Ice builds up at furnace exhaust

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Old 01-29-15, 09:46 AM
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Ice builds up at furnace exhaust

I've read about high efficiency furnace and aware that they release more moisture upon combustion. This situation have me worried though. Not sure if you can make out, but there is a bunch of ice under the flue cap and I've seen much worse than the picture shows. The ice often blocks part of the PVC. My guess is that ice is getting trapped on the screen.

I'm planning to go up there and cut the screen on that side of the cap, plus extend the PVC by a few inches, perhaps increasing the diameter with an adapter. Any other suggestions?



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Old 01-29-15, 10:08 AM
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All furnaces that burn fuel (oil, propane or natural gas) produce moisture in their exhaust. The efficiency of the furnace has nothing to do with it. A high efficiency furnace extracts more heat from the exhaust so the moisture may condense or freeze when it exits the chimney or exhaust.

Is the ice you are seeing causing any problem? Is it blocking the exhaust flow? You've said that ice blocks the exhaust but have you actually been up there and seen it or has it prevented your furnace from working?
 
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Old 01-29-15, 10:20 AM
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No problems to date, but with carbon monoxide, when there is a problem, it is usually fatal. I am not sure if the furnace will shut off when there is a blockage regardless if it has that feature or not. I have not been up there yet, but seen that the ice forms significantly.
 
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Old 01-29-15, 12:26 PM
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There are saftey features, specifically a draft proving switch, to prevent the furnace from firing if the ice blocks the vent. I don't think the exhaust pipe is high enough above the top of the chimney. If you have the installation manual it will tell you in the venting section.
 
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Old 01-30-15, 07:07 AM
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Well, here's the manual http://dms.hvacpartners.com/docs/100...5cav-060-5.pdf.

It is not clear to me if there is a clearance requirement related to the chimney (page 33). There is a 24 inch requirement above roof level or 12 inch above snow level. However, it is still appears in my situation even though the clearance is met, the snow and the moisture get trapped in the flue cap.
 
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Old 01-30-15, 08:13 AM
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I am curious if you chimney has been like this for some time and you have not had problems what is prompting your concern now? Last winter was extremely cold. I assume you had icing then and it did not cause a problem.
 
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Old 01-30-15, 09:47 AM
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I'm just trying to be proactive here. Why wait to wake up to a freezing house or not wake up at all? Especially if this was not done right and the experts tell me there is a need to remedy the situation.
 
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Old 01-30-15, 11:22 AM
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Yes the cap and cage around the top of your chimney seems more appropriate for a wood burning fireplace. I usually expect to see something like this. It's smaller and the cage to keep out animals is closer to the flue where it can be better heated by the exhaust gasses. When looking for this picture I came across several others very similar to yours with ice encrusting a large chimney cap so you're not alone.

 
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Old 01-30-15, 12:52 PM
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I suggest eliminating the cap & making sure the top of the chimney is sealed. The way yours is installed, if a 2 pipe installation, would create exhaust gas recirculation into the combustion air. NOT GOOD
 
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Old 01-30-15, 02:21 PM
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The two elbows are pointing in the opposite direction. Do you think that would cause the air to recirculate? If I remove the cap, could I just leave this set up? Could I plug the top of the chimney with foam board insulation?
 
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Old 01-30-15, 06:28 PM
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Are therre two pipes & if so, do both the intake & exhaust have elbows?
 
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Old 01-30-15, 06:40 PM
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From what installer said during the job, "there are two elbows pointing in the opposite directions".
 
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Old 01-30-15, 07:04 PM
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Your vent system is installed improperly. See fig. 36 & text on pg. 39 of the manual.
 
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Old 01-30-15, 09:18 PM
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so to fix it I need to remove the cap, point the exhaust up and add another elbow to intake? What is that special piece on the end of the exhaust?
 
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Old 01-30-15, 11:24 PM
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The moisture is hitting the cap, condensing and freezing.

The install manual should have info on distance between exhaust and intake and more.
 
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Old 01-31-15, 02:51 PM
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The only special piece I am aware of is if you are using a concentric vent, which you are not.

The intake & exhaust pipes are supposed to be insulated. Refer to Table 6 for how thick the insulation needs to be. It's not going to be easy to get insulation past couplings. Your installer should have insulated the pipes during installation. If the installation is less than a year old, I suggest talking to the installation company. They should make it right.
 
 

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