This is Lesson 7 of our informal online course: Home Energy 101All lessons by Philip Drader, Home Energy Coach.

Replacing a furnace can be a fairly straightforward affair. You call a couple heating, ventilation and air conditioning companies (commonly referred to as HVAC companies) and they give you quotes to replace it.

That’s great if you know you need a new furnace because your current one has reached the end of its life. But what if your current furnace is still working fine. Will a new furnace help make your home more energy efficient or improve uncomfortable rooms? Here’s my advice to help you decide.

vented water heater and furnaceWhen replacing your furnace is an easy decision

A good rule of thumb is that if your furnace has a pilot light, or has a metal vent with a gap just above the furnace (see photo) then go ahead and replace it before winter.

Keep using your high efficiency furnace

If the label on the side of your furnace says that it’s greater than 90% efficient to start with, replace it when it dies since you already have a high efficiency furnace. You won’t save enough gas with a higher efficiency furnace to pay for the upgrade.

Determine how much money you can save money with a new furnace

If your furnace efficiency is below 90%, it’s useful to know how much you can lower your heating costs to determine if you should replace your furnace right away or if you could wait. You’ll want to determine if you can save enough money in fuel costs to justify replacing the upgrade.

  1. Start with your yearly gas bill to figure out much how much you’re paying for the natural gas used to heat your hot water for the whole year.

Most people use about the same amount of hot water for showering, dish washing, and washing clothes all year round, so you need to figure out how much natural gas you use monthly to heat water.

  1. Look up your bill for a warm weather month when you were home with the furnace off but not frequently using a natural gas barbeque.
  2. Take that monthly amount and multiply it by 12 to get your yearly cost to heat water.
  3. Then subtract your water heating costs from what you paid over the year for natural gas.
  4. furnace Energyguide labelTake your resulting number and multiply it by your furnace efficiency. It’s usually listed on the side of your furnace (or boiler). Turn it into a decimal. If your furnace it 83% efficient, it would be .83.

That number you have is the absolute minimum cost for a fuel-fired appliance (for example natural gas or propane) if it were 100% efficient.

  1. You can then reuse the formula with your new furnace options.
  2. Use results of Step 6 to calculate your savings by replacing your furnace with each option.

Taking time to do this math can help you determine if the savings on your fuel costs are enough to upgrade your furnace.

example of calculation to determine savings with a new furnace

What if you have uncomfortable rooms?

If you have rooms that are uncomfortably warm or cold, first check to make sure that all the vents in the house are open. Then take some time to “balance” the house. Open vents in rooms that are used and close the ones in rooms that are not used. If you had to adjust more than a couple, give the house a day or two to settle in to its new temperature profile.

The vents might also have a little circular adjustment plate inside the duct. Some are accessible from the vent, and some are accessible on the main duct coming out of the furnace in the basement. If your home has them, you can try using them to adjust air flow.

takeoff from main duct

takeoff from main air duct

If there are still rooms that are uncomfortable, the next step would be to tape up wherever there is a takeoff from the main duct such as to take air upstairs.

takeoff with foil tape

foil tape around connection with main duct

The duct is probably pretty dirty, so clean it first so the dust doesn’t prevent the tape from sticking. Then use a high-quality aluminized tape to seal around each takeoff.

Another option – though it isn’t the most efficient option – would be to run your furnace fan continuously instead of on auto. But doing so means using electricity and it decreases the time until the motor needs to be replaced.

Some people will have settings on their vent cover’s diffuser to tell them what setting to choose for summer vs winter. Your basement for instance, will rarely need any air conditioning in the summer, so all the vents should be closed. However, it will probably need to be open for full heating in the winter.

You might need to change your water heater too

If you have an exterior chimney and a water heater that is vented into the chimney, you should replace your water heater with a sealed-combustion unit at the same time that you are doing the furnace replacement. Otherwise, the chimney can cool down when there’s no longer the furnace venting through it, and the venting from the water heater could possibly backdraft and let combustion products into the home and potentially lead to carbon monoxide poisoning.

You can do so fairly inexpensively, if like most people, you rent your water heater. If for some reason you can’t replace your water heater at the same time, ensure you have carbon monoxide detectors in your home. Everyone needs to have these detectors according to a fairly recent law.

What if you don’t need a new furnace?

If your home gets uncomfortably cold but you don’t need a new furnace and you’ve explored the other options I’ve suggested to improve uncomfortable rooms, you should get a home energy evaluation to help you develop a game plan to make your home energy efficient and comfortable.

The other lessons in this series may also be helpful.

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