Flue gas condensation

[Canuk]

Hello folks, a newby to the site here with a question about flue gas condensation.

I am hoping someone can help explain the effects of flue gas condensation to me such that I can both understand it and determine if my system is being effected by it.

Briefly, I have indoor Kerr boiler in my shop heating the infloor of both my shop and my house. During the real cold months of January and February the system is slightly undersized and requires frequent loading, even then my water/glycol will drop to as low as 110-120 degrees overnight.
In an effort to increase water temps (and reduce flue gas condensation), we have an electric boiler as well to boost the temps above 130 degrees during times of low wood boiler output.

I have read anything I can get my hands on to try to understand the relation of water temps to flu gas temps and the corrosive effects it has on the system. My user manual for the boiler clearly states the water temps must be above 130 degrees at all times in order to reduce/eliminate corrosion caused by flu gas condensation.

I (think) I understand the concept, but I still have some questions that I hope the more experienced here can help me get my head around.
So here are few questions that if I understood better, I might then finally be able to determine what to do.

My SS chimney doesn't seem to have any condensation running down inside the pipe. Does this mean I don't have flue gas condensation?

Would I be able to see if this is happening?

I read that the corrosion will shorten the life of the boiler, if it is starting to have an effect, would I be able to determine this by looking at my boiler....ie see rust?

Does the corrosion only effect the chimney pipes?

I ask all these questions because the cost of the electric boiler is killing me ($1600 per yr) and I am trying to determine if I might be better off accepting the shorter boiler life vs paying the electric costs and just let it run at the lower temps.

Can anyone help me understand if/how my lower water temps are effecting my situation?

Thanks Canuk

 

[Nofossil]

Welcome to the boiler room. That's a bunch of pretty good questions, and I'll try to answer them. Hopefully, someone who actually knows what they're talking about will correct any mistakes ;-)

There are two potential problems: condensation in the boiler itself and condensation in the flue. The more common problem is condensation in the boiler. This happens when the incoming water is cold enough so that the water jacket is colder than the 'dew point' of the combustion gases in the boiler. When this happens, you'll get condensation inside the boiler. You won't likely be able to see it while the fire's going, but you'll get corrosion inside the boiler on the visible (flame / smoke) side of the water jacket. This will be near the cold inlet. The corrosion may not be dramatic, but if that area is a little rusty of flaky, then you have a problem.

The typical way to protect boilers against this is called 'inlet temperature protection' and is usually accomplished with a bypass loop and either a big mixing/tempering valve, a bypass circulator, or a bypass valve. In any event, it allows a portion of the water from the boiler outlet to circulate directly back to the boiler inlet.

Flue condensation usually happens as a result of a boiler that's VERY efficient at extracting heat from the flue gas, possibly combined with a poorly insulated flue. It can cause deterioration of the flue (ceramic or metal) over time. You need flue gas below about 150 degrees F to have this problem - not likely for most of us.

 

[Canuk]

Thank you nofossil for the information, I think I am slowly getting it. If you don't mind can I ask a few more questions to be sure I have it?I have attached a picture of my set up so that I can try to be understood. My terminology is probably going to be way off, but here goes my best attempt.

You commented:
"The more common problem is condensation in the boiler. This happens when the incoming water is cold enough so that the water jacket is colder than the ‘dew point’ of the combustion gases in the boiler. When this happens, you’ll get condensation inside the boiler."

When the system was designed we installed what we called a Primary Loop, as part of this loop we are recirculating heated water back to the boiler input to try and maintain balanced in/out water temps.
The difference we referred to as the 'delta". You can see the primary loop spralled all over the wall behind the boiler. I have a valve that I can use to regulate the delta. I monitor the temps with stats at the locations indicated by numbers 1,2,3 & 5.

Will maintaining a proper delta of approx 20 degrees help prevent condensation?

Will I have condensation inside my firebox during times of low water temps (say 100 degrees with a delta of 20 degrees), while my fire has also burned low? Or will this only happen when my fire is high and the water temps are low, regardless of delta?
My manual says below 130 degree water temp is where "flue gas condensation will occur.

Until I started to understand it the way you are explaining it I was under the impression I would get corrosion on the outside of the boiler where the flue pipe exits (at #4). Is this of any concern? BTW, I don't have any condensation running back down the chimney pipe.

Lastly (maybe), like most of these fossil burning units, my fire box is caked with hard black sludge. Will this sludge prevent me from seeing if I have any corrosion from condensation, or is there another way to tell if it is happening?

Much appreciate your thought's and comments.

cheers canuk

 

[Nofossil]

Delta T doesn't matter - the real issue is the temperature of the water entering the boiler. That really doesn't want to be less that 130 degrees for any length of time. Realistically, you'll always have that for a few minutes at startup, but the goal is to minimize that time.

It's probably more critical in the first half or 2/3 of the fire. Towards the end, most of the hydrocarbons have been pyrolized and you have mostly charcoal - nearly pure carbon. When that burns, it gives off carbon dioxide but no water. No water means no condensation. During the earlier phases of the fire there's still plenty of hydrogen in the wood. When that burns you get water. Any water that's in the wood itself is also boiled off in the earlier phases of the fire.

The area that's at risk is the water jacket in the general area of the inlet pipe. On most boilers, that's in the rear and near the bottom. May be hard to see depending on the design of the boiler. Bright lights and a mirror on a stick might be necessary. I've seen a Tarm that was ruined by condensation.

 

[Canuk]

Thanks again for your expert advise, I finally get it.

cheers Canuk

 

[Gooserider]

To chime in a bit, NoFo has it right on the problem area and the cause - The delta isn't critical in this case, what is important is the temperature of the water returning to the boiler. What seems to be the most common setup is to have a bypass loop, or something like your primary loop that mixes hot water coming out of the boiler with the return water in order to keep it hot enough, which you are sort of doing with your "delta" valve. But rather than using a manually controlled valve, people use some kind of thermostatic valve that automatically changes the mix of water in order to maintain the right temperature... The downside is that obviously the more heated water you return to the boiler inlet, the less you will have going to the load.

Bottom line is you either have to be willing to accept running your heat load at a lower temperature, or come up with another way of supplementing your heat output like you have been with your electric boiler... I can understand not wanting to do electric, is there some way to add a second wood boiler or upgrade the one you have? Another option that MIGHT work depending on how you run your system now is to add some storage so that you can run the boiler really hard during the day and charge the storage, then run off the combination of boiler and storage at night. (It will also help your efficiency during the shoulder season)

Gooserider

 

[Canuk]

Thank you both for all your advise, I now understand what needs to be done...not sure just yet what approach to take?

What a great site and a great bunch of helpful folks...thanks!!

 

[jebatty]

Your boiler mfr recommends minimum 130F return water to provide corrosion protection. Other sources recommend 140F to as high as 160F. The point may be to not go short on return water protection. It is important.