Merv 5 – How High Can I Go Before Restricting Flow Too Much?

by Don Munn

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Question: I have a Water Furnace and currently use a pleated filter with a MERV 5 rating.  how high can I go before I start restricting the flow too much and damage the system?

A Merv 5 filter is really not very efficient.  If you have a properly sized duct system and you change your air filter regularly I see no reason not to use an air filter with as high as a Merv 12 rating.  as Merv ratings increase so does restriction to air flow, so you always have to be careful.  The reason I mention a properly sized duct system is because if the duct system is undersized it will restrict air flow dramatically and is a common problem in many homes.  A very efficient air filter and an undersized duct system are not a good combination.

There are a couple of signs that may indicate an undersized duct system:

-  The air conditioner coil ices up when the filter becomes dirty.  Most systems will do this if the filter becomes extremely dirty but you should not have this problem if the filter becomes only moderately dirty and you change it regularly.

- Whistling sounds coming from around the air handler access panels can be an indication of an undersized duct system.  This can also indicate poorly fitting access panels.

- If your furnace trips of on the high limit switch, this could indicate an undersized duct system.

There are other things that could cause the problems listed above but they are also signs of restricted air flow.

If you have your equipment serviced regularly ask your contractor if the duct is sized properly.  Opinions vary among contractors regarding filters with high Merv ratings.  Your other option is to try a higher Merv rating and monitor equipment operation closely.

Don Munn About Don Munn
Don has been working in the HVAC industry for more than 25 years and shares his experience and knowledge on this site. If you find the information presented here helpful please use the buttons below and share with your friends.



{ 4 comments… read them below or add one }

Barbara

I received an Air-Care electra silver filter which I am using in my Lennox G26 gas furnace. I have found one corner of it raised when I have checked it. I secured it with small wood blocks. Is this filter meant for an air conditioning unit only; or may it be used in a furnace as well? Am I damaging the furnace by securing the filter with blocks to hold the clamps thus the filter in place? Thank you

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DonaldT

I assume you mean that the filter was being lifted from the filter rack due to air flow?

If so, it is possibly an indication that the filter is restricting air flow and there is quite a pressure drop through the filter.

If the filter is not heavy it’s probably not a big deal. If the filter is heavy and this is happening it indicates more of a pressure drop which could be a problem.

I have seen many pleated filters do this and don’t consider it to be a problem because they are very lightweight.

I’m not really familiar with these particular filters but I think they should be fine for a furnace. The best thing to do would be to contact the supplier or manufacturer. They should be able to provide you with more accurate information.

Holding the filter down with small blocks of wood is fine as long as they are not in the air stream and causing more restriction to air flow. Holding the filter firmly in place helps to ensure all air is going through the filter and not bypassing it.

It’s also very important to clean these filters monthly.

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ralph coreno

a contractor installed a 16/24 air filter instead of a 20/24. Is this filter just as good?

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Don Munn

Hi Ralph,

First I have to say that you should always install the biggest filter possible in your HVAC equipment because a bigger filter means less air flow restriction.

My concern is that if your furnace will accept a 20 X 24 X 2 filter then the 16 X 24 X 2 can’t be filling the entire duct or air handler opening. If this is the case there is a lot of air bypassing the filter and that’s a problem. If the extra 4 inches of opening is blocked off in some way to prevent air from bypassing the filter then it is probably restricting air flow.

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