Direct Venting of Gas Furnace/Water Heater


Old 04-09-04, 10:01 AM
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Direct Venting of Gas Furnace/Water Heater

I'm in the process of having my HVAC system replaced. I'm also going to replace the hot water heater at the same time because of it's age and the fact that I'm tired of cold showers after my kids take showers before school... it's a 40 gal electric with what feels like a 24 hour recovery cycle.

My contractor is telling me that if I go up to 90% efficient units I can go with direct venting. Right now I only have a 4" flue to draw off the furnace. I'm thinking about replacing the hot water heater with a gas unit. So obviously a 4" flue couldn't handle two 80% efficent units.

The distance from the hot water heater to the exterior wall is about 15 feet. Is it possible to vent through a perfectly horizontal pipe that far?

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Old 04-09-04, 10:29 AM
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Hello: NTAC

Direct venting units can be and usually are vented to the outside via a double walled vent. Both the combustion air and the exhaust fumes are vented within the same both walled vent.

Therefore, only one larger hole is needed for the larger venting pipe, the angle can be 90 degrees and no inside air is required for combustion air.

Only requirement is both units cannot be combo vented into the same common venting pipe. Each is seperate. Based upon all those I have seen and those which are code legal locally.

Another fact to consider is any power venting of the furnace. If the unit is power vented, which is likely, it cannot be combined vented into the water heater.

When the furnaces venting fan turns on and while it is running, the fumes or some of the fumes will be forced back into the vent pipe of the water heater. Thus comming out the downdraft divertor or being forced back into the intake or exhaust of the water heater.

Thus combined venting of the two appliances is not likely to be or meet code nor recommended nor suggested by either mfg'er.

15 feet is likely too far for a non power vented water heater. Usually such appliance come with a very short combo dual walled vent system which is completely pre built. Minor assembly is than needed.
Old 04-09-04, 10:40 AM
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Thanks for the advice. He only said they couldn't be combined. It was my assumption that it was a problem of a 4" pipe handling the volume of two appliances.

I think I'll go with a gas hot water and pay the extra to relocate the unit next to an outer wall.
Old 04-14-04, 07:56 AM
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You can get a power vent water heater and vent that through the wall (2" PVC usually), and use the existing flue to vent the furnace.
Old 05-20-04, 03:03 PM
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Heater venting

I just had a new furnace installed and the furnace is vented to the outside using a plastic pipe. Is there anything on the market, that is a little more attractive, atleast for teh outside of the house?
Old 05-23-04, 09:01 AM
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You could paint the PVC to match your house then it wouldn't stand out as much.
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