Derating Gas Boiler

Mike T., Swampeast MO

"No safe way." They didn't mention if they meant "safe to equipment" or "safe to occupants."

Mike T., Swampeast MO

Condensing/modulating boiler (particularly the one I REALLY want) just ain't in the works for a while.

Friendly heat/reefer man did his semi-annual check of my system--he keeps telling me I'm in the wrong business by the way but agrees I'd starve unless I moved--and had a suggestion for my severly oversized WM CGM.

Plug the orifices then drill out slightly smaller. He says he's done this on commercial FA equipment with good results.

I'm not at all familiar with the actual combustion end of the process so he said he'd come by in March when business is slow to get me going.

Do I have anything to loose--except maybe being forced to get a new boiler sooner than expected?

[Deleted User]

I would

be real careful not to reduce the input too much & create a condensation problem. W/A units are a little more forgiving than cast iron.

We have derated oversized boilers by removing the burner tubes at both ends of the boiler and plugging the orifice taps. Always. Always - ran tests before and after. Tinkering w/ orifices can result in something you don't want. CO. Too little gas for the burner tube can be as bad, or worse, than too much. Once again. Careful.

Mike Kusiak

Im not sure its a good idea to mess with the firing rate of a gas boiler. The heat exchanger passages are designed a certain size to provide the correct amount of excess air at the rated input. If you reduce the input without reducing the size of the passages, you will be including a greater percentage of excess air, which will reduce the combustion efficiency of the boiler. Your fuel consumption might actually increase due to the decrease in efficiency. How much is the boiler acually oversize?

Mike T., Swampeast MO

About 2.25x oversized

Boiler from 1970s. No condensation issues at current despite the fact that it only hits 140° when BELOW design temp outside!

Yours was similar to the issue I raised when talking about this to him.

Main problem I have is quite short cycles--about 2 minute, 20 second burn time. Presume that this is not particularly efficient. Since it's running constant circulation with TRVs and reset burn time stays remarkably consistent when maintaining inside temp--it's the interval between firings that changes with outside temp. Have considered buffering and primary/secondary, but don't believe it worthwhile given that it won't be needed with new boiler.

Have adjusted the bypass for longest burn time with least total burn time in a given period. At least it seems to work that way

One "good" thing I find about the way oversize boiler is that with the way oversized rads and TRVs I can get amazingly quick response. Hope to retain this to a smaller degree with a condensing/modulating boiler by oversizing a BIT and relying on modulation to give the "boost."

Steamhead

Mike, why not check with W-M

for their take on it? Tell them what you have and how much it's oversized, and see what they have to say. Bill Wright frequents the Wall, his e-mail is bwright@weil-mclain.com .

Ron already came up with the second part of my post, but it's worth repeating: TEST IT before and after. With a modern digital combustion analyzer you can see if the stack temp gets too low or the excess air or CO too high. But you knew that.

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Mike Kusiak

I sounds to me that you have the system operating as efficiently as it can. If by the bypass adjustment you have minimized the total run time of the burner while maximizing the cycle length, I dont know how you could do much better, except by going to the condensing boiler. Your system seems like an ideal application for condensing. Can you increase the temperature differential between low and high limit to increase cycle length? ( longer cycles but less frequent ) It sounds like you have so much thermal mass in your system a bigger differential would never be noticed.

Mike T., Swampeast MO

Mechanical reset

with fixed (20°) total differential.

Certainly agree that a condensing boiler should be ideal. Have wrung as much conservation/efficiency out of the current system as I can imagine with insulation/weatherization/TRVs etc.

Would really like to hear of someone with a similar system (iron rads, constant circulation, reset, TRVs) who has switched from plain cast iron atmospheric to condensing so I can get some idea of whether or not the increased efficiency is enough to justify the initial cost (and presumed higher maintenance/shorter equipment life). PLEASE--no offense meant by that statement but I seriously doubt anyone really expects condensing/modulating boilers to last 30-50 years with little or no maintenance beyond regular burner tube cleaning/checkups.