[snupytcb]

as long as everything tested out fine and ignition is smooth everytime I don't think it will hurt. certain conditions just require certain set ups. I have wondered too.

 

[snupytcb]

Usually pressure min is 3.2.

Adjusting below that even with lowering blower speed/CFM can cause condensation in the heat exchanger.

Trane has a bulletin about adjusting manifold pressure outside of factory spec.

what if it was oversized quite a bit, like by 40kbtu? could the temp rise drop too much if ductwork is in good shape?

 

[snupytcb]

Yes.

Plus, if you need a 60,000 BTU input. And have a 100,000 BTU input. The inducer is still drawing air in for 100,000 BTUs of input when you reduce gas pressure so its only 60,000.

I see. what about a natural draft, no assist? would the flue temp consistently be too low?