|> couple of degrees of warmth, not to get the first few.  If your heat pump
|> isn't going to be able to get the job done, it will be at the top end not the
|> bottom end of the temperature scale that it will need help.

You are (mostly) right.
The things which are important are:

	- As the outside air cools, not only does the efficiency of the
	  heat pump decrease, but the heat exchanger ices up, which really
	  reduces the efficiency :-)

	- Heating contractors are a mean as anyone else, and generaly only
	  install the cheapest stuff possible. In the case of the thermostat,
	  they install a real cheapo one which has a coiled bi-metal strip, which
	  tilts *two* mercury switches. The second switch has a slightly different
	  angle, and "comes on" a few degrees later than the first.

	  Number one controls the heat pump - the idea is that the heating is
	  generally on all of the time - when it cools, the heat pump comes on,
	  and it will restore the temperature, or, at worst, stabilise it.

	  If the temperature continues to fall, switch number two goes on,
	  and switches on the resistive heating.

I was looking for a programable thermostat to use with my system.
They just don't seem to exist - well, let me re-phrase that - some
DO exist, but they are just electronic versions of the system described
above - you set a "trip" temperature (or several of them) and the heat pump
goes on at that temperature. At a pre-set number of degrees (3 ?) below that,
the secondary heating kicks in.

I think that installing one of these would be a complete waste.
What you win by reducing the temperature at night, you lose when
daytime comes, and the stupid thing switches on the RESISTIVE heating
to get you back upto daytime temperature (less three degrees).

It just needs some intelligence appied, both to the design and the
thermostat !

Switch on ONLY the heat pump, and if the rate of temperature rise
over X minutes is less than (???), THEN kick in the backup system.

If anyone knows of a reasonably designed item like this - PLEASE let
me know ... otherwise I may just go into buisness making the things !

Philip