How to Quiet a Noisy Furnace Blower

Make Sure It Really is the Blower

All that racket coming out of the furnace vents is, invariably, blamed on the furnace and, by extension, the blower. Because, as far as you know, it is the only moving part in there. Make sure you check it out first. But be prepared to discover that the blame lies somewhere else. 

 

The Mechanics of a Forced Air Heating System

Your heating system is a complex combination of incoming fuel, incoming air, recycled cool air, and warm air blown into the places where it is wanted and needed. The heart of the entire system is the furnace. It is installed by people the construction industry calls ‘tin bashers’. The name refers to someone who is usually highly skilled in, and knowledgeable about, a multitude of things most of us know very little about.

They must ensure that the system they are installing has the correct amount of fuel mixed with the proper amount of oxygen (both returned from inside the house and brought in from outside) to produce the required amount of heat safely–and then be distributed throughout the building. All we do is turn up the thermostat; then wait for the heat to arrive. When noise arrives with the heat, we either put up with it or try to make it quieter.

 

How to Reduce Moving Parts Noise

The only true moving parts in your furnace are the blower fan and motor. But they run a lot and move thousands of cubic feet of air every year. Eventually things wear, or get loose, and will begin making noise that is carried into your living spaces. This is called flanking noise–sounds traveling through the system that will reach into each room. (This type of noise is different than Airborne Noise or Impact Noise which generally travel through the adjacent walls, floors, or ceilings.)

These noises can be described as squealing, humming, scraping, thumping, banging (usually on start-up), rattling, and rumbling. 

• Squealing – caused by lack of lubricant.  Older electric motors did not have sealed bearings. They need to have oil added annually–in the oil ports at each end of the shaft. If it does not have oil ports do not oil the shaft. That will only leach the grease out of the bearings. Older blower wheels will also likely require oil. (Really old ones might have grease cups that need to be filled with bearing grease.) You will probably need to remove the access cover to get at them. Use 3 in 1 Motor Oil. It is made for electric motors.

• Squealing – caused by a loose belt. If the belt is loose, the pulley on the motor will turn faster than the belt is moving. The resultant rubbing will cause your squealing noise. There should be a method to tighten the belt to solve this problem. (But while you are down there why not loosen it enough to remove the belt and inspect it. Replace it with the correct size if it is cracked or frayed. Also make sure that the pulleys on the motor and blower fan are tight and lined up. A loose pulley is as inefficient as a loose belt and will shorten the life span of both your belt and shaft.)

 

• Humming – could be caused by a loud transformer, the capacitor, or the fan itself. You might not want to stick your head inside while it is running to figure out which–if any–of them are making the noise. But neither the capacitor nor the transformer are all that expensive.You can change them one at a time if you are not certain. If neither is the problem you will still help the motor run more efficiently. Humming from the blower fan can be caused by dirt build-up, bent blades, or worn bearings causing it to oscillate. If you decide to change it–and it would be my last option–you will have to remove the entire squirrel cage (blower assembly and housing) to get at it. There are many available wheel sizes. Make sure you order the correct one.

 

• Scraping – the sound of metal rubbing on metal. This can be caused by a loose or broken motor mount, a loose or broken blower wheel, or loose pulleys that are touching the motor housing or squirrel cage housing. Inspect all of these. Make sure all parts are tight. Replace anything that is broken. If  the pulley wheels have been rubbing for a long time make sure you replace them with the same size.

 

• Thumping – often caused by an unbalanced blower wheel. This is often caused by dirt build-up, or bent, broken, missing blades, or wear on the shaft. Again, you will have to remove the entire squirrel cage–first to diagnose the problem, then to repair it.To my knowledge, you cannot replace individual blades of the blower wheel so you will have to replace the whole unit. Purchase the correct part and make sure before you re-install it into the cabinet that everything is tight, lubricated (if necessary), and you do not have any spare parts lying around. You do not want to take it out again.

 

• Banging (also whistling or ‘whooshing’) – usually caused by undersized ducts, closed, or plugged vents, or a clogged filter. Although these noises do not strictly happen in the furnace compartment, you might hear them and wonder what is falling off down there. Undersized ducts–both warm air ducts and return air ducts–usually are a product of a new blower or new furnace with more capacity than the system was designed to handle.The air cannot get through fast enough to satisfy the blower forcing pipes to move around where possible, and the plenum (the metal chamber attached to the top of the furnace) to expand and contract. Closing or plugging some air vents will force more heat into rooms with open vents. It will also annoy hell out of the furnace causing it to make banging noises expressing its displeasure. This can also cause a dangerous heat build up. For filters, please see the section below.

 

• Rattling – most often caused by vibration. Check for loose parts that can be anywhere or anything. Blower motor mount, squirrel cage, plenum, filter too small, or the furnace itself vibrating. Installing an acoustic rubber pad under the furnace (specially if it is sitting on concrete) will damp the vibration you hear through the pipes.Although the blower fan and motor are the most obvious parts to check, take a look at the complete unit. Pipe brackets, burner parts, cold air returns, access panels into the burner chamber and blower chamber, or even the plenum itself could be loose. The more vibration you have had over the years; the better chance of more things becoming loose.

