When is a 2 stage system good ?

[adamk]

What's good about a 2 stage vs a 1 stage. A friend got a 2 stage system - with a gas furnace. I recently purchased a single stage heat pump.

Thx.

[pstu]

One thing that's good is long runtimes on 1st stage, usually leads to greater comfort. If you have no complaints about comfort, that might not apply to you but it applies to many homes. Another good thing is a 2-stage AC such as the Trane 19i will have considerably better energy efficiency on 1st stage. I am a homeowner and like mine.

[adamk]

How is the 2 stage system sized? Is the first stage sized to 90% design temp and the second for the rest ? So its more for comfort than for energy efficiency ?

Thanks.

[pstu]

For all that I know about, the 2nd stage is sized to the design load. Then whatever fraction 1st stage is, is decided by the engineers that designed the thing in the first place. The Trane 19i has two entirely separate compressors, and the 1st stage seems to be chosen about 55% of the 2nd stage size. The Trane 16i and many others have one compressor that can operate at two capacities, they seem to usually have 1st stage sized about 65-70% of 2nd stage.

Because these units are generally only made in whole ton sizes, there is a greater tendency to oversize. Contractors always say to me it doesn't matter, and for sizing purposes they probably are right. One angle though is airflow thru existing ducts, often the original ducts are undersized for the original system, and become even more so when the system is upsized. Any bad effects would not show up on 1st stage but only on 2nd.

These AC systems are *sold* more for comfort purposes than efficiency, because the high initial cost makes it hard to justify on a dollars and cents basis. However they are top of the line and almost always have a better SEER number than single stage units.

There are 2-stage furnaces too, but we are not talking about those are we?

Hope this helps -- Pstu

[billva]

A two stage system, be it a furnace, condenser or heat pump is designed for comfort only. (more even temps, more humidity removal in summer, more humidity supplied in winter, more filtration, etc...)

As far as efficiency goes, everything is equal, your system will need to add or remove BTU's,if it does it at a slower rate it will run longer, but still use the same amount of fuel.

i.e. a two stage 82% furnace will have the same AFUE rating as a single stage.

[bmathews]

Price is one obvious difference. Both will probably keep you very comfortable. It's like a BMW compared to a Ford. Neither are bad, but one is a bit nicer with more features.

[beenthere]

2 stage is sized the same as a single stage.

[adrianf]

Anytime the sun rises in the East and sets in the West. When ever the load diversity exceeds 30%. When ever you want to reduce the percentage of the time the system is actually oversized. When ever zoning is needed.