What Size Gas Heater Do I Need?

The size gas heater you need for a small room (29m³ – 42m³) has a heat output of 2.9 – 4.2kW.

A medium room (42m³ – 66m³) needs a gas heater size with 4.3 – 6.6kW.

Large rooms (66m³ – 82m³) need a gas heater size with a heat output of 6.7 to 8.2kW.

Do not go by MJ ratings, as heat output can vary greatly based on gas heater efficiency.

Using kW is the net heat output that you are looking for.

These gas heater sizes are all approximations and your situation may be different based on climate, insulation, ceiling height, the number of fenestrations, window coverings, drafts, floor coverings and sub-flooring.

Rinnai has a very good gas heater sizing calculator that takes many of these variables into consideration.

It is important to get the right size gas heater for your home.

You want to select the ‘Goldilocks’ heater that is just right for your gas heating needs.

Gas Heating

Gas heating is the heating of a room or outdoor area using a gas space heater fuelled by natural gas, LPG (propane or butane) or biogas.

Indoor gas heater types include unflued portable gas heaters, flued gas heaters, radiant gas heaters, convector gas heaters, radiant-convectors, and gas fireplaces.

Gas Heater

A gas heater is a space heater used for room heating or outdoor area heating fuelled by natural gas (including CNG and LNG) or LPG (propane, butane or a mixture of the two).

The two general types of home gas heaters are flued gas heaters (vented) and unflued gas heaters (unvented).

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

Courtesy of Rinnai

The required kW output required is primarily dependent upon the area to be heater and the climate zone in which you live.  (Please see climate zone map above)

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

Adjustment Factors

The indicated gas 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

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 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.

Portable LPG heaters, with a gas bayonet, tend to be more energy efficient than flued gas heaters.

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.

It is always wise to follow the manufacturers’ advice.