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The shower in my master bath is tiny. When I removed some dry wall to see if I could widen it, I found a large ~10" round vent running through the space (must be the furnace).

Now that I have a high efficiency furnace it seems I no longer need such a large vent since only the hot water heater uses it. I think the furnace installers inserted a chimney liner to reduce the draw.

If I could get by with a 4" or so vent pipe, I would be able to expand the wall by 5" inches or more (not sure if there are code restrictions on how close framing can get to a gas vent). Another idea is to vent the hot water heater out the exterior of the house, but I"m not sure if this is even possible.

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  • @FreeMan's edit dropped the fact that the existing furnace is a HE one which presumably is not vented through the ~10" vent. The poster should update the question to confirm that this is indeed the case.
    – Eli Iser
    Commented Feb 5 at 14:54
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    It still says that, @EliIser, right at the beginning of the 2nd paragraph. Of course, this is also a 12 year old question that I only edited because it was brought to the top of the list by someone's new answer cough cough. ;)
    – FreeMan
    Commented Feb 5 at 14:57

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This is one of those questions that is very difficult to answer properly with the info given. Although it sounds like you may be able to downsize the vent/flue because to the change over to a high efficiency furnace, there are lots of considerations depending on the exact equipment you have. The water heater is a different application as well.

In your particular case, I think it would be wise to bring in a qualified HVAC tech to analyze the situation and give you a safe plan ideal for your needs and location. Never cut corners when fire and poisonous gas hazards can result. Good Luck

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    +1- If you have the install manual for your furnace it should say how it needs be vented. With the model number usually you can Google to get a manual. I agree with Shirlock, you should consult an HVAC expert because there are venting issues - techniques you may not know about.
    – SteveR
    Commented Mar 11, 2012 at 15:50
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You can replace your water heater with a high efficiency model that, like your furnace, will use low temperature exhaust pipes that are typically vented through the wall (rather than going up through the roof). This will allow you to completely remove the existing flue/exhaust pipe but will restrict you (and future buyers) to only being able to install high efficiency water heater and furnace (or electric versions).

This is the setup I have in my house (HE furnace and water heater vented through the wall) and there is no flue going to the roof (I suspect the house originally had a heat pump and electric water heater and wasn't connected to a gas service when built).

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