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Question DetailsAsked on 4/20/2016

are there any benefits of two stage air conditioner And two stage gas furnace for house?

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This sort of question has been asked here a number of times, but I only found one previoous one answered - here -

Basically speaking, two-stage or continuously variable output units save you energy in three ways, in theory -

1) by only outputting the amount of heat (or cooling) that is needed for the conditions at hand - so if a lot of temperature difference is called for, or if the temperature differential between the air to be heated/cooled and the desired temperature is large, then the unit runs at full power. But if demand is less, and commonly also towards the end of the heating/cooling cycle it steps down to a lower output, saving the energy that would have been used running the unit as full power until the desired thermostat temperature has been reached. This results in savings from the lower power/energy used at the reduced power cycle.

2) Because the staging minimizes (or with variable output commonly almost eliminates) overheating/overcooling, it reduces the temperature swings in the house - both by dropping to lower output as the thermostat approaches shutoff point, and also because it will commonly operate more often at lower power to maintain a narrower temperature range in the house than with a single-stage unit.

3) Because the multi-speed units tend to run the fan a lot more of the time, you tend to get more uniform heating/cooling through the house, so if the thermostat(s) are in the appropriate location generally you can set the thermostat several degree lower and still feel comfortable throughout the house.

However, that being said -

4) for residential purposes, generally the savings will be maybe 10% to at most probably 15 to MAYBE 20% energy savings (fuel plus electricity), so except in very heavy use areas (especially with A/C) generally your savings are in the low to mid hundreds of $/year savings, with typically a $3-5,000 capital cost difference - so the payoff time for the multi-stage unit tends to be long - over 10-15 years typically. So - unless you intend to stay in the house indefinitely and have no prospects of transferring or moving to a new job, the variable output units will commonly not pay off for the person who pays to have them installed because new home buyers will rarely "pay up" for a multi-speed unit.

5) One other factor working against their economics is in moderate heating/cooling situations the staged performance can be useful - by not using more fuel/electricity than needed for a nominal change in house temperature. However, because in that environment the total heating/cooling bill is not extremely high, the savings from a multi-speed unit is also limited. But in the extreme zones, where techincally they should save the most money, it turns out that is commonly not the case for the simple reason that in extreme heating/cooling conditions the unit has to run quite frequently anyway because of the alrge temperature diffaerntial between indoors and outdoors - so with staged or variable running it commonly ends up running almost continuously rather than a few times an hour, which ends up wasting energyor at leasat burning up much of the savings in the long run.

As BayAreaAC said, the variable or staged units tended to have a lot more operating and maintenance issues - in my experience this has NOT improved as they get more common. Another MAJOR issue is that with all the added sensors and computerized controls, most of the techs have not been up-trained in diagnosing them, so a minor failed or inoperative sensor which should cost $100-200 to replace commonly ends up with a total board and multiple sensor replacement for $1000 or more. Had one friend pay THREE times for a new board at around $1500 each time, and one of those times the manufacturer statewide warranty technician looked at it too and said the board was shot too. Turned out one of the sensor wires rubbed against a sharp sheet metal housing edge and had been partly stripped bare so was shorting out to the frame at sporadic times, causing the unit to shut down.

A LOT of them also just plain have a ton of additional sensors and dampers and cdontrol circuits and such that compound the failure rate - the classic example of put in more parts that can fail and your mean time between failure gets shorter and shorter - commonly more often than once a year with many units I have seen put in over the past 5 years or so. In fact, my go-to HVAC company used to provide annual maintenance agreements for service (plus parts) and will not do so for multi-stage or variable units now because the number of service calls became to frequent with those units. And of course, with the more complex units and more frequent failures you get more heating or cooling outages, which can be critical in some areas or if there is someone in the house in poor health who cannot tolerate loss of service for a few days in the peak service call season.

So - as you can tell - I generally do NOT recommend them, especially if not in an extreme A/C zone where a two-stage A/C unit might actually pay for itself. And not at all in heating zones - I just have not seen justification to roughly double the installed cost of a furnace or boiler to get staged output.

Answered 4 years ago by LCD

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