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  1. #1
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,180

    Exclamation water heater killer?

    Homeowner had been in house for 5 yrs: Cat.I fan assisted 80% gas furnace common vented into a manifold shared with a 40/40 gas DHW heater. Complained the WH pilot kept dropping out. Replaced TC a few times, got fed up, went to Home Depot, bought a GE WH and had "plumber" friend install it into manifold. Pilot on new WH still dropping out. Gas utility tested with match near top of unit blew out match so Red Tagged. Called me to check for obstruction.

    When I disconnected vent connector from furnace, I noticed flex. aluminum liner from manifold into breeching but it was 100% blocked with chimney rubble. How did the liner fill with rubble? Pulled breeching and found the liner was about 3 feet long! They turned the end upwards and stuffed it into the breeching. With that furance, it didn't take long to rot the chimney and the debris fell down filling the liner stub. With the vent blocked, both appliances vented out the draft hood. Note, the furnace safeties never tripped because the draft hood allowed it to vent out, thus bypassing primary safety controls. There were no signs of condensation in the exhaust plenum, no signs of flame rollout on the flame shield above the burners and the vacuum, fusible link, and two spill switches were all intact. The plastic bushings on top the new WH were melted.

    The two appliances were located in a basement with an 8" masonry curb around them to protect from floods. This curb trapped the CO2 spilling from the draft hood as it settled low and displaced the O2 thus choking off the pilot. I still don't know why they are alive. The "plumber" who installed the WH never inspected the breeching but assumed it was ok because he saw aluminum flex attached to the manifold inside the wall. First of all, never install galvanized pipe in a masonry wall. Always inspect the breeching and flue every time you change out appliances then last, perform a combustion and depressurization test. BTW, the furnace tech never suspected a problem because the furnace was venting nicely through the WH draft hood. Look at how the acids ate up what was left of that aluminum liner.

    Before replacing the WH, a simple test with a combustion analyzer would have shown a falling O2 in the pit around the heaters, which would indicate CO2 displacing the O2. CO production isn't far behind.

    I recommended a full, thorough inspection and testing of the furnace, esp. HX prior to resumed use and same for WH and a full length listed stainless steel liner. Since common venting will defeat the furnace safeties, I recommended a power vented WH with just the furnace to the liner and add a low level CO monitor .

    Interesting case,
    Hearthman
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  2. #2
    Join Date
    Apr 2008
    Posts
    16
    Is it usual to have two heating appliances sharing a common flue pipe like this? Seems like a kludgy way to do it, even if it is more expedient. If both appliances are burning gas at the same time, could there be an issue with the pipe being too restrictive?

  3. #3
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Nice thread Bob!

    To think everyone wonders why I am such a proponent of banning common vented applications?
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  4. #4
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,180

    Cool

    I'd say 90-95% of the homes in the Philly area have two heating appliances common vented. You typically have an oil or gas boiler in older homes, then oil furnaces then gas furnaces as the homes get 30 y/o or less. They both usually have a gas water heater. Most have the WH vent connector above the heater connector breeching but they still common vent in the same flue. About 5-10% have any sort of a cleanout below the breeching.

    Manifolding is allowed by both gas and mechanical codes. It can work great with boilers and WHs when done properly. However, as soon as you common vent a furnace with an atmospherically vented appliance that has a relief opening such as a barometric damper or draft hood, you effectively bypass the primary safety controls on the furnace. Even if the WH is vented into a separate breeching, it can still relieve blocked flues and vent into the home through the draft hood as it did here.

    If you install spill switches on the WH draft hood, such as a Field Controls GSK-3, they instruct you to test to see which side spillage is more prone on then install two switches. The problem comes in which appliance(s) to interrupt. If you connect the GSK-3 to just the furnace, the WH can still spill while the furnace is firing. If you connect it to the WH, the furnace can keep firing. Since TCs operate on 25-30 mv, adding these switches to WHs can make it more prone to nuisance trips, callbacks and irate homeowners.

    The better solution is to chimney vent the furnace (if they won't spring for a Cat. IV condensing furnace that vents out with PVC) and replace the WH with a power vented model.

    In all cases, you need to test the CAZ for depressuization & spillage and combustion testing of the appliances.

    David, I was hoping you saw this thread!

    Hearthman

  5. #5
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Quote Originally Posted by hearthman View Post
    I'd say 90-95% of the homes in the Philly area have two heating appliances common vented. You typically have an oil or gas boiler in older homes, then oil furnaces then gas furnaces as the homes get 30 y/o or less. They both usually have a gas water heater. Most have the WH vent connector above the heater connector breeching but they still common vent in the same flue. About 5-10% have any sort of a cleanout below the breeching.

    Manifolding is allowed by both gas and mechanical codes. It can work great with boilers and WHs when done properly. However, as soon as you common vent a furnace with an atmospherically vented appliance that has a relief opening such as a barometric damper or draft hood, you effectively bypass the primary safety controls on the furnace. Even if the WH is vented into a separate breeching, it can still relieve blocked flues and vent into the home through the draft hood as it did here.

    If you install spill switches on the WH draft hood, such as a Field Controls GSK-3, they instruct you to test to see which side spillage is more prone on then install two switches. The problem comes in which appliance(s) to interrupt. If you connect the GSK-3 to just the furnace, the WH can still spill while the furnace is firing. If you connect it to the WH, the furnace can keep firing. Since TCs operate on 25-30 mv, adding these switches to WHs can make it more prone to nuisance trips, callbacks and irate homeowners.

    The better solution is to chimney vent the furnace (if they won't spring for a Cat. IV condensing furnace that vents out with PVC) and replace the WH with a power vented model.

    In all cases, you need to test the CAZ for depressuization & spillage and combustion testing of the appliances.

    David, I was hoping you saw this thread!

    Hearthman
    The situation you mention with the GSK's can be remedied using two double acting barometrics on the furnace and the water heater.
    The water heater would need a manual reset spill switch tied into a thermocouple junction block while the furnace would need an auto reset spill switch tied in with the pressure switch.

    Somebody mentioned a capillary type switch which was bendable that could be wrapped around a draft hood without much voltage drop to keep the thermocouple from dropping out.
    Wish I could remember where it was.

    When I see common venting on multiple draft hood equipped appliances I get nervous as the likelihood of any of them venting properly is slim to none.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  6. #6
    Join Date
    Mar 2008
    Location
    Northern Virginia
    Posts
    209
    thats a funky pitch on the tank

  7. #7
    Join Date
    Mar 2008
    Posts
    279
    in michigan you can have a water heater and furnace share the same common flue.

  8. #8
    Join Date
    Dec 2007
    Location
    Northeast Wisconsin
    Posts
    482
    The National Fuel Code book says you can share a common vent. Of course there are stipulations. And local code can be more restrictive.

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