Bryan asked:
Hi. This website is great; there is really good information. My question is regarding large filters. My furnace requires a 5-inch pleated filter, but because these are hard to find I have placed 5 1-inch pleated filters from 3M. Is this going to restrict too much airflow? Should I buy a 5-inch or cut back to 3 or 4 1-inch filters? Thank you so much.
Hi Bryan,
You should remove 4 of those filters immediately because they will have a negative impact on air flow and operating efficiency will be reduced. Stacking filters in that way will have the same effect on your system as a dirty air filter. Dirty air filters and poor duct design are two of the main causes of poor air flow.
Residential heating and air conditioning systems are not designed to accommodate more than one filter and five pleated filters stacked could cause major problems for you.
Potential problems with low air flow in an air conditioning system
Air conditioning systems are designed to have 400 CFM of air flow to operate at maximum efficiency. 450 CFM is required for heat pumps. There are a number of potential problems that can result from reduced air flow when stacking air filters in an air conditioning system.
- Reduced air flow which can cause poor air distribution throughout you home.
- On an air conditioning system the evaporator coil can ice up.
- Operating efficiency will be reduced which means longer run times and higher operating costs.
Potential problems with low air flow in gas or oil furnaces
- Poor air flow is one of the primary causes of heat exchanger failures. When heat exchangers fail carbon monoxide can escape into your home and cause carbon monoxide poisoning.
- Uneven temperatures or cold areas in your home
- Lower operating efficiency
A 5 inch filter can be left in the system much longer than a 1 inch filter, 6 months or longer depending on a number of factors such as pets, traffic in and out, kids and so on. Each home would be different. Also a 5 inch filter would have less impact on air flow than a 1 inch filter.
Best,
P.S. I’m not sure what size 5″ filter you require but the links below will take you to 3 of the most common sizes.
16 x 25 x 5
20 x 20 x 5
20 x 25 x 5
Nicole Asked:
I am a tenant living in a duplex and here recently the blower motor caught on fire. I use MERV 8 filters and have changed them every three months.
The HVAC technician that came out to the home said that the filter caused this blower to catch on fire. The issue I have is that it was not sucked into the blower but was still in the filter cabinet and moderately clean (with a little smoke damage).
Is this common for filters to be the cause of blowers to catch on fire or is it more likely that a bearing got overheated / was not lubricated? Remind you that these units have not been serviced for over three years. If you could give me your knowledge and information based off that I would appreciate it.
Hi Nicole,
You are correct in your analysis of what happened. I really doubt that your furnace filter caused the motor to catch on fire, although neglecting your furnace filter can contribute to some of the factors that cause this to happen. That sort of thing is caused by overheating and a MERV 8 filter is not going to have enough impact on air flow to cause that to happen.
Motor overheating can be caused by several things like low voltage, high voltage, tight bearings, shorted windings in the motor and probably a couple of other things that don”t come to mind at the moment.
If you haven’t had the motor replaced yet, make sure the technician checks for proper supply voltage and other problems. I would really recommend a different contractor.
Some Common Causes of Blower Motors Over Heating
Although unlikely, blower motors do occasionally catch fire. As with any fire this is a dangerous situation and is one more reason HVAC equipment needs to be installed and maintained properly, by a qualified service technician.
- Dirt collecting in and around the motor. – Over time dust can build up on the inside and outside of the motor and act as insulation. This reduces the amount of air passing through and around the motor and causes overheating. Inefficient filters can contribute to this problem.
- Sustained low voltage or high voltage. – Motors are designed to operate within a certain voltage range. Improper voltage causes motors to overheat. The usual cause of low voltage is poor electrical connections. High voltage is a distribution problem.
- Tight and worn motor bearings. – This is usually caused by a lack of
lubrication. Not all bearings are designed to be lubricated, some are sealed and can’t be accessed for lubrication. Check your motor to see if there are oil ports on the ends of the motor. They can be difficult to access but if the motor is oiled it could potentially last years longer. When bearings become dry they wear and overheat.
- Shorted motor windings will cause a motor to fail and can cause the motor to catch on fire, although, the motor will usually fail without catching on fire. Shorted motor windings are usually caused by weak winding insulation which is the result of over heating caused by the problems listed above.
Best,
