On Jan 3, 3:22 pm, ATJaguarX wrote:
On Jan 3, 3:00 pm, Heathcliff wrote:



On Jan 3, 11:33 am, dpb wrote:

Heathcliff wrote:

...

It is true that for best efficiency, the furnace should be sized so
that it is just able to keep up on the coldest days. An oversized
furnace that cycles frequently will waste energy. ...

How, precisely, do you come to that conclusion? Given two otherwise
comparative units, how does size alone affect efficiency?

--

Fair enough, here are some references:

http://www.cmhc-schl.gc.ca/publicati.../03-109-e.html
(canada mortgage and housing corporation technical bulletin)

http://www.alpinehomeair.com/Furnace...fm(homeheating
supply co.)

and it's not just efficiency, but also early failure of the unit is a
possibility. -- H

I can understand the oversized furnace issue. What I don't understand
is why my furnace will shut off and fire back up 10 mins later when I
have my thermostat set to 69 degrees. I would think that the furnace
should be able to stay off for longer then that. I've noticed that
the thermostat never drops a single degree before it kicks the furnace
back on again. Is there something internal on the thermostat that
measures fractions of a degree?

I think most thermostats have an adjustment that determines how far
off the setpoint it allows the temp to drift before taking action. So
for example if you set it to 68, will it allow the temperature to
drift down to 67.5, or 67, or 66 before kicking on the furnace,
depends on this adjustment. I know my old cheapo Honeywell round
thermostat had this, but you had to take the cover off to monkey with
it. If you feel it is cycling on an off too much, adjusting the drift
setting may help with that.

"Setback" usually refers to a feature on the thermostat that allows
you to set the temp to different values for different periods of the
day. So for example you can set it to go down at night, and perhaps
during the middle of the day if no one is home then, but be toasty in
the morning and evening when you are up and around. -- H