This winter, we have being using our minisplit as our primary heating source. It is rated to provide heating down to an outside temperature of -5 degrees Fahrenheit (-20.5 degrees Celsius).
But once we were railroaded by the Siberian express and temperatures dropped below -5 degrees Fahrenheit on January 29th, I turned the minisplit off and fired up the boiler to power our radiant heating system.
Even though everything seemed to be humming along just fine, I heeded the recommendation by the City of Chicago to proactively check on the mechanical systems. It held the promise of soothing my nervousness that arose from the record cold temperatures that remained below (and sometimes far below) 0 degree Fahrenheit (-17 Celcius) for 49 straight hours.
That’s how I discovered that our Trinity LX 150 boiler must have shut down sometime during night #2 of our polar vortex. Because of the insulation, our building takes a long time to cool down, and it may have been another half a day before we noticed that the heat was off.
One of the safety features on our boiler is a combustion sensor, which jumped into action because it detected insufficient airflow into the combustion chamber (see image above).
Outside I found some ice build-up on the exhaust, which is expected because of the water vapor that is in the exhaust. What I did not expect was to find that the air intake pipe had frozen up. It is a little hard to see, but you may be able to make it out in the image below.
That ice build up literally choked our boiler and shut it down.
I whipped the hair dryer out to warm up the pipe and melt the ice, which was a completely futile exercise at -20 degrees Fahrenheit (-29 degrees Celsius). I didn’t even get the ice on the pipe to melt, let alone the ice in the pipe!
Cathy had a better idea and handed me our heating pad, a towel, and a bungee. That did the job within a few minutes, and the relatively thin ice block in the pipe collapsed so I could scrape it out.
Hunting for the cause
With the boiler up and running again, I started to search for the probable cause of the ice blockage, and was handed a “duh” moment.
The wind blew the exhaust loaded with water vapor across the fresh air intake, where it froze up. Cathy pointed out that this installation (having the exhaust blow across the intake) did not make any sense. I checked with the installation manual, and she was right!
The recommendation is a minimum 18 inches vertical separation between exhaust and intake. I used a short piece of PVC pipe and duct tape for a quick fix.
And it makes perfect sense, because the hot exhaust fumes tend to rise up fairly quickly. Having the air intake at least 18 inches lower almost eliminates the risk of exhaust fumes being sucked back into the boiler. I just have to come back and make this fix permanent.
What really bothers me is that I had assumed from the time the boiler was installed that the exhaust and air intake met the manufacturers recommendations. God knows how many times I had the boiler run inefficiently because southerly winds blew the exhaust right across the air intake!
It’s that fine line: Do you micro-manage installations by your contractors, or do you trust that they know how to do things right?
That picture of you with the hair dryer on the vent brings back memories! We ran into issues with our condensing water heater for the same reason, but even with a pipe extension we had problems because the boiler exhaust was just a couple feet away. Our boiler has a concentric vent and has been issue-free, but the water heater was separate, like yours, and frosted up numerous times. Ultimately we re-routed it to a concentric vent out the roof, which also stepped up the pipe diameter from 2″ to 3″, making it even less likely we’ll have frost up again.