Advice to consider before replacing furnace

Here are issues and information to consider when you are thinking about installing an energy-efficient furnace.

A more expensive, energy-efficient furnace might not pay back that extra cost, but it probably will make your home more comfortable.

A furnace must be at least 95 percent efficient if you are going to qualify for the $500 federal energy tax credit.

The credit expires at the end of this year, but James Dulley, a mechanical engineer who writes a syndicated column that appears in the Daily Herald, expects it will be reinstated.

Jim Nanni, manager for appliances and home improvement for Consumer Reports, points out you would have to have a significant savings in energy costs to make it financially worthwhile to throw out a working furnace.

The furnace plus installation might cost $5,000, he estimates.

If you can't expect to get the money back in less than 10 years, it's not considered a good business investment.

Nanni pointed out that furnaces have an average life of about 18 years. One that is still operating properly and is inspected every year was probably bought no earlier than the mid-1980s and thus should be 80 percent efficient, so the financial savings in replacing it would not be there, Nanni said.

One dangerous thing a contractor checks for is a crack in the heat exchanger that could put carbon monoxide into the home.

When you do have to replace your furnace, then you can compare the cost of a regular one to a high-efficiency one. The additional cost would be what you would want to recoup with gas savings.

The high-efficiency furnaces are more expensive because the secondary heat exchangers and direct venting are more complex.

Any furnace you buy today is at least 80 percent efficient due to a 1992 law, but 80 percent efficient ones were made much earlier than that.

If you have an old furnace with a 60 percent efficiency and are replacing it with an 80 percent furnace, you might need to put a stainless steel liner in your chimney. That additional expense might make it worthwhile to instead choose the 95 percent furnace, Dulley said.

However, he points out if your home is very well insulated it might not make sense to spend the extra money for a high-efficiency furnace because you are not using much gas anyway.

The 90 percent efficient or condensing furnaces put out exhaust so cool it goes through a plastic pipe and cannot use your chimney.

A two-stage furnace operates at low-heat output, which is more efficient, when the outdoor temperature is moderate, Dulley explains.

On a bitter night, the high-heat output will kick in.

Two-stage furnaces are more comfortable and quieter than one-stage because they run more continuously, starting and stopping less frequently. They are more expensive, however.

Modulating furnaces are like two-stage, but they go up or down by a smaller percentage rather than just high and low. You might need a modulating thermostat, too, to get the best performance.

Variable engines for the fans are important for comfort if you have a two-stage or modulating furnace, he said.

Dulley is one of the people who does not think there is a lot of difference between the major brands of furnaces.

They all have life-time warranties on the heat exchangers, he said.

Trane says its new furnace is 96.7 percent efficient, but Dulley does not find that impressively better than those that are 95 percent efficient.

Randy Scott, vice president of product management for Trane, disagrees, saying it is important to continue to push the envelope of energy efficiency.

If you don't have a sheet or label that tells how efficient your current furnace was when it was installed, ask your contractor to look up the model number.

Furnaces in the mid 90 percent range have been available for 15 to 20 years, said Dulley, but not many of them were purchased in the early years.

Sizing the furnace is very important, said Dulley. The contractor should check your home's insulation and can calculate the right size. Your home might be more or less efficient than one across the street that looks just like it.

Replacing your furnace with one of the same size is the wrong way to go, Nanni said.

"We known most furnaces are oversized by a factor of almost two." That means they are almost twice the size they should be.

And that's before you take into account improvements the owner might have made in the home since the old furnace was installed -- new windows and added insulation, for example.

If one room is too hot or too cold the duct system might need to be reworked, said Nanni.

If you put in a two-stage furnace you might want to change your air conditioner to a two stage also, Dulley said, especially to help control humidity.

No matter what type furnace you select, you should make sure your duct joints are well sealed and the ducts are insulated in unheated space.

Otherwise you will lose heat or at least all rooms will not be getting the desired amount of heat.

Dulley seals his joints with Gorilla Glue brand duct tape.

Dulley likes electric heat pumps. These provide air conditioning in the summer and heat during the spring and fall. But in the winter a gas furnace would still be required. Heat pumps cost $300 to $500 more than an air conditioner, he said.

Andy Armstrong, director of marketing for Johnson Controls Unitary Products, which makes York, says the company has a furnace that prevents temperature swings and works with a traditional thermostat.

Heating contractors are busiest the first few months of cold and heat seasons, said Armstrong.

That means you might be able to get a good price in January or February when business might be slow.

Manufacturers have sales generally from September through November and march to May, he said.

Lennox plans a new line of variable-speed furnaces in January that will be 95 percent efficient, said Ken Ely, senior heating product manager.

It is difficult to get manufacturer's recommended prices for furnaces, but Consumers Digest, based in Deerfield, does that when it prints its furnace story every other year. The next one will come out in September.

Rich Dzierwa, editor, believes manufacturers want their dealers to be able to make as much money as possible.

The differences between different brands of furnaces generally are not deal breakers, he said.

"You don't have to worry about the brand. It has a lot to do with installation. The important question is whether a furnace is sized and installed properly."