I have a situation where I need to reduce the input to a FAU natural gas heater.
It is rated for 100k BTU input at 3.7" WC.
I read somewhere that a heater rated at 3.5" WC (100%) is reduced to 0.6" WC for 40%.
I did not find out how this is calculated... ie. pressure in inches water column vs. percent of BTU input.
I am trying for: 100% - 3.7" WC 60% - ?" WC and 40% - ?" WC. I may need to make changes in my percentages so I would like to know how the pressure is calculated based on 100% - 100k BTU.
Is there a formula for calculating this? Is it based on the orfice size in the burner?
Thanks... Bill M

By reducing the design operating gas pressures you might be setting yourself up for a big loss of efficiency. Burner tube length.orifice size,heat exchanger,air mix adjustments,and many other design parameters are involved in the proper design of a heating appliance. If you want to play with the unit to try to save energy or for any other reason I suggest you purchase or use a nice CO and temp check meter to see exactly what you are doing. All else is guess work.

Thanks for the response!
I fully realize that if the heater is NOT designed for anything OTHER than 100kBTU, that all the items you mentioned will be comprimised AND efficiency will suffer.
This is for a research project that I am doing and I will not be using this for normal operation.
I have been trying to get this information and have been rather unsuccessful that is why I am asking here as there is a wealth of information on this forum.
I would appreciate any information about this.
Thank YOU!

Another quickie...
The furnaces that do have multi stages of heat input... how is that accomplished? Is the gas pressure varied? Are the number of burners turned on and off? Is the gas valve pulse width modulated? Any other methods used?
Thanks again.

Two stage valving

So if two stage valving is used, are the pressures different for each valve? i.e.. Stage 1 valve is set at, say, 2.0" WC and stage 2 is 3.5" WC?

You should only have one valve but the gas pressure should be set with high fire (second stage) and in turn sets the pressure for first stage. Yes first stage is at a lower manifold pressure but if the pressure is to low you will mess up a good furnace in a hurry.