Condenser size for home AC

[sageoldmann]

My sister is having her home AC replaced. The HVAC guy is trying to talk her into a much larger unit than she now has. He claims it will be more efficient with shorter run times, saving her energy. A friend suggested she get a smaller unit. His claim is that it will be more efficient, use less power, and it will remove more humidity making the house feel cooler at a higher thermostat setting, thereby using less energy. Who is right?

[The Viking]

Neither.
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A correctly sized unit will be most efficient!

The highest current/lest efficient running condition is when the system starts up. So increasing the number of starts will not be efficient.

An undersized unit will just not cope.

Ideally what she needs (assuming the old unit used to cope with the load) is a unit with the same capacity as her old unit. BUT, she do want a unit with higher COP/better efficiency.
It should be inverter driven and it should run on a gas called R410a.

Note: The best unit is rarely the cheapest.

[paul_h]

Won't your sister have an idea of how well the current system coped? If there was no problems with the current size, maybe an ungrade in size not needed?

Is the "friend" in the HVAC business? If not, why listen to them? edit: Maybe get a quote from another HVAC company instead of tossing up these two current opinions.

Size the indoor to the load, there's a lot more that just capacity, there's also airflow ratings of the fan, duct length it needs to pump through, the amount of outlets or zones run together etc. Sometimes you need a bigger unit just to have the airflow required and the larger than the old system is just to improve that if requirments have changed.

[sageoldmann]

Viking, what's "inverter driven" mean? Is that some kind of compressor?

[paul_h]

You haven't heard of them in the US?
It's compressor that is fully variable speed. They use DC voltage, so they "invert" AC voltage to DC voltage to have a mutlispeed compressor, meaning it can slow down or speed up to meet the current load. Costs more, but if you run the A/C a lot, saves in a lot of power consumption. Worth looking into as I've heard power is pricey in some states in the USA