Hello,
IMO, Both answers are true. To utilize SEERS over 14 you need perfect duct sizing. Few systems are that good. The newer SEERS require very low static pressures to achive the ratings. Otherwise you are paying for something that never happens. One advantage of a variable speed system, is most brands adjust for static pressure autromatically and reduce air flow. This makes the variable speed accomodate poor designed duct systems better than fixed airflows. The problem then is the reduced capacity of the system. You no longer have a system rated at the calculated size. For hot climates this is not good. For cooler climates, it may be ok?

The house is 25 years old. I'd guess that the ducts are reasonably well designed for something of that age. I live in a hot, humid area (Austin TX) so cooling and de-humidifying are both important.

The improvement from 14 to 15 SEER is only a couple of percent of the energy bill. It seems strange that a change this small would require such a difference in ductwork.

Would a lower tonnage 15 SEER unit work effectively? My contractor has proposed a 3-ton, 14 SEER unit but I think a 2-ton unit would probably be sufficient. It will be cooling a 1200 foot lower floor with conditioned space above, two very large double paned west-facing windows and no other major heat sources, located in Austin TX with summer temperatures averaging 95+ and high humidity. Unfortunately the label on the old unit is unreadable so we can't tell what tonnage it was.

I live in Austin also, 1330 Sqft. and have 2.5 tons.
Assuming the upper floor is not being served by
that unit 2.OT would just squeak by except for a
few days per year when we top 105f.

I'm no heating and cooling expert by any means, but my understanding is that the actual SEER ratings are similar to the EPA fuel ratings for your car - More hypothetical than actual fact because they are influenced by so many 'unknown' factors (your duct sizing and condition being a couple of the many) that what you actually get in everday usage is some other number, almost always lower than the advertised number and sometimes MUCH lower

Buying more efficient equipment still pays some dividends, however. Likely the 10 SEER system you had some time back was probably only operating at 9 or 9.5 if you were lucky and changing that out for a 12 SEER system and making no other changes probably bumped you up to 10.5 or 11, so there was SOME benefit to the better equipment even though it won't operate at it's published efficiency in your particular application

I bought 13 SEER equipment this go around and the outside unit is physically MUCH larger than the 12 SEER unit I had in my other house (larger condensing coil) and I would bet that I'm getting very near a full point increase over what I would have if I had bought another 12 system, so buying higher efficiency helps . . . . to a point - So long as it doesn't cost you too much more for the equipment, it's still a 'good deal'

In my research, there seems to be a 'sweet spot' on cost VS energy usage right around the 13 to 14 SEER units. Professionals I have asked about this tell me that going much above 13 without installing a complete new system, ducts and all, properly designed for the house probably doesn't give you the best bang for your buck. If you want a new system but intend to use your old ductwork I would probably buy something in the 13 to 14 range and let it go at that - To get the absolute maximum from the extra you're paying for the really high efficiency units, you'll probably need to install new everything, specifically designed for high efficiency

You can probably make a substantial savings on your bill by buying one of the new programmable thermostats and setting it for the way your family uses the house, backing it down a little when nobody is at home and then bringing it back to the comfortable level just before you're scheduled to arrive home. I know we're saving a good bit this winter doing that with our furnace, as we actually prefer sleeping in a colder house with an extra blanket but we don't like getting out of bed in a cold house so the new thermostat gets things just right as the alarm clock goes off in the morning - You can program some pretty exacting scenarios in these new thermostats . . . . the one I have allows you to take it off the wall and do the programming from your easy chair ;-)

Just another layman's opinion . . . . maybe not worth the 2 cents, so no charge ;-)

Don

Hello,
Every house is different. There is no such thing as one size fits all with HVAC. Unless you have done extensive remodeling and added insulation, buying a smaller unit than what is present should only be done after a manual J is calculated. Using a smaller system causes premature failure, and not enough cooling. Any knowledgable HVAC guy can guess at the tonnage from the amp draw. He can also tell from the inside coil label. But the inside coil will take about 30 minutes to disassemble and read the label. This action will cost money. Austin has rebates that the rest of Texas doesn't have, on energy saving installations, you might want to see if you qualify and what is needed.

It sounds like 2 tons should be plenty for my lower floor, especially since I've got conditioned space overhead. Unfortunately none of my three bidders were capable of performing a Manual J load calculation. They each used various rules of thumb to estimate my needs at 2.0 tons, 2.5 tons, and 3.0 tons.

I'll probably use 14 SEER since it just doesn't seem worth the extra expense and duct worries of moving to 15 SEER for only a couple percent improvement. That extra money could be much better spent on window treatments and some better weatherstripping on the doors. Or perhaps even a few fancy nights on the town with the wife.

Just for context, my house has four units. There is an unknown capacity, unknown SEER unit on the first floor (1200 feet), 2.5 tons 14 SEER for most of the main floor (approx 1200 feet), 2 tons 14 SEER for the master bedroom/bathroom suite (approx 600 feet), and 2 tons 14 SEER for a small third floor area (approx 650 feet). The four cooling zones are GREAT for my utility bills but then again I've got the expense and hassle of maintaining four different units. I don't use the third floor much, and only cool it on an as-needed basis because all the heat from the house is channeled up from the first and second floors.

Hello,
From your basic description, you sound way oversized. Depending on if the system borrows air from different systems to cascade the cooling, the 2-600 sqft rooms sound way oversized.

A local manufacturer of water cooled equipment may be beneficial to you. Mac is very knowledgable, and has a product made for Texas.

http://www.alliedenergysystems.com/thermal.html
.

2 tons should be plenty. When a system is oversized, it cycles on and off more often, runs for shorter periods, and removes less humidity, because sensible heat changes before latent heat. The air gets colder, but the water vapor does not condense as it moves across the coil. With higher humidity you set the tstat to a colder temp to adjust for comfort, and the system runs a little more.
Also, with a 25 year old duct system, I would go with the cheaper system and spend the savings on replacing and repairing the duct, regardless of how good you think it might be. You could easily be losing as much as 20% (from that system) to unconditioned air spaces. This has become such a big problem that California has written duct leak inspection into the code, and has methods of enforcing it. This is not only because of poor installation practices, but because of the nature of the beast, and it has gone unrecognized.

I haven't heard anybody telling you to get a competent contractor who can do the heatload calcs to DETERMINE just what you need-- a small investment BEFORE you plunge into anything.

Don21,
Do you have a special programable thermostat for your heat pump? We used to shut our oil furnace down to 60 or so at night and while at work but have it programmed to be 68 when we got out of bed or home from work. I was told not to do this with just any programmable thermostat with a heat pump because such a large swing would result in the emergenncy heat firing to warm the house and it would actually cost more. Does your thermostat somehow know to gradually bring the temp up?

I'll second!
Keep shopping & find a competent hvac company that can do load calcs on your home. use this info to determine if you would be better off replacing some of the smaller units that are so oversized with a true zoned system..one unit with zoned dampers and ...one unit. age of units would be a factor, but as austin has some great incentives it would offset some of the cost.

but you need some competent baselines, keep shopping for contractors.
and on duct leakage..it is unbelieveable! on new construction I routinely find 1 1/2 to 2 tons of duct leakage on a 5 ton install.
install is the key to performance. not just for ducts but for the whole system. keep this in mind.
and best of luck.