Can you reuse the ducts on a gas to reverse cycle swap?

Why reusing gas ducts is a close call

The two systems move different amounts of air to deliver the same heat, because gas runs a far hotter supply air temperature.

Here's what the manual confirms. On gas ducted, the first heat exchanger over temperature limit (HX OT1) usually sits at 110°C. The outlet thermistor faults if it reads above 60°C at start-up. So air coming out well above 50°C is normal for gas. Gas ducted design airflow is 50 to 70 L/s per ceiling outlet on a 150 mm outlet.

Reverse cycle supplies air at roughly 35 to 45°C in heating and 12 to 15°C in cooling. Because the air isn't as hot, you need a lot more of it to deliver the same kW. On top of that, the fan has to push it through a filter and an indoor coil, and both add resistance (static pressure).

The best proof is already in the gas manual. On a Braemar TQ3X30, adding a filter takes the return grille from 900 x 400 mm to 1200 x 400 mm. A reverse cycle indoor unit always has a filter and a coil.

What actually goes wrong, in order of how often it bites

1. Return air too smallThe single most common failure. The gas era return grille and return duct won't pass reverse cycle design airflow with a filter and a coil in the way.
2. Supply ducts too small, too long, or bent too tightYou get noise, air that doesn't reach across the room (poor throw), and zones that don't balance. The gas manual's own rules still apply, and old installs break them all the time. Go up 50 mm in diameter beyond 6 m and 100 mm beyond 9 m. Minimum bend radius is 1.5 times the duct diameter for duct up to 300 mm, and 2 times for 350 to 400 mm duct.
3. Ducts not insulated for cooling, so they sweatGas ducted ductwork was never expected to run cold. In cooling, duct that isn't properly insulated and vapour sealed sweats and drips into ceilings. The gas manual only ever asked for insulation for the heating case.
4. Tired flexible ductTrade view, not an official one. Reuse is sometimes fine on a straightforward replacement, but you have to physically inspect it. Older, deteriorated flex usually needs replacing.
5. Electrical capacityTrade claim, unverified. Older homes on low amperage supplies sometimes need a switchboard upgrade, quoted at $1,500 to $2,500 on top.
6. Wrong outlet typeGas ducted often uses floor registers, which should blow sideways. Floor outlets are fine for heating but poor for cooling. For cooling you want ceiling outlets blowing mostly straight down. This is the failure nobody warns the customer about.
7. Zoning and the common zoneGas ducted needs one outlet always fully open, and the return in a common zone that can't be shut off. Reverse cycle zoning has its own minimum airflow rules, and the old dampers rarely line up with them.

What I'd do on the quote

Lead with the return air check. Don't promise duct reuse before you've been in the roof, and don't judge it by eye. Measure airflow against the new unit's design airflow, not against how the ducts look.

And be straight about floor outlets if the house has them. A floor outlet gas system converted to reverse cycle typically cools poorly even when the airflow is fine. Saying that at quoting stage costs you some jobs. Not saying it costs you the ones you win.