Question
A home high-efficiency gas furnace exhausts through a basement window via a
piece of PVC pipe.  The exhaust creates loud tones at 517 and  to a
slightly lesser extent at 1034 hz that can be heard several blocks away.
My question:  Can a resonant tube be attached to the PVC pipe to absorb the
noise?  Or will it be necessary to run the exhaust through a muffler?
Thanks in advance–JM

Answers
I would talk to the furnace installation people first of all. Perhaps
adding a larger diameter pipe onto the end will slow down the velocity,
as well as flairing the output someway? Get rid of the noise. Don’t muffle
it!

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• Answer:Sounds like a poor installation. Something they installed to loose and vibrating. I assume they warranty their work. Give the contractor a call and have them fix it.

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• Answer:HVAC Tech.: The hum you hear might be the purge blower running, this should happen first in the light up process. the sequence is staged so that the purge is first, the unit lights off, when it heats up enough the blower comes on and warm air comes out. When the heat setting is reached the fire goes out, then as the thing cools down then the blower turns off last. Hope that helps you!

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• Answer:PROBABLY ONE OF THE PLENUM WALLS ‘OIL CANNING ‘ OR SUCKING IN AS THE SYSTEM COOLS DOWN.

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• Answer:It depends on what about the furnace unit is making the noise. Typically it is the fan motor, or associated moving parts. Sometimes it is the housing vibrating from the spinning of the blower fan. It may be the plenum or metal (or other) box that the air blows through to get to the ductwork. Lets look at all reasons it can make noise and eliminate things First, the fan. If the furnace/ac kicks on and there is a rattling or grinding or whining or other noise that accelerates as the fan picksup speed, the bearings in the fan blades or motor are going bad. This is not good/expensive to fix maybe. IF the noise is worse when the fan is at full speed and stays constant while the unit is blowing air it could be that the belt is loose or if no belt the linkage from motor to blower fan is loose or worn out. This may be fixable with a little tinkering but not for someone not mechanically inclined. The mounting for the motor or fan blower may be loose, try to tighten the screws or bolts that hold them down. If the noise starts slowly after the fan is at full speed it could be that there is resistance in the plenum or ductwork/air ducts/vents. This usually isn’t that noisy, but it can be bad. You should go throughout the house in each room and make sure that the ducts are clear and vents are all open to the full. Usually there are fins that direct the flow to a certain direction these should be open wide, also behind them may be a flow restrictor fins. These are used typically with a lever that opens or close the flow off completely to ‘zone control’ the room. These should be open wide as can be too for the testing. Should be so the air ducts and vents in each room are open wide and blowing air full when the unit is operating. Be thorough as you search for vents in all rooms, you may miss a vent or two in odd rooms, behind furniture or in larger rooms. If any air vents are open wide and don’t blow air with good force(should be similar from vent to vent and from room to room, decreasing slightly the farther from the furnace unit you get) then the ducts(tubes) are blocked somehow. if they are in the attic, they may have been crushed by the cable guy or the christmas ornaments box you shoved up there. or maybe you have squirrels building nests within them or something ( similar in the basement depending where the ductwork runs.) If they are smashed, reshape as best you can and see if that gets the air going again. (careful not to tear them open or cause leaks in the duct itself) If too much air is restricted from coming through the ducts it can make the unit more noisy. If the ducts leak into the attic or basement you lose heat/cool into that space instead of the room and could cause water damage from condensation. Seal leaks with duct tape or replace the duct. Same goes for the return air intake(s) usually there is one at the furnace unit on the door to the unit or an adjacent wall. The filter usually goes there. the filter should be clean and fit properly for the opening so all air goes through it not around the edges. Other return air ducts can be placed throughout the house in larger homes so check that the same as the air ducts and clear any blockages. Sometime when the unit is installed, the unit is too powerful for the size of ductwork existing and this can backpressure the unit causing excessive noise and resultant loss of efficiency of the entire unit. It is working too hard to force air through it. This is hard to calculate, a bigger plenum and more ductwork is required. 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In that case, there are other things to do to help quiet the environment of your living space. If the furnace is in a wall that adjoins the bedroom or living areas, you might try to place larger pieces of furniture infront of the area where it is. Like putting your big dresser on that wall right where the furnace is located. Maybe hang a tapestry on that wall as well will help quiet it some. Move the bed or your sitting chair to the other side of the room from the furnace. Another more extensive fix can be to double up the drywall board around where the furnace is to quiet it down. this seems radical, but it will reduce a noisey unit significantly if done properly. (last resort stuff) In short, Eliminate any vibration noise, air flow restriction noise, or structural weakening, use sound barriers in the rooms or consider replacing the furnace altogether. An existing furnace that is over 5 years old (the pros say) is inefficient and can cost up to 50% more in energy cost than a new unit. A new unit can be much quieter, more efficient and sometimes you can move it to another area that is less annoying noise-wise. Hope this helps! Good luck!

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• Answer:it sounds like something is either about to break loose or has already broken loose.I was told a little vibrating is normal. call the repair man and do your best to tell him what the problem is . if you have the same luck as me when he gets there it will run smoothly no noise at all LOL.and the minuter he back’s out of your driveway it will start all over again. LOL

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• Answer:Wow, there is a million noises I have heard from both a water heater and a furnace. Sort of hard without you making the actual noise. If I had to guess, I usually hear higher pitched noises from a blower wheel in a furnace due to bearings. See if it does while the actual heat is on, or fan, ect. If not, Assuming the work you have done to gas logs, Gas pressure might be an issue. Sometimes, gas gets thrown off when anyone does work on a similar appliance. The gas that passes through the burner chambers on the furnace whistles and whirls, because it has a low burn off rate. (All that means is it cant burn the gas fast enough, or the gas pressure is too low, causing it to ‘dance around’.) The last thing I see often, is a piece of tape, duct work or something close to the actual furnace has come a little loose. It can be a litlle chore trying to hunt for it. Though you will know it as soon as you put your finger on it. I will garentee you, one of these three issues will be your problem. I hope it works out for you, good luck & merry christmas.

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• Answer:Mine used to do it and all it needed was to cleaned and have the filter replaced and it stopped. Mine was never dangerous but you never know probably best to call someone to work on it.

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