How do I prevent harmonic resonance hum in PVC vent of newly installed hot water heater?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

I can answer this for those that experience this with the HTP PH-LD. these are vary nice stainless steel machines. It can be corrected with an offset adjustment on the gas valve. An experienced technician can adjust this to run perfectly and not have any sound. The fan speed can also be turned down in the program but don't turn it down for the noise. This has a strong fan for long distance venting.

To get the gas valve adjusted just right, first set it to low fire. Press and hold the top two buttons for 5 seconds until it enters service mode. Use the left button to take it to low fire and adjust the offset to achieve CO² 8.5% to 9%. Take it to high fire and set the throttle screw to .2% to .3% higher then what it was on low fire. For example: Low fire 9%, high fire 9.2%. If there is still some sound, start over on low fire and increase the suggested setting richer till it vanishes. These units must have a combustion test performed to dial them in correctly as the manual states for trouble free long life that needs no service. These advanced instructions are not in the manual but works on other high performance Ultra low NOx products as well.

Answer 4

I had the same problem with a recently installed HTP Phoenix Light Duty. When it would fire up to high, there would be a reverberation that sounded kind of like it was coming from the exhaust pipe. It was especially bad at the vent exit from the garage. The noise was periodic, meaning that it would come on for a few seconds and then go off. Did not have the problem on low fire.

With some prompting, my contractor did a combustion analysis on the unit, made some adjustments to the gas valve, and now the noise is gone.