Five-Inch Furnace Filter?


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Old 04-01-21, 05:57 AM
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Five-Inch Furnace Filter?

Recently had my annual furnace maintenance, at which time the technician recommended replacing my 1" filter with a 5" filter. This would require some modifications to my intake (and some cost). My furnace is a Lennox G61MPV-60C-091-12.

I had never heard of a 5" filter system on a furnace. Is this recommended, or just an unnecessary expense?

Thanks.

Wayne





 
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Old 04-01-21, 07:03 AM
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Did he say why you should upgrade to a 5"? What does the manufacture recommend? What originally came with the furnace?

Here's the thing...What do you think a furnace filter does?

1. Does it filter the air you breath?
2. Does it protect the internal mechanism of the furnace?
3. Does provide protection from allergens?

The only real correct answer is #2!

Yes, #1 and #3 are factors but only to a small degree.

Consider, that small box in your basement, with a spinning fan moves air through metal tubes. Do you really think that can FILTER the air you breath. With all your windows, and doors and spaces between framing and the duct work?

In a hospital, a room considered to be air tight and filtered for those with breathing problem are just that. Air tight. With special machines to filter out oxygen from other gases.

To be sure, if you have a mild allergy, or an extreme dust problem, more expensive or higher rated filters will help to some degree. But they cannot "FILTER' the air in your house. You can buy room filters that use an ozone producing mechanism. But again it's for one room and will only help to a certain degree.

The furnace filter is there to keep out large particulates such as cat and dog hair, large dust particles and debris that can cause your motor, gas jets, electronics and other moving parts from being damaged.

They are a necessary item and should be used and changed regularly.
 

Last edited by Norm201; 04-01-21 at 09:44 AM.
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Old 04-01-21, 08:38 AM
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I had my filter upgraded to a 5" from 1" but this was when they were replacing the furnace so they had to do some duct work in any case.

A 5 inch filter has better air flow and need replacing less often.
I was replacing the 1" four times a year whereas the 5" only needs replacing twice a year but it is still more expensive.

I would not bother upgrading unless you are having problems or replacing the furnace.
 
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Old 04-02-21, 06:30 AM
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Did he say why you should upgrade to a 5"?
I asked him, and this is what he wrote:
A 1" filter restricts total air flow if it has any reasonable level of filtration.
A five inch filter of the same rating will flow more air, at a lower velocity.
Generally my rule of thumb is all units over 60,000 BTU input get a 5" filter because units larger than that have target airflows of 1200 CFM or more, and few 1" filters can flow that much.
What does the manufacture recommend?
The only filters recommended by Lennox in the user manual are 1". However, I've noticed that the filter gets deformed in toward the furnace side after a short period, so there's probably some truth to what he says.

What originally came with the furnace?
1"

When he was here, he also said that, because this furnace seems oversized for my home, I might want to change the furnace configuration so that it runs at only the lower speed. I would think that if I did that, the need for a larger filter would be reduced, since the volume of air moved per minute would be lower. I suspect that he's right about the furnace being oversized: I have my programmable thermostat set to drop the temp at night by about 5 degrees F, and I don't think it ever comes on until the morning, even when it's -20 F outside, and the house is very well insulated. Running it at the lower speed would also make it less noisy when running.

Thanks.
 
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Old 04-02-21, 07:07 AM
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Here's the thing...What do you think a furnace filter does?

1. Does it filter the air you breath?
2. Does it protect the internal mechanism of the furnace?
3. Does provide protection from allergens?
So, I feel it does all of the above to a certain degree!

Over the homes we've built, I have installed the big cartridge filters you mention, we even had an electrostatic setup once and of course the standard 1" filters.

Out of all the options we never found one that was like "wow this is the best" because the truth is that your furnace only runs for short durations of time so unless you install a low speed motor to constantly circulate the air it simply doesn't make a big difference what you install!

BTW, I use pretty basic 1" filters, buy in bulk and change every few months!
 
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Old 04-02-21, 07:09 AM
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FWIW...at our store we sell FILTRATE filters, 1" and many 3"-5". And we sell the cheepo fiberglass non pleated ones. We also have product fair every year when our vendors will set up a booth and demonstration model for many products. 3M-Filtrate is one of the vendors to set up a manometer and air pressure and a MPV vs MERV demonstration. It seems very convincing while they show you how air is forced and filtered. What they don't show you is that most houses do not have perfect "lab" set-up. Nor are the duct work in perfect shape or configured to "lab" conditions. Nor the fact that in many homes the return air ducts are blocked or nearly blocked. And many outlet ductwork may also be behind furniture or curtains. In other words real life conditions.

They will use any gimmick to sell product. They even have a WiFi app that will tell you when your filter is ready to change out. Some have built in whistle that will sound when clogged. There are also electrostatic filters. AND they all cost $$.

Unless you have a particular problem go with what the manufacturing engineers designed for the unit.

If your interested see Filtrete MPR vs MERV

If money were a non-issue I would buy the very best, just because I could. But I would not expect the value to meet the expense.

If you have an allergy or an extreme dust problem, then experiment with a high allergen filter until you see a difference in your household.

Think of this way. We spend nearly 93% of our life indoors. Much of it at work, some in stores, cars and perhaps about half that amount in our homes. And the outdoors (at least in the urban areas) is not top quality. So do you think a better more expensive furnace filter is going truly increase your breathable air during your life?

I'm not saying a larger than 1" filter is worthless. I'm saying you need to balance the very best with what is practical and cost effective.

I would try going with the lower fan speed just to see what happens. But I ask the question why did you have a maintenance guy there and did you in fact have a problem this past heating season? I guess I saying, if it ain't broke, don't fix it.
 
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Old 04-02-21, 09:30 AM
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Thanks for those comments. To clarify a bit, I'm not personally looking to the filter to cleanse the air, just ensure that contaminants don't gum up the furnace motor or other workings.

The primary reason I wondered whether I should change the filter size was to ensure that the furnace motor is not working harder than it needs to, shortening the life of the motor.
 
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Old 04-02-21, 10:17 AM
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A 1" filter restricts total air flow if it has any reasonable level of filtration.
A five inch filter of the same rating will flow more air, at a lower velocity.
I'm not an engineer or a HVAC expert.

But my thinking is...take a fan, any fan and put a cover over the intake. It will stall and burn up. Put a cover over the outlet and it must push air around or through it. Add tiny holes on the outlet cover and it becomes a bit easier to force air through it. Continue to make the holes bigger or add more holes. OR put a piece of cloth over it. The fan blows through it collecting pieces of dust and debris. Take that same cloth or filter and put it in front of the fan but not right up next to the blades (leave a bit of room). Now you will collect debris before the it hits the blade or the fan motor. Now add pleats to the filter to add more surface area to collect more debris. Now stack several pleated filters to the first one. At what point will you starve the fan from getting air? Well the answer is talk to the engineer who designed the system. In this case the engineers said 1 one inch will do the job at best efficiency. Add more or thicker filter get more filtration at lower efficiency.
 
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Old 04-02-21, 05:55 PM
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Logically, a filter is the cheapest part of a furnace. So, if a different filter actually would improve the function of a furnace, I'm sure the manufacturer would spend the extra few cents to factory install it and brag about it in every ad, to increase sales. Since they didn't...
 
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Old 04-03-21, 05:20 AM
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The filters should not be deforming.
What MERV rating are you using?

Did the tech check the air pressures with a monometer?
 
 

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