Does relative humidity increase efficiency in condensing boilers?

steamfitter

If a condensing boiler uses outside air for combustion, and it is very humid outside, will the boiler condense more water vapor in the flue gas when the hot water return temperature is lower than 130 F?

Hypothetically, if you have two identical condensing boilers, both with hw return temps at, let's say 100 F, and one boiler is in an environment of 95% RH, while the other is in a dryer environment, say 30% RH, will the boiler using the "high humidity" combustion air condense more at its HEX?, and therefore be more efficient while the weather remains in that state? Just curious.

My thoughts are that it would be an insignificant amount of increase in efficiency, if any at all. However, I was just thinking how the effects of a very humid climate might change the numbers regarding AFUE on larger condensing boilers.

Any thoughts on this are greatly appreciated!

Jamie Hall

No. exhaust gas will be at 100% relative humidity anyway...

steamfitter

Jamie,
Is the amount of water vapor in the flue gas related in any way to the type of fuel and/or combustion air?
100% RH of exhaust gas in condensing boilers at EWT below 130 F? does the EWT dictate the % of RH?

Gordy

No. What @Jamie Hall said. If that were the case manufactures would find a way to introduce water vapor into the intake air.

Gordy

Yes.

steamfitter

Thank you Gordy!
The chart shows the 130 F dewpoint for natural gas @ 30% excess air. Is 30 % excess air a standard? Is that why articles about condensing boilers refer to this temp as the minimum condensing temp for boilers that run on natural gas?

RayWohlfarth

The barometric pressure will change the oxygen content of the air for the burner. For example, if you set up a burner with 15% excess air at a barometric pressure of 29 and the barometric pressure drops by 1%, the oxygen percentage of the combustion air drop from15% to 11% ( A dangerous area).
Combustion air temperature also affects the oxygen percentage. For example, if we set up the burner for 15% O2 at 0 degrees outside temperature, the oxygen percentage drops to 4.6% when the temperature raises to 40 degrees F combustion air temperature which is very dangerous. The colder the air, the more oxygen is in it. Hope it helps.

bob_46

Ray, I don't believe the chemical makeup of air changes with changes in barometer or temperature. The density changes but the ratio of oxygen to nitrogen stays the same.

RayWohlfarth

Bob,
I respectfully disagree.
I do not believe the chemical makeup changes. I have found the o2 percentage inside combustion air changes according to the barometric conditions and temperatures of the combustion air. Many of the industrial boilers use an oxygen trim system which adjust the fuel to air ratio by measuring the oxygen percentage of the flue gases. If the oxygen percentage stayed the same, they would simply be able to use a modulating burner and get the same results. Thanks for the feedback.
Ray

bob_46

Oxy-trim measures flue gas o2 not combustion air o2.

Gordy

At a given altitude the % of oxygen in combustion air does not change with barometric pressure. As been said what changes is density, and pressure.
The thread is actually about humidity, and it's effects on efficiency in combustion air.

Gordy

A simple gas analyzer will read 20.9% O2 all day everyday no matter the barometric pressure, or temperature at a given altitude.

Jamie Hall

It's helpful as has been said to wander off the wall from time to time -- in this case to aviation, and the bad old days of piston engines. Every pilot learned -- or should have learned -- about density altitude and the effect of humidity on engines.

Density altitude is related to your physical altitude (how high you are above sea level) and the temperature (the warmer it is, the higher the density altitude is. Pilots worry about density altitude, because the higher your density altitude the less power a normally aspirated piston engine will produce -- and the less power you have the longer your takeoff run and the slower your rate of climb once you do get off the ground, and nothing ruins your day faster than running out of either runway or altitude.

You also learn that as the density altitude increases, the less fuel your engine can burn -- so you lean the mixture (only you really don't; what you are doing is reducing the fuel flow rate for a given throttle setting) back to maximum power. Why less fuel? It's not that the fraction of oxygen in the air changes -- it doesn't -- but that there simply is less air getting into the engine.

Humidity does have a small effect, too -- on very humid days, there is a change in the air composition (the water vapour displaces some of the air, so the effective oxygen concentration is slightly less). Again, lean -- but this time, only slightly.

