Replacing our AC and Furnace

[jamd]

We have lived in our single story home of about 1600 sq ft with a walk-out basement near Kansas City for 15 years. It has a GE 150,000 btu furnace and Janitrol 4 ton AC that are both approximately 30 years old. They've worked well for us but have required increased maintenance the last few years and we've decided it's time to replace them.

We've used the same small local company for regular preventative maintenance (as well as the occasional emergency repair) and they have done a great job of keeping these systems going. We want them to do the replacement. They came out and did a load calculation and advised that we should go with a smaller furnace and AC. They sell Amana and made the following recommendation.

ASX14042 3.5 ton 14 seer
AMH80905C 80% 2 stage 90,000 btu
matched cooling coil, TXV, line set and base pad
Aprilaire #8466 thermostat

(They originally recommended a 95% furnace but our house is rock and there is no easy way to run the pvc out of a wall.)

There is a lifetime warranty on the furnace and AC (original owner) and 10 year parts and labor.

Does this system seem to be a reasonable balance between cost, quality and efficiency?

Thank You

[beenthere]

The equipment is ok. 45 BTUs of heat per sq ft seems like a lot even for Kansas.

[George2]

I would recommend (and I'm very surprised they didn't) a variable-speed blower with a thermostat to control the dehumidification and the humidifier.

[motoguy128]

I would recommend a load calculation be done and Beenthere pointed out. Unless the house is all windows with no shade, KC is a hto and cold climate, but wow, that's a huge furnace and AC for that size home. WIth good insualtion and Low-E storm windows to window, moderate air leakage and tpyical construction you should be clsoer ot 2.5-3.5 Tons with a 80-100kBTU furnace.

IF it was very efficeint with good shade and low air leakage, you might need as little as 2 tons and a 60k BTU furnace. I've learned frmo expereince. Smaller is better as long as it maintains 75F at design temperature without a setback (setbacks with AC don't save much if any money, they do save some with a furnace). It will pull down more humidity and be cheaper to operate with longer run times.

Design temp in Kansas City is 93F, 75F dewpoint, 4F winter design. About the same as where I'm at. That means when it's 100F like it was yesterday a properly sized system will lose a little ground (1-2F depending on the size of the house, shading and thermal mass) in the late afternoon, but catch back up by evening.

If its' 5-6F above design temperature and your unit is cycling on and off in the late afternoon... it's too big. The difference can be as much as 5-8% RH or more without even slowing the blower speed. That's enough that you will be comfortable at a temp 1-2F warmer... which means your saving energy. It takes 10 minutes for a system to reach full efficiency. A properly sized system will run that long probably almost 50% of the time. More if it's 2 stage. LArger units also need larger ductwork. A 3 ton unit installed on ductowr originally size for 4 tons could save around 50-100 watts when running. I think that's close to 0.5 SEER.

[jamd]

An additional factor which I should have stated up front is that our living room has 18 ft vaulted ceilings and about half of one wall (East) is glass (which we've since tinted). What we don't have in square footage we make up for in cubic feet.

Based on the calculations he performed, our contractor decided that the original system was too big, hence the change from 150k btu to 90k btu and 4 ton to 3.5 ton.

I will ask about upgrading to the variable speed furnace instead of the multi speed. I'm not sure if he was trying to save us money but upgrading should only add about 10% to the total cost. Should the fact that he didn't suggest this make me leery?

Thank You!!!