My gosh, that's older than I am, it must be about as efficient as a camp fire.
Seriously, that can be done with your combustion analyzer. In states where you are allowed to do so, it is a handy way to put a number on a very important piece of data. Sometimes the HVAC tech will have left that information from his/her last tune up, but I don't find it very often.
Here is just one link, I didn't read it all, that discusses combustion efficiency:
PS, I would guess 50 to 60%
Thank you for the information. I will look the link over carefully. Of course the efficiency of the combustion process is important but this is only a part of the equation. Over time scale (rust) etc will build up on the heat exchanger and this will cause more heat to go up the stack rather than transfer to the water. A well serviced unit with high combustion efficiency would still have degraded over time. There is a service record that indicated an 82% efficiency with 10% CO2. Not sure how this can used to evaluate if the original AFUE of .65 should be degraded.
Having the combustion efficiency gives you the big part of the AFUE number. From there you know that the AFUE is lower. At least you have a basis for determining the combustion efficiency that is better than guessing.
If you have an adjusted heat loss for the home that reflects the actual living conditions and their actual energy used, then you have another path to estimate AFUE, but it is riddled with guesswork.
A typical cast iron boiler, pre 70, would be around 60%. Yours should be less. As for the 82% on the service record, being able to generate your own number would increase your confidence in that number. In my experience, a tech has to get a good number or s/he did not tune the boiler properly, so they get a good number, whether it is correct or not. Who is checking???
ASHRAE of course has their methods, but not something I have tackled. In Maine we are prohibited from getting too involved with anything HVAC. We can run CAZ and the rest is hands off. We can't even drill a hole in the flue if there isn't one, LOL. Here we don't have a line between HVAC and Energy Auditing, we have a concrete wall.
Hi Matthew. The boilers and steampipes in these pictures are from the original boilers in this hospital when it was built in 1940 or 1941. I coated these in 1989. It dropped the skin temperature from 267F down to 155F. The engineers said this was a 35% reduction in heat loss through the metals and insulation (Asbestos). If this could help your project, let me know.
I think boiler AFUE is a really tough thing to narrow down. If a boiler is grossly oversized it will tend to go very cold. Cycling losses will be huge. So measuring combustion only tells a very small part of the picture.
Here's an article I found very interesting that you might find helpful: http://bit.ly/oSyma4
The boilers in the pictures of my post have a low fire / high fire system. They were always on low fire until the temp came down and the high fire mode kicked on for however long. The only time one would be turned off was for annual servicing, re-mortaring the fire bricks inside, etc,
There was a much smaller and newer boiler that served the kitchen area for hot water ony. That was a off and on system there.
Not sure if on / off systemes existed back in the 40's.
I think this is getting away from the OP question though, which I suspect refers to a residential system (this is Home Energy Pros).
I could be wrong, usually am at least once a day and hope to keep that up for a while. :-)