National Radiator gas boiler

Steamhead

can be expressed in several ways depending on how it is being measured.

Combustion efficiency= how well the burners turn fuel into heat. This is what you're getting from the combustion analysis.

Steady-state efficiency= how well the boiler and burner generate heat and transfer it to the water, when the boiler is running constantly. This is what you're getting from the ratings and the stack temps.

Annual efficiency (a.k.a. Annual Fuel Utilization Efficiency, or AFUE) takes losses into account when the boiler is off. This is where most boilers that age lose efficiency, especially atmospheric gas boilers. That's the basis for you being told your boiler is only about 60% efficient.

New boilers must have an AFUE of at least 80%, and some are much higher. It may pay to replace that old National on that basis.

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Mike Kusiak

Tie bolts

While cleaning a National gas boiler in a hot water system built around 1954, I noticed that the long tie bolts that hold the cast iron sections together were completely loose. I have heard that these bolts should not be too tight to allow for thermal expansion, but these 1/2 inch diameter bolts could be turned with the fingers, without a wrench. Not wanting to crack the castings, I snugged them up slightly. Does anyone have any experience with how tight to make these bolts? I assume that they loosened due to 50 years of expansion and contraction. Suprisingly, there is no sign of leakage at any of the push nipples.
Another question that I have is about the efficiency of this boiler. The rating plate shows an AGA input rating of 178,500 BTU, and an AGA output rating of 950 Sq ft water, which translates to about 142,000 BTU. Does this rating include a 1.15 allowance, as it seems much more efficient than the ratings of modern boilers? In operation it appears to be quite efficient, as the stack temp before the draft diverter is only 375 degrees with water temp at 160, and after the draft diverter it is only about 180 under the same conditions. I have compared this to a modern Burnham boiler with only 130,000 BTU input, and I get temps of 500 degrees and 240 degrees at the same points. Can it be that a 50 year old boiler is so much more efficient than a modern one? The National uses vertical flues about 26 inches long with upward zig-zag offsets vs the Burnham pin grid design.

jerrygb46

tie-bolts

Finger tight is all you need. been that way a long time.Good cast iron boiler ground and tapered push nipples!Now their a term i have't herd in a long time.

Mike Kusiak

National boiler

Thanks for the advice on tightening the bolts. Looks like I can get a few more years out of this old timer. I am still confused as to the efficiency of this boiler as most people say gas boilers this age are only about 60% efficient, yet from its ratings and stack temperature it looks to me closer to 80% or more.