Flue gas temperature comparison

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.

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.

@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.

@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.

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.

@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.

@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?

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.

We consistently run 400 gross stack temp and 10-12% CO2, with 82-85% efficiency.

But our biggest problem is low system efficiency because these boilers are so oversized.

@Bob Harper said:

That's not a true efficiency. Take the NCI course and you'll see its actually much lower. 

My understanding is that the 82-85% "efficiency" range I mentioned for our boilers is the "dry gas" efficiency which our boiler tech determines from stack temperature and CO2 concentration. From those parameters, we know how much thermal energy is being carried away by the "dry" flue gas, as a percent of the total energy in fuel oil combustion.

That "dry gas" efficiency number does not include the latent heat energy being lost as hot water vapor, which is around 7% for oil. So when we subtract that from an 82% dry gas efficiency, the total combustion efficiency is only 75%.

So for every gallon of oil I burn, 25% of its energy is lost as hot gas and water vapor up the chimney. The other 75% of the energy is absorbed by the water and the boiler metal itself.

Most of that 75% absorbed energy gets transferred to the water circulating through the boiler and gets carried out to the pipes and radiators that heat up and radiate/convect into the house envelope. But after boiler shutdown, some of the heat remains in the water inside the boiler, and in the several hundred pounds of cast iron heat exchanger. I recently installed a thermal post-purge aquastat that continues running the circulator after boiler shutdown in an attempt to extract more of that residual heat from the boiler. I don't know exactly how much residual heat remains in the boiler after post-purge, but let's say it's 5%, and let's assume that this 5% then gets lost up the flue as the boiler cools.

So we lost 25% of the energy during combustion as dry gas and vapor latent heat loss, and then we lost another 5% of residual heat from the boiler up the flue as it cooled after shutdown. So we lost 30% total, which means our overall system efficiency was 70%.

That's how I do our efficiency bookkeeping. I understand some people will split hairs over jacket losses, piping losses, etc. All I care about is how much energy stayed in the building vs. how much escaped up the flue, and since we have cold start boilers, the flue loss is simply the combustion efficiency loss of 25% (dry gas plus vapor), plus the residual heat loss up the flue after shutdown. The jacket "loss" and the piping "loss" all happen within the building envelope, so as far as I'm concerned, that is heat that is released within the building and therefore isn't a loss.

@hot_rod It seems strange to say this about a 100-year-old house with cast iron radiators and massive old gravity piping, but we're already halfway to your near-perfect hydronic system. We have so much radiation that we could heat the entire winter with SWT of 120 or less.

So if we just had your mod con boiler running ODR, we could be running 110-120 SWT all winter, condensing 100% of the time.

So right away that would take us from around 70% overall efficiency to 90%+.

If I didn't have 3 other condo owners to deal with, I might just do it!