Calculating Boiler Efficiency

Aquaman

We are attempting to calculate boiler efficiency on a residential hot water cast iron boiler.
We know the natural gas input and Delta T but do not know gpm.
We already know the calcualtion for efficiency is ouput/input but we need to calulate the actual output with the figures measured as described above.
Any tried and proven Engineers out there want to take a stab at this?

don_201

Without

Without knowing flow it will only be a guess.Maybe one day the people that makes the circ would install some test port.

Untill then you have to pipe in some guages.Be it with two
guages or one guage and two ball valves.

Brad White_191

Don is right.

You need to know the flow.

You might be able to triangulate with an approximation but when you leave a variable undefined, the entire equation is suspect.

Jim Hilpipre

Efficiency calculation

If you clocked the meter to determine BTUH input and adjust for efficiency couldn't you determine the GPM by the delta T?

Jim Hilpipre

Brad White_191

Not accurately

Clocking the meter might get you to the ballpark but it measures cubic feet of gas, not the heat content of that gas.

To adjust for efficiency is not useful because the efficiency is what is being sought (unless I mis-understand you here of course).

The delta-T has to take in all of the "net, above the line factors" but without knowing the flow, one cannot reconcile it accurately. You can guess based on pipe size but then the answer is based on a guess, it is as good as the least reliable variable.

Unknown

What if you measured the pressure on each side of the circulator to get the head loss of the system and then looked at pump curve to get the corresponding GPM? I just tried a mental calculation of this on my own system and seemed to get a reasonable result.

I have a head loss of about 7 Ft across the pump. According to the B&G 100 curve, that corresponds to a flow of 12 GPM. With my 22 degree Delta T, I should have about 11000 BTU/GPM or a boiler heat output of 131,000 BTU/Hr.
By clocking the gas meter I get a heat input of 162,000 BTU/Hr. Dividing the output by the input gives me a net steady state efficiency of about 80%, which is just about right for an atmospheric gas boiler.

D107

Interesting paper on the subject

Not sure if this covers all the bases, but seems relevant. see attached.

Brad White_191

That is on track, Mike

Indeed that goes to flow and is along the lines of what Don said above. Simple "Pete's Plugs" on each side of the circulator and a very detailed, clear pump curve would get you as close as anything.

Now, not to be a skunk at the garden party, but if the pump has run for years, the impeller could be worn and the actual flow could be less.

But still, probably the best way for the least effort.