Under-fired furnace

[ajz]

My old Clare-Hecla furnace with a single large burner is under-fired. It's rating is 112kBTU but I clocked the gas meter and got 93kBTU. Then I measured the manifold pressure and it is 2.8" WC. Temperature rise is about 80-90F.
Does under-firing a furnace have any negative consequences on the operation of the furnace?

[amickracing]

Yup, it can and usually is just as bad for the heat exchanger as over firing. Plus condensing in the venting, not running as effciently as it could be is another problem.

That being said I'm not 100% sure of the effects of under/over firing a furnace that's older like the model you have.

If you're getting that much temp rise it sounds like they jacked with the fan settings too.

[Edited by amickracing on 01-24-2006 at 09:53 PM]

[Jultzya]

Originally posted by amickracing
If you're getting that much temp rise it sounds like they jacked with the fan settings too.

It's not uncommon for older units to have 70-100 Delta T specs.