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I just had a new power vented water heater installed (HTProducts Phoenix Light Duty) in our crawlspace. The contractor that put it in used part of the existing 3" concentric vent from the furnace we removed used.

The issue is as the exhaust fan gets up to higher speed a loud hum starts in the vent that sounds a bit like a (albeit quiet) fog horn. This noise is nothing like the fan noise close to the water heater. It's low frequency and loud enough to annoy us in our bedroom on the second floor and most likely our neighbor.

I am fairly certain the issue is to do with some kind of natural harmonic happening in the exhaust vent and this is reinforced in that if you momentarily cup your hand over the vent exhaust (blocking the air flow) the hum stops and it goes back to the quiet expected noise for maybe 10-20 seconds, sometimes longer, depending on what the fan is doing then.

Has anyone come across this or have a solution? Total length of pipe isn't really an option as we can't move the water heater position though the length of where the smaller pvc pipe from the water heater exhaust joins into the 3" may be.

EDIT Thanks for the suggestions. Further info: The fan is built into the water heater unit and as it is new and is controlled by the water heater cpu and I don't want to void the warranty messing with the fan is out. The manufacturer said they haven't heard of this. Based on the noise and feeling the vent pipe I think the standing wave is caused by the fan beating the air and there is minimal vibration on the vent itself. Also the hvac guy that put it in has three ninety degree angles in the 2inch pipe before it goes into the 3inch which then goes straight outside.

UPDATE So after installing 3inch pvc pipe the whole way from the water heater to the concentric vent and still being puzzled at the hum the contractor started going back over everything and after adjusting the combustion mix it appears the hum was caused by too lean a mix.

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Organ pipe is probably a good concept for what's happening. –  Ecnerwal Dec 17 '13 at 18:57
    
Is the vent pipe connected directly to the fan, or is there some form of decoupling in place? If there is no decoupling in place, it's very possible that any vibration in the fan is transferred directly to the pipe. When the noise is occurring, try grabbing hold of the pipe (somewhere in the middle) and see if the sound stops (or at least lessens). How is the pipe supported along it's length? –  Tester101 Dec 18 '13 at 12:05
    
Self answers are welcome here, feel free to post and accept the solution to your problem. –  BMitch Jan 1 at 0:22

2 Answers 2

Couple options come to mind:

  1. Change the fan.
  2. Change the fan speed.
  3. Change the fan connection to the vent.
  4. Dampen the vent.
  5. Change the size or length of the vent.

If you can't or don't want to change the fan, then try dampening the vent by clamping it somewhere to the building structure and possibly adding some padding or pipe insulation where it makes those connections. This changes the resonance length without changing the actual length of the pipe.

If dampening the vent doesn't work, then you can change the length of the vent. You mention that you can't make it shorter, but perhaps you can make it longer, or larger. Check the manufacturers requirements on the maximum length, number of elbows, minimum size, and required slope before making any changes like this.

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Are you thinking the fan chassis vibration is traveling up the pipe? I was actually thinking that the fan is beating the air just right to trigger resonance in the pipe, not of the pipe. They use a squirrel cage blower in those models, so I'm not sure it's chassis vibration. –  antiduh Dec 17 '13 at 19:38
    
That was my thought, but I could be wrong and wouldn't discount your suggestion that it's the fan beating the air itself. –  BMitch Dec 17 '13 at 19:43
    
I've edited the original post as I believe it is the fan beating the air rather than the fan chassis vibration from both the sound and feeling the vent pipe. It sounds like a digeridoo. –  Rob L Dec 17 '13 at 22:25
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@RobL added one more option, make the pipe longer. Shorter isn't the only option to break a resonance frequency. –  BMitch Dec 18 '13 at 12:17

This is an incomplete idea, and I'm not sure whether or not it will violate code, but you can take a page from automotive engineering and install a muffler on the end of the pipe. This will damp out resonance in the air stream by buffering the air stream.

It seems a few people have have had this idea - this guy made one by taking a 2" pipe, perforating it, wrapping fiberglass around it, and then surrounding it with a 4" pipe. The perforations and glass pack will absorb sound, like a silencer for a gun:
http://www.youtube.com/watch?v=LPN2uUfx7C4

Here's an example from a particularly designed car exhaust:
http://www.myrideisme.com/Blog/wp-content/uploads/2010/05/howto.car.mufflers002.jpg

Alternatively, your best bet might be to run 2" PVC all the way out of the house - if my physics classes taught me anything, that transition from 2" to 3" might be acting as a resonance chamber; some very high end tuned exhaust systems do this on purpose to set up resonance in the exhaust to suck air out of the engine just before the exhaust valves close.

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The youtube video seems like the same issue however in this case there is more of the hum and less motor/fan noise. I will check the local code and the manual as to whether we can use 2 inch all the way. –  Rob L Dec 17 '13 at 22:31

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