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My furnace turns on, and sometimes, has blow back through the tube leading back to the water heater, causing condensation on top of the water heater, blowing out the pilot light. Then 9 out of 10 times, the furnace vents out properly. I don't understand how this can happen.

The HVAC repair men are stumped also.They replaced the old water heater that was doing the same thing. Then they changed the vent shaft from a "y" to a "t" connection leading to the vent to the roof. It works 9 out of 10 times perfect, then the furnace turns on, and the vent leading back to the water heater gets hot all the way back to the water heater, and the top gets soak and wet, and the pilot goes out. Frustrating as hell. The vent on the roof seems clear as the steam is clearly coming out .

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    When a vented fuel furnace with an exhaust stack does something vent related sometimes and not other times, and it seems completely random, it may be the wind or weather.
    – Harper - Reinstate Monica
    Nov 14 '17 at 3:27
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    Hello, and welcome to Stack Exchange. Rather than add information in a comment, you should edit it into your original question (I'll do this for you).
    – Daniel Griscom
    Nov 14 '17 at 3:32
  • A picture of your venting pipes might be in order. It sounds like a restriction in your venting pipe to the roof . However the wording of your question is a little confusing try putting things in a step by step sequence of events 1: Changed Water heater which blew back to furnace, changed vent from Y to T - problem resolved for Water heater blow back. (or is it only working 9 out of 10 times - except when furnace turns on ?? 2: Furnace now has blow back to new water heater .. Please try to clarify your question so we know exactly what you have and we are on the same page as you are.
    – Ken
    Nov 14 '17 at 4:02
  • This happens when the smaller vent pipe is not pitched sufficiently. Make sure each unit has it's own barometric/atmospheric damper, and if the tee improved over the wYe make sure the open end of the tee is pointed straight down, then extend it with a short section.
    – herb guy
    Mar 6 '18 at 15:20
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As Harper says, I think you have an atmospheric problem. Some times just lengthening the vent line 3 or 4 feet will fix this. Sometimes you need a power booster to overcome the down draft. P.

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I had three HVAC/Plumbers to look at the occasional blow back I was getting with my gas H/W heater vent. They replaced H/W heater, installed a smaller 4” vent pipe (was 6” when furnace was attached to it). That helped on average. But every now and then when I would walk by it, I would get an exhaust odor. Never enough to set of an alarm. Finally, one evening I could smell some of the odor. I took a look on the roof and noticed some leaves caught in a web on the pipe of the vent Were moving, but no wind. Noticed the attic fan was higher up the roof line (and in code) than the 4” vent pipe. I turned off the attic fan, checked the draw on the vent pipe from the H/W heater and there was great draw. Turn on the attic fan to perform the same test and it generated enough blow back to blow out the lighter flame I was using to check it. Turn off the attic fan again and checked outside on the leaves in the web and they were laying still. Extended the vent pipe above attic fan and all is good now.

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