Gas and Oil Home Heating Furnaces - Gas Furnace venting problems

Hi,

We just had a home inspection prior to selling our house. We have a gas water heater & gas furnace tied together for the exhaust venting. The inspector noticed that there is noticeable backdraft into the gas water vent when the gas furnace is running. We took apart the metal vent ducts & no blockage was present. There is also a sizable ice build up on the chimney vent. What else could be the problem? The inspector assumed that something was blocking the vent but we found nothing on the inside. Could something be blocking at the cap? Due to the weather, we have not yet trekked onto the roof to check.

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Thanks

Doug

If your basement atmospheric pressure is lower than up at the roof, the furnace may take the path of least resistance to the low pressure and have it wind up going down the fork to the water heater. If this is the case we must figure out why this is.

I had a back draft recently at a commercial building I maintain. Combination of ice build up on the roof vent and low pressure in the establishemnt due to bath fan left on all the time with no other source for air make-up! I solved the problem.

If you run your dryer, let's say, you are pulling air out of your basement and putting it under low pressure.

Buildings with multiple chimneys often have air venting problems due to one chimney pulling air up while the other chimney lets air back down.

DaveBoy, thanks for the reply.
This seems to be the case all the time, with nothing else on. I even opened up the basement window to balance the pressure, same results. What else can I try? I'm hoping get be able to get on the roof tomorrow.

Thanks

Out of the box here. But can be that the flue gas from the furnace has a lot of moisture in it and did form ice up there on the vent pipe cap.
One other thing. The pictures didnt show if the vent pipe to the roof was the right size for that furnace vent and the water heater if it goes into a B gas vent pipe flue.

Ed, the gas being backdrafted from the furnace through the water heater is definetly moist. I'm wondering if we have a birds nest or hornets nest that is trapping the moisture & allowing it to freeze? The ice is actually formed onto the vent, although you can still see some smoke still being vented. Would not be the 1st time for a hornets nest, had same problem is bathroom exhaust vent & water dripping back through.