Slow Furnace Fan Down?

[bad_idea]

This is a house question, sorry all. I imagine the info should help some folks with their garage furnace though.

I have central heat and air in the house. The furnace fan is exceptionally loud. I had an HVAC guy come out and replaced the blower wheel on it. That quieted it down a good bit. I would like it quieter. The evaporator and fan are located in a closet in the back of the house. The closet itself is the 'case' for the unit with the air coming in through the ceiling piped from the returns and the air goes out the floor of the closet. The closet door is part of the 'case'. I am thinking the door transmits a lot of noise and I am sure the walls of the closet are not insulated.

I am considering pulling the sheetrock out of the closet, install some Roxul, and then install two layers of soundproof drywall. Before I go to all of that trouble, I would like to look at simpler options. Is it possible to slow the blower motor down? Would slowing the blower down impact heating and cooling costs? Other thoughts?

As a quick fix, I am considering these panels: https://www.amazon.com/50Pack-Acoustic-Panels-Sound-proofing-Absorption/dp/B07DC9PK3T/ref=sr_1_10?crid=18IPT5UQ688VB&keywords=sound+deadening+panels+for+home&qid=1576411167&sprefix=sound+deadening+panels%2Caps%2C155&sr=8-10 Would these do well exposed to the large volume of air passing through the closet? Would they muffle the sound going out into the house?

 

[larry4406]

I'm not an HVAC guy but I think if the fan speed is set too slow while in AC mode, the coil can freeze up.

 

[Showkey]

The fan in many systems is set to low speed for heat and high speed for AC.

Simple check of the wiring on the blower will confirm the speeds.

Some times they set the blower higher to force air to distant rooms. Especially if the duct design was poor. The higher the fan speed the lower the temperature rise will be.

 

[fitter30]

Solid door? Try a piece of drywall inside the door.

 

[ant.foste]

Slowing the fan down will limit the btu transfer and subsequent evaporation or boiling of the liquid refrigerant entering the indoor evaporator coil. Too much reduction will lead to you sending liquid refrigerant back to the compressor outside and causing damage, will lead to icing and freezing of the indoor coil and had possible damage, and can lead to reducing heating effectiveness in the winter but actually better dehumidification in the summer months.

More advisable alternatives are to install sound insulation on the exterior of the furnace and accessible ducting, then insulate the the walls, ceiling, and door of this closet. A more expensive and less "hack" solution is to upgrade your HVAC system to a higher grade model from a known name brand, as "quiet" is part of the engineering that's built into the machines to please the more demanding consumer.

 

[SHIFT]

This is a house question, sorry all. I imagine the info should help some folks with their garage furnace though.

I have central heat and air in the house. The furnace fan is exceptionally loud. I had an HVAC guy come out and replaced the blower wheel on it. That quieted it down a good bit. I would like it quieter. The evaporator and fan are located in a closet in the back of the house. The closet itself is the 'case' for the unit with the air coming in through the ceiling piped from the returns and the air goes out the floor of the closet. The closet door is part of the 'case'. I am thinking the door transmits a lot of noise and I am sure the walls of the closet are not insulated.

I am considering pulling the sheetrock out of the closet, install some Roxul, and then install two layers of soundproof drywall. Before I go to all of that trouble, I would like to look at simpler options. Is it possible to slow the blower motor down? Would slowing the blower down impact heating and cooling costs? Other thoughts?

From your description, it sounds like you have a down-flow (or counter-flow) furnace, with supply air (S/A) only being distributed from below the furnace. Is that correct?

 

[bad_idea]

From your description, it sounds like you have a down-flow (or counter-flow) furnace, with supply air (S/A) only being distributed from below the furnace. Is that correct?

Yup. Air comes in the top, goes out the bottom.

 

[SHIFT]

Yup. Air comes in the top, goes out the bottom.

I've never seen it that way, generally the space under the furnace is used as a return-air (R/A) plenum, and an up-flow furnace is installed with S/A distributed throughout to each room via ductwork in the attic. It is hard to believe you get much cooling/heating if the S/A is coming from a single place in the house.

Regardless of whether it is uplow or downlow, a furnace in a closet with the grilles below it will always be noisy. I don't think further insulating the closet is going to help as the source of the noise is the fan moving air across the grilles below it. Here are a couple of things that you can look into.

1. Insulate the space under the furnace. It won't help much, but it will help a little.

2. Check to see if you have a stamped or bar-type grille. If you have a stamped grille, replace it with a bar-type. On a stamped grille (one-piece) the individual openings will be smaller than a bar-type (multiple pieces), which will contribute to the noise. The bar-type grille will be much quieter because it will provide less resistance to the air moving across it. You can use a screwdriver to increase the openings on a stamped grille, but it won't come out pretty....

3. Check the grille size and see if they are properly sized for the amount of CFM being pushed through them. As I noted above, every residential furnace installed in a closet that I have seen is always drawing air through the space under it. The size of the grilles is the #1 contributer to noise, usually because the grilles installed are way to small. The general rule of thumb is one square foot of grille per ton, but I have seen that being met maybe 20% of the time. Since your grilles are supply air, it is a different situation but I still think larger grilles may help. You may want to talk to an HVAC contractor about this, specifically their in-house Engineer.

Ultimately, the quietest A/C system install will always be a horizontal system installed in the attic space with as much duct on the R/A line as possible and the biggest R/A grille you can fit. Keep that in mind when it is time to upgrade.