wrote:

I have a Heil brand gas forced hot ait furnace. It comes on & heats
up, but doesn't blow hot air in the house. The blower comes on, but
usually doesn't blow very hot air.

I had a tech come out four times. He said it is heating up to the right
temp of 140 degrees. He replaced the fan control switch, and then the
electronic thermostac. It is still doing it. Now he says that we do
not have enough vents and wants to run another return.

The problem I have with that is the furnace is 10 years old and we
lived her 4 years. It was never a problem before. Until this winter
it would get very hot. I'd be sweating at 75 degrees. Now it feels
cold and damp at 75, if it reaches that temp. I'll have it set
for 78 degrees and it will never show more than 69 or 70 at night.
During the day it gets warmer, but I think that's from the sun coming
in the windows.

Could sombody please share an opinion of what to do. It's going to
start getting colder at night here in New Jersey. Thanks.


If the air temp is 140-F leaving the supply plenum, "then it is losing
its heat before it exits the diffusers, or it is delivering very low
airflow."
Very low airflow could cause the limit switch to shut off the gas valve;
the blower would continue to run, - the low airflow would result in an
insufficient delivery of BTUH heat to the rooms.
On colder nights continuous operation would not keep up to the rate of
the heat loss!

As long as the Room TH cycles & keeps the heat & blower on, it is NOT
the source of the problem.
The room TH has nothing to do with airflow volume or heat loss in the ducts.

Is the filter clean, is the blower wheel blades free of lint or debris,
is the evaporator coil plugged with lint?

Check the ductwork & airflow system for proper performance, for help -
read all the linked pages below. - udarrell

--
PROPER A/C UNIT & DUCTWORK Sizing is over 70% of Operating Efficiency
http://www.udarrell.com/proper_cfm_b...syste ms.html

Air Conditioning Installations Now Produce 95% Failure Rate
http://www.eeba.org/conference/2004/...s/Proctor1.pdf

Solving - External Static Pressure (ESP)
http://www.udarrell.com/udarrell-air-conditioning.html