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From: ccrse@ucdavis.UUCP (0058)
Newsgroups: net.consumers
Subject: Re: Heat pump/water heater energy conservation
Message-ID: <476@ucdavis.UUCP>
Date: Fri, 13-Dec-85 14:39:44 EST
Article-I.D.: ucdavis.476
Posted: Fri Dec 13 14:39:44 1985
Date-Received: Sat, 14-Dec-85 23:56:15 EST
References: <237@neurad.UUCP>
Organization: University of California, Davis
Lines: 36

>                                 ....Is it more economical to set back the
> temperature at night, and during the day when no one is home, or to leave
> the temperature adjustment alone?  Is there something unique to heat pump
> technology in this regard?....
> 
> Finally, with regards to the hot water heater, is it advisable to install a
> timer so that it only runs when people are likely to be home.
> In other words, is the energy needed to reheat the water more or less
> than that required to maintain the water temperature for a prolonged period of
> no hot water use. My hot water usage is quite modest, yet I think this
> appliance is quite an energy hog.

If I remember correctly from when my father installed his heat pump, the
explanation runs something like this:

Heat pumps modify the air temperature only very slightly, i.e. the difference
between the inlet and outlet temperatures may be only a few degrees.  They
are able to maintain a given temperature quite efficiently by moving rather
large volumes of air through the system, but for this reason will require a VERY
long time to make significant changes in air temperature.  (For this same
reason, it is not unusual to find that the system will run almost continuously
under extreme temperature conditions, whether hot or cold.)  When large changes
are required, it is more desireable from the viewpoints of both efficiency
and speed to use resistance electric heat to make the change, then let the
heat pump take over to maintain it.  While the resistance unit may be more
faster and more efficient at making larger temperature changes, though, it
is still no where near as efficient as just letting the heat pump maintain
a given setting.  This is why you are counseled to 'set it and forget it'.

As regards your second question, unless you are gone a great deal, I suspect
you would be better off to leave the water heater on continuously.  Considering
how long an electric water heater can take to come up to temperature, you pro-
bably would only have it off a very few hours each day.  While this *might* save
you a small amount of energy, the additional stress on the heater from the
heating and cooling might cause it to wear out prematurely, thus costing you
as much or more in the long run.