On Dec 2, 1:31*am, Deodiaus wrote:
Yes, it is a gas furnace. *There are 4 such units for the house.
Yes, it is a big house, but the cost of living here is cheap. *I am
out of work for a long while, so really tight with money. *We bought
the house when we both were working and got a great deal on it.
I checked the vents and they all seem open and unobstructed (ceiling
vents). *I can feel a bit of air coming out, but don't know if that is
enough.
I changed the air filters.
The room that is problematic is downstairs.
I swapped out the high limit switch with another one, just to make
sure that the one there was not faulty.
The other one (from an upstairs unit) is
Texas Instruments
pat no 5144273
1NT17L-1875
L160F-30
025-29041-004
9413

It shuts off 10 deg F colder.

Maybe I should try to find one that cuts off at 180F

I watched the unit with the cover off.
It seems to work fine just fine for the first 10 mins.
The furnace turns off after 5 min, presumably when the high limit
switch protects the unit from over heating.
It blows air for about 2 mins, and reignites and heats up and shuts
down.
It seems to operate, but its just that after a while it stops heating
up.
I have the thermostat set at 74F, while as it only gets up to 68F.
It seems to behave as if had reached the desired temp.
Maybe I should check to see if there is still a call for heat by
seeing if there is current across the C-Y leads [call for heat by the
downstairs thermostat.

You can also test by simply maxing the thermostat and then observing
the unit. I presume it has previously worked and this is a new
problem. It is possible that the air flow is too low. Most units do
not shutdown the fan when there is an overtemp situation, they just
shut down the gas. You can also meter the overtemp, most commonly
they are in series with the controls and open when there is a fault.
That means you will measure no voltage across them when the unit is
operating normally and there will be voltage across them when they
have tripped. You need to look at the schemantic to see how they are
wired. The most common wiring is also in series with the flame
rollout switch if there is one so you want to make sure that is not
the problem. You should find a schematic inside the unit. As others
have mentioned the airflow needs to be correct as well. If the
blower is starting up ok then about the only mechanical thing to check
is the blower wheel, make sure it is not loose on the blower mtor.
Usually they rattle when thisis the case but you can usually reach in
and check that with the blower compartment cover off. Confirming the
correct airflow is best done with a thermometer in the outlet side.
The paperwork with the unit will give you a temp range that it should
be inside of. You may find a hole taped over where the installation
tech checked it. If it is too hot and you have confirmed that the
blower wheel is ok and there are no obstruction in the ductwork you
can change the fan speed, usually at the control board by plugging a
different blower lead into it. But the fan speed should ahve been set
properly on installation and there is normally no reason to change
it. If you don't have a meter or you're not confortable following
schematics then it might be time to call the service guy.