How to Use a BTU Calculator

A BTU calculator is a powerful tool in the right hands, but like all tools, its usefulness depends upon the hands using it.

If we know how to use it correctly, a BTU calculator is an invaluable aid to working out the heating requirements of our home.

If on the other hand we blunder through it, the consequences could range from our radiators not putting out enough heat and needing to be replaced, all the way to needing a completely new central heating system installed!

So how do we avoid these disasters and get the correct calculation every time? Read on.

The key to using any tool properly is to understand it before we even start work, and a BTU calculator is no different to a wrench or spanner in that respect.

There are many different websites out there, each offering slight variations on their BTU calculators, and it's easy to become confused.

However each will use the same core factors regardless, and these factors are as follows:

The Dimensions of the Room

• Width
• Length
• Height

The Room's Type

• Dining Room/Lounge
• Bedroom
• Kitchen/Common Area

Additional Factors

• North-facing
• French Windows
• Double Glazing

Let's examine these in more depth.

The BTU requirement will be for the whole room, so we need to calculate the total amount of space the room covers. To calculate this, we simply multiply the Width by the Length by the Height to get our measurement in cubic feet.

To get a basic BTU requirement, we then multiply the cubic feet by

• 3 for a Kitchen or Common Room
• 4 for a Bedroom
• 5 for a Dining Room or Lounge.

These numbers reflect the fact that we are likely to spend less of our waking hours in the kitchen than the lounge.

This BTU requirement is then modified by

• +15% if the room has a north facing wall
• +20% if the room has French windows
• -10% if the room has double glazing installed

This is because north facing walls receive the least direct sunlight and are prone to damprot, French windows allow in sunlight but allow more heat to escape through the glass and double glazing is a tried and proven method to reduce heat loss from windows.

Now it's all very well talking about this, but let's put some realism into this.

Let's say we have a living room. It is 16 feet long, 16 feet wide and 8 feet high.

This gives us 16 x 16 x 8 = 2048 cubic feet of air that will need heating up.

Entering these figures into a standard BTU calculator will give us a total BTU requirement of

10240 BTU/hour

Now let's add double glazing (-10%) and a French window (+20%) into the mix.

We now get a total of

11264 BTU/hr

This will be the total amount of BTUs we need to fully heat this room.

BUT

It's not as simple as using this figure to work out the amount of BTUs we need from our radiators, and radiators aren't the only things that generate BTUs, as any Matrix fan will know.

"The human body generates more bio-electricity than a 120-volt battery and over 25,000 BTU's of body heat." - Morpheus

Yep, human beings do. We generate heat just like any good electric heating device, and depending on the activity we're doing, generate different amounts of heat; generally the more active we are, the more heat we generate.

Here are some examples of the BTU the human body generates while performing the following activities:

• Sleeping - 250
• Relaxing - 350
• Office work (seated) - 420
• Office work (standing) - 640
• Ten-Pin Bowling - 960
• Walking (3mph) - 1040
• Factory work (heavy) - 1600
• Exercise (heavy) - 1800

In our example, we're talking about a living room, where we expect people to relax and not be jogging on the spot (well, unless they have a Wii!) so we'll use the 350 BTU figure per person.

We have a partner and two children, so the amount of heat being passively generated by all our bodies is

350 x 4 = 1400 BTU

This is heat that is already warming the room, so we can deduct this total from the BTU calculator's earlier result of 11264 BTU to give us a final total figure of

11264 - 1400 = 9864 BTU/hr

This is the total BTU that our radiators and other heating devices will need to generate to fully heat this room, assuming that the 4 human beings are all relaxed and that there are no other heat sources in the room.

It is very important to remember that this figure given by the BTU calculator is what our radiators will need to put out under ideal conditions.

If we live in an unusually warm climate, on top of an hill where we are completely exposed to the elements or if we live in a region where snow is commonplace, we will have to compensate for these factors.

Remember, it is always better to overestimate our needs than underestimate. We can lower a radiator's maximum heat output but we can't force it to generate more heat than it is physically or chemically capable of!