Filter sizes and other questions about my new system.

[the_emi_guy]

Greetings folks,
My 25 year old 3 ton AC is dead. Decided to have 15 year old gas furnace replaced while I am at it. Furnace is on floor in basement with 20x12 sheet metal return duct.
Have several quotes.

My biggest concern is filter size.
All quotes so far except one are just copying the same 16x25 filter size from original system. Furnace was replaced 15 years ago by previous owner (Bryant Plus 90). It runs 1570cfm through 16x25 filter that just slid over 20x12 duct inside of unit. Thus airflow through filter was restricted to 1.67 square feet. This is a velocity through filter of 1570/240 = 940fpm. Every time I have replaced this filter in last 10 years, it has been bent out of shape by force of the air flow.
From what I am learning online, for 3 ton I need 1200cfm, and for 1200cfm I need 600 square inches of filter area. This would be a 20x30 or 25x25 filter.

This seems like a big filter. Should I insist that contractor put in 20x30 filter? If so, how would this work. 30 inches is bigger than the depth of the furnace. Is there a way to angle the filter?

One contractor mentioned that he would do something to increase filter size, but he also wanted to upsize me to 4 ton. I am reluctant to do this without some formal manual J calculation.

My second concern is with the cfm rating of air handler. So far all quotes have air handler flow rate well above the 3ton 1200fpm. One contractor quoted a Bryant furnace with a 1950cfm rate while cooling (59sc2A100S21-14). Is this a concern, either for operation of AC or capacity of ducts and filter? Do these "variable speed" furnaces change speed in AC mode to maintain proper rate?

A third question. One contractor recommended that I put in a low efficiency "old style" furnace since I have a 6" flue right there from original furnace when home was new (15 years ago water heater was orphaned when high efficiency unit was installed). He said it would be more reliable and repairs would be less expensive.
I am ok with the efficiency hit if the unit will be more reliable (calculated it would cost me $60/year). Does he have a valid point?

Thanks

[BaldLoonie]

Personally I'd put in a media air cleaner, those have the 4-5" thick filters, not 1". More effective and less restrictive.

If a water heater is orphaned in a chimney, drop a chimney liner down. I wouldn't put an 80% in a chimney without a liner either so his idea of staying 80% to avoid a liner may cause more trouble than it solves.

[the_emi_guy]

BaldLoonie,
Thanks for the response.
I will look into media air cleaner!

It is not a stone chimney, it is a 6 inch diameter sheet metal flue that goes from basement right up through roof. It originally vented furnace and hot water heater, so I'm assuming it could still do so.

[the_emi_guy]

Inspired by BaldLoonie, I went to Home Depot and noticed that all of their 4 inch thick filters are the same price. So I am thinking that my new system should accomodate the largest 20x25x4. Before I approach my bidding contractors with this idea, does anyone here have any ideas how I could make this work. I have included photo of my old system. See attached photo. 12x20 return duct comes down vertically on left and is attached to 12x20 section that runs horizontally into furnace. Would it be better to have this large filter mounted horizontally?

[catmanacman]

Should be a easy retrofit for a skilled instaler

[the_emi_guy]

Update on filter,
One contractor is offering to put in an Aprilaire #2410 which looks nice but the replacement filters are not generic, I would be locked into using Aprilaire filters. I would like to use a standard size filter (if such a thing exists, I'm only going by what I see at Home Depot). I would like it to be in a good, sealed, chassis because I don't want negative pressure right next to gas hot water heater. Can someone recommend a product? I'm doing Internet searches but I really don't know this industry.
Thanks

[SBKold]

I would use something like the aprilaire over the 3m filters u saw at HD.

Keep inmind a high quality contractor grade unit can last up to a year.

What size system are you getting. We need to know airflow requirements and then the contractor needs to calculate the rest of your existing duct systems losses to make sure you will have proper airflow to match the size of the unit.

[George2]

Thanks for the picture. It tells us alot.

Where do I start? First, I love the light switch (vs. a SSY). Second, it always amazes me why a contractor wants to install a larger unit than one that has been doing (I assume)the job for 15 years.

Your return air drop needs to be redone which will be easy during the new installation. I would recommend a 4-5" filter rack arrangement that will use the "big box" store pleated filter. Ask the contractor. If he says it can't be done, keep looking.

[BaldLoonie]

The 2410 is a great filter. One of the least restrictive. You could get a Honeywell or Air Bear but if you use a generic filter in it, you could be right back to restrictive. Personally I would pay the price of the Aprilaire media.

Dealer sizing is one of the biggest issues. We see gross oversizing quotes all the time. My cousin needs new equipment for a 700 bedroom wing. Amazing the huge furnaces dealers think are needed to provide 25,000 BTU of heat at 0° out.

[the_emi_guy]

Thanks folks, I have decided to go with the 2410.
I have one other issue. Previously, the 1/4" sump pump hose was bundled with the copper tubes and electrical wires to the outside. Thus it drains onto the ground from the open hose about 3 feet off of the ground. We had a really bad winter a couple years ago, snow covered hose, froze water, blocked sump, and float valve shut down furnace. In my area regulations do not permit me to use DWV to drain furnace.
I have mentioned this to all contractors and have had several inputs:
1 - Use bigger hose on exterior
2 - Drain into sump pump in mudroom

I was considering #2 but am having second thoughts about pumping water into this mudroom all summer due to mold concerns.

Any thoughts?
Thanks