Condensing boilers above sea level

Mike Kusiak

I have read that the condensing temperature (dew point) of the flue gas of natural gas burned with zero excess air is 135 F. What is the condensing temp with the usual 50% or so excess air normally found in an atmospheric gas burner? I think this temp would be more relevant to the minimum return temp of a cast iron boiler necessary to prevent condensation on the heat exchanger. I also wonder the condensing temp of the flue gas after the draft diverter, after it has been diluted with another 100% or so of air. I would seem that the condensing temp would be far lower than the usually mentioned 135F after all this dilution. Does anyone know where I can find this info?

Jay Stein

Condensing Boilers Above Sea Level

Did you know that elevation affects the temperature at which a condensing boiler will condense flue gas water vapor? Turns out that at sea level, the temperature at which condensate will form on a boiler heat exchanger is 135 degrees F. At 6000 feet the condensing temperature is 126 degrees F. That's no problem if you plan for a lower return water temperature at higher elevation, but I wonder how many designers do so. When researching this issue, most of the boiler manufacturers I spoke with told me that elevation had no effect on condensing temperature. The equations in ASHRAE fundamentals are unequivocal however: the higher the elevation, the lower the condensing temperature. Are hydronic designers aware of this effect? Do they properly compensate for it at higher elevation? I'd be interested in hearing from some folks in the know.

kf_2

Not many that...

I've seen do. The other thing that they don't take into consideration is that the 135 degree temp is lower at anything less than a 10.5% CO2. Not alot of burners can achieve this.

kf

Mike Kusiak

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Unknown

Mike

As a boiler manufacturer, we introduce approximately 50% excess air though the boiler passageways and draft hood. The dew point of a boiler burning natural gas is lowered to approximately 127°F by doing so while still maintaining an average of about 7% CO2. Here is a chart showing differing dillution air ratios with different fuels. As far as the question regarding the dew point changing at different altitudes, I believe that is already figured into the equipment by taking into effect the different jetting that is done for high altitude conditions. Our products are already factory set and identified as such for high altitude conditions. Hope this helps.

Glenn Stanton

Burnham Hydronics

[Deleted User]

Conflicting statements...

Does everyone remember the hackles that were raised by P&M magazines article on Plain Vanilla Systems?

http://www.pmmag.com/pm/cda/articleinformation/features/bnp__features__item/0,2379,64795,00.html

Their article stated that the operation of boilers at high altitudes suppressed formation of condensate. According to the Doc here, it enhances it to a lowered temperature. It could be semantics and interpretations too...

Let's face it, EVERY boiler is a condensing boiler at one point in time or another. It's just a question of how long it condenses for, and whether it is designed to condens or not.

I see the Doc's point though. If you plan on a condensing situation, and it doesn't, was it worth the additional investment?

I also think this opens up a whole new can of worms as it relates to overall thermal efficiency of certain appliances. Care to comment CaptainCO???

Good to see you again Doctor Jay. Stick around and teach us something, will ya??

ME

Mike Kusiak

Thank you Glenn, for the information and chart on dew point vs temperature. This is exactly what I was looking for.


I took a few measurements on a Burnham 205 and found an excess oxygen reading of about 7% to 7.5% before the draft hood, which closely corresponds to the 50% excess air design spec. I then measured the oxygen level after the draft hood and found about 15% O2. This appears to indicate that another 100% additional air is introduced by the draft hood. This would seem to be confirmed by the flue gas temperature before the hood of 475 F and a stack temp of 260 F after the hood. I realize that these measurements are strongly dependent on draft conditions and venting and may not be typical.

My conclusion from all this is that while condensation would occur on the heat exchanger at 127 F, condensation would not occur in the vent pipe or chimney until about 110 F. Is this correct, or am I missing something in the analysis?

By the way, were you associated with Burnham when they were located in Irvington, NY? I live about a mile away from the old plant on the Hudson. I worked for a while for a company which rented space from Burnham, in a building that was described to me as the old boiler testing lab.

Don Seymour

never mind

Mike Kusiak

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Unknown

MIke

We are basicly trying to establish a better environment within the flue passageways that will be less conductive to condensation while at the same time maintaining a near constant CO2 level in the flue byproducts. The excess air also helps draft somewhat but that will always be a varying factor based on chimney conditions, draft, available combustion air, etc. I have been with the company for 4-1/2 years now. I was affiliated with Burnham for another 18 years before that at the distributor sales level. I believe that the move from Irvington to Lancaster occured shortly after the company purchased American Standard boiler back in the mid to late 1970's. Hope this helps.

Glenn