Whats it all about? Well, simply put, its like an ''E'' check for your car, except performed on your boiler or waterheater. Why we do this and why we place such emphasis on it is because we can learn alot about how your boiler or waterheater is performing by whats in its exhaust gas.
I have personally seen on many occasions a boiler that checks out mechanically and visually but then combustion testing shows its producing 2000 parts per million of carbon monoxide, a deadly amount. Fortunately when I've seen this the chimneys were always working and carrying the carbon monoxide outside. Now what if birds built a nest in those chimneys during the summer? would those people still be alive today?
See, when something goes wrong and people die of carbon monoxide from a boiler or waterheater, two things had to happen. First- the unit had to be producing deadly levels of carbon monoxide. Second- Why didn't it vent up the chimney? So hopefully now you see why its important for a unit to be burning cleanly. If birds built a nest in my chimney today, I would still be fine since my waterheater and furnace burn so cleanly that there is less carbon monoxide produced than is allowed for an unvented space heater. But I know this because I test them!
We test for oxygen levels, carbon monoxide levels, temperature, and draft. By knowing these numbers we can tell if more work is required. Combustion testing needs to be done whenever the unit is serviced and at least annually. If it is not done, the service is incomplete.
Bacharach Company makes test instruments and has some fantastic information available on their web site, which I encourage you to investigate. A link to Bacharach's training area is provided here.