What is high draft?   

Combustion gas spillage out of any fossil fuel-burning appliance is dangerous. Depending on the home or building situation, spillage can happen in a couple ways. Many HVAC technicians who service heating equipment miss this problem. I’ve witnessed it on the job quite a few times first hand. Recently I came across a Valiant natural gas atmospheric vented steam boiler that had a high draft. Take a look at how I initially discovered it by simply performing a combustion analysis with draft test and then followed up with the repair. You’ll be shocked!

High draft

A high draft means that there is good combustion air in the boiler room and a good chimney. Although having too much of a good thing can cause problems. Too much air in the room and external building pressures can cause combustion gas spillage out of the draft hood resulting in a dangerous situation. It can also affect the efficiency by removing the heat transfer out of the boiler by under-firing.

Now let me get back to the Valiant boiler. After cleaning and checking it, I performed a combustion analysis and draft test simultaneously. Upon starting the combustion test the carbon monoxide spiked from 0 to 3,853 PPM. I knew the draft was high by looking at my draft gauge, 0.7in WC, or water column pressure. The normal is 0.1in to 0.2in WC. After doing a few testing procedures, I determined that 1. the home was on a high level, and 2. it had an excessive draft. In this particular case, the excess room air moved forcibly from the rear of the basement into the front boiler room and up into the front of the boiler’s draft box. The air and the combustion gas collided. The air blocked off the combustion gas and caused it to spill into the room. It was a dangerous problem that could have been deadly.

The homeowner explained, he had a HVAC company service the steam boiler before he called me in February. According to what I found and the homeowner telling me, the technician serviced the boiler last September. He then returned later when the home owner started using the boiler in late September because of the CO detector going off and the boiler spill switch tripping causing a no heat situation. He told the owner he could not find what the problem was causing the CO in the house and replaced the spill switch and left. This February I get a call for a service request on a noisy steam boiler making banging sounds and irregularly adding water into the boiler.

What happens ?

Flue gas is made up of carbon monoxide (CO) and carbon dioxide (CO2). If the CO2 can’t leave the boiler, it displaces oxygen at the burners and over time increasing carbon monoxide to high levels. This is the most hazardous problem in the HVAC field today. From what I discovered is the spill switch must have went off because of the venting problem and high draft pulling excess room air up the draft hood, which created back pressure. The high heat from the combustion gas set off the spill switch in the draft box because it had no place to go but out of the draft box and into boiler room.

The repair

To fix this particular situation I had to modify the boilers draft box and install a gas Field Controls MG1 Draft Control. The video below shows the testing and the repair.

Prevent high draft

Signs of high draft problems would be spill switches being replaced often and carbon monoxide detectors going off. Other signs of venting problems would be white powder on draft hoods and burners and black soot on the burner cover. A high draft can also cause less efficiency by low firing and cooling off the heat exchanger. To prevent this, I recommend having an annual inspection and combustion analysis every year. It’s better to be safe than sorry. Don’t gamble with your family’s lives.

Don’t Guess, Test

If you have any doubts about your situation, and want to get an accurate boiler inspection, book it now. Vince doesn’t guess, he tests!

Carbon Monoxide Safety Check

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