Knowledge/Systems/How an indirect gas ducted heater works
How an indirect gas ducted heater works
Indirect means the burner heats a heat exchanger and the house air passes over the outside of it, so combustion products never mix with the air in the house. A sensor in the duct (the duct thermistor) sets the fan speed to hold the outlet temperature. If the airflow drops, the over temperature switches shut it down.
- Natural gas inlet
- 1.13 kPaminimum, at the heater
- Propane inlet
- 2.75 kPaminimum, at the heater
- Gas connection
- 3/4 inch BSPBritish Standard Pipe, female flare nut, all models
- Flame signal
- Over 2 mVDCwhere applicable
What indirect actually means
Two air streams that never touch. Gas burns inside a sealed heat exchanger and the products of combustion go up the flue. The house air is blown over the outside of that exchanger, picks up the heat, and goes down the ducts.
That's the whole safety case for the appliance, and it's why a cracked heat exchanger is the failure that matters. A crack is the one thing that lets those two streams meet.
It's also why the carbon monoxide (CO) spillage test exists. You're checking that the flue is doing its job and the combustion products are going where they're meant to.
What controls the outlet temperature
Not a thermostat on the burner. The duct thermistor speeds the fan up or slows it down to hold the outlet temperature, and the over temperature switches are the backstop. If the air gets too hot, the fan speeds up. If it can't keep up, a switch opens and the heater stops.
Which is why there's no published temperature rise range for these appliances, and why a heater that keeps cutting out on overheat is nearly always an airflow problem rather than a gas one.
| Switch | What it does | Typical set point |
|---|---|---|
| P-55 | Pressure switch, normally open | Closes at 55 Pa |
| P-COND | Condensate pressure switch, normally open | Closes at 18 Pa |
| HX OT1 | First heat exchanger over temperature | 110°C on most models |
| HX OT2 | Second heat exchanger over temperature | 130°C across the range |
| OT3 | Flame rollout, manual reset | 130°C across the range |
The gas side figures
Size the gas pipe so the heater inlet gets at least 1.13 kPa on natural gas or 2.75 kPa on propane. That has to hold with the heater and every other gas appliance in the house running flat out. Size the pipe to AS/NZS 5601.
Burner and manifold test point pressure is model specific, and there's no universal figure. The nominal test point pressure is on the data plate under the appliance lid. When you finish servicing, make sure the pressure test points are closed and not leaking.
A diagnostic worth having in your head
If 24 VAC is present and there's no test point pressure, replace the valve. If 24 VAC isn't present, check OT3, the manual rollout switch, is reset and there's continuity across the terminals. And on modulating valves, check the gas rate setting on the controller before you condemn anything. Below 100 per cent, the nominal test point pressure won't be reached.
What good looks like at handover
- Stable ignition
- Flame carries over reliably across all burners
- Flame signal over 2 mVDC where applicable
- No flame rollout, lifting, yellow tipping or instability
- Around 230 VAC supply and around 24 VAC transformer output, stable with other site loads
- Correct polarity and earth continuity
That list is out of the service guide and it's a decent commissioning sheet on its own. Yellow tipping in particular is the one people look past, and it's telling you something about the combustion.
Weighing up what to put in front of a customer?
Most of the hard calls aren't technical. They're about what you can stand behind when the customer rings you in February.
Kevin's been having that conversation for thirty years. Ask him.
Who this is for: small and medium HVAC contractors in Victoria. If you're a homeowner after a rebate or a repair, you need a licensed contractor, not us, and yours can point you the right way.
Related
- Can you reuse the ducts on a gas to reverse cycle swap?The honest version. No engineering document covers reusing the ducts, so here's what's checked, what's our best judgement, and the seven things that go wrong.
- Hydronic heating, and what the rental rule does to itSteady radiant warmth, slow to respond. And under the 2027 rental rule, a failed gas boiler points to reverse cycle rather than an air to water heat pump.
- Does a Victorian gas heater have to be replaced with electric?If the owner lives there, a dead gas ducted heater doesn't have to go electric. Space heating was dropped from the rules. Rentals are different from 2027.
- How evaporative cooling actually worksThe gap between dry bulb and wet bulb sets how cold it can get. Efficiency decