Search the forums

Loading

How to reduce furnace noise

Chris_Zito's picture

*
The general noise level produced by my furnace is too loud. I believe the main culprit is just the sheer volumetric flowrate being pushed through the ductwork. The blower by itself sounds normal. The burners put out a good bit of noise, but that is unavoidable I imagine. My house is 1-1/2 yrs old with a Rheem "Criterion II" natural gas forced air furnace. 125,000 btu/hr. Blower is 9.5 amps, 3/4 Hp. I think the blower is capable of 4 speeds, since a table printed on the panel shows the airflows for the 4 speeds. My house is 2700 sf, 2 stories, 10 registers up, 10 registers down, 1 zone.

Whenever my furnace turns on, the blower starts in a slower speed (without flame) for about 45 seconds, then the burners light and the blower shifts to a medium speed for about 30 seconds, then finally shifts to fast speed.

Is it possible to make the blower run at a lower speed? Is it safe (delta-T would increase)? Would the efficiency be changed? How can I accomplish the speed change?

Thanks.

(post #166994, reply #1 of 9)

*
Chris,

What you are hearing when you furnace starts is first the combustion air fan and power exhauster starts. The gas control circuit has a "flow prover" switch that checks to make sure adequate flow is present, then opens the gas valve and starts the burner. At this point no air should be coming out of the registers. Once the burner heats up the heat exchanger to a preset minimum temp (usually about 180 degF) then the furnace blower starts. This limits the amount of cold air that comes out of the duct work at start up.

The blower that you described should have about a max rating of 2000 cfm @ 0.5" w.c., which is a 5 ton unit. If the furnace is a two stage furnace, the it will operate at a low-fire setting (reduced heat output) with a lower blower speed and hi output on a higher speed. Changing factory set speeds would be il advised for many reasons. Some high end units have 4 speeds that can be set, usually as follows: low-low for contiuous ventilation w/o heat or cooling; low-med- low fired heating; high-med, high fire , max heating, low cooling and high-high for max cooling.

The causes of noise in a forced air system can be simple or very complex. Starting with the simple, air velocity in the duct work. If the air velocity in the ducts is less than 1500 fpm, it should be fairly quiet. If the velocity is reasonable, then isolating the duct work from "amplifiers" is the next step. Make sure that it is supported and just lying on top of ceiling joists. Wraping the ducts will help some both with sound attenuation and insulation.

Some layout problems can cause noise as well. Look for any "bull head" tees, tight turns, radical offsets, etc.. If it looks restrictive, it probably is.

(post #166994, reply #2 of 9)

*
Tim, thanks for the reply.

You're right. The blower only runs at one speed (although it seems capable of 4) and it kicks on after the heat exchanger has reached a certain temperature.

All the supply ducts are well insulated and have reasonably uncomplicated flowpaths (no tight turns, etc). I'm pretty sure most of the bothersome noise is from the return plenum, which comes down from the attic, through a framed-in chase, then into the top of the furnace in the garage. The return chase makes a complicated horizontal jog about halfway down the second story wall (a bedroom wall), where the sound level is excessive. I also think the return plenum is undersized, but there's not much that can be done about it now.

Would it be safe if a licensed HVAC technician adjusted the system so that the blower ran at a slower speed AND ADJUSTED THE BURNER OUTPUT appropriately? I know the furnace would run longer, but quieter.
Is it possible with my particular model furnace?

You mention changing the speed is ill-advised for many reasons. What are they?

(post #166994, reply #3 of 9)

*
Chris,

I don't know if it is possible to modify either the fan speed or the burner capacity of your furnace.

