Before you even get a quote to replace a boiler or furnace!

[Who]

99% of boilers and furnaces are oversized and almost that number are vastly oversized.

1. Don't assume that the old boiler or furnace was the proper size.

2. Don't assume that the contractor can just look at your house and know.

3. Don't assume that the contractor cares about sizing it properly. Only the best do and they are way too rare and should charge to do you a quote since it will take a few hours of their time which costs way more than your spare time.

4. Don't assume that a bigger boiler or furnace is better for your house or doesn't have to work as hard. Wrong!!!

5. Don't assume that you are helpless to know what size you need.


The heatloss calculation is definitely something that you can do yourself.

The best investment you can make when replacing your boiler or furnace is a the couple of hours it takes to do a heatloss calculation and anyone can do one it. It just takes time. The 3 or more contractors you call over will not invest 2 hours to do one for you for nothing, nor should they. They'll often sell you the same sized boiler or furnace as you already have, which is already oversized... I can guarantee you that it is, or worse yet, the next BIGGEST size, or even bigger.

Our homes leak heat at different rates depending on outside air temperature, wind infiltration, etc. The boiler or furnace starts and stops to replace that heat that leaks out. If the boiler or furnace is too big, it starts and stops much more frequently and that hurts its efficiency and shortens its life. It also reduces your comfort by heating the house TOO fast.

Imagine driving a car with an on/off switch instead of a gas pedal. Would you want an engine that can just get you up to speed or do you want to have to frequently flick it on and off? That's how (non-modulating) boilers and furnaces work. Even modulating boilers and furnaces work that way when its minimum output is greater than the current rate of heatloss.

Do the heatloss first. Don't let anyone sell you a bigger boiler or furnace than you need.

With A/C they same deal applies but it is called a heat gain calculation.

A properly sized boiler means a more comfortable house, a longer lasting boiler and will use less fuel. Smaller boilers also cost less.

SlantFin has a free heatloss calculator software package on their web site and their are other online resources. The heatloss calculation is known in the industry as a Manual J calculation.

As optimizers, please keep in mind that your "true" heatloss is only about 65 to 75% of the Manual J calculation so you can size a boiler or furnace with a net rated output of about that range. At worst, if you go a bit too small, you'll need to dress a bit warmer in the house if you hit record lows... and you can feel smug you're better sized for the rest of the year.