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posted: 2/4/2011 1:00 AM

High-efficiency furnaces forgo traditional chimney

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By James Dulley

Q. The repairman says I need a new gas furnace. He recommends a super-efficient one because my chimney also needs repairs. What are my options for the best comfort and why is the chimney a factor?

A. The condition of the chimney is a factor because new super-efficient gas furnaces do not vent up the chimney. They are vented outdoors horizontally through a wall by a small plastic pipe. The cost to repair a chimney is often greater than the cost to upgrade from a new medium-efficient to a super-efficient gas furnace.

Even if your chimney were still in excellent condition, it makes economic sense for almost everyone to install a super-efficient furnace. Always have the contractor do a payback analysis for you for furnaces of various efficiencies and prices.

A lower-cost, lower-efficiency gas furnace may make sense in mild climates or in super-efficient houses. The reason for including super-efficient houses is their heating bills are so low anyway, the small utility bills savings may not pay back the higher cost of a super-efficient model.

You have quite a few super-efficient gas furnace options. Any of them you select should cut your heating bills by 40 percent or more. Even if you do not need a super-efficient model for heating, its air handler and controls may be needed for the most efficient and comfortable air-conditioning.

The efficiency rating of a furnace is called AFUE (annual fuel utilization efficiency). Your old furnace probably has an AFUE in the 60-percent range. All of the new (no chimney) furnaces will have an AFUE greater than 90 percent. Some are as high as 98 percent. This means only two percent of the gas you pay for is lost out the flue as compared to 40 percent now.

For the best overall comfort, a new modulating gas furnace is best. This type of furnace constantly calculates the instantaneous heating needs of your house and adjusts the heat output accordingly. This can range from as little as 15 percent to maximum heat in just several-percent increments. With this type of furnace, the room temperature stays extremely constant at your thermostat setting.

Another relatively new option is a three-stage gas furnace. Instead of providing fully modulating heat output, it has three heat outputs. These typically are 40-, 65- and 100-percent heating. This design also provides excellent comfort and steady temperatures. Two-stage gas furnaces have been around for many years. With just a low- and high-output heat settings, they should provide adequate comfort for most people.

With any of these multistage furnaces, they run longer and cycle on and off less often. This is because they run at a lower-than-maximum heat output the majority of the time. This contributes to the steady room temperatures, less noisy cycling, and more effective humidity control. Keep in mind, a furnace-mounted humidifier and air cleaner are effective only when the furnace is running.

The least expensive super-efficient furnaces use a single-stage design. These still have AFUE's greater than 90 percent and have been used for decades. When coupled with a variable-speed blower, the comfort level is good. I use a single-stage, variable-speed blower system in my own home. This blower also greatly improves comfort for summertime air-conditioning.

Q. I have a tankless water heater that is located far from the master bedroom. It takes a long while to get hot water there. Will one of those rapid hot water pump kits work here?

A. Rapid hot water kits use a high volume pump to bring hot water through the pipes very fast to your faucet. No water is wasted because all the water stays in the plumbing system until hot water gets to the faucet.

The maximum heating output of your tankless water heater will determine how well a kit works. Since a high flow rate is pumped through the tankless water heater, the water may not initially come out as hot as you would like.

• Write to James Dulley at 6906 Royalgreen Drive, Cincinnati, OH 45244, or visit