About a month ago, one of my techs went on a “no heat” call. The house is a rectory for the church next door. The call came in, and after he ran it, I assumed the problem was solved since I hadn’t heard otherwise. 

A few weeks later, they called back, saying the boiler was out again. It was a boiler that we installed a few years ago. When I heard we had a callback, I decided to stick my nose in and figure out what exactly was going on. I looked over the call notes and noticed that the spill switch on the draft hood was tripped.  He had reset the switch and chalked it up to a windy day. That obviously wasn’t the case.  

The boiler was installed and running for several years and had worked fine without an issue. What could have changed? While on my way to the call, I started reviewing what I knew about combustion and building pressures. The first thing I did when I walked in the door was to ask, “Has anything changed with the house? Was anything remodeled?”

First, I wanted to visually inspect the boiler, heat exchanger, draft hood, and flue pipe. I removed the boiler's top panel and inspected the heat exchanger from above. I also pulled the plate above the burners and stuck my phone inside the combustion chamber, recording a video of the condition of the bottom of the heat exchanger. And I wonder why I go through cell phones so often, big surprise. Anyway, everything looked perfect. No soot in the boiler, no bird in the flue, no rust, no dirt. I found absolutely nothing.

Next, I fired the boiler and waited for it to heat up while closely watching my combustion analyzer. The only thing that was slightly off was that the stack temperature was on the low side of the acceptable range. I increased the gas pressure while closely monitoring the combustion. Everything looked perfect.

I ran out to my truck to scratch my head and grabbed my DG-8 micro-manometer. The first thing I wanted to do was measure the basement pressure with reference to (WRT) the main body of the house and then measure the house pressure WRT the outside. The basement was a little negative, running less than 1 Pa differential. I repeated the test with the dryer and the water heater running. The water heater had an impact, but it was nothing to write home about. I knew it was not the problem. When measuring the house, I noticed it was also slightly negative and would bounce around -3 Pa WRT outside.  

I used the DG-8 to measure the draft in the flue pipe. No matter what I did, it was always running in the normal draft pressure range for a natural draft boiler, which is -0.01” to -0.02” WC. The boiler was running perfectly, and none of this made sense. My mind started racing, and I questioned what would happen if it were colder outside. Why was this boiler drafting now?

I walked around this big old house looking for clues. I found a few old-fashioned bath fans. You know, the ones with the chrome grille. Whatever I turned on had little to no impact on the draft pressure. I climbed the staircase to the second floor and turned the corner with my head down, ready to accept defeat, but I looked up, and there it was: the smoking gun. I felt excitement in the pit of my stomach. But what are the chances this guy was crazy enough to run a whole house fan during the middle of a Chicago winter? 

I headed back down the stairs, looking for the customer. “SIR! SIR!! By any chance, have you run the whole house fan recently?”  

He said, “As a matter of fact, I have. I know some people like to put plastic over their windows and seal their houses up airtight. I have a thing for fresh air.”  

“Sir, I think I might know the problem, but I want to prove it. Can you do me a favor and turn on the whole house fan? I want to simulate your normal routine while running the fan. Close any doors you normally would have closed and open or close any windows, respectively.”  

The fan kicked on, and I ran down the stairs to the basement. The first thing I checked was the building pressure WRT outside. The house suddenly went VERY negative. It went from -3 Pa to -10 Pa.

I ran over to the boiler and hooked up the DG-8 once again to measure the draft. When I looked down at the manometer, and I saw the numbers bouncing between 0.00″ WC and -0.005″ WC. I felt my cheeks rise as a smile formed across my face. I figured out the drafting issue and was able to prove it. 

How would you classify this problem besides pure stupidity? I remembered the article I read by John Tooley that was written in the 1980s called “Pressure Balancing a House & Mad-Air.” I am not sure if this instance was a classic case of Mad-Air, but it was definitely a pressure issue that was mechanically driven. It was a case where improper use of mechanical ventilation could have turned into a very dangerous situation. 

I don’t know what would have happened if the boiler hadn’t had a spill switch installed, but let's just say I wouldn’t recommend trying it out. If I would have had only focused on the equipment, I wouldn’t have been able to solve the underlying issue. 

This case is a perfect example of why you should look at the house as the system and the HVAC as just a single component. As my friend Chris Stephens would say, “You need to look at the big picture.” So, next time you're working on something that has you scratching your head, consider pressing pause and zooming out. Solving problems is much easier when you take the blinders off.

—Adam Mufich