The reasons that I would not attempt to modify these parameters is:
- I doubt that the equipment has that level of flexibility built into it. Changing motor speeds is not that simple, though I am not an electrician. Most smaller HVAC equipment is direct drive and runs at the speed of the motor. The speed at which motors spin is a function of the electrical service available (60 Hz) and how it is wound and where taps are placed in the windings.
-IF it were possible to adjust the speed of the blower, then there would be the issue that you brought up about temperature rise across the heat exchanger. The potential to damage the heat exchanger is a concern, though not that great, really, and voiding any equipment warranty you might have is a definite.
-IF you could also reduce the heat output of the burner, all would be OK, but I would bet the cost of such work would be more than the benefit.
-If the adjustments were possible and IF the cost was reasonable, then you would still not be addressing the real problem. I believe that you have it pinned down in the return plenum. One way to address this would be to remove the return and replace it with one properly sized. If that is not an option, surrounding it with fiberglass batts (regular R-11 wall insulation) would deaden the sound, maybe to an acceptable level. If access to the duct is not available and you are unwilling to tear into your house to get it, you might be able to reduce the air flow through the system by closing down dampers (if the are installed) close to the furnace. If you do not have dampers in the system, you could (have someone)install them where ducts are accessible. This would also cause the temperature rise to go up.

Another option would be to add a new return in the system closer to the furnace so the furnace sees the same air flow (+/-) but the noisy return duct has less air flowing through it.

(post #166994, reply #4 of 9)

*
Chris,

One more option to consider. Check out your furnace and look for the following: inside the bottom access panel where the blower is located, check out the wiring (shut it down first). There should be 6 wires coming from the blower motor to a terminal block under where all the thermostat wires connect to another terminal block. There should be 3 wires going into the motor terminal block from the controls. A white one (common) a black one (heating blower power) and a red one (cooling blower power) Look at this and note which wires are connected where. The block should have numbers for the 2 colored wires and a "c" where the white wire is connected. If things are as I described, you may be able to change the blower speed.

(post #166994, reply #5 of 9)

*
Tim,

"Another option would be to add a new return in the system closer to the furnace so the furnace sees the same air flow (+/-) but the noisy return duct has less air flowing through it."

Be sure to follow any restrictions on putting returns ina restricted area (i.e., if your furnace is in a "furnace room" or closet, there should not be a return register in the closet to compete with the draft.)

I suspect there are intricacies involved here beyong that general rule.

(post #166994, reply #6 of 9)

*
With a sealed combustion(one with a PVC piped supply and exhaust) furnace, which I think is about the only option available these days, it is not a concern. With other gas fired appliances, like a water heater with a power vent (not typically a sealed combustion device) in close proximity, the potential exists. Depends on lots of factors, size of the space, amount of ventilation available to the space and proximity of the two. Good advice to follow nonetheless.

(post #166994, reply #7 of 9)

*
1 Duct work might not be sized right . 2 RA grill upstairs might be humming due to air flow remove it see if noise is better. 3 do not adjust blower speed unless you know what your doing 4 for noise the furnace blower compartment can have ductliner installed and the ducts if they are sized for it . 5 canvas connector can be used on the risers and RA connections to keep furnace from rattling duct work

(post #166994, reply #8 of 9)

*
O this furnace comes W / ins . how big our your ducts and metal ductbord or what ?

(post #166994, reply #9 of 9)

*
The general noise level produced by my furnace is too loud. I believe the main culprit is just the sheer volumetric flowrate being pushed through the ductwork. The blower by itself sounds normal. The burners put out a good bit of noise, but that is unavoidable I imagine. My house is 1-1/2 yrs old with a Rheem "Criterion II" natural gas forced air furnace. 125,000 btu/hr. Blower is 9.5 amps, 3/4 Hp. I think the blower is capable of 4 speeds, since a table printed on the panel shows the airflows for the 4 speeds. My house is 2700 sf, 2 stories, 10 registers up, 10 registers down, 1 zone.

Whenever my furnace turns on, the blower starts in a slower speed (without flame) for about 45 seconds, then the burners light and the blower shifts to a medium speed for about 30 seconds, then finally shifts to fast speed.

Is it possible to make the blower run at a lower speed? Is it safe (delta-T would increase)? Would the efficiency be changed? How can I accomplish the speed change?

Thanks.