A Megajoule or MJ Probably Isn't What You Think

Megajoule is a unit of energy typically represented by the symbol “MJ”. The prefix “mega” infers one million and megajoule equals 1,000,000 joules, an SI unit of energy with the plural “megajoules”.

What is a megajoule and why does it matter when I buy a gas heater?

What size gas heater should I buy?

Many people think they know but you might be surprised by the real answer…

What is MJ - Megajoule?

♦ How is it pronounced? Say it as if it were spelled "mega jewel" or "mega jewels", not "mega jowl".

Keep reading for the details on all of the above…

A MJ is NOT Heat Output

I recently watched a product video produced by a major Australian retailer.

In it, a “Product Expert” reviews a Rinnai gas heater.

The problem was that the presenter lost me in the second sentence when he said:  “That’s 13MJ of heat.”

This is an incorrect statement and perpetuates a common misconception that MJ represents heater output, as opposed to gas input.

Gas Consumption is in "MJ/hr" — Not Just "MJ"

Megajoules per Hour (MJ/hr) ratings on appliances actually indicate the gas consumption of the appliance, not the heat output.

On the other hand, heat output must also take into account the relative energy efficiency ratings of the models being compared.

The MJ/hr gas consumption rate is often erroneously expressed as just "MJ".

This is how MJ is frequently used in reference to gas appliances.

Heat Output = kW

Kilowatt (kW) is how gas heater output is measured.  3.6 MJ of input equals 1kW of output at 100% efficiency.

However, no gas appliance is 100% efficient.

That is why we have Star Ratings, so we can compare relative efficiency between models.

Appliance output is often expressed as just kW.

Energy Efficiency - Star Ratings

Star ratings were developed to provide consumers with an easy way of comparing the energy efficiency of different models.

All States now require both LPG and Natural Gas heaters to be tested and certified, under AS4553, by independent testing organisations, such as the AGA and SAI Global.

The Star Rating is based on net heater efficiency.

Net heater efficiency is calculated on the basis of a combination of efficiency measures and takes into account all gas and electrical inputs.

It basically compares the amount of raw energy input consumed by the heater in comparison to the heat energy output provided.

This is calculated as a percentage and then translated into the corresponding Star Rating.

What to Consider

If you buy too small a heater, it will not be able to adequately heat the intended area.
Selecting a heater that is too large will cost you extra money and oversizing can be unsafe.
A number of things need to be considered when determining your heater sizing:
♦  The volume of the area to be heated including consideration of ceiling height.
♦  The climate zone that you live in.
♦  The physical features of your home including wall & ceiling insulation, window coverings and carpeting.
♦  The kW output of the gas heater, not to be confused with the gas Mj input.

Heating Area Volume & Climate Zone

The required kW output required is primarily dependent upon the area to be heater and the climate zone in which you live.
The following rough guide assumes your ceilings are no higher than 2.4M:

Very Cold Zone: 1kW output required for each 8.5m²

Cold Zone: 1 kW output required for each 10m²
Cool Zone: 1kW output required for each 13 m²
Mild Zone: 1kW output required for each 16m²

The indicated heating areas would be adjusted down by 5% for each of the following conditions: house built on pillars (non-slab), no carpets on floors, no drapes on windows or a ceiling height exceeding 2.4m.
Deduct an additional 10% if the area does not have ceiling insulation.
Please note that this is only a guide and individual home designs and situations may vary.

Gas Heater Sizing Example #1:

You live in the Cold Zone and you want to heat an area 5m X 10m, which equals 50m².
Because 1kW will heat 10m², you should need a heater with a 5kW output.

Gas Heater Sizing Example #2:

Same as the previous example but in this case your ceilings are over 2.4m and you have wood flooring instead of carpeting.
You would need to deduct 5% for each of these two items.
So, instead of 1 kW heating 10m², it would heat 9m² (10% total reduction).
This means you would need a heater with 5.6 kW of output (50 ÷ 9).

Use kW not MJ

As previously explained, Megajoules per Hour (MJ/hr) ratings on appliances actually indicate the gas consumption of the appliance, not the heat output.
It is often expressed as just MJ.
Kilowatt (kW) is how gas heater output is actually measured.

Energy Efficiency Matters - Star Ratings

So, remember to use kW as a comparison and not MJ.
The efficiency of the heater, in converting gas energy to heat, is a key factor.
For example, a 25MJ heater with a 5.8 Star rating has an output of 6.2kW.
A 25MJ heater with a 2.8 Star rating only has an output of 5.0kW.
As a result, in a Cold Zone climate, they would heat 62m² and 50m², respectively.
This means that the 5.8 Star heater will heat an area 24% larger while using the same amount of gas.
Please refer to the manufacturers’ specifications to determine the exact kW output for each heater you are considering.

Minimum Room Sizing

Some States specifiy the minimum room sizing (volume of room in M³)  and minimum ventilation requirements for unflued heaters.
Minimum room sizing also varies between thermostatically controlled and manually controlled heaters.
Portable heaters should not be used in bedrooms, bathrooms or hallways.
All installations must comply with Australian Standard AS5601 - Gas Installations.
Your gas fitter will be able to guide you on many of these points.

Manufacturer Sizing Charts

To make it even easier, most manufacturers have sizing charts, for their heaters, based on climate zone maps.
Take the time to measure the area you wish to heat and use these charts and maps to get the correctly sized heater.

Compare Indoor Gas Heater Prices

Now you can compare prices for all the major brands of indoor gas heaters including:

Rinnai, Everdure, Paloma, Braemar and Cannon

Click on your category of interest to compare prices: