Trained Eye

 
 
 

Noisy New Furnace

Question:

 I recently replaced the low efficiency furnace in my 1050 square foot bungalow with a two-stage, high efficiency furnace.  The ductwork, which dates back to 1953 when the house was built, was not changed.  While the furnace motor runs more quietly than the one in the old furnace, it forces the heated air through the ducts at a much greater velocity.  As a result, there is a significant amount of noise. This is best described as a whooshing sound as the air exits the registers.  I find the noise to be annoying, especially at night when the house is otherwise quiet.

Do you have any recommendations on how to reduce this noise?  The installation guide for the furnace briefly mentions that the blower speed can be decreased or increased by changing the motor's wiring.  Is this something that I should ask the company that installed the furnace to do?

Answer:

It is often quite surprising that changing one component in a home can affect the entire home to such a degree that it either dramatically improves or causes major concerns to the comfort level of the occupants. In this case, your new furnace has done both. The heightened efficiency of the blower in the furnace has made the heated air move through the home much more quickly, effectively heating and cooling the home better.

Unfortunately, this higher air speed has caused a major annoyance. I will try to offer some suggestions to help with the problem.

The interesting thing about your issue is that it illustrates an excellent point about the way we view our homes. Most people will view their home as a living area, which contains many smaller systems, each operating independent of one another. Home inspectors are trained to treat the entire home as one large system. In that way, we can analyze the way each individual component or system within the home affect the overall function of the building. It is very important to understand how a defect or failure in one component may affect other parts of the home. In your situation, an increase in air movement within the home may help minimize moisture buildup and condensation, provide more even heat distribution and increase the comfort level. What has not been taken into consideration is that the high volume of air moved with the new furnace blower may negatively affect the performance of the older ducting in your house.

The reason for the greater air speed and noise is simply a better designed and more powerful blower in the furnace. You are correct that it may be possible to turn the speed down by simply rewiring the control panel in the furnace, but I would not recommend this as the first solution to your problem. This furnace is designed to operate with the fan on the highest level and lowering the fan speed may defeat part of the purpose of upgrading your heating system. Changing the fan speed will not affect the furnace itself, but it may require the unit to cycle for a longer period of time until the thermostat is satisfied. This may raise your heating bills, only slightly, but may also reduce the effectiveness of the newer furnace.

The first solution may be to check the ducting to see if any components or parts are loose or have gaps or holes. The whistling noises you are hearing may be caused by air leaking out small areas in the ducting or from friction with parts of the ducting or registers. Check to ensure that your registers are securely fastened within the heating boots in the rooms and that there are no loose or broken parts. Simple duct tape and small amounts of sealant or a few screws and pieces of sheet metal may help close up any troublesome openings.

The problem with the new noise may not be as much with the newly installed furnace, but your familiarity with the old unit. By this I mean that you never noticed the noise from the old furnace, because you became accustomed to it and stopped noticing anything unusual, over time. I suggest that this may happen with your nice new unit, as well, over time. What may help is to have your heating contactor adjust the fan speed to a lower level, and gradually adjust it upward over a period of time. This may acclimatize you to the new noise of the higher volume of air moving through the ducts, and the problem, which may still be present, will not be as noticeable.

Another thing that may help is to run the furnace fan continuously on low speed. This will help you get used to the air movement at low speed and when the thermostat calls for heat, the furnace will automatically kick into high speed. This will also improve the air movement in the system and may produce slightly shorter cycling times, as well.

Whatever the outcome, we should always be prepared to have some period of adaptability to new components installed in our older homes. Mostly they should improve the quality of comfort and convenience within our homes, but sometimes they create unforeseen problems. When a major upgrade like a furnace is undertaken, we should be prepared to make alterations or modifications in other systems to balance out these changes.