Installing central air might not require electrical upgrade


Bob and Sue Locke are planning to have central air conditioning installed in their 55-year-old Cape Cod home. It will be a single-zone system, with the air handler situated in the attic. Their question is whether the central air installation will require them to have their electrical service upgraded. The heating and air-conditioning contractors from whom they've gotten estimates have differing opinions.

Their house has gas heat, a gas range, a gas clothes dryer and a gas-fired water heater. The existing electrical service is 100 amps, but the service panel is full - no open positions for additional breakers.

One contractor's advice is to upgrade the electrical to 200 amps. Another says the 100-amp service is adequate and proposes adding a subpanel to make room for the air conditioner's circuit breakers.

A third contractor doesn't even think a subpanel is needed, since space could be made in the panel by replacing some of the breakers with "double breakers" (often called tandem breakers). The Lockes are trying to decide which plan to go with.

A heavy upgrade from a 100-amp electrical service is often recommended when installing central air, and it can be a good idea, even if not absolutely necessary. Many houses with 100-amp services have the service loaded up with 240-volt major appliances even before air conditioning is installed. Those appliances include electric cooktops and ovens, electric water heaters, and electric clothes dryers. If those appliances are present, they can use in the neighborhood of 80 amps if running simultaneously.

Adding central air to an existing 100-amp service already loaded up with appliances could exceed the capacity of the service or, more likely, cause nuisances like dimming lights when the air conditioner starts up.

In the Lockes' house, natural gas is used to fuel the major energy-consuming appliances, so the existing 100-amp service is not fully used. The new central air conditioning will use less than 30 amps of 240-volt power, which would leave adequate capacity for other demands.

So, while upgrading the service to 150 or 200 amps would be just great, it does not appear to be necessary to go to that expense. If it turns out later that the 100-amp service isn't satisfactory, it can always be upgraded.

The lack of space that they have identified in their service panel will have to be dealt with. It may be possible to make room for the new breakers in the panel by retrofitting tandem breakers, as one contractor proposed.

The contractor should confirm that the panel is rated for tandem breakers by the manufacturer. If it is, and enough tandem breakers can be installed to connect the air conditioning with room to spare, that's the most economical approach. But if the panel is full, they'll have to spend several hundred more dollars to have a subpanel installed, or make the larger investment in an upgrade to a larger service.

Cleaning the air

Returning to the topic of high-efficiency air cleaners discussed here a couple of weeks ago, Mike Hendricks, owner of Environmental Systems Associates in Columbia, e-mailed that installation and maintenance costs on pleated media filters are less than I said.

His company's installed price for an Aprilaire/Spacegard unit is $400, and replacement filter elements (replaced about once a year) are $42 plus tax.

Those figures place pleated media filters at an even greater cost advantage over electronic air cleaners than I'd indicated.

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