On Saturday, March 1, 2014 6:45:14 AM UTC-5, Stormin Mormon wrote:
On 3/1/2014 12:22 AM, wrote:

Closing a cold air return in an unheated room won't impact the combustion

air intake.



No, but it MAY cause the furnace to "short cycle" because the plenum

temperature will rize too high due to restricted air flow. This causes

the high limit switch to shut off the burner untill the blower sucks

the heat out of the heat exchanger, when it will relight. This makes

the efficiency of the furnace drop SIGNIFICANTLY.



Heat can go into one of a couple places. Into the

house, into the cellar, up the chimney. When

efficiency drops SIGNIFIGANTLY, the heat is lost

some where.

Up the stack. When a furnace short cycles it is running in the

in-efficient zone for more of the time. The heat goes up the stack

instead of into the house.

SM: So, the air handler blower is running, the gas is off, and the heat

is going up the stack, you say? I'd think the heat is going into the

building.





So the burner shuts off, and the blower still runs.

How does this make the furnace less EFFICIENT? I

don't picture it. Shutting off the gas lowers

the efficiency? Really?



At church we have converted oil furnaces, which

now run NG. They have a high limit sensor in the

discharge air, when it gets to 110 or 120 it turns

off the flame. The three blowers keep running.



So, turning off the gas now and again makes the

efficiency of the furnace drop SIGNIFICANTLY?

How's that, again?

see:

http://www.ehow.com/info_12081271_sh...-problems.html



SM: Says decresed efficiency, but doesn't explain

the process, and where the heat goes.



http://www.ask.com/question/why-does...ing-on-and-off

http://books.google.ca/books?id=Vdxw...ciency&f=false



SM: This page is not available, or you have exceeded

your limit for the day (my limit seems to be zero).



SM: I still don't see it, sorry.




A few things. In any correctly designed and installed forced air
furnace, you'd have to close down a lot more than one or two vents
to have the furnace shut down due to exceeding the high temp limit.
And in any recent vintage furnace, I don't think it's just going to
restart. More likely it's going to stay off with a fault code flashing.
If you choke off enough air flow, you also run the risk of burning out
the blower motor, depending on what kind it is.

As to the loss in efficiency, exactly how much you would lose IDK.
But you do lose efficiency if a furnace keeps going on and off instead
of running continually. For one thing, modern draft inducer models
run the blower to purge the system for like 30 secs before starting.
Whatever heat there was, it's blowing cold air through it for that period
and the heat is being vented outside with the air.
At the very least, you're losing that heat. Also, if it does recycle,
while it's sitting there cooling off from the over temp, some of that
heat is also flowing out the combustion air path, losing heat. Or being
lost to an unheated basement, etc.