Thread: orphaned water heater

In an inspection yesterday I found an orphaned gas water heater. I did not post photos, it is the typical scenario where the original cat I furnace was replaced last year with a cat IV. The new furnace has a new plastic vent pipe, and the metal Y connector in the original metal vent pipe has been capped where the old furnace vent pipe connected. This leaves the WH venting into a 6 inch vent pipe.

I live in Arizona. I know what orphan means, and have found orphaned backdrafting water heaters in basements. But in this case the WH is in a garage and the vent pipe has a short, straight vertical run. So if this is a real concern, why hasn't it always been a concern for the 8 months every year that the furnace is not operating? I suppose the hot weather/attic in the summer could affect venting (compared to a very cold attic), but there are plenty of days here that are cool, but not enough to operate the furnace.

No client has ever asked me this, and I recommended a licensed plumber. But in explaining why it's a concern (the WH doesn't generate enough heat to properly draft in such a large vent pipe), I think it's a logical question to ask why this would not be a concern anytime the furnace sharing the vent pipe was not operating.

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Randy, the orphaned water heater can be the most hazardous appliance in a house. Even when common vented with a Cat I furnace, they are still a hazard but even worse alone. Summertime is often fraught with flow reversals and backdrafting. It can be next to impossible to establish a draft on hot days. If there is a forced air system in the CAZ with sufficient return leaks to further depressurize the CAZ, you can have, in effect, a carbon monoxide distribution system.

The problem with draft hoods is they work as designed. When there is a flow reversal, they allow flue gas spillage. If the chimney becomes blocked the appliance will happily continue firing oblivious to the hazard. The National Comfort Institute teaches a method of correcting this by removing the draft hood replacing it with a bullhead tee and double acting barometric damper set with a manometer then install a spill switch wired to the thermocouple. This allows dilution air to reduce condensation while providing a reasonable level of protection against backdraft. I've made this modification many times and every AHJ I've shown it to has approved it.

While much has been made of the orphaned WH problem, little has been done to explain just what the issues are. When a heater first fires, there is a column of cooler, denser air and gases in the flue. Since it is heavier than the flue or the CAZ, it tends to push down or flow reversal at standby. You have to overcome this reversal using the lift from the lighter flue gases. This results in some spillage out the draft hood, so it takes longer to 'prime' and get going. Now, aside from just lifting this dense column of gases, you want the walls of the vent warmed up and above dewpoint so it doesn't wick away energy that could be lifting those cool gases. A hot flue is a happy flue. If you have a thin-walled vent such as B-vent, it takes very little energy to dry and warm this inner liner. If you have a large, cold, wet masonry flue, it could take the entire burn cycle and never really prime. If the wall of the flue is wet the temperature of the wall cannot rise above 212°F until it has dried. A soggy masonry flue takes longer to dry. That mass of masonry takes forever to warm up as it continues wicking heat away until the entire wall thickness has reached equilibrium if.....there is the requisite 1/2-4" air space around the tiles. If they're cemented in, it might not ever reach it. Now, if this is a cold exterior flue in winter, again it will struggle to 'prime'.

Reducing the flue size to match the total firing rate is a bit of a problem for several reasons but consider two cases:
1) 40 MBH water heater vented into 6"x 5ft B-vent. All gas appliances are tested at only 5' BTW.
2) same WH venting into a nominal 8x8 terra cotta flue tile lined masonry chimney say 12 feet tall. This was common vented with a 60K furnace but is now orphaned. What's the difference?
The B-vent in this case would have a flue volume of 1,695 cubic inches with an inner wall surface area of 360 square inches of thin gauge aluminum to heat up.
By comparison, the ubiquitous tile flue would have a volume of 6,084 cubic inches and an inner wall surface area of 3,744 square inches of cold wet tile to warm up. Now, since the cold corners don't contribute to flow, you really only have an effective flue area of 4,775.04 inches. You have to overcome more volume but have less providing lift for you. The B-vent has 27% of the volume of the tile flue with 47% of the inner wall surface to warm up.

Now, having said all that just understand that once you get it 'primed' and venting up& out, the larger flue volume will provide more lift. Think of the amount of lift you get from a kid's balloon vs. a hot air balloon you ride in. More volume of hot air = more lift. So, that oversized B-vent isn't really all that terrible but it would be if masonry. HTH

Thank you, Bob. I did understand what you were saying. I knew that without knowing the 'formulas' that you used. In my first post I said I explain to my clients "the WH doesn't generate enough heat to properly draft in such a large vent pipe". Which is what you said, boiled down to one sentence that a typical client would understand.

And you did answer my question. The WH sharing a metal vent pipe with a cat I furnace can (hopefully) create a draft by itself. So here is what I should have asked. If a WH can draft without the shared appliance operating, it should also draft when orphaned. So do you report on orphaned gas appliances? Assuming a properly installed metal vent pipe, masonry flues are another thing.

Originally Posted by Randy West

Thank you, Bob. I did understand what you were saying. I knew that without knowing the 'formulas' that you used. In my first post I said I explain to my clients "the WH doesn't generate enough heat to properly draft in such a large vent pipe". Which is what you said, boiled down to one sentence that a typical client would understand.

And you did answer my question. The WH sharing a metal vent pipe with a cat I furnace can (hopefully) create a draft by itself. So here is what I should have asked. If a WH can draft without the shared appliance operating, it should also draft when orphaned. So do you report on orphaned gas appliances? Assuming a properly installed metal vent pipe, masonry flues are another thing.

I perform a Worst Case Depressurization Test to determine the House Depressurization Limits. Armed with that information along with combustion analysis I can advise my client. A listed metal vent or chimney has a huge advantage over masonry as I outlined but they ain't bulletproof by a long shot. Local conditions in the CAZ are just as important. It all has to work together or not at all. If the WH has much age on it or is venting into a masonry flue I try to steer them towards a power vented unit. While double the cost of a chimney vented gas WH, the cost of a liner more than offsets the cost of the power vented model. If the depressurization is too bad and the client unwilling to correct it (weatherize attic level, seal duct work, balance ducts, tight windows closed on upper floors, makeup air for large exhaust fans slaved to them) then I recommend power vented even if the chimney vented WH was just installed minutes before I arrived.

Remember that any testing or inspection is just a snapshot in time. Conditions can and will change. A WH may vent fine at standby on a still day but backdraft like crazy in a little wind or depending upon wind direction.