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I recently changed my HVAC filter and followed the same airflow direction (pointed arrow direction) as was on the previous filter. Almost immediately after replacing it, I keep hearing a whistling sound come from the HVAC whenever it's on. Is it because I chose a too-small pore size? I put in filters with MERV 11, which is much smaller than was there before, but we have a baby at home and I want clean air!

Picture below if that helps.

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  • Is the new filter a more effective type? The cheapest filters are the least restrictive of air flow. The better the filtration the more restriction of flow which can lead to whistling – Kris Sep 14 at 16:51
  • Yeah it is much more effective (due to baby) – David Sep 14 at 16:51
  • Is the whistling coming from the filter housing or elsewhere in the ducts? – isherwood Sep 14 at 16:52
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    Check the filter housing. I suspect you will find leakage that is now being exacerbated by the more restrictive filter. Seal up the leaks and you should notice the whistling diminish. – jwh20 Sep 14 at 16:56
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    David, I would be looking into an electrostatic filter to retrofit in your system. I normally install large particulate filters on the intakes (the cheap fiber glass 1” ones) then in the furnace room on the intake side of the furnace I install electrostatic filters these for a big system are around $1200.00 in the US. They take smoke out of the air where a MERV 11 can’t do that. I have a granddaughter that has severe asthma and with the air handler running 24/7 that keeps the air clean even at times like Oregon is going through now. I pull the elements and hose them in the shower monthly . – Ed Beal Sep 14 at 17:12
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Yes changing to a finer filter can cause serious damage to your system. I try to always mention this. Each system is set up for a fairly narrow range of back-pressure.

By putting in a a filter that takes smaller particles out it increases the back pressure on the system and can even cause motor failure. How can this cause motor failure motors cool themselves with internal fans if the load it out of range the motor will star overheating once the motor gets to about 180 external temp the internal temps are much higher and they usually start shorting out 1 coil to the next more heat and soon you see and smell the magic smoke that is escaping.

The other thing that can happen is overheating the fire box lucky there is a safety to prevent a meltdown but it can shorten the furnace life if used for heat while the higher rated filter is in place.

Last the air exchanges are reduced so less air is getting filtered, so not only is size important but also what the system was designed for. High MERV filters require a little more beefy baffle to hold the filter securely or they buckle and air goes around them.

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  • Most of the systems don't give you any info on what MERV values you can/should use..... at least they didn't use to. +1 – JACK Sep 14 at 17:55
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    I put it on them when I assemble the duct work , most at least write the suggested filter type on the cover or access port. The air handler should have its max pressure that’s what we design around. – Ed Beal Sep 14 at 18:22

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