I decided on the 80% AFUE...2 stage York furnace simply because: #1)Our home is located 2 miles from the beach ( Santa Monica California )...and I feel it should be sufficient. with our (usually) mild climate ( Am I wrong ?) #2) A 90% AFUE furnace would cost an additional $1000.00...In your opinion should I spend the extra money for 90% AFUE ??? The contractor will be installing next week. Please reply ASAP As an update...It looks like I will not be able to add the central air system :...the cost is HUGE because I would have to REPLACE AND MOVE my entire electrical box from its original location . It will not accomodate an additional 50 AMP breaker !!...Cost would be (approx)an extra $ 4000.00 plus whatever city permits and code upgrades may cost!! To summarize...York Gas Furnace... New ducts (cal flex silver jack 6.0 ) Thermostat : Honeywell 8000 model Air Filtration : Aprilaire 2250 TOTAL COST $ 4800. I have one more question...please:....My original register return for gas furnace is located on wall near floor level in my hallway...I was told by contractor that this is not the correct location for it....It should be moved to ceiling of my hallway..Unless I hear otherwise from this forum...I will have it moved as he suggests...SHOULD I seal up the old register return or let it remain but it will not be functioning ??... Thanks all !!

Not a PRO, but I believe that TDR stands for time delay relay, which might affect fan operation. To expand on what Tiger said, different brands of modulating furnaces behave differently with regard to staging (heat levels). Speaking as a Rheem (mod) owner, the thermostat 'knows' what the set point is, but also, the indoor temp. So, let's say, that on recovering from a setback, knowing how many degrees it needs to heat, it will start out at as high a level as it needs to in order to heat things up. As the home approaches set point, its heat loss lessen, and thus the heat input will progressively modulate LOWER. Throughout the day, responding to indoor or outdoor conditions, the stat will respond by directing the heat input higher or lower (without regard to previous cycles). So leaving the doors/windows open, or running multiple ovens will elicit response (up or down) at any time. Other brands may differ in their response. They will start out at a given rate (some based on previous cycles) and ramp up over time until the stat is satisfied. Not good for a large setback. They may not have the ability to ramp down during the same cycle. Thus the up/down scheme. Without being specific as to brand, your mileage may vary. V

Cesar, I have 122 year old ducts in my home (I have seen the original bill of sale for the furnace and know the brick ducts are original--the only thing that has been replaced is the feeder ducts in the basement that were replaced when the original gravity furnace was replaced. I now have a heat pump with A/C (I didn't install it, bought it with it installed). We have one room upstairs that doesn't have a duct, and we put a window unit in there (my bedroom, I like it cold, so it's great). The only problem we have is that there is a 10 degree difference between downstairs and upstairs (I think this has to do more with hot air rising than the PUSH of the system though)...and we have no return from the 2nd floor, just on the landing of the stairs. It can get a little warm in the summer as we try to go light on the A/C for $$ reasons but since we put in the blown-in insulation in the attic, it's been good. We've been here 2.5 years and are comfortable. I'm glad we didn't have to install it. I don't know if just because it works for us it works for everyone... our ducts are large, probably 8-10 inches square, with large louvered grates, and the longest run is probably 40 feet.

We were told by the technician not to use the "best" filters because they don't allow as much air through the unit and make it work harder, but don't use the least expensive, either. He also mentioned, if you deal with a number of pets and lots of pet hair in your home, you may need something specific to handle that, and change it at least once a month. At one point during allergy season, we left our unit fan on constant to keep the allergens filtering through the filter and made sure we changed it at least once a month due to the additional use. It's not a one size-fits-all thing.