[lcn96]

Am installing a comfort aire 75000 btu high efficiency gas furnace installed in the horizontal position
Installation is complete and done according to manual and codes.
Used 2" exhaust and intake with minimal elbows and length. Have 2 10" returns and 5- 6" supply. the problem is that the rollout and limits keep opening after a short period I have tried speeding up the fan, checked all ductwork for flow. The flame in the burner compartment seems to be large, erratic, and close to the limit switches. This is a new gas service and I have not yet verified the gas pressure, that will be my next step.

Any other suggestions would be greatly appreciated.

LCN

[mchughpd]

ck all your sizings, 75,000 btu requires more than what i see here

[tony_pullen23@yahoo.com]

is this natural or lp gas? do you have the correct jet installed?
also five 6" ducts can only move about 560 cfm and that isn't nearly enough air flow. most likely your problem. why do you have two 10" returns and only 5-6inch ducts? is this ten inch duct or ten inch grills?

[lenbernhardt]

Originally Posted by lcn96

Am installing a comfort aire 75000 btu high efficiency gas furnace installed in the horizontal position
Installation is complete and done according to manual and codes.
Used 2" exhaust and intake with minimal elbows and length. Have 2 10" returns and 5- 6" supply. the problem is that the rollout and limits keep opening after a short period I have tried speeding up the fan, checked all ductwork for flow. The flame in the burner compartment seems to be large, erratic, and close to the limit switches. This is a new gas service and I have not yet verified the gas pressure, that will be my next step.

Any other suggestions would be greatly appreciated.

LCN

Well. A 75000 Btu gas furnace is more than likely equipped with a 3 ton a/c blower which requires 1200 CFM. 5- 6" Supplies and 2-10" returns will only handle around 800 (just a guess) Also 75000 btu should be vented with a minimum 3" exhaust. Sounds to me like you have too much furnace and not enough duct.

[lenbernhardt]

Originally Posted by lenbernhardt

Well. A 75000 Btu gas furnace is more than likely equipped with a 3 ton a/c blower which requires 1200 CFM. 5- 6" Supplies and 2-10" returns will only handle around 800 (just a guess) Also 75000 btu should be vented with a minimum 3" exhaust. Sounds to me like you have too much furnace and not enough duct.

First thing I would do is remove the exhaust vent pipe and watch your flame.