You want to think twice about this idea, which can backfire on you.

Buying a used furnace will save you money and might be quicker than buying a new one, but there are several reasons to beware.

Safety is just one. How well do you know the furnace’s history and condition?

Also, remember:

• Many HVAC contractors are leery of installing used equipment. They prefer working with new equipment from established manufacturers.

• New furnaces generally have a 10-year warranty while the used furnace probably doesn’t have an applicable warranty. Most manufacturers limit warranties to the original owner and the original building where it was installed, so the used furnace’s warranty probably is void.

• A furnace’s average lifespan is anywhere between 15 and 30 years. How old is the furnace?

Remember, older furnaces are less fuel-efficient than a modern furnace, which will cost more up-front but will do a better job for less money monthly.

And a used furnace’s components already have at least some wear and tear on them, meaning repairs may be in the near future.

• Is the used furnace the right size for your home? The proper size is needed to maximize efficiency and minimize costs.

• Used furnaces make home-buyers and insurance agents nervous. If you’re selling your home you probably will have to answer questions about the furnace. Ditto for buying home insurance.

Bottom line: Buying a used furnace saves money but increases the likelihood of expensive problems.

For furnace repairs and installation in NYC and LI, call Premier HVAC. We guarantee our work.