Set blower speed higher, then use more restrictive high MERV filter to cancel out?

**startover909** · 10-19-2020, 05:54 PM

My contractor installed a furnace (Carrier 58SB0A090E21--20) for me that has a multi-speed ECM blower that's capable of up to 1980 CFM air flow. My AC unit is just 3-ton, so it only needs around 1200 CFM.

The tech adjusted the blower to spec and cooling performance is now good. I'm currently using a non-restrictive MERV4 filter.

The tech told me my specific furnace model cannot adjust the fan speed automatically based on static pressure, even though it has a ECM fan. Not sure if this is true.

So I'm just wondering, since the blower is capable of higher speed, can I opt to use a very high MERV rated furnace filter which gives good air filtration performance, then have the tech increase the blower speed a bit to counter the extra pressure? Would this do harm except for some increased energy usage by the blower? Or it does not work this way?

**JayMan7** · 10-19-2020, 05:59 PM

Are you asking about using like a 4" filter in place of a 1" filter? Because a 4" filter provides so much extra surface area you'd probably be fine (and they are high merv). As far as fan speed it would seem that although you have a ecm motor it is not true variable speed it just has speed taps so yes it sounds like the tech is probably correct.

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**startover909** · 10-19-2020, 06:05 PM

Originally Posted by JayMan7

Are you asking about using like a 4" filter in place of a 1" filter? Because a 4" filter provides so much extra surface area you'd probably be fine (and they are high merv). As far as fan speed it would seem that although you have a ecm motor it is not true variable speed it just has speed taps so yes it sounds like the tech is probably correct.

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No I did not have the 4" media filter enclosure installed (regretting that choice). I'm just using the standard 20x25x1 filter.

Previously I always used low-MERV filters to avoid restricting air flow as recommended by others. Now I wonder if I may actually use a high MERV one to improve air quality if the blower can be adjusted to counter the pressure drop.

**BaldLoonie** · 10-19-2020, 06:14 PM

Get the media cleaner put in, so much better than 1" and less restrictive. Your dealer can take a static pressure reading at get down into the 3 ton airflow territory. From what I see, probably going to end higher than 3 ton or lower, the chart didn't have a speed tap that would give you right in the 1200 CFM range. I don't know why dealers don't look at the size of the blower of the furnace they put in. Your furnace is a good one for a hot climate where you don't need that much heat but need 5 tons of air to cool.

**startover909** · 10-19-2020, 06:25 PM

Originally Posted by BaldLoonie

Get the media cleaner put in, so much better than 1" and less restrictive. Your dealer can take a static pressure reading at get down into the 3 ton airflow territory. From what I see, probably going to end higher than 3 ton or lower, the chart didn't have a speed tap that would give you right in the 1200 CFM range. I don't know why dealers don't look at the size of the blower of the furnace they put in. Your furnace is a good one for a hot climate where you don't need that much heat but need 5 tons of air to cool.

They told me it would cost a lot more to install the media filter after the fact. They previously offered to install it for $ during furnace installation, which I declined because I did not do enough research at the time for its benefits (and hated the idea of changing filters in the attic), and they did not really push it either.

So you're saying for this furnace, it's impossible to tune it to the exact 1200CFM range. Suppose it's set to a bit higher than 1200CFM, would it make sense to use a high MERV restrictive 1" filter to reduce the air flow a bit to even things out?

**BaldLoonie** · 10-19-2020, 06:33 PM

From what I saw, there's a 200 CFM gap between the 2 speeds on either side of 1200 CFM. Put your filter in and ask you dealer to take an external static pressure reading of the system. Then he can look at the airflow chart Carrier provides and set accordingly. If in a humid climate, go under 1200 but not much under 1050 CFM. In a dry climate, going over 1200 is fine.

**rider77** · 10-19-2020, 06:35 PM

Are you trying to use your new furnace as an indoor air cleaner?

**startover909** · 10-19-2020, 06:41 PM

Originally Posted by rider77

Are you trying to use your new furnace as an indoor air cleaner?

Lol no. But I do want less dander and dusts etc without having to use a purifier. I understand the AC filter will never work as well as an air purifier, but I guess it can still help?

**JayMan7** · 10-19-2020, 06:49 PM

Originally Posted by startover909

No I did not have the 4" media filter enclosure installed (regretting that choice). I'm just using the standard 20x25x1 filter.

Previously I always used low-MERV filters to avoid restricting air flow as recommended by others. Now I wonder if I may actually use a high MERV one to improve air quality if the blower can be adjusted to counter the pressure drop.

When I said 4" filter, I was actually assuming you did not have a 4" housing. What I was talking about is something like this...

They can restrict the air flow some depending on the design of the duct they are sticking into, but realistically I doubt it would cause you a problem. We sell the ones that have foam around the outside edge to seal them better but I can't remember what the brand is.

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**startover909** · 10-19-2020, 07:10 PM

Originally Posted by JayMan7

When I said 4" filter, I was actually assuming you did not have a 4" housing. What I was talking about is something like this...

They can restrict the air flow some depending on the design of the duct they are sticking into, but realistically I doubt it would cause you a problem. We sell the ones that have foam around the outside edge to seal them better but I can't remember what the brand is.

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Whoa thanks. I did not know filters like this existed. This seems to be the best of both worlds. 4" filter and easy replacement right at return vent? Maybe I made the right decision not to install that $ housing lol!

Do you mind giving me the good brand name?

**JayMan7** · 10-19-2020, 07:23 PM

Yes I will try and get it tomorrow morning at work. What is your current filter size?

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**startover909** · 10-19-2020, 07:27 PM

Originally Posted by JayMan7

Yes I will try and get it tomorrow morning at work. What is your current filter size?

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20 x 25 x 1.

Thank you sir!

**beenthere** · 10-20-2020, 04:38 AM

20x25x4

Honeywell, FC40R1011

**JayMan7** · 10-20-2020, 07:47 AM

.

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**beenthere** · 10-20-2020, 08:54 AM

https://www.googleadservices.com/pag...gQQ0Qx6BAgDEAE

**startover909** · 10-20-2020, 10:51 AM

Thanks y'all. I was pretty excited to find out the existence of these filters, especially the foamed model @JayMan7 linked. Then I remembered something about my return and opened it to double check...

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