 

• Rumbling – is almost invariably a sign of a burner problem or fuel ignition problem known as ‘Roll-Out’.  Roll-Out is a small explosion or burst of flame on start-up that could be caused by a cracked heat exchanger. This can be a dangerous situation. So, unless you really know what you are doing–and most of us don’t–have a professional furnace installer inspect it, and deal with it if necessary. Everyone in the house will sleep better.

NOTE: Before checking into any of these potential problems or doing any repairs–TURN OFF THE FURNACE BREAKER , TURN THE THERMOSTAT COMPLETELY DOWN, TURN THE GAS OFF– and threaten everyone in the house with physical harm if they change any of your settings while you are working on the furnace.

If, after all of your diagnostics, you decide that a new blower motor is the answer, make sure that the new one has the same horsepower rating as the old one. There is a distinct possibility that replacing your 1/3 horsepower motor with a 3/4 horsepower motor (because you want to move more heat into the living areas) will exacerbate some of the problems you are trying to eliminate. More power and speed can overpower the system because it is not designed for a larger sized motor, and you will get more banging, or humming. Be careful what you wish for.

 

How to Reduce Heating Cabinet Noise

My furnace, which is 13 years old, has 3/8″ acoustic soundproofing inside the blower cabinet with a cut-out for incoming air. The blower cabinet in most older furnaces is essentially a hollow tin box with a motor and fan hanging from the top. The access panel is quite often too loose allowing it to rattle; or too tight allowing you to break fingernails removing it. And the slot holding the filter was not manufactured using a micrometer either, allowing the filter to move when the fan goes on or off.

Types of Filters–and Why They are Important

There is a huge range of furnace filters–from cheap fiberglass to washable, re-usable, electrostatic. Also various sizes and thicknesses. The one you choose must be sized for your furnace. It also must allow good air flow; yet filter out as much dirt, dust, bacteria, and microbes as possible. Without decent air flow your blower motor will have to work harder, you will not get as much air into the rooms, and you could be trying to track down noises coming from the vents. If you are not filtering out most of the dust and dirt, not to mention anything else, what is the point of even having a filter?

After much experimentation over many years we settled on the Filtrete 1900. It has very low air flow resistance (like cheap fiberglass) but has a very high filtration efficiency. No, I do not use the Smart Phone App. (Turning my cell phone on is usually an accomplishment.) I follow the directions provided, or change it when my wife tells me to.

Soundproofing the Cabinet

New furnace motors and blower fans are very quiet and most new furnaces have soundproofing material in the blower compartment. Older furnaces are usually just metal boxes housing the equipment. I would suggest lining it with Automotive sound proofing mil automotive sound deadening mat. The backside is fully self-adhesive which is one of the main attractions. Screwing through the metal cabinet kind of defeats the soundproofing program, and you might make things way worse by drilling into something important–like your fuel line.

View on Amazon

Full disclosure. I have never used the product but from the information and reviews, I believe it will do the job. As long as the glue holds up. Having a piece of insulation sucked into your blower will not turn out well. Given my history and product knowledge, I would reach for a box of self-impaling insulation clips, glue them on the cabinet walls about 6″ apart (special adhesive sold with the clips), install my acoustic barrier and the exterior washers, and sleep well at night knowing nothing will ever come adrift.

The downside of this method is cost. You only need about 20 or 30 clips and washers. Most places will not sell them in quantities of less than 100, and the adhesive is sold by the gallon (enough to do over 500 clips). You might find a siding and/or insulation company that has some leftover from a job and is willing to sell you what you need.

 

Noise From Peripheral Attachments

As mentioned, the heart of your heating system is the furnace–consisting of the heat chamber, blower chamber, attached plenum, and miscellaneous piping leading to, and from, most of the rooms in the house. Just because you hear noise from your heat vent does not mean it is caused by the furnace. And even if it does come from the furnace it is caused by flanking noise–sounds travelling through the system that will reach into each room. 

Soundproofing the Plenum

The plenum is the large metal hat attached to the top of your furnace. It collects the warm air and directs it through the duct system. Almost invariable it is screwed to the furnace. If you feel that sound is escaping from the joint you can add more screws, run duct tape or foil tape over the seam, or seal it with high heat Silicone sealant like Silicone Sealant.

My preference would be to remove all of the screws, raise the plenum at least 1/8″, fill the gap with sealant, then lower it in place, and re-screw it–adding extra screws if necessary. (Although the pressure is fairly low, it is building up inside the plenum. Always try to have the caulking on the pressure side of anything you are sealing, or between two materials. Works better to have air or water pressing it into the gap than away from the gap.)

Soundproofing the Air Pipes

Faulty duct installation can produce a banging or popping sound. When the furnace blows warm air the metal ducts expand from the heat. If they cannot move lengthwise you will have something called ‘oil canning’. The pipe will wrinkle, or buckle, in and out because it is suddenly too long for the space it is occupying. (Think vinyl siding that is nailed too tight. The same term is used for the wrinkles in it.) I have seen a suggestion that you hammer a dent in the pipe where you think this is happening. Seems counterproductive to me. Especially if it does not solve the problem.

