Domestic HWH diverter missing, or just different?

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Permalink Reply by Rob Buchanan on September 25, 2012 at 9:38am

I was back in the residence today and took a look at the underside of the box in question. It is most certainly a draft diverter, but it looks as if it is decidedly unsafe. Based on the stack effect, the warm, moist air would circulate at the top of the box and cause the box to rust. I took a picture, and lo and behold that's exactly what's happening. I assume the box will eventually rust out, and the diverter will fail or the box will be compromised.

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Permalink Reply by Robert Riversong on September 25, 2012 at 3:01pm

The primary purpose of a draft hood is to maintain a constant, and not excessive, draft from the appliance, much like the barometric damper on a furnace flue.

In many jurisdictions, you can't vent a gas appliance into the same flue used by any non-gas combustion appliance and the smaller BTU/hr burner must enter below the larger burner if two gas units share a common chimney.

Also, a large chimney flue may not generate enough draft speed to properly evacuate the water heater gasses, which may be why there is moisture accumulation in that intermediate draft hood, which is also far enough from the water heater to allow flue gasses to cool. The long horizontal run and the number of elbows will also significantly slow the flue gas speed.

I would recommend a complete rebuild of that flue system, using B-vent directly to the outside, with a proper draft hood at the water heater.

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Permalink Reply by Sean Lintow Sr on September 25, 2012 at 10:40am

I am by no means an expert in this area & you should call one in to look at it but the applicable code is:

IRC 2009 - G2407.3 (304.3) Draft hood/regulator location. Where used, a draft hood or a barometric draft regulator shall be installed in the same room or enclosure as the appliance served so as to prevent any difference in pressure between the hood or regulator and the combustion air supply.

As for seeing this "technology" in the future - lets hope not as atmosperic devices should be going bye bye & replaced with direct vent

You also have to recall that a draft hoods main purpose is not to ensure a draft but to act as a safety feature in case the vent is blocked by allowing the gasses to escape into the room - Safety Sunday: Water Heaters, the Combustion Process & the Draf...

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Permalink Reply by Rob Buchanan on September 25, 2012 at 11:07am

So, if I'm reading that correctly, as long as there is no pressure differential between the combustion air supply and draft hood, it is technically in compliance. The combustion air intake and draft hood are in the same room.

As far as "future," I meant in other houses, not as new units are installed, of course. I would be stunned to see something this silly in new installation.

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Permalink Reply by Timothy DeLuca on September 25, 2012 at 7:36pm

Have you done any CAZ testing under worst case situations to test for combustion gas spillage? I agree that this horizontal venting holds a lot of potential for dangerous CO spillage. What are the rise, run and sizes of the flue pipes, including where and size of the flue the DWH is tied into?

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Permalink Reply by tedkidd on September 27, 2012 at 10:16am

Looks almost perfectly horizontal.  Think they took a level to it (lol)?

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Permalink Reply by Rob Buchanan on September 27, 2012 at 11:43am

Strangely, it does slope within normal limits. I hadn't thought about flue gas condensation being a problem, though. On the plus side, there are a handful of CO detectors around the house.

The next step will be a full CAZ. Having not identified the box as the draft diverter, I didn't have a chance to do so.

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Permalink Reply by Cory Chovanec on September 27, 2012 at 6:23pm

Rob-

It might be a direct vent mobile home water heater installed in a house. This is only a guess but check underneath the water heater and look for a opening.

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