What Size Furnace Do I Need?

When you starting thinking about the size of furnace you need, you may feel like you are asking the age old ‘how long is a piece of string?’ question. It can also start to feel a bit like Goldilocks and the three bears - some too hot, some too cold, some too sweet, nothing right! And we all know why making the right decision is crucial, like with Goldilocks and her porridge, making the wrong decision is just not palatable.

The size of your furnace is very important to the overall efficiency of your home’s heating system. You don’t want a small furnace that will leave your family reaching for the blankets on a cold day, but equally you don’t want a huge furnace needing large duct work, that’s wasting energy as it constantly cycles off and on. Your system needs to be as customized as possible to the needs of your home, so how can you work out what this is and what size you need?

That’s where we can help. Making the right decision about the size of furnace you need is about being well equipped with the knowledge to make an informed decision. You will need to rely somewhat on the professional you charge with installing your furnace, as well as some whizzy software packages, but knowing the criteria will help you along the way.

First top tip - don’t simply replace your old furnace with a similar model. Never assume you’ve got it right in the past. This is an investment that you probably won’t make for another ten years, and it may cost you in the region of $10,000, so review it and get it right.

Second top tip - know about the manual J load calculation. This calculation looks at the size of your home, the ductwork, and your home’s overall energy efficiency. It will give you the information you need to get the right size furnace in BTUs (British Thermal Units per hour). In non-jargon, what needs to be considered are things like the size of your home, what direction does it face (for example, homes that face south may get more heat from the sun in winter), how open is it, how insulated is it (is it well protected from winter winds by vast amounts of trees?). You also need to think about where you live. A home in Florida will need less heating power than a Midwest home.

Third top tip - when looking at furnaces make sure you consider their size, but also their efficiency in relation to their size. It won’t be a surprise to you that furnaces have come on in leaps and bounds in the last ten years since you last brought one. By going for a higher rating, for example 93% compared to 80% efficiency, you may be able to buy a smaller unit, but get the same amount of heat, as it’s much more efficient - bigger size doesn’t always mean better heat these days.

So, think about all of these factors and you can come up with your own size estimate. Ultimately you will need a professional’s view and a software program, which will input these factors and give you an accurate estimate on the size of furnace you need, but giving yourself a good understanding is a great first step.