First I would make sure that the pipes are loose in the hangars, and are not strapped tight to the joists. Then I would remove all of the tape from the joints and make sure they are joined as tightly as possible. Make sure you re-tape all joints. You should do this anyway to ensure no air, or whistling noise is escaping from them.

You can use duct tape or foil tape. As a last resort you can remove the boot (the part at the end of the duct that gets the air into each room) and cut the pipe shorter by about 1/2″ – 3/4″. That should solve the problem. I know–it is way easier to bang a few dents into each duct.

Note: Although it has been used for decades to seal duct joints, some people in the industry are beginning to question the efficiency of duct tape. I am not able to judge that, other than to make the observation that it is much thinner than it was.

To truly soundproof your ducting, you can take them down and wrap them with a heat reflective insulation (which will also help prevent heat loss from the pipes), or Mass Loaded Vinyl–a great soundproofing barrier. (Both of these are a fairly expensive and time-consuming option.) When you re-install the duct work make sure that you hang them properly. I would use Woven Duct Strap instead of the common metal strapping. It eliminates any kind of metal on metal rubbing, or tearing of pipe wrapping.

To be honest, the duct noise would have to sound like a marching band before I would take my ducts down, wrap them, and re-install them.

Return air ducts are almost always quiet. The only place there could be noise is at the furnace filter. See Types of Filters section above.

 

Noise From the Furnace Room

Noise from the furnace blower can be very annoying in adjacent rooms. In an unfinished basement there are no adjacent rooms to be concerned about. But there will be some airborne noise into the room above.

Soundproofing the ceiling can be a nightmare as you can see from the attached picture of my utility room. It is way more efficient to soundproof the floor above. (Please see our article ‘How to Soundproof a Floor‘ to help you deal with that.) Mobile and modular homes quite often have the furnace on the main floor, producing all kinds of opportunity for Airborne and Impact Noise.

Soundproofing Basement Furnace Rooms

Most modern building codes require that the furnace is in a separate room, or that the house has a door at the bottom or the stairs to separate the heating system from the living area. But in many houses the furnace is just sitting in an open basement. If you decide to enclose your furnace and/or utility room, soundproofing is fairly easy. You can frame the room, then insulate the walls and ceiling (if easily accessible) with soundproofing ProRox SL 960 Rockwool, drywall and finish as required.

If a room exists–whether the basement is finished or unfinished–I think that hanging sound suppressing blankets on the  walls, and a soundproofing curtain over the doorway are really your only viable solutions. Moving blankets will do the job. You will probably have to cut, and tape, the blankets around any ducting, plumbing fixtures, electrical wires and boxes. If you want any ceiling soundproofing to be effective, I think you will have to address it by soundproofing the floor above.

Soundproofing Main Floor Furnace Rooms

Main floor furnace rooms have the added problem of Impact Noise because the furnace is sitting on your framing–not a concrete floor below the living space. Sound and vibrations will travel along both the ducting and framing. After doing everything possible to quiet the blower fan and motor, and the cabinet (see options listed above), you could consider trying to lift the furnace enough to get a sound and vibration suppressing membrane under to contact points. (The good news is–the plenum is not attached to the top of the furnace. The bad news is–the plenum passes through the floor under the furnace.)

Most main floor furnace rooms are designed to use as little space as possible. Therefore, working inside them is a struggle–at best. And the walls are likely finished with drywall or paneling. So, again, your best bet is to hang soundproofing blankets on the walls. You should also soundproof the ceiling to keep noise out of the attic (if there is one) because sound will also travel up there and you could hear the noise in other parts of the house. Bubos Acoustic Panels are not only a sound absorbing product but also fire resistant and flame retardant. Install them with X Fasten double sided mounting tape.

If I did not care about removing them without destroying the ceiling finish I would also use Liquid Nails construction adhesive. (Note: Be absolutely certain you want the panels to be on the ceiling, or walls, permanently because once the adhesive sets removal becomes a demolition project.)

Some main floor furnace rooms have louvered doors because that is the cold air return for the furnace. You cannot completely blanket the door or change the slab to something solid without adversely affecting the operation of your furnace. You may be able to blanket off some of the louvered section–if the door is fully vented–and still get enough air to enable the furnace to function as designed.

If you decide to take this route, go slowly. Blanket off a small portion (at the top) and check how things work. If you are satisfied everything is still OK, then continue to add until you have as much soundproofing as the furnace can handle. If only a section of the door has louvers, don’t mess with it. The door is probably engineered to supply the correct amount of return air to the furnace to provide maximum efficiency.

HVAC (Heating, Ventilation, Air Conditioning) Information & Education

If you want way more information than you will ever need about HVAC systems, one of the best sites I have found is Healthy Heating. The professor tends to pontificate a bit but there are articles on most types of heating systems. Very little on the site is about soundproofing. If there was–I would not send you there.

Final Note: Make sure you have your heating system cleaned every year. Eliminating built-up dust and dirt (not to mention the kid’s Lego pieces) only makes it more efficient–and quieter. It is also possible that your duct cleaning contractor will point out any potential problems that he/she encounters.