Can Gas Heating Ducts Be Reused for Reverse Cycle Air Conditioning?

The short answer

For a new ducted reverse cycle installation by Macedon Ranges Gas & Air, no. Every ducted reverse cycle system we install gets new, correctly insulated ductwork, sized and laid out for that system. We do not connect a new reverse cycle unit to the ductwork that served a gas heater. The reasons are below, and they are the same reasons a quote that says “connect new unit to existing ducts” is worth questioning.

The exception is a like-for-like gas heater replacement, where the existing gas-heating ducts can stay if we have inspected them and they are in suitable condition. That is a different job with different airflow, covered on the gas heater replacement page.

Why gas heating ducts are often the wrong size

A gas ducted heater delivers a relatively small volume of very warm air. A reverse cycle system delivers a much larger volume of moderately warm or cool air. The duct system that served the gas heater was sized for the gas heater’s airflow. Push the reverse cycle system’s airflow through it and one of three things happens: the fan works too hard and the system is noisy, the far rooms are starved because the air cannot get there, or the installer turns the fan down to make it quiet and the system never delivers its rated capacity. The equipment is fine; the ductwork is strangling it.

Why insulation matters more for cooling

Gas heating ducts were often installed with insulation that was adequate for winter heating. In summer, a Ranges roof space can sit above 50 degrees. Cool air travelling through thinly insulated ducts arrives warmer than it left, and condensation can form on the duct surface. Reverse cycle ductwork needs a higher grade of insulation than many older gas heating systems have.

Why the outlets and layout often do not suit

Outlet size and position were chosen for a gas heater’s air volume. With reverse cycle, undersized outlets become noisy and draughty, and outlets placed for heating alone can be in the wrong spots for cooling. Return air, which many gas heating systems undersize, becomes critical, because the reverse cycle system needs to draw back everything it delivers. Layouts with long, branching runs that a gas heater tolerated can starve the ends of the system.

What “new ductwork” actually means

New insulated duct runs sized for the system’s airflow, new outlets sized and positioned for cooling as well as heating, a return air that can actually feed the unit, and the whole system supported, sealed and commissioned. It is a larger job than a swap, and it is the reason a properly designed conversion performs on the hottest and coldest days rather than only in mild weather.

How to read a quote

1. Does it say the ductwork is new, and what size and insulation?
2. Where does the return air go, and how big is it?
3. Are the outlets sized and placed for cooling, or are they the heating outlets that were already there?
4. Is the reasoning in the quote, or is it a guess?

The Braemar Dominator is a different question

The Braemar Dominator is a specific product designed as a direct replacement for a gas ducted heater that may, where the existing system is suitable, reuse existing components. That is the manufacturer’s pathway for that product and it has its own suitability conditions. It is not a rule that applies to conventional ducted reverse cycle conversions, and it should not be used to justify reusing ductwork with equipment it was never designed around.

Our approach

Design the installation properly rather than compromise a new system by connecting it to infrastructure built for a different appliance. New ducted reverse cycle means new correctly insulated ductwork, every time. It is part of the MRGA Standard, and it is explained on the ducted reverse cycle and VEU gas to electric pages.