Definition of condensing boilers (revisited)

Steve Ebels

Between the two gas burners, which of them is operating at a higher efficiency and why the differnce in condensation temps? Would that have to do with excess air? This post makes me think about an old boiler I serviced a couple weeks ago. It was a steel fire tube design and made no condensation at 115* degree water temps. Flue temp was just shy of 300* if I remember correctly. This BTW, had a natural gas power burner on it. The boiler was originally coal/wood fired, converted to oil and finally Nat gas in the 60's.

kf_2

Definition of condensing boilers (revisited)

I just finished reading the post further down The Wall with this same title, and wanted to add some things that seem to have been missed.

What is a condesning boiler?
I once heard an industry proffesional reply to this question by saying "it's a boiler that condenses".

I can make just about any boiler condense, but that doesn't make it a CONDENSING boiler. I can make a boiler form condensation on the heat exchanger by lowering the water temperature below the dewpoint of the fuel. I can make condensation form in the venting by oversizing the chimmney or by somehow cooling the chimmney. Neither of these make them condensing boilers.

A condensing boiler must be able to achieve combustion efficiencies of 90% or better and have a means of exchanging the latent heat in the flue gas to the water in the vessel.

One of the main rules of thermodynamics is that heat goes to cold in any direction, and the greater the difference between the two, the faster the heat will travel to the cold.

In a condensing boiler, as the COOL return water passes by the HOT flue gasses (they are still hot at this point)the heat from the flue gas is given up to the return water, elevating the return water. Now we just raised the temperature of the boiler water without the burners help.

Think of a flat plate heat exchanger, taking heat from one side and transferring it to the other side. This is no different except that one is a water-to-water heat exchanger and the other is an air-to-water heat exchanger.

It was mentioned, in that earlier post, that the lower stack temperatures result in the higher efficiencies, when it is just the opposite. The higher efficiencies result in the lower stack temperatures. Just like the condesation is not the cause of the high efficiencies, it is the effect of them.

Flue gas condensation can start any time combustion efficiencies are above 85% I can have pretty low stack temps in a grossly oversized chimmney but with no way to transfer that heat back into the system water, its just condensation down the drain.

In response to the questions about water heaters condensing. They sometimes will. This is why most water heater manufacturers use stainless steel burners, and pilots instead of electronic ignition. It has not been a huge problem in water heaters because, as Heatboy pointed out, the CO2 levels are usually pretty low, this is becasue of an uncontrolled amount of excess air. As to to Jim's statement of higher CO2 levels I would be curious to know if that test was taken before or after the draft hood. It is possible that the air into the burner is restricted, raising the CO2, however this would raise the levels of CO to a possibly dangerous level.

SOME FACTS:

Condensing temperature of fuels:

Natural gas with atmospheric burner = 118 deg. F. @ 7% CO2

Natural gas with power burner = 136 deg. F. @ 10.5% CO2

#2 Fuel Oil = 117 deg. F. @ 13% CO2


Sorry about the Novel but I'm pretty passionate about the subject.

Thanks for listening,
kf

kf_2

Steve

Steve,

Correct, the reason is the excess air in the combustion process. You will notice that beside the dewpoint temp I have listed the CO2 level as well. As you know, if we reduced the excess air into the atmospheric burners we would raise the CO2 level. This would in turn raise the dewpint temp. However, without the ability to properly dial in the air to fuel ratio on a typical atmospheric burner (like can be done with a power burner) you would run the risk of also raising the O2 levels.

Because of this, you will see higher combustion efficiencies on power burners.


kf