Flue gas temperature comparison
Kaos just posted about a Weil McLain CGI 4 gas boiler that's condensing flue gases. This is a subject I'm trying to understand better, because I've been warned repeatedly here that our WGO-5 oil boilers starting cold and running low water temps are in danger of condensing, yet they've run for almost 30 years with no signs of condensing. So I'm trying to understand why Kaos's boiler is condensing and ours are not.
The condensing boiler he's troubleshooting is described here:
Weil McLain CGI 4 cast iron boiler
Natural gas, 90 MBH input
Natural draft (no forced combustion air)
120 degree flue gas temp near boiler (after 20 minutes run time)
Our boilers:
Weil McLain WGO-5 cast iron boilers
Oil fired, 165 MBH input
Forced draft combustion
400 degree flue gas temp near boiler
What jumps out at me when comparing these two boilers is the vast difference in flue gas temp. Our flue gas temp is always around 400 degrees near the boilers regardless of return water temp, which is why I keep saying it's impossible for our boilers to be condensing, and the roughly 30 years during which they've been operating this way (without any evidence of condensing) bears this out.
By contrast, the CGI 4 boiler mentioned above by Kaos has 120 degree (!) flue gas temp near the boiler, so no wonder it's condensing. So why is the flue gas temp so much lower on this boiler than on our WGO-5? One obvious difference: it's a natural draft furnace, as opposed to our forced combustion air furnace.
I don't know the physics of the CGI 4 boiler design, but I'm assuming the much lower flue gas temperature is at least partly a result of the natural draft design. Does the natural draft design entrain a larger volume of air in order to create the buoyancy effect needed to create the draft? That would help explain in part why the flue gas temperature is so low. If it's marginal to start with because of air dilution, then I can understand how a low water return temperature could push it "over the edge" and cause condensing.
If so, these two boilers have two completely different combustion regimes with different physics. Our WGO-5's have forced combustion air, with a relatively small volume of air being injected at high speed into the combustion chamber. As such, there's no dilution from excess air until downstream of the barometric damper, and even after that the flue gas is still quite hot. By contrast, does the CGI 4 boiler with a natural draft ingest a larger volume of air at slower speed, thereby diluting the flue gas temperature and making it more prone to condensing?
Comments
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As GroundUp Posted:
Sounds like it's over-radiated and needs either a bypass or p/s because the thermostat is being satisfied before the boiler can reach temp
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Our WGO-5's are over-radiated as well. Our water temp starts at 65 and hits maybe 120-130 at the end of the burn, but the boilers don't condense.
So my question still stands: why does the CGI 4 have such a low flue temperature that a low return water temp can make it condense, while our WGO-5's (which are similarly over-radiated) have 400 degree flue temps even with low return water temp?
I suspect it may be a result of the difference between natural draft combustion and induced draft combustion but would like to hear from someone who knows the answer.
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You are looking at 30 years of small incremental efficiency improvements. Higher efficiency means colder exhaust, no way around it.
Something to keep in mind if you ever need to replace the existing units, you'll probably run into similar issues.
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I don't follow you. Our existing boilers are 86% AFUE rated, running at 82% actual efficiency. Your CGI 4 boiler is 84% AFUE. So basically the same efficiency. But your boiler is condensing and mine isn't, with drastically different flue temps.
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You are looking at two different fuels. they burn differently and have different amounts of water vapor in the byproducts of combustion. Another factor is the exhaust temperature from one appliance is 120° and the exhaust temperature of the other appliance is over 400°
And the WM CGi does have a combustion air fan on the exhaust side of the heat exchanger. So both are forced combustion air systems.
Condensation has to do with the due point of the gas. combustion byproducts have a high water vapor content as they exit the flame. when the flame is over 1000° that water is a vapor. as the heat is absorbed form the flue gas as it passes over the colder water inside the cast iron heat exchanger, the flue gas temperature drops. When that temperature gets too low, the gasses reach the due point, then the water vapor condenses. Your Oil Boiler may have some of that problem, but you flame starts out hotter and the exhaust does not drop to 120°.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan said:
Another factor is the exhaust temperature from one appliance is 120° and the exhaust temperature of the other appliance is over 400°
This is what I'm trying to understand, why the exhaust temps are so different. That seems to be the crux of why the other boiler is condensing and mine is not. Even though my flue gas has less vapor, no doubt it would be condensing too if my flue gas temp was only 120.
And the WM CGi does have a combustion air fan on the exhaust side of the heat exchanger. So both are forced combustion air systems.
Thanks, I didn't realize that. I thought "natural draft" meant there was no fan involved.
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@jesmed1, Natural gas flames max out at about 1800°F. while Fuel oil flames max out at about 2500°. I don't know where I know that from but it sticks in my mind from long ago. Adding excess air in order to keep the flame from turning into a sooty mess will result in a lower temperature flame. Since Fuel oil has a metered air supply from the combustion fan adjustment, the excess air can be kept to a minimum. Natural draft and fan assisted draft gas burners seem to add more excess air by design. Power gas burners are a little hotter flame. But your are comparing the CGi and the WGO boilers so that is what I going to go with.
The CGi flame with all the excess air, may get to be 1200 degrees. At 80% combustion efficiency that would make the exhaust temperature about 240°
A fuel oil flame from a Beckett AFG burner properly set up with a zero smoke may get to be 2000° At 80% that would leave an exhaust temperature of 400°. That is the long and short of it.
1200° flame temperature in, minus (-) 960° absorbed by the boiler = 240° stack temperature out. that is 80% efficient combustion.
