Firing Oil Furance Below Spec - OK?

[calhome]

Am replacing an old oil furnace with a new Bryant. New units come in 2 sizes (each with three firing rates). Luck would have it that my required size of 90K btu output falls smack in the middle of the two (smaller unit max output is 85K, larger unit min output is 97K). My contrator recommends going with the larger unit -- says it can be fired below 97K output by putting in a smaller nozzle (.75 gph for 85K) if necessary (the Beckett burner is the same in the big and small unit). I'd appreciate any insight re the pros/cons of doing so if the installation at 97K is oversized. No a/c involved, just heat in a mild climate. Am also replacing some v.old ductwork, so will be picking up efficiency enroute from the basement to the registers.

[beenthere]

Use the larger furnace, and fire it at its minimum, not below.
You'll be oversized by less then 7%, not a big deal.
If you underfire it, you could cause condensation in the chimney.

Have you improved your weatherization, are there any doors, or windows you could change to improve your air infiltration.
Can you add insulation to your attice.

If so, then the smaller furnace could be used, and statify the stat at your outdoor design, since its only 5% under your posted load.

[casturbo]

Downfiring an oil furnace a little bit is no problem whatsoever. It brings the efficiency up alittle bit too.

[wallynut]

This burner can be down fired with no problem but check
manufacturer specs because i think the turbulator might
require an F-3 Head and also baffle.

[tyreman]

Read the minimum nozzle specifications on the rating plate.
start there.
do a stack temperature test to ascertain its not to cool a stack temperature.
remember to subtract basement air temperature if a test with a thermometer like a bacharach manual insertion stack thermometer is used thats then the net stack temperature.
Some people have even gone to putting a different retention end cone for a next firing rate lower if so applicible to the burner and hopping up the system but this NOT recommended unless a technician decides to do this at his own accord on his unit.
modifying a supplied package violates in all likelyhood a/any future warranty.

[wallynut]

"modifying a supplied package violates in all likelyhood a/any future warranty."
This is not true,ask manufacturer rep.
I have had this situation before with Beckett Burner.

[tyreman]

Oh okay..in Canada here it wasn't looked to great on at one time but since I am retired it may have changed.

[beenthere]

You posted that the smaller furnaces max output is 85000, and the larger furnace with a .75 nozzle is 85000, so its the smae amount of heat, no difference.

Use the larger model, and down go below manufacturers ratings, it can cause chimney problems down the road 5 to 10 years.

[Al Gregory]

I have been repairing and installing oil burners for 25 years. Downfiring a furnace one size will not harm it. It will probably increase effeciency and the furnace will probably last much longer. It will also run quieter too.

[beenthere]

Been down firing them for years also.

He's asking about down firng a multi fire rate furnace, lower then its recommended down firing, usually the stack runs alittle too cool for the chimneys benifit on these that I have done.

Can only slow the blower down so much.