Now on a boiler we don't usually worry about altitude changes -- but we do worry about intake air temperature changes and barometric pressure changes. Unless they are extreme, however, they won't affect the efficiency of combustion much. They will have a small effect, as the fuel flow is presumably pretty constant, so on a hot day with low barometric pressure, the mixture will be slightly rich. Conversely, on a cold day with high pressure, it will be slightly lean.

Gordy

@Jamie Hall you must have your pilots license. I took flying lessons a long time ago until I figured out the expense out weighed my thirst for flight

Jamie Hall

Gordy said:

@Jamie Hall you must have your pilots license. I took flying lessons a long time ago until I figured out the expense out weighed my thirst for flight

I do -- ASELI. And until recently I had a lovely Piper Arrow, but had to give it up -- I don't really see quite well enough to be as sharp as I'd like to be any more. There is nothing as satisfying as flying, and once you have flown you will always want to again.

As you say, though, it doesn't come cheap!

Gordy

My point is it is a minimal variance. Not enough to really effect anything substantially.

It's not like we are moving the boiler from installed altitude to 10000 feet. We would be dealing strictly with barometric pressure, and temperature.

Normal barometric pressure is 29 bar. Rarely does it go up or down more than 1" unless we have some wild super Storm.

With that in mind read this chart.

RayWohlfarth

Gentlemen Perhaps I am saying it wrong and I definitely do not have the concentration to be a pilot. My concern is I was taught the best time to perform a combustion analysis is during the coldest time of the year. This allows you to operate the flame for 15 minutes prior to adjustment of the fuel to air ratio. This allows the flame to stabilize. What concerns me is the fuel to air ratio changes with the combustion air temperature. If you set up your burner using cold outside air, the fuel to air ratio will change when the air warms. It could become dangerous. I believe a boiler using outside vented combustion air should have the fuel to ratio tested regularly.

Gordy

@RayWohlfarth I'm having a hard time following this. What about people that live where winter temps can vary from -25 to 45, or worse. If you do a combustion test at -25, or 45 isn't any deviation from intake air temperature the same? I understand getting a stable flame.

Zman

The OP has an interesting question about condensing boilers and humidity. My understanding of this is that the energy recovered in a condensing boiler is from the condensate that is a byproduct of combustion which would normally go up the flue. My gut on this is that adding moisture will not make the boiler more efficient.

The discussion went down another path about the temp of the combustion air and it's relationship to efficiency. This one always seems like a double edge sword. Sure the boiler will burn more efficiently on a zero degree day if you feed it 50 degree air. The question is what does it cost to get all that zero degree air up to 50 degrees? The notion that some folks have that pulling air out of the conditioned space is a good idea is, in my opinion absurd. All that does is randomly make the building pressure negative to atmosphere whenever the boiler fires. That's just bad building science. It can also lead to inconsistent combustion, everything changes when someone leaves a window open....

RayWohlfarth

I have searched all over the place for this reference. It was on my old Pc that died. When I was doing research for a book, I found this and it changed the way I thought about how temperature and barometric conditions affect combustion. It is by Johnston Boiler.

Gordy

Interesting however the lowest the table goes is 40 degrees. I would be inclined to believe that any heating appliances intending to use outside air would take into consideration these variances.

This is intriguing since the temperature of outside combustion air can vary wildly not only with outdoor temps, but intake vent length. Shorter intake length would not warm intake air as much as a longer intake length, and even indoor air can be used.

I also find it intriguing the initial settings in the table used 80 degree air temp as a base line. How often is that the case? Even if indoor air were used.

RayWohlfarth

Gordy,
I think it is based on 80 degrees F because these boilers are usually inside a hot boiler room. It really made me think about using outside air for combustion.

Zman

I have to assume those numbers are for atmospheric boilers?
I have never seen nor can I imagine those kinds of number variations where you have a power burner in control.

bob_46

Ray, "The barometric pressure will change the oxygen content of the air for the burner". This sentence is why I jumped on you and either you misspoke or I misinterpreted this sentence. Everything else you posted including the chart makes perfect sense and is right on.

RayWohlfarth

Zman they only sell boilers with power burners
Bob Thanks It is kinda scary how it changes I like to be challenged It makes me better