2000° flame temperature in, minus (-) 1600° absorbed by the boiler = 400° stack temperature out. That is also 80% combustion efficiency.
Does that help with understanding the difference?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan Thanks for the detailed explanation. That makes sense. So basically the lower flue gas temp of the natural gas burner with more excess air, along with higher vapor content, makes it easier to condense with lower return water temps than the flue gas from oil burners.
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Looking up the flame temperatures for different fuels does not look like there is much of a difference between natural gas and kerosene
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Ever get wet paper doing a smoke test? Forget the boiler, what does the flue pipe and chimney look like?
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OK, thanks for that reference. So if the flame temperature is roughly similar, there must be another factor causing such a low flue gas temperature in the problem boiler that @Kaos is troubleshooting. Maybe it's excess air dilution?
Another interesting factor is the effect of water vapor % in the flue gas. The following graph nicely shows the dew point of flue gas vs. water vapor content. Flue gas from a natural gas boiler has about 15% vapor and a dew point of around 130 degrees. But flue gas from an oil boiler has only about 7% vapor, so the dew point is lower, closer to 100 degrees.
That still doesn't explain why the flue gas temp in the natural gas boiler is so low. Another factor is the different molecular makeup of a natural gas boiler's flue gas vs. the oil boiler. My oil boiler can be 84% efficient at close to 400 degree flue gas temp, but a natural gas boiler has to be closer to 330 degrees flue temp to be 84% efficient, according to the graph in this link:
So apparently the natural gas boiler needs a lower flue gas temperature than an oil boiler for the same % efficiency. That makes sense, because with more energy being lost to vapor latent heat, the dry gas temp has to be lower in order to extract the same overall % of total energy.
That's another piece of the puzzle, but it still doesn't explain why the flue gas in Kaos' 84% natural gas boiler is going from 330 degrees to 120 degrees, while my 84% oil boiler's flue gas stays at 400 degrees, even with the same low temperature return water.
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I'm just a homeowner who hasn't done the combustion analysis myself, but the boiler techs who adjust our boilers have never mentioned a problem. And the flue pipe and chimney look fine. We recently had the chimney inspected for a possible gas burner conversion (which we decided against) and the chimney guy said it was in good condition.
Also, we have an internal chimney that is not being cooled by outside air until the chimney gets up into the vented attic above two floors and a basement. So the chimney itself is less prone to condensing than an exterior chimney.
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Like Ed said, there's a difference in the fuels. Also the type of heat emitters and the water volume in the piping makes a huge difference. An oil fired boiler heating baseboards shouldn't ever see any issues from condensation. But any boiler that heats a high water volume cast iron radiator system is going to need some type of boiler protection.
Also your WGO might already have some sort of boiler protection built in if it's equipped with a modern aquastat like the Hydrolevel 3250, this aquastat will not run the circulator when the boiler water is at condensing temperature. It might also have a triple aquastat for this same reason.
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@SuperTech said:
But any boiler that heats a high water volume cast iron radiator system is going to need some type of boiler protection.
I wish I had a dollar for every time someone on this forum told me that.
And yet, after 30 years of operation, from cold start and ending with 120-140 degree supply water temps, with 150 gallons of water in each system and cast iron radiators, these two WGO-5's are doing fine with no bypass, no low-limit aquastat, nothing. Based on 30 years of oil consumption, I figure they have run close to 15,000 hours each, nearly all of which were under cold-water conditions that everyone says should have destroyed the boilers.
Now, if I were installing the same boilers new, yes, I would pipe a bypass in exactly like the manual says. But when I keep asking the pros here "why are these WGO-5's magically defying the laws of physics by not condensing when everyone says they should be condensing?" no one has a good answer.
Also your WGO might already have some sort of boiler protection built in if it's equipped with a modern aquastat like the Hydrolevel 3250, this aquastat will not run the circulator when the boiler water is at condensing temperature. It might also have a triple aquastat for this same reason.
Like I said above, there is no aquastat protection, no bypass, nothing. The boilers have run approximately 15,000 hours each with practically stone-cold return water. And they're fine.
Believe me, I wish they would just die so we had a good reason to get smaller, properly sized boilers with better system efficiency. But these things just refuse to croak. And they refuse to condense.
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What @EdTheHeaterMan said. Gas has a large amount of water vapor in it. Oil does not.
On a cold day an 80% gas boiler you will see it condensing as it exits the chimney. Not with oil.
This was the reason (before condensing boilers) that gas can never = the combustion efficiency of oil. When gas burns the water vapor it contains has to be turned to steam (970btu/lb of water) it goes up the chimney and that extra heat is lost as it condenses outside in the air.
The old rule of thumb with non condensing boilers it that the minimum flue temp must be at least 100 degrees over the steam, water or air temp depending on weather it is steam hot water or warm air
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@EBEBRATT-Ed said:
The old rule of thumb with non condensing boilers it that the minimum flue temp must be at least 100 degrees over the steam, water or air temp depending on weather it is steam hot water or warm air.
Interesting, I had not come across that bit of wisdom. So are you saying that, for a hot water boiler, if the boiler water temp was, say, 120 degrees, the rule of thumb was that the flue gas temp should be a minimum of 220?
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Before we repipe anything, we need to see if that boiler is under-fired.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
I guess that could help explain why the CGI 4 is condensing, if it's under-fired?
We've downfired our WGO-5's to 80% of rated input, which is as low as Weil McLain would approve, but we still have 400 degree flue temps.
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