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I run a 20x25x4 media in mine. It's a 2 stage two ton and a 2 stage 45K furnace.

If you are clocking 56k input on that 1978 furance and it's able to keep the house warm, then you know the heat loss is LESS than 56k. If the furnace is running 60% efficiency (not uncommon for an old furnace, especially when derated to 1/2 gas input) you're at 33k output, or about what a modern 80% 40k furance can do. Manual J is great, but keep in mind it had a considerable "fudge factor" built in. I've been eyeballing the Thermalzone 45k for my own 1600sq ft house to replace the 88k 80% that the builder installed in 1999. 95% efficient, 2 stage, variable speed. Ruud/Rheem makes them and they are sold through United Refrigeration.

Originally Posted by 54regcab

Since you have the advantage of being able to time the run cycles with your current system do so. It will give you a good idea of the REAL heating requirements for your house. If you get a design day, see what percentage your current system runs. Keep in mind if you are even 30% oversized a 2 stage does you no good compared to a smaller single stage system. If your house only has a 40k heat loss and you install a 60k furnace, it will never kick onto 2nd stage. If you are close to the next smallest size you may be able to tighten up the house to get there.

Thanks for your advice. Since you brought it up, I do plan on trying to do a Manual J at some point and taking a close look at my ductwork before I decide on what to get. My house was built in '78. It has the original 108k btu, natural draft, GE furnace in it. Shortly after I bought the house about 10 years ago I noticed the burner short cycling on the limit switch all of the time. I rigged the fan to run on high speed and lowered the gas pressure until it would get me a more acceptable temperature rise and run with out tripping out. I just left it go like this for the last several years. Just yesterday I clocked my gas meter and found it was only sucking down about 56k btu worth of gas.

I think it is supposed to be a 65-70% efficient furnace, but I am sure it is running much lower.

So the air filter is probably the least of my worries, but it is something I have been wondering about. Once again, thanks.

Since you have the advantage of being able to time the run cycles with your current system do so. It will give you a good idea of the REAL heating requirements for your house. If you get a design day, see what percentage your current system runs. Keep in mind if you are even 30% oversized a 2 stage does you no good compared to a smaller single stage system. If your house only has a 40k heat loss and you install a 60k furnace, it will never kick onto 2nd stage. If you are close to the next smallest size you may be able to tighten up the house to get there.

Originally Posted by 54regcab

Check out the link I posted, it shows the differnce in media area vs. filter size and also the static pressure drop. How many CFM are you moving and what filter size is "native" to the furance you are considering?

Thanks 54, that is some good reading material. I haven't decided on which furnace I will get yet. I don't plan on doing it for another couple of months from now. I know that I want something that is variable speed, probably 2 stages of gas heat, around 60k btu, and I will be re-using my old 2 ton A/C (for now).

Also thanks to everyone else who has replied so far.

Taller X Wider X Thicker X (+11 merv) = Better
24 X 24 X 4" is the smallest filter I would use.
I you have a mold problem because your filter is big, you have a big moisture problem.
For indoor air quality, assure a fresh air change in 4-5 hours, maintain <50%RH throughout the home, and merv 11 air filter.
Regards TB

Check out the link I posted, it shows the differnce in media area vs. filter size and also the static pressure drop. How many CFM are you moving and what filter size is "native" to the furance you are considering?

Originally Posted by BigBacardi

carrier EZ FLEX fi;ter only way to go in my opinion

What is so great about these? I just did a quick internet search and it seems that I could get a whole case (12) of 2" filters for about the same as it would cost for me to get 2 of the 4" EZ Flexes.

I guess I am on the fence right now between going with a 2" or a 4" filter. I am still not convinced on which would be the most sensible choice. By the way, this will be for installing right at the furnace, not in a filter grill, if that makes any difference.

carrier EZ FLEX fi;ter only way to go in my opinion

Two 20"x20"x4" ZL Merv 8 media filters are what I use in my own home for my 2 ton AC. Buy them from local supply house, priced less than the 1" retail packaged filters at the big box stores. Last about 6 months, best value for the $$ IMHO. http://www.glasfloss.com/Pleated%2020121.pdf

Aprilaire 1210 is the newer version if the 2200. Merv 13 if you use a 213 filter. They seem to work very well.

Back in my residential days, we always pushed the space gard filter at the time it was a model 2200. Those are no longer made, newer models are lower profile and higher merv..I think. At any rate, as a technician and seeing the inside of 1,000's of blower compartments, motors, blower wheels, secondary heat exchangers and evaporators, I can honestly say when a space gard was installed initially and in service for years no dirt was ever present. Not the case when other filters where installed including electronic. I think electronics could do a good job if kept clean, but who religiously yanks them once a month and cleans them ? There is more choices now than when I was in the residential market, but I think if you got a space gard or comparable style filter you will be in good shape. Your favorite wholesaler will have the filters and replacement media.

I agree with the two inch thick pleats, four inch will start to grow mold long before they are used up, and are a bit of waste.

I always go with the biggest filter i can fit in the hallway. 30x25 or 30x36 is not uncommon for me. I know it may be overkill but my customers love how they can barely hear the air moving due to its size. I am a big fan of 2" merv 11 pleats because they only cost me 2$ more (5$ total per filter 30x25)than a 1" and they last 2-3 times as long. Its not uncommon for a customer that has a 3000 sqft house to get 4-6 months out of them. I buy all my filters at a dedicated filter place, better price and quality than big box stores.

What filter size would you get in your house?

I am planning on installing a new furnace in my house soon. I would like opinions on what filter thickness/merv rating I should plan on going with.

I am looking for the best all around combination of not too restrictive, good at insuring that my heat exchanger/evaporator stay clean, low cost, etc. I am not looking at trying to turn my house into a clean room, I just want something that will do a decent job at a fairly inexpensive price.

I don't necessarily care how often it needs to be changed. What I am wondering about is how much of a difference do different sizes/merv ratings make on how often a filter needs to be replaced, and what the overall cost would be? For example, which filter would be least expensive per year, a 1" pleat that needs to be replaced every month, a 2" that needs to be replaced every 2-3 months, or a 5" that needs to be replaced twice per year?

Also, what is the best place to get air filters. Should I get them from a place like Grainger or J-Stone, or should I check into online sites like Amazon or a dedicated filter place?