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Just learned a house we purchased has a heat pump vs a traditional heater. The heat pump / AC unit configuration outside, blower inside.

We live in the phoenix AZ area, and an efficient AC unit is critical. AC works pretty well, heat pump works fine too. But is this combined unit more or less efficient than a traditional AC unit and seperate electric heater?

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I ask because its probably 20 years old and needs replaced. Wondering if i replace with a new model, or go AC only and add in an electric heater. –  Matt Adams Nov 13 '12 at 18:54
    
Almost anything will be more efficient than a 20 year old appliance. –  DA01 Nov 14 '12 at 8:28
    
@DA01 of course, just wanted to replace it with the most efficient set up. Be that a combo system or a single AC and add in an electric heater. –  Matt Adams Nov 14 '12 at 15:47
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2 Answers 2

up vote 3 down vote accepted

Warm climates are good candidates for heat pumps.

Heat pumps have more efficiency in heating mode when the ambient temperature is warmer, less efficiency when the ambient temperature is colder.

If the outside temp falls too low, the heat pump stops providing useful heat. This is why heat pumps are often supplemented by "emergency" electric heaters that kick in when the outside temp falls too low.

Heat pumps can be more efficient than electric resistance heaters because they only need to move heat energy around, rather than create heat. When the outside temp falls too low, however, it takes more energy to move around the very small amount of available outside heat than it would to just create the heat with an electrical resistance heater.

According to wikipedia:

This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. When it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater rather than to overload an air-source heat pump.

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Cold air will not be an issue in Phoenix, but generally speaking, let's not forget there are other sources of free heat than outdoor air. It will obviously cost more to access it, but can be less expensive than relying on resistive backup heat. I'm thinking primarily ground water or the ground itself as heat sources, but solar heated mass could be a possibility. –  bcworkz Nov 13 '12 at 20:37
    
Good notes here. Now any thoughts as to the AC unit in the summer? Would a combo unit be less efficient than a dedicated AC? or is not that different? –  Matt Adams Nov 13 '12 at 20:56
    
@bcworkz indeed. Ground source or geothermal heat pumps are fantastic, but not yet in widespread use, therefore my assumption was that the OP was contrasting a "conventional" air source heat pump with resistive electric heat. There are certainly alternatives that are more efficient than either of these two options. –  mac Nov 13 '12 at 20:57
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@MattAdams: a heat pump and a dedicated A/C are so similar in "cooling" operation as to not warrant any further thought. –  mac Nov 13 '12 at 20:58
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A heat pump should meet your heating needs fine in Arizona. Note that you can get them in different efficiencies. The AC performance will be rated in SEER, and the heat pump performance rated in HSPF. For example, with Trane units, you have options from 14.5 SEER/8.5 HSPF up to 19 SEER/9.0 HSPF. The higher-end units may also be quieter. Check with your local utility to see if they have rebates; that can also effect the ovearll price.

You can find online calculators that let you compare the heating cost of the various options.

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