Furnance Filter SIze

[j1cs]

I have a Carrier 90% furnace downflow return- 4" 16x25 filter on top. What are thoughts on replacing it with a 1" filter changing it every month?

[tipsrfine]

It could cause air flow problems. A 1" does not have as much surface area as that 4" media filter. Especially if you use one of those 1" 3m filters. Do you have a central return, or individual returns to each room?

[j1cs]

Central return from hallway to furnace ranch style home

[tipsrfine]

Central return from hallway to furnace ranch style home

I'm assuming you want to go to the 1" because it is cheaper? I would want to take some static pressure readings before making any decisions. If you have good air flow, but your media filter is getting to dirty too fast, you may get away with replacing your central return grill with a return filter grill, and install a 1" fibergrass filter there to add longevity to your media filter.

[j1cs]

Yes, the filter has been in use for 10 months, not to dirty, just thought because itis easy to get to changing each month with the 1" pleated filter would not be a problem. I have three 1" pleated filters I bought, what if I stack and use all three at one time and the replaace with the 4" that is in there now.

[tipsrfine]

I wouldn't do that. They wouldn't fit snug enough and you'd have dirty bypassing the filter. 10 months life for a filter is great. Try and find someone that sells them cheaper. I belive home depot sells them.

[54regcab]

Look at your HVAC supply for the Glassfloss ZL 4" filters. They aren't any more than the 1" 3M's at the big box store. That's what I run at my house.

http://www.glasfloss.com/products/z_pleated.cfm

[catmanacman]

I would just change the 4 inch every 6 months if you have ac and heat once a year if you have heat only

[SBKold]

the 4" filter was an upgrade over a simple 1" filter. It provides you with more efficient a/c operation. Stacking 3 1" filters is about the worst thing you could do. IMO.

My question is - if it lasts 10months why not use it? I guarantee it has done a good service to you by keeping the expensive unit clean while....hopefully..... (depending on your unit size) not restricting airflow like a 1" filter would.

I say hopefully because sometimes even 4" filters restrict air flow. But nowhere as bad as a 1" 16x25 pleat.

It would be like someone having a K&N filter in their car and not wanting to use it!

[beenthere]

Yes, the filter has been in use for 10 months, not to dirty, just thought because itis easy to get to changing each month with the 1" pleated filter would not be a problem. I have three 1" pleated filters I bought, what if I stack and use all three at one time and the replaace with the 4" that is in there now.

If one of those pleated filters has a resistance of .07" to air flow. Then 3 of them in parallel will have a combined resistance of .21", and you will need to change them every 2 weeks at least.

The 4" while more expensive up front will be less resistive to air flow, and cost less per year then what you want to do.

[udarrell]

I have a Carrier 90% furnace downflow return- 4" 16x25 filter on top. What are thoughts on replacing it with a 1" filter changing it every month?

First, it is critical to know what the heating & cooling CFMs will be so we know how to figure the filter sizing equation.

If you switch to a 1" deep blue throw-away filter, the filter should be sized for an initial 300-fpm velocity. The 4" pleated filter may get by using 500-fpm initial velocity through it.

If you go to a 1" throw-away filter for a 3-Ton system use the following formula: 3-Ton use 1050-cfm / 300-fpm velocity is 4.166 sq.ft (Ak) free-air-area, not physical size.

Figuring 60% free-area, that's 4.0166 * 1.4-required increased physical area is 5.833-sq.ft or * 144 or 840-physical sq.ins.

I'd use two 25X25, actual less two inches, or 23X 23 filter area is 529 * 2 = 1058-sq.ins of total physical size. That is more than the 840-sq.ins needed, but more is always better.

What CFM does your system require for both heating & A/C?

If you use 4" pleated filters I would still not use over 400-fpm initial velocity through those filters.

This is good filter information; however all appear to be pleated, even the 1" filter, that is a lot of pressure drop for a clean filter:
http://www.glasfloss.com/Pleated%202010.pdf

[surenuff]

First, it is critical to know what the heating & cooling CFMs will be so we know how to figure the filter sizing equation.

If you switch to a 1" deep blue throw-away filter, the filter should be sized for an initial 300-fpm velocity. The 4" pleated filter may get by using 500-fpm initial velocity through it.

If you go to a 1" throw-away filter for a 3-Ton system use the following formula: 3-Ton use 1050-cfm / 300-fpm velocity is 4.166 sq.ft (Ak) free-air-area, not physical size.

Figuring 60% free-area, that's 4.0166 * 1.4-required increased physical area is 5.833-sq.ft or * 144 or 840-physical sq.ins.

I'd use two 25X25, actual less two inches, or 23X 23 filter area is 529 * 2 = 1058-sq.ins of total physical size. That is more than the 840-sq.ins needed, but more is always better.

What CFM does your system require for both heating & A/C?

If you use 4" pleated filters I would still not use over 400-fpm initial velocity through those filters.

This is good filter information; however all appear to be pleated, even the 1" filter, that is a lot of pressure drop for a clean filter:
http://www.glasfloss.com/Pleated%202010.pdf

always good to hear from you Udarrell ....... a fellow airflow junkie. Most do not realize that those published pressure drop rates are only rated at 300 FPM and 1200 CFM. If we up the FPM, we crusify airflow with high PD. Oh, by the way, they gave me the * by my name within 24hrs of the application. That was fast.

[udarrell]

always good to hear from you Udarrell ....... a fellow airflow junkie. Most do not realize that those published pressure drop rates are only rated at 300 FPM and 1200 CFM. If we up the FPM, we crusify airflow with high PD. Oh, by the way, they gave me the * by my name within 24hrs of the application. That was fast.

Glad to see U got the *

The filter area is very important toward keeping things clean & efficient, also it's critical to indoor air quality.

These days, furnaces tend not to have filter racks in them, therefore a sharp 90-ell with filter nearly up against it is very common.

Usually the filter area is not nearly large enough & in that position there should always be turning-vanes in that 90-ell, otherwise the bulk of the airflow going through the filter will be toward the bottom half of it, with greatly accelerated velocity through that portion.

IMO we should be putting a couple of large filter grilles in the main air conditioned areas, or in all the rooms, except bathrooms & kitchens.

This could make a big difference in airflow efficiency, reduced noise levels & general human comfort conditions.

[surenuff]

We all know that thr best answer would be to have a return in each room, but we also know that is just not going to hapen on any kind of regular basis. Cost and other reasons will always keep it from happening that way. The two large RAFG in conditioned areas of the house is the next best thing, but only if they are sized correctly with the duct work sized correctly for the retrun side as well. There are too many people who still do not understand that 144 sq in. per ton is not enough for a return air grill, and certainly not enough for a RAFG. Capacity and effeciency are bound so tightly to proper airflow.

[54regcab]

Yeah, the builder used 12" RA ducts on 20x20 RAFG's for both of my returns. They didn't factor one was 20' away and the other 5'. Needless to say the closer one pulls a LOT more air.