Combustion testing a Dunkirk Q-90-75

The Steam Whisperer (Formerly Boilerpro)

Its been a while since I had to test one of these.  I have the target combustion numbers, but I wanted to double check where to place the probe.  It seems to me there may have been some type of air bypass built in to the unit (I never could see where) that would through the oxygen levels off.  Any ideas?  Factory tech doesn't know about combustion testing these units,  "just set the gas to 2.5 inches", they say.  That was how it was set up from the factory when new and continued to eat ignitors until I set it up with an anlayzer.

Tim McElwain

Dave let me do a

little checking around, I will get back to you. The 2.5" W.C. pressure does not sound right to me?

Charlie from wmass

The 2.5 is correct IF

you have 1000 btu natural gas. it needs altered depending on the fuel. I can not see a port for analyzing though. I think they figured no one would bother. You may need to install one in the exhaust pipe.

Tim McElwain

Yes the 2.5" W.C. is correct

Dave what I do is go about 18 to 24 inches up  the exhaust PVC and drill a hole in the 2" big enough to fit my probe and do my testing there. The 18 to 24 inches gets you away from any turbulence from the combustion air blower. I then seal the hole with some high temperature RTV sealant. I have also tapped a pvc fitting and then put a stainless steel bolt in with some sealant to seal the hole.



Hope that helps.

The Steam Whisperer (Formerly Boilerpro)

Thanks guys.....

apparently there is no bypass air in the exhaust.  I tested at the outdoor vent and got 8.8% O2 and CO up near 100PPM, so I think I have an air leak somewhere probably past the burner but before the inducer.  Target is about 4.5 % O2 depending on intake air temp and boiler temp.

The Steam Whisperer (Formerly Boilerpro)

Testing

I was back on the jobsite and found the air bypass that mixes fresh air with the exhaust.  This unit cannot be tested on the vent pipe, but samples must be taken from a port located just before the fan.  Replacing a gasket and a little tweaking got the combustion stable once again.   Setting this unit at 2.5 inches gas pressure resulted in it firing at 85,000 input, when the unit is rated at 75,000 input.  Make sure to clock your meters and check your combustion numbers.