Does slightly underfire=longer cycle=save money?

Patchogue Phil_2

Wondering about squeezing the last usable BTU dollars out of a oil fired boiler.



My own not very efficient boiler,  New Yorker S-118 AP with a Beckett burner.  At 140psi calls for a 0.85 gph 80*B nozzle  I believe is effectively close to the 1.0gph factory rating at 100psi???  The S-131-AP calls for 1.0gph nozzle,  with exact same physical dimensions of the S-118.



If the heating load requirement is somewhere midpoint of those two nozzles,  would the smaller nozzle give more usable heat per oil dollar?    I'm thinking that the larger nozzle would waste/send more heat out the flue.  And the smaller nozzle would have longer burn times,  longer cycles which is supposed to be more efficient too,  right?

Mike Kusiak_2

Underfiring

In general, reducing the firing rate within reason will increase the combustion efficiency of the boiler. This intuitively makes sense,  because while the surface area of the boiler heat exchanger remains constant, the BTU input is reduced and therefore relatively more heat is extracted from the fuel burned compared to the heat up the flue. As you mentioned, the lower stack temperature indicates more heat is being captured by the exchanger and proportionally less goes up the chimney. This assumes that the other combustion parameters remain constant, excess air, CO2, etc. In addition,  longer firing cycles, occuring less often should also contribute to increased efficiency.





There is a limit on how low you can fire to avoid condensation problems in the flue and chimney. In general, you probably should not go below about 350F stack temp to avoid trouble. Really, the best way to confirm proper operation at any firing rate is to set the boiler up with a good combustion analyzer, making sure all combustion parameters are within spec.

Brad White

In principle, yes

But of course any lower firing rate has to take stack temperature into consideration and that you can maintain space temperature, of course.



But provided you are within the stack and space temperature parameters, the difference between your previously higher firing rate and your lower working firing rate, is energy money in the bank.