Do different filters require different ductwork?

[tucson]

We are getting ready to install a new 4-ton Lennox gas/electric rooftop package unit. Right now the return is in the furnace closet door so that will be replaced with one in the LR ceiling. We’re also getting rid of a rooftop swamp cooler. The house is brick on a concrete slab in Tucson, Arizona.

Since we’re also replacing most of the ductwork, can our system be set up so we have the option to try different filters? Or would that create air flow problems? I don't really understand how that works.

If a standard 1” return filter works, that would be great but if that's not enough we'd like the option to try others. Anything from a 1” MERV 12 pleated filter to a 5” MERV 8, 5" MERV 10 or 5" MERV 16 media filter.

Our dealer said he can build a box above the return in the LR ceiling for a 5" media filter. Lennox makes one thats MERV 10 and one that's MERV 16. I'm pretty sure we could also put those in the Lennox unit itself up on the roof.

As always, thanks for your help.

Sue

[seatonheating]

What they need to do is size the ductwork for the different filter options. Changing filter type is easy if this done. It is just a matter of changing blower speeds to adapt to each filter.

For example:

Restrictive filter: high fan speed

Mid-range filter: Med-high

Hi-flow filter: Med or Med-low

Something like that, just want to make sure you understand it. I'm sure others on here will get a little more descript.

[breathe easy]

I would suggest you have them install a standard 4" filter box. This should accomodate a standard 4" (nominal) commercial filter. Replacement filters for this are inexpensive, readily available and come in a variety of different efficiencies and resistances. A 5" filter is more expensive, harder to find and gives you less options.

[dash]

It's not quite as simple as that,with a PSC motor,take a look at a typical fan chart.

.65 with .25 of it being the filter ,high speed=1760cfms

.55 with .15 being the filter.med speed=1630 cfms


Best bet is pick the static of the best filter and design for it,if you use a lesser filter,you can then look at lower speeds,add resistance if needed to get the correct cfms.

Even better go with a variable sped fan,design for best filter,and it'll deliver the correct cfms automatically if you use a lesser filter.

[hvaclover]

Yeah dash is dead on.

Make sure the duct system is designed for good airflow with very little restriction to air movement. We use a factor of .1 for the total resistance of the duct system. The higher the factor the more resistance to proper air movement. This will result in crappy cooling an heating, higher utilities, and frequent loss of blower motors and premature heat exchanger failure.

The more CFM (cubic feet per minute) with the least amount of resistance is the best design for any duct system.

[t527ed]

the 15 seer unit she mentioned in her other thread is a v drive blower.

[sam_hikes]

Hey seatonheating:

I recently had a xl14i with a vs blower and wondered what is the determining factor for changing the blower speed? Is there a technical method as I've played with them all and wonder what the proper method was?

What they need to do is size the ductwork for the different filter options. Changing filter type is easy if this done. It is just a matter of changing blower speeds to adapt to each filter.

For example:

Restrictive filter: high fan speed

Mid-range filter: Med-high

Hi-flow filter: Med or Med-low

Something like that, just want to make sure you understand it. I'm sure others on here will get a little more descript.

[sam_hikes]

sorry to jump in here on this... but my new system uses 5 inch Trane Perfect Fit media.

[tucson]

Thanks everyone for the excellent info. That seals it on getting the VS unit. I'll ask my dealer about also designing for the most restrictive filter.

Breathe Easy: the media filters that have been suggested so far are all 5". Which ones are you talking about that are 4"? Do you know a website where I can see them? We have spring allergies which is why we're thinking we might want a high MERV.

hvaclover: That's interesting about .1 total duct resistance. Is that hard to achieve? We have a gabled roof so there's plenty of room in the attic.

Thanks again everyone for helping me out.

Sue

[breathe easy]

Do a search for MERV 11 4 inch filters. You will have plenty of options.

Even with allergies you do not need a higher MERV than 11. Anything higher is buying into the "MERV race" we are seeing today. Little if any extra benefit but increased cost and more air flow restriction.

[seatonheating]

Hey seatonheating:

I recently had a xl14i with a vs blower and wondered what is the determining factor for changing the blower speed? Is there a technical method as I've played with them all and wonder what the proper method was?

Sam,

You need to have your local tech come out and determine if it is needed. He has all the tools necessary to measure this.

[sam_hikes]

The reason I asked is the tech told me there weren't any regular tools for this but whether I was having hot/cold spots? This is same tech who hasn't checked static pressures either.

What tools should I suggest they bring to properly determine if a change is needed?

Sam,

You need to have your local tech come out and determine if it is needed. He has all the tools necessary to measure this.

[tucson]

breathe easy, thanks for the info. I'll check out the 4" MERV 11 filters.

Sue