The gas input is the estimated amount of btus/hr that is passing through the gas valve to be combusted. The heat of the flame and the flue gases is transferred to the water circulating through the boiler. How well those btus/hr are transferred gives us the output rating. On this tag, the output rating is hard to understand because the value changed from btus/hr to “SQ. FT. H.W. RAD.”. What does that mean?
Does it mean the square footage of a house with hot water radiators? If it did, what would be the factor? I often rough estimate a boiler size from the square footage of the heated space and the condition of the insulation. Most contractors will know if the building is loose, tight or somewhere in between.
I typically use 50 btus per square foot of heated space for a very loose building. So a worst case scenario would be the 1066 output times 50, which gives us only 53,300 btus/hr output from the 200,000 btus/hr input. Something isn’t right there. That rating doesn’t seem to relate to the square footage of the heated space.
The 20% Rule
Gas-fired cast iron sectional boilers with atmospheric draft used what seems like the exact same factor to calculate their output until government regulations got in the way. Almost every one I’ve seen from the ‘50s and ‘60s took 20% off the input to calculate output. On this tag, since it is from that era, the output in btus/hr should be 160,000 btus/hr, or 20% less than 200,000 btus/hr.
But it doesn’t say that, so let’s try another interpretation. Cast iron radiator sections have always been rated by their manufacturer. The rating commonly used is what we now call EDR, or Equivalent Direct Radiation. Since most cast iron radiators can be used for a steam system or a hot water system, the EDR rating could then be used easily to calculate the output of a radiator based on the steam pressure in the radiator or the water temperature in the radiator.
What is inside the radiator determines how much heat it can produce. The same size radiator produces much less heat with 180°F water circulating through it than if the radiator was filled with low pressure steam. For example, a 60 sq. ft. EDR radiator heated with steam produces 240 btus/hr per sq. ft. EDR, or 60 times 240 equals 14,400 btus/hr. The same radiator heated with hot water at 180°F produces 150 btus/hr per sq. ft. EDR, or 60 times 150 equals 9,000 htus/hr. That is quite a difference.
If we now try multiplying the 1066 shown on the tag times the factor for hot water at 180°F, which is 150, the product is 159,900 btus/hr. That is very close to the 160,000 btus/hr I would have expected if the tag gave us the output in btus/hr. Once again, output is 20% less than input, or in combustion efficiency terms, right at 80%.
Recommended Capacity
The last bit of information also relates to the boiler output and is labeled “Manufacturers Recommended Capacity.” The value is 800, which is 25% less than the output at 1066. It is also based on the amount of hot water radiation that it can heat up, but for some reason is now labeled “SQ. FT. 150 B.T.U. HOT WATER RAD.” So they are now being specific about what factor was used to calculate EDR to btus/hr.
Why would they reduce the gross output by 25% to get the recommended capacity? I have a couple of explanations. The first is understood by the way systems used to operate, which was manually on/off without thermostats. If the building got cold before the next coal fire was started, the extra capacity got the building warmed up quicker. Sometimes it is referred to as the piping and pickup factor, or the cover the backside factor.
Another is understood by the weather. If the temperature outside would drop suddenly, the extra capacity would be better able to maintain inside temperature. The cynical explanation is the manufacturers made more money by selling bigger than necessary boilers to increase profits. The psychological explanation is “bigger is better.” They can all lead to oversizing, which in today’s world means less efficient. Use an accurate heat loss to size the next boiler for best efficiency on hot water systems.
Medical Update
My surgery was a week ago and I feel better than anticipated. The pathology report showed 5 out of 56 lymph nodes with cancer. The oncologist wants to do a dozen rounds of chemo, two days on a pump, 12 off, repeat 11 more times. Ugh! Trying to stay positive knowing others have been through much worse.
Patrick Linhardt is a forty-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.
