Boiler Exhaust Freezing
Climate Zone 7
Installed a new boiler last summer (baseboard heat and water heater). Ran intake & exhaust at 3″ diameter for at around 16ft in the attic and out the gable end wall (diameter and length is well within manufacturer specifications) . I chose to go through the wall instead of the roof to minimize the roof penetrations.
Intake and exhaust are sloped back towards the boiler in excess of the 1/4″ per foot slope specified back towards the boiler.
Kind of kicking myself for not insulating the exhaust while I had the attic open.
I ran the exhaust long out past the wall and didn’t get around to finishing the “proper termination” before winter. I am seeing significant freezing in the exhaust when the vapor condenses. It hasn’t fully plugged up yet, but I am fearful it will.
The orange line in the attached photo shows roughly where the ice is.
– Has anyone had luck mitigating this situation?
-Attached is the manufacture’s detail for the proper termination. My intuition tells me that 90deg up and 45deg outwards is going to also not work.
-Idea #1 is cutting the exhaust closer to the house and use a sanitary tee with bird screens on top/bottom.
– I don’t believe the boiler is short cycling while heating, but it does fire then stop often when hot water is called for in the house. ( kids, hand washing, dishes, etc.)
Thank you!
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Replies
I would try sloping the pipe that penetrates your exterior wall away from your house that way the moisture that condenses drains out of the pipe instead of back in. If the pipe is in fact sloping back towards the house this could also invite rain water that lands on the pipe to drain back towards the house. I don't believe it is critical that the pipe slopes the whole way back to the boiler. I'd say the idea behind sloping it is that the condensation needs to drain somewhere and not get trapped in the pipe, so if it drains outside or back to the boiler either one is fine, so long as it doesn't get trapped.
I would also try shortening the exhaust pipe as well where it sticks out of the wall. With that long of a pipe it gives the exhaust more time inside the pipe to cool and condense on the walls of the pipe. I dunno if your installation manual provides a minimum distance required from the house or not but it looks like it gives a maximum distance of 12". Does your exhaust pipe exceed 12"?
Why did you run it up into the attic and not just out the wall on the same elevation? The reason I ask is that the longer pipe runs give more moisture more time to cool and condense inside the pipes so you'll get more condensation to remove at the boiler.
The instruction manuals for condensing furnaces I have read require the exhaust to terminate using a 90° elbow pointed down or out the roof. This makes the water drip off before it can freeze.
Your photo shows how an intake should look.
Note I think it is a mistake to operate sealed combustion units as exhaust only as your was done. Consider have then come back and run the inlet pipe. What your are doing is using a fan to blow conditioned air out of your house and forcing unconditioned air to leak in somewhere.
Walta
You don't want to pitch anything outside. Condensate is highly corrosive and will chew up anything it splashes onto.
Won't be fun to get up there, but you need to insulate the exhaust pipe. Make sure to insulate the run inside the attic as well.
So, I operate 3 condensing appliances every year, two furnaces that are 25+ years old and a boiler at home. The ones at work were installed with intake, exhaust and a drain. Somehow the drain didn't cause the previous tenant trouble but it would make a ten foot icicle and clog, shutting off the furnace. They drained it into my shop sink, so I put a tube full of limestone under it, but, hey not my pipes. I guess they don't run those like that anymore, but the exhaust has a fair icicle when it is cold nonetheless
At home I have a boiler that needed to extend the exhaust ~6 feet to clear a deck. Insulated the exterior portion because the manufacturer had no provision for such
Has not frozen or had an icicle for 15 years
Makes me wonder, is the boiler set up correctly. So much moisture still in the exhaust
There seems to be a lot of modcons out there not doing much condensation, irks me to no end as most of the time it is simple configuration that is off.
BTW. The exhaust on a condensing unit is pretty much 100% RH, a unit that is running well will have much colder exhaust (still at 100% RH) so it will now hold less water.