Cohousing-L Mailing List: Re: Best Practices: Geothermal HVAC

[Michael Barrett (mbarrett]

> When we moved in our HVAC company said  the most economical way to use 
> the units was to set a temperature and leave it.

When the thermostat is raised, whether manually or under automatic control, the 
unit works to achieve the new higher temperature.  If the temperature 
difference between ambient and desired is larger than some manufacturer's 
preset number or if the unit has to run for more than a manufacturer's preset 
time (on its way to achieving the new temperature) the manufacturer has 
designed the equipment to turn on back-up electric heat. This back-up electric 
heat is the hugely dominant factor in increased operating costs.

If the backup electric heat is turned off (i.e. so the thermostat cannot turn 
it on), and you don't mind waiting longer for the house to warm up in the 
morning, your heating bill will be smaller ! Yes it will!  How much will depend 
on how much the set-back in degrees is, for how long in hours, and how well 
insulated your house is.   Yes, the system will run for a long time in the 
morning - but still less than the sum of all the short bursts it used to run 
during the night before you set the thermostat back. Only experiment and 
reading the electric meter night and morning for a few days in the two modes 
will tell you if it is worthwhile in $s.  Since electric heating of hot water 
is the dominant electric power usage in homes not heated by electricity the 
effects may be swamped and confused by, say, uneven numbers of showers taken. 
[Turning off my back-up electric heat requires going inside the box to flip a 
tiny switch on a circuit board]

It would be too time consuming and complicated to lay out the thermal heat flow 
equations here, with time factored in, but I am confident any thermal engineer 
will agree.

So the HVAC company's advice to leave the thermostat at a constant temperature 
for economy is true only if  using a night time setback causes the system needs 
to kick-in with the very expensive back-up electric heat.  (My thermostat has 
an indicator showing when the back-up heat is on.)  Turn the back-up heat off, 
and set the ground source heat to start earlier and you can still be 
comfortable for less. And sleeping cooler is more natural.

Sleep well,
Michael Barrett
Liberty Village, Maryland