I’m planning on replacing an old 40 gal gas water heater that is presently vented up the chimney, with a 50 gal Power Vented unit, venting it out the side of the house via 2 inch PVC for about 15 ft of run. This way I can reclaim some space in the bathroom upstairs since the chimney wont be needed. The installation manual states that the water heater venting needs to be 10 feet horizontally from any forced air inlet, such as a high efficient furnace intake. I had planned on using the joist bay next to the one that the furnace uses for the intake and exhaust venting. This would be at best only 30 inches of separation between the water heater and furnace venting. I’m guessing the 10 ft rule is to prevent the carbon monoxide being discharged by the water heater from being drawn back into the furnace. But if that the case, why is it that the intake and exhaust for the furnace can be so close to one another. From what I’ve seen, they are typically piped so that the intake is about a foot above the exhaust. Doesn’t this break it’s own rule? Any suggestions about the vent spacing or installation methods would be appreciated. Thanks….
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Sorry, if the manual says 10' minimum, that's what you have to go by.
There are 2 concerns.
First, even if the water heater is making minimal CO (say 10 ppm - which is low but not rare) the primary combustion gas is CO2.
If that CO2 gets pulled as combustion air for another gas appliance, that CO2 will displace oxygen in the combustion air for that appliance which could cause it to produce significant CO.
(The same if the first is producing high CO already - but for ####condensing furnace that air isn't mixing with the house air - although you still don't want that situation.)
Second, flue gases contain significant water vapor. If they get pulled into the air intake in very cold weather, they can ice up the air intake valve and screw up the combustion of the 2nd unit.
(See attached pic - where the exhaust for a 90+ was mounted low and the air intake above it -- which is backwards: the flue is supposed to be above the air inlet becasue flue gasses are hot and will rise from the flue.)
10' seems like a lot, especially for a power vent water heater, but anything less and you're taking an unquantifiable risk.
Sojourners: Christians for Justice and Peace
Edited 9/23/2004 9:08 am ET by Bob Walker
Seems to me that you're saying the way that furnaces are vented is wrong, so why not make the situation worse?
I think that Rich and Bob are righ, and you shouldn't put them too close together.
BTW - With a name like bobv, it makes me wonder if you're Bob Vila finally trying to learn something...(-:
Bumpersticker: Cover me. I'm changing lanes.
Each manufacturer specifies the spacing of the vent and air inlet; most require that the vent be at least 1' above the air inlet (the flue gases are hot and rise as they exit the vent.)
I've done CO testing on a number of 90+ furnaces which are vented that way and haven't found any problems from the flue gases getting recycled. As I said, if they were recycled, the co levels would rise during operation.
I don't know why the water heater manufacturer is calling for 10'; maybe it figures most water heaters are going to be installed by DIYers and it wants a margin of safety figuring folks won't follow the instructions exactly?
Sojourners: Christians for Justice and Peace
I too vote to follow the installation manual. Another issue with direct vent water heaters is that you need an outlet that produces no less than 114v. Some basements have this circuit after the washer, sump pump etc. so the power is dirty and provides low votage which will throw the controler on the water heater into a shut down/trouble mode. Found this out the hard way once. DanT
No, I'm not Bob Vila, but I am trying to get the right info....
I miss spoke before regarding proper furnace venting. The exhaust is definitely above the intake venting, so recirc of exhaust gases shouldn't occur. As far as the electrical supply goes, I have installed a dedicated 15 Amp circuit for the unit, so proper voltage should not be a problem.
Placing the water heater's exhaust 10 feet horizontally from the furnace intake, forces me to have part of the venting hang below the floor joist in the basement, something I wanted to avoid. But I'd rather error on the side of caution. Doing so will create a run of about 25 ft, plus three 90 degree, and two 45 degree elbows adding too much restriction for a 2 inch installation. Using 3 inch elbows add another 20 ft, but the total 45 ft is still within the limit for 3 inch venting.
Another item the manual recommends is not to terminate the vent pipe directly on brick or masonry, but instead, use a metal 2'x2' backing plate behind the vent. Any ideas why? Presently my furnace is vented directly against the brick. Bob V
The condensate from the exhaust is acidic and can destroy the mortar.