Quiet PVC exhaust?
October 9, 2007 3:34 PM   Subscribe

How do I eliminate noise from 20ft of 3in PVC exhaust pipe without reducing pipe diameter or airflow?

I recently had a new hot water tank installed. It is one of the energy efficient gas types which has an electric exhaust. The plumber had to run about 20ft of PVC to get the exhaust gases outdoors. It's all good, but the problem is that the exhaust makes a low whuuuur noise that is amplified by the time it reaches the outside. I can't change the diameter of the exhaust pipe as this restricts the flow too much and the heater shuts down. What device can I build that will eliminate this noise without restricting the airflow?
posted by kuujjuarapik to Home & Garden (8 answers total)
A larger pipe, or one with some sophisticated baffles.
posted by rhizome at 4:00 PM on October 9, 2007

What about adding some elbow joints to work as a sound baffle?
posted by trinity8-director at 4:31 PM on October 9, 2007

Well, there are 3 elbows along the length that don't lessen the noise. I also do not want to replace the pipe, and I've never heard of PVC pipe with sophisticated baffles.
posted by kuujjuarapik at 4:38 PM on October 9, 2007

Glass pack-type muffler. Check out the pot-growing websites. Those guys are ingenious with reducing airflow noise.
posted by i_am_a_Jedi at 4:50 PM on October 9, 2007

Are your PVC pipes white plastic or the black foam-core DVW (drain-vent-waste) type? I have one of these water heaters, and with black pipes, I don't have noise problems.

You might check to see whether the noise is amplified because of the resonance of the pipes (correlated to the pipe volume, and consequently the diameter) or because the pipes themselves are vibrating and transmitting sound that way. If the pipes are vibrating, you can probably improve things by strapping them down tighter, or by putting a little padding around the pipes where the straps are. If it's a resonance issue, you might improve things by putting a section of larger-diameter pipe in the intake. That shouldn't affect the heater's operation (but I am not an HVAC contractor).

Another thing to try is calling the heater manufacturer. Those heaters are expensive; it's likely the manufacturer is very interested in making sure customers are happy.
posted by spacewrench at 4:57 PM on October 9, 2007

The automotive muffler suggestion above sounds good, but combustion gas contains acidic water that will corrode all but the noblest of metals.
posted by hortense at 6:59 PM on October 9, 2007

It's possible that your outlet pipe length lines up perfectly with the pitch of the exhaust fan, thus creating a loud speaker for your exhaust fan. An easy way to check would be to stick a 2-3ft extension on the end of the pipe and see if that helps. Depending upon the frequency of the fan you may need to whack of a half inch or so at a time to get the maximum effect.

Really, you may only need to add a couple inches to shut it up. If lengthing the pipe doesn't help, try a different fan. The current one may be out of balance or have a bad bushing thats causing the noise and a replacement could fix it.
posted by TheJoven at 7:46 PM on October 9, 2007

Thanks mefites for the suggestions. I'll let you know what works.
posted by kuujjuarapik at 3:55 PM on October 10, 2007

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