You pretty much found the source of the impetus for the air cleaner install. My issues with air cleaners are that they can remove what they take in, but much of what can cause respiratory problems is direct source (having a dog sleeping next to your face or a cat on top of your head) or large particles that settle on the floor, couch, bed, etc. Get a bright light point it at your choice of furniture or flooring and give it a whap. Heck, go into your closet and give your clothes a good shake.
An air cleaner just wont help with that. The biggest benefits are keeping the critters and other allergens out of the house as much as possible, cleaning as much as possible, and even then I think you're just mitigating the potential problems somewhat.
As far as the usual allergen reduction pleated filters, I got fairly unanimous feedback from HVAC pro's in person and online: they get to do a lot of repairs as a result of their use straining system fans through excessive static pressure and damaged coils/heat exchangers from reduced air flow. Plus those old systems dont run often enough except during the hottest and coldest days of the year to do much air filtration.
So, per the wife, when she's sitting in a room with an air purifier, it makes her feel better. I clean their filters, so I know they arent really catching that much. The prefilter takes a nice thin layer of fuzzy dust over a months use and I generally get 3 years out of a hepa filter before the air flow starts dropping.
Since our three big room air purifiers were nearing the end of their service lives, I had the choice of peeling off a thousand bucks for three new ones, going through a couple of hundred bucks of filters and maintenance on them per year, and having what I know is a practically useless "solution". Oh, and they cost me about $250 a year in electricity to run.
Or, since we had an aging furnace, I figured I'd try drinking the HVAC marketing koolaid and try one of these centrally mounted units coupled with the low flow constant on variable fan furnace. Generally the cost was about $1500 for this combo over a non variable fan and no air cleaner. The electricity costs to run that fan and purifier for a full year at my rates is about $130. I remove and vacuum, wash or blow out the prefilter and collection cells and those should last between 5 and 10 years. No filter costs for the furnace or the air purifiers.
I'm getting another small benefit in equipment lifespan by virtue of having the filter sitting directly under the furnace rather than at the return grills inside the house. Theres no such thing as a perfectly sealed duct (well, the aeroseal guys will give me some trouble about that) so those returns are pulling at least a small portion of their air from the attic or crawlspace. Plus the dust and crud thats in that air, which in my old system ended up catching on what was the best filter in the house: the wet a/c coil, reducing its flow and efficiency and requiring cleaning.
So economically I come out way ahead. The wifes happy. It actually seems to be doing something useful for us where the old room units werent doing much of anything other than blowing a little air around. Its good for the equipment.
This system is a 90k btu furnace, four ton, 80% afue. I didnt go for the higher efficiency gas burner for a bunch of reasons. Our winters are so mild I'd get about a $30-40 savings (max) per year...it'd take me ten years to get something approaching a payback for a 92% AFUE. The >80% units use a PVC vent instead of the old 4-5" duct and I had nowhere to vent it since the unit is in the garage with two floors overhead, a very visible side of the house on one side and a lot of expensive landscaping on the other side, both of those with large roof overhangs. So I'd have had to have a black pipe sticking 3' out of one of those sides for venting. Oh, and the 90+ furnaces have a secondary heat exchanger and a lot more junk in them that can break, and theres a lot of anecdotal evidence in the HVAC community that they're not as reliable in the long run and need more maintenance.
Sam, as far as your setback question...I dont think thats the case. A furnace uses a certain number of therms of gas to create a certain number of btus of heat to create a certain amount of degrees of change. Whether its using a full flame for an hour or the half flame for two hours, its the same. The only difference is run time and comfort level. You'll get some additional duct heating/cooling loss in the longer running first stage, since the ducts are r-6 or at best r-8. So its plausible that setting back not only saves you some energy at night, running it up in the second stage in the morning might be MORE efficient. If your ducts were 100% efficient with no loss, it'd probably be a toss-up.
I couldnt find a guy to do a manual J either. Manual J discussions tend to run like our annuity conversations and there is merit on both sides. A full manual J takes about 3-4 hours to run, and in general for most ordinary homes, might change a seasoned installers opinion by a half ton or so. Where you get into strange stuff is older homes that lack insulation, lots windows, drafty homes, funny basements with odd ventilation, homes with lots of vaulted ceilings, etc. Then some environmental aspects outside the home can also make a lot of effect like lots of trees and shrubs, adding solar screens to the windows, storm windows, yada yada yada.
I had one guy measure all the rooms with a laser tape (which I immediately wanted) and do a 20 minute back of the envelope calc and he came up with a 3.5ish number, but said the 4 ton would be recommended for the higher air flow in the summer when the heats blazing. Four other guys came back with 4 ton. One guy suggested a 5 ton, but I know my ducts wouldnt take that much air flow...the house would sound like the boston pops warming up for a concert when the fan was running on high.
The bottom line for the 3-4 people still eating popcorn and reading this is that an oversized system will waste a little money up front for higher equipment costs, run short run times, which is inefficient, will blast out lots of very hot and cold air, which is uncomfortable, and not run often enough to balance air temps. A system thats too small will be unable to keep up on the hottest and coldest days, but up to a point a system thats slightly too small is way better than one thats slightly too big.
I got into a stalemate with two companies. They had their guy come out and eat up 90 minutes of my time telling me how good their company was without disclosing what equipment they'd use or recommend ("it depends!"), sizing or approximate cost. I told them 4-5 times that they were sitting there because I'd already vetted them and come up with only people that were recommended and didnt seem to have a lot of problems. Which didnt stop their sales pitch. We then got into a race condition after that where they wouldnt cough up any details until I was willing to give them the business. No way.
Interesting reading some of the hvac web sites. These folks seem to have a strong disdain for the homeowner and anyone wanting to do any maintenance and repair on the unit themselves - quite a difference from our own general philosophy here. They want the hvac stuff to be a mysterious black box that only a pro touches. They often compare their learned skills to be the equivalent of an MD and worthy of the same price/hour. In short, they remind me of financial planners. But of course, not the really good ones like we have here
I think if I were going to do it again, I'd skip the slick willies and the large mill shops and just find a company with the word "sheet metal" in their name (there were plenty) or the guy and his two brothers, do a quickie load calc on my own, tell them what I'd like them to install, and let them do the mechanical install. Make sure I get a FACTORY transferable parts and labor warranty.
I found some anecdotal operational and reliability data that backed up what CR came up with...that carrier/bryant/payne and trane/american standard might make slightly better gear and goodman/amana/janitrol was a little behind the pack. But a good system badly installed wont do as well as a slightly below average system installed well.
Based on product cost and availability, plus long warranty I might install one of the goodman variable two stage units with a higher seer a/c than i got. The equipment is enough cheaper that I could get that for less than I paid for this. Everyone fixes goodman, there are plenty of places both B&M and online to get parts and whole units, and they've got one of the longest warranties in the business.
My original intent was to go that route, but I had the damnedest time getting someone to give me a quote on the product. Everyone listed goodman, but bad mouthed the product and downplayed the cost savings on their way to quoting me a carrier or trane system. I think the installer industry in general hates goodman for the way they sell to everyone and their price policies...doesnt make for a happy exclusive marketing arrangement for the retailer/installer.
At the end of it, I was just tired of the whole thing, didnt want to fight city hall to save $500, and figured it might be money well spent to put in one of the major brand names and make it a minor benefit to a buyer on resale.
You may now return to your regularly scheduled program, War and Peace...