Why more A/C tonnage downstairs instead of upstairs?

[red-beard]

Originally Posted by GH85Carrera

I always wonder why the AC guys have the "formula" that just is OK for moderate days but sucks for the hot part of summer. At my house when the heat really cranks up the unit just runs non stop and the inside temp will creep up, especially if I have several guests.

A friend had a house custom built for him. You could play volley ball in his living-dining room. He told the HVAC folks that he wanted to keep his house 70 degrees on a 110 degree day while having a house full of people coming and going. He ended up with two of the largest home units on the market. I don't know what his electric bill is but when my wife & I go over there in the summer my wife dresses for winter.

This is why I'd like to go with either on the of the Variable Frequency Drive units (Nordyne IQ-Drive) or at least a 2 stage system.

The 2 Stage is basically 2 A/C units in a single package. You can run the small unit, the large unit or both, giving you some turndown but also some big cooling capability when you need it.

The VFD unit uses a different type of compressor and a variable speed control on the compressor, along with a variable expansion valve. The unit can run from 60% of nameplate capacity to around 115%. So a nominal 4 ton unit runs as a 2.5 ton unit all the way up to a 5 ton unit. The VFD also soft starts the unit, preventing a huge inrush current.

[masraum]

Wait, you guys have a multi-story house with multiple AC units, and you're complaining? I've got a 2 story, 2000sf house built in '67 with crappy insulation. I've got one unit. I thinkits either 3.5 or maybe 3 tons. I run the fan in the "on" position from Mar until Nov to keep the temp differential between up and downstairs from being extreme. You guys should consider yourselves lucky.

Here's some interesting tidbits from the 'net.

To achieve optimal efficiency & the highest operating SEER Rating long runtimes are essential.
It takes a lot of amps during startup & it takes around 5 minute to reach optimal cooling performance.

Optimal efficiencies:
We could cut residential heating and cooling equipment size in America by 30% to 50% if Contractor's would perform honest Manual J calculations, and provided full credit for every load reducing element or detail they could do prior to equipment sizing, when doing the initial load calculation audit.

Air infiltration rate, can be half the load, & should be checked & reduced. Ductwork & airflow must be checked & optimised for full nominal BTUH performance.

Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.

Then undersize equipment just a little, while optimizing the ductwork & thus reducing blower MTR HP & its heat, while optimizing airflow through the evaporator coil.

The comfort level is never as good with short cycling oversized units; & it is very hard on equipment.

If your AC is cylcing on/off very often, it is likely that your AC is over-sized i.e. providing much more BTUs than your house needs. The short run time doesn't allow time to condense moisture and results in high humidity in humidity climate. You can increase the "temperature swing" on your thermostat (or replace it with one having this). The default energy-star setting is 2 degrees F i.e the AC would turn on 1 degree above the set-point and turned off when the temperature reaches 1-degree below the set-point (2 degrees "swing"). Older thermostats usually have 1-degree swing.

[kach22i]

Originally Posted by EarlyPorsche

.......... and basement

Sounds like you are cooling the basement to control the humidity, so first floor plus basement = 2 x 2nd floor SF area.

Like you said, heat rises and the roof has all that surface area exposed to the hot sun (verses cold earth of bsm't).

Sounds like to me things are sized right, but you need some adjustments made to balance the system.

[turbo6bar]

Originally Posted by GH85Carrera

I always wonder why the AC guys have the "formula" that just is OK for moderate days but sucks for the hot part of summer. At my house when the heat really cranks up the unit just runs non stop and the inside temp will creep up, especially if I have several guests.

Peak loads are only rarely seen. If you size for peak load, you'll be oversized 95+% of the time. In many parts of the country, humidity is a concern, so you want the system running as long as possible. However, multi-stage equipment and inverter-drive compressors have eased that design limitation.

I would be hesitant to simply throw a bigger unit at the problem. Unless ductwork is upsized, you'll may not see the desired results. Before throwing more tonnage, make sure the ductwork is sealed and insulated properly. Consider adding radiant barrier to the attic, or convert to an unvented attic. When the ductwork is in a vented attic, you can lose as much at 20% of your heat and A/C output. Also, those losses are greatest when you need the conditioned air the most.

If, after addressing efficiency and waste, you find a need for more output, consider a ductless mini-split. Your systems will be staged to maximize efficiency and flexibility.
jurgen