When the weather gets really cold, the condensation around our vent pipe ices up and the septic gas smell backs up in the house. My husband has to go on top of the roof and chip the ice away which isn't easy when the roof is icy too. I have never had this happen in any other house and when we describe this to people, they have never dealt with this problem. Why does it do this and what can we do to keep it happening every winter?

How far does the roof vent stick out of the roof line and its location to another roof pitch if there is any?
How big in dia. Is the pipe coming out of the roof?
For a whole house system it should be 3" vent pipe
How far down the vent does it freeze and if you look down the vent is there an angled fitting near the roof line and is that getting chipped out? You can out a Pvc 90 on top of the vent to keep icy water from getting in the pipe.
The 90 elbow on too of the vent will also help hold some heat in the pipe longer slowing freezing and it will still vent
One temporary solution is to put some heat tape around it for now.

Signed 21 Boat

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Hot and moist sewer gases rise through the vent to the roof. They freeze in upper portion of the vent creating ice clog. You can get elec. Blanket, wrap it around the pipe directly below the roof in the attic several times, secure it with duct tape and plug it in. Let it on through out the winter . You can plug it into a timer and have it go On and Off several times a day. It will keep the end of the pipe couple of degrees warmer disallowing moist sewer gas to freeze.

When the weather gets really cold, the condensation around our vent pipe ices up and the septic gas smell backs up in the house. My husband has to go on top of the roof and chip the ice away which isn't easy when the roof is icy too. I have never had this happen in any other house and when we discribe this to people, they have never dealt with this problem. Why does it do this and what can we do to keep it happening every winter?

Sounds like a Artic Vent,(see image) would solve your problem. Check them out at; Water Line Heat Trace Freeze Protection For Cottage, Home, Commercial Water Pipe Applications - Arctic Vent (http://www.heatline.com/arcticprod.htm) Good luck, tom