Modulation of boiler fire...good or bad?

knkreb

So in one of my other discussions, it was brought up that a boiler runs best on high fire and will be most efficient. 

That particular boiler that I was discussing was a Bryan RV boiler oil fired hot water. 

So, is high fire best?  These boilers are oversized like crazy. They cycle on low fire in the coldest of weather. 

My thinking is it best to run at low fire and run a constant rate to satisfy the load. I'm thinking steady running state is happiest for everything...kinda like cruise control in your car. 

I have another site that has a gas fired Cleaver Brooks CB200. It's a great running boiler but it has a lot of it's load striped out in the last few years. It lives on pretty much low fire and maintains a constant steam pressure. At 48 years of service, it runs well. 

Cycling to me seems harder on the equipment. Let me know your thoughts. 

hot_rod

What do you mean by runs best?

Steady state operation will be the most fuel efficient running condition

The design of the boiler HX has a lot to do with operating conditions

A mod con boiler for example runs more efficient the lower the return temperature. A oversized mod con on low fire can be very efficient as you have a lot of HX surface area to condense. Viessmann has some excellent training on condensing boiler operation.

All boilers run more efficiently at low operating temperatures, but conventional non cons need to run above condensing temperatures and adequate flue temperature to prevent problems

knkreb

@hot_rod
So, this thread comes from another thread that I had posted about minimum stack temperature.

https://forum.heatinghelp.com/discussion/185715/min-stack-temp-for-oil-fired-boilers#latest

@captainco had said that he had a system at a school that he recommended run on high fire, with fuel savings. This was not making sense in my head, so I wanted to learn more. I'm talking about conventional boilers, rather than the new condensing gas type.

ScottSecor

I am referring to modulating commercial burners (oil and natural gas) mounted to non condensing boilers.  Running the boiler (if single) at low firing rates for extended periods of time works best.  With multiple boilers (often lead-lag) the same holds true.  You have to be careful of minimum stack temp, CO2 levels, smoke levels with oil, flue gas condensation with oil or gas, etc.  On most systems we work on, the minimum firing rate is likely double or triple the required firing rate needed on a moderate day.  As such, the burner will still cycle on warmer days.

To add to @hot_rod comments,  steady state works best from my experience.   At steady state, there's no losses from pre and post purge, heated surfaces are already warm, less trials for ignition,  stack is already warm, etc.  Perhaps most importantly is with a well maintained building comfort can also operate at a steady state.