Modulating boilers......................

John Ruhnke

From my experience, any boiler that is under fired has too much excess air in the flue gas. This excess air carries heat up the chimney and away from the boiler. If you modulate the gas to the boiler you are under firing it. This causes a huge waste in combustion efficiency do to the excess air traveling up the chimney. To gain in combustion efficiency I find that you almost always need to richen the fuel mixture not reduce it. I am sorry guys but I am not completly sold on modulating boilers.

I understand that in the warmer spring or fall weather when the boiler is way oversized, the short cycleing is greatly reduced in a modulating gas style boiler. I will agree that you get a fuel savings from that.

The big question is weather or not the fuel savings from the longer cycles, out ways the extra fuel costs from underfiring in the combustion process.

I predict that in two years when I am able to give a stop watch to all of the wallies we will answer questions like that and more. The DOE (Department of Energy) can't save the world as much energy as the wallies could.

With my patent pending stop watch, (Heating Efficiency Formula) we are going to save the world billions of dollars in energy in a way that the DOE will never ever be capable of doing.

JR

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LEAD PIPE

Do they

Do they adjust the intake air accordingly for low fire? When I tested mine the O2 in the flue gas as about the same for low & high fire, both around 5%

chuck shaw

yes,

they adjust the air as well as the gas. The Munchkin does it by changing the blower speed, when the blower is moving slower, it brings in less total gas mixture, and keeps the fuel to air raito the same.

Chuck Shaw
Technical Support Department
Heat Transfer Products

John Ruhnke

Forced flue gas..............

That makes sense to me when dealing with a forced flue gas,it is easy to regulate the excess air by speeding up or down the flue gas fan motor, but what about in atmospheric equipment, you can't do that can you?

I guess modulating gas makes sense in a forced combustion boiler as found in direct vent or condensing boilers. In atmospheric equipment if you drop the fuel you can't drop the air intake in the same required amount. So you end up with a lean fuel mixture and bad efficiency. Do you think modulating boilers make sense on atmospheric equipment?

JR

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Brad White_2

I agree with Chuck

that the air-fuel mixture is sacrosanct. Fan speed is an ideal way, if not a damper. Large industrial boilers (fire tube scotch marine and cast iron types) have burners with cam operated dampers to do similar work. Where flue gas output becomes a draft issue, I specify Exhausto draft inducers but that is another story beyond your question.

You are 100% correct, John, in that you cannot play with one and not the other. Atmospheric boilers? Interesting. At first I was going to say "no" but then the draft (and dilution) is a function of fuel input, so in part of theory, there may be some compensation there. In the end, sealed combustion is the only real way to control it.

The image I have of reducing flow to an atmospheric burner is the "red tip" of a propane torch at low gas flow.

RC

Hi-Lo Fire

What about high-low firing rate like on Raypak & Lochinvar atmospheric boilers?

Brad White_2

Good Question

Only a guess on my part (and has me expanding on my first message) that with true fully modulating boilers, you set your combustion ratios and check your CO, CO2 etc. at the maximum and the minimum settings. Between those, proportionality is assumed (unless you have real O2 trim in which case it is independent and more complicated).

I think your point is well taken in that fuel input and draft are related (as in my first posting). My guess is that with 2-stage burners they test and certify each stage with perhaps some compromise at one end of the other. Just a guess, I really do not know. Perhaps one of the manufacturers can chime in? Got me curious now.

Unknown

We have had

Atmospheric gas burners used when converting old coal systems that actually modulated and at the same time opened and closed an air shutter to control secondary/excess air. It has not been done in years as equipment has gone the route of cheap is better.

There are ways to control both gas and air and do it efficiently and effectively.