I have a 3200 sqft house with a 117k BTU furnace. The 3 kids bedrooms
are at the ends of the trunk line (usually with 9-10 branches before
them) and always seem to be 1-3 degrees colder then the thermostat.
This 1-3 degree difference was only achieved by closing almost ALL of
the dampers 1/2 way. My concern is that I have too many of the
dampers closed off too much and was concerned that I would be hurting
the system. My thought was to add a duct booster fan to the baby's
room (and possibly the other 2 kids rooms) so that I could open up the
other dampers more. At the very least, I want the baby's room to be
the same temperature as the main living area.
I agree, it is better to improve flow by lowering resistance than it is by
increasing resistance. That said you may be able to compensate by bumping
up the fan speed. There is a temp range you need to stay inside of. You
should be able to find a sealed hole a little ways down from the system
where the installer put a temp meter to check. If you're going to fool
around with it much you should find out the range it is supposed to be in
and check it. You can cause big problems by straying outside that range.
How big is the main line? Is it solid walled? (sheet metal/duct board or a
bigger piece of flex?) What size flex is running from the main line? What
is the R of the lines?
Sometimes you can improve flow by upping to 8 inch flex on 6 inch runs that
have problems. You can put an 8 to 6 reducer at the vent but you do want to
change the takeoff to 8 inch. Lowers the resistance.
Also long runs can lose a lot of heat. Switch to r6 if you have r4 at the
same time you go to 8". Wrap the main line if it is solid walled.
A lot of people consider those booster fans to be a crutch for a badly setup
or installed system.
You an also call your service guy and see what suggestions he has.
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