# Ice buildup on High Eff Furnace Vent



## Garasaki (Apr 14, 2008)

High efficiency gas appliances work by "borrowing" some of the exhaust heat and using it to heat the air/water a little bit more.

The products of gas combustion are very "wet". 

In a normal efficiency appliance, the temperature of the exhaust is high, and as a result the dew point is high. So as this exhaust leaves the house, it takes a little bit for the cold air to cool it down, make it "dew" or condense, and then the dew freezes. But by this time, the air has floated away from your house.

In a high eff. device, the exhaust gas is equally "wet" but not nearly as hot (since the heat as been "borrowed" and put back in your house) - therefore it's dew point is quite low. The result is that the moisture condenses out as soon as the pipe leaves your house. This moisture sits on the pipe - the cold air hits the moisture, it freezes. The next batch of moisture does the same thing (except sooner because the pipe is now colder since its got frozen water on it already). So it eventually builds up a lot of ice.

It's a common problem. I don't know what the remedy is (or if one exists). Hopefully someone else can comment on that. But now hopefully you understand what's happening, why it's happening, and that it's not unusual.


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## SKIP4661 (Dec 3, 2008)

You will want to take a look at the pvc venting. There should be at least 1/4" per foot of fall toward the furnace. This allows the condensation to drain to the furnace where it csn be handled by the condensate drain. A high spot or reversed pitch in the venting could allow the condensate to drain outside and cause the problem you are describing.


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## drtbk4ever (Dec 29, 2008)

Thanks for the information guys.

Garaski, your explanation helps to explain a lot of things.

Skip, I'll check the slope of the exhaust pipe tonight. 

I'll let you know what i find out with the slope of the exhaust vent.


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## Garasaki (Apr 14, 2008)

I didn't even notice your screen name. Garasaki is a combination of Gary Fisher (moutain bikes) and Kawasaki - from back in the day when I had a KX125.

I've been w/o bike for about a year now unfortunately. Last bike was a KTM 250sx I rode in enduro's with. Sure miss the knobbies...


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## drtbk4ever (Dec 29, 2008)

Garasaki said:


> I didn't even notice your screen name. Garasaki is a combination of Gary Fisher (moutain bikes) and Kawasaki - from back in the day when I had a KX125.
> I've been w/o bike for about a year now unfortunately. Last bike was a KTM 250sx I rode in enduro's with. Sure miss the knobbies...


Hey Garasaki,

I got my first dual sport bike when I was 15. Still have it today although it sat in my Mom's garage for 22 years. An old 1981 Honda XL 125. I picked it up one day, fired it up (it took a lot of kicking with 20 year old gas still in it), and had the bug again.

So two years ago I picked up a used Yamaha WR450. Best move I ever made. Although initially it was way toooo much bike for me. Hmm, probably still is too much bike. No racing for me though.

You are right, there is nothing more theraputic than feeling that back tire chewing on some dirt.


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## Garasaki (Apr 14, 2008)

I rode a buddy's KTM 525SX. Man that bike was a blast. Those big thumpers are soooooooooooooo easy to ride. Don't even have to worry about what gear you are in. Was debating about something like that (really any new 450 would be in the same boat) or I was really enjoying some time on a different buddys KTM200 - that bike was easy to ride because it felt like riding a BMX bike - no weight to it at all. Would have been great for enduros or hare scrambles. Unfortunately around here racing is the only option to ride on decent terrain. Very few public trails in Iowa. Or maybe a 300 2 stroke. Or get a 125 and bore it out to 144 - super fun bike on the cheap. :laughing: 

First bike was a Suzuki DS125 - I think maybe 1980. 4 stroke 125 - probably like yours?


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## drtbk4ever (Dec 29, 2008)

Garasaki said:


> I rode a buddy's KTM 525SX. Man that bike was a blast. Those big thumpers are soooooooooooooo easy to ride. Don't even have to worry about what gear you are in.


No kidding. I compare my 450 to a tractor. Pick a gear and chug on through. I rode a newer CRF450X for a half hour or so this summer. Much lighter than my WR, but I couldn't wait to get back on my tractor. 



Garasaki said:


> First bike was a Suzuki DS125 - I think maybe 1980. 4 stroke 125 - probably like yours?


Yeah those old 4 strokes were/are tough. I know my Honda still runs after 27 years, 2 oil changes and on 20 year old gas.


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## drtbk4ever (Dec 29, 2008)

SKIP4661 said:


> You will want to take a look at the pvc venting. There should be at least 1/4" per foot of fall toward the furnace. This allows the condensation to drain to the furnace where it csn be handled by the condensate drain. A high spot or reversed pitch in the venting could allow the condensate to drain outside and cause the problem you are describing.


Hey Skip,

I put a level on the PVC venting and found that is does in fact slope towards the furnace. I didn't measure exact rate of fall exactly but it seemed to be at least 1/4 inch of fall per foot. So I am darn sure any condensate in the PVC will flow back to the furnace. 

I guess the ice forming is something I have to live with.


Anybody have a solution?


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