Duct sizing, bends and return air on gas ducted heating

What are the supply side rules?

1. Airflow per outletCeiling 50 to 70 L/s, floor 45 to 55 L/s, both based on a 150 mm diameter outlet. Floor outlets should blow sideways rather than straight up.
2. Upsize long runsIf the duct run is over 6 metres, go up 50 mm on the spigot and the duct. Over 9 metres, go up 100 mm. Do it for the full length of the run, not just the far end.
3. Respect the bend radiiMinimum bend radius is 1.5 times the duct diameter for 150 to 300 mm duct, and 2 times the diameter for 350 to 400 mm duct. A tight bend chokes the airflow (it adds static pressure), and you pay for it every minute the system runs.
4. Leave one outlet openOn a zoned system, keep at least one outlet fully open whenever the heater is running. If the zoned system has a single controller, the return has to be in the common zone, the one that's never shut off.

How big does the return air grille need to be?

This is one of the most useful tables in the manual, and it rarely gets quoted. Add a filter and the return grille has to be a lot bigger than most people expect.

Keep that table in mind on a conversion quote. A reverse cycle indoor unit always has a filter and a coil, so the old gas return grille is the first thing that won't be big enough.

The rest of the return rules

• Return air duct at least 3 metres long, with a bend in it to cut noise
• Grille at low level, in a central hallway
• Not in a bathroom or a laundry
• Not recommended above 2.7 metres
• This heater isn't suitable for outside return air
• A door relief grille, or a big enough gap under any door that would block the path from outlet back to return

What should you tell the customer about the filter?

The manual asks for the filter to be cleaned every two weeks and replaced every year. Few householders do either.

That matters to you because a blocked return filter is a common cause of the overheat fault family. When you hand over a new system, showing the customer where the filter is and how to pull it out saves you a callout you'd probably end up wearing.

What are the flue and condensate rules?

• 100 mm metal flue, approved by the AGA (Australian Gas Association)
• Twin wall for any enclosed part, and recommended beyond 2 metres
• Maximum vertical rise 5 metres
• 500 mm from the nearest opening, and from an electricity meter or fuse box
• 1000 mm horizontally, in the direction of discharge, from an openable window, door or non-mechanical air inlet
• Condensing models need a condensate drain with continuous fall, unobstructed, draining freely and holding no water

External clearances follow the Australian and New Zealand Standard AS/NZS 5601. Size the gas pipe so the heater inlet still gets 1.13 kPa on natural gas, or 2.75 kPa on propane, with every other gas appliance in the house running flat out.