downfired gas boiler

luke

A year ago, I installed a new steam boiler in a one pipe system. The unit was short cycling so the contractor got from the manufacturer new orifices which reduced the firing. The gas consumption went from 210,000 to about 150,000 and it this seems like the right size as the unit is not short cycling at all, and is able to keep up on the very cold days. From what I have read, I need to be concerned about flue gases condensing if the unit has been downfired this much. How would I know if this is happening?

lchmb

stack temp

Generally if the manufacturer help's to downfire a unit the have an idea of what it will do with their unit. I would however check the stack temperature to make sure it is running high enough.. Call the company back and see what they recommend for a minimum stack...:)

Unknown

Have a combustion analysis

done on the boiler by a professional. I am not a big fan of derating (downsizing) boilers. I would ask the manufacturer what the recommended perimters for CO2, O2, draft, Flue gas and stack temps, and what level of carbon monoxide it should have.

What make of boiler is this? It has been my experience that steam boilers do not like being downsized. Short cycling can be caused by other things.

Why was it oversized to begin with? Did anyone measure the square feet of radiation? Was a heat loss of any kind done?

Mad Dog

The other guys are right

It is not a good idea, but I have been in that situation once or twice. I would have it checked at least every two years for the sections clogging up with soot. I have seen this be a problem more on gas hot water boilers. I think with the higher temperature of steam there is less of a chance of vastly cooler sections. Mad Dog

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luke

thank you

With regard to Tim's question, the boiler it replaced with even larger (300,000 btus) I had 3 contractors look at the job and 2 of the 3 gave me estimates to replace the old unit with another 300,000 unit. The third (the one that I went with), said that unit is way too large and did an estimate of the sq footage but the exposed pipes may have thrown off the calculations and I told the contractor that I was planning on adding radiators.

This is a brownstone home and over the years many radiators were removed. I guess in the old days, the windows were open all time!

The house used to be too cold but ironically at the smaller firing rate, the house is much warmer and more comfortable as the radiators stay hotter longer--fewer temperature swings --and the heating bill is down. So, there's really not need to add radiators.

So basically it was mistake but less of a mistake than replacing it with another 300,000 unit.

I found the short cycling annoying (air vents opening and closing every few minutes) and was concerned that I was going to kill the boiler prematurely with that number of cycles. I've checked and the air valves in the mains are properly venting so I have concluded the unit is just too large.

I never really noticed the old boiler short cycling maybe because it was leaking and never really built up pressure-- just wasted fuel. Or maybe I never paid that much attention to it. It was sooted up though.

I will get a combustion test done--and if necessary, fire it back up and live with the short cycling.

You can use my story when homeowners try to convince you that they need a larger unit. Now that I understand this better, I realize that a too large unit is like an air conditioning split system in which inside and outside unit are not properly matched.