BTU vs Jet Size

Michael Stoic

How do the manufacturers get different BTUs out of similar sized furnaces? The boxes look to be the same with different ratings, like 90K, 100K, 110K, etc.

On my 100K BTU Bryant, I noticed that the gas jets that supply the gas from the manifold into each burner are marked with a #42. Will #41 or #40 jets in the same furnace bring the BTUs down? It seems that they will, but I am interested in how to calculate this.

Maybe the burners themselves are different (shorter or with smaller fin clearances), I don’t know. I can see how it could be that a smaller jet with a smaller burner will produce a smaller flame and a smaller overall rating, but on their own, the smaller jet in place of a larger one should also bring the output down. I want to know if it’s possible to calculate this. Perhaps a table exists where for a given furnace chassis (assuming a constant diameter for the supply manifold) the output correlated to different jets has been calculated.

The logic seems similar to that behind having different jet sizes available in carburetors, where different jets can be installed to produce varying degrees of fuel delivery and power output. I am aware that there comes a point where to further increase a jet size will require an additional supply of air and an increased flow for the exhaust, but many systems will have a range within which close-by jet sizes can be accommodated without requiring other modifications.

There are several reasons I am asking these questions:

With the type of third rate installation and maintenance of the HVAC equipment I am discovering in my recently purchased house, I would not be surprised to discover that the furnace was selected on a fat rule of thumb. Also, I am now adding insulation to the outside walls and will install new doors and windows. I am going to do the heat loss calculations and suspect the existing boiler might turn out to be oversized. It would be just so satisfying at that point to swap in the four smaller jets and bring the furnace output down. Whether this is possible or not, I like the challenge of considering this hypothesis. I go through many such exercises when helping someone diagnose some performance problem related to their vintage motorcycle (I run honda305.com) so this is a natural sort of thing to consider for me. And finally, I ask because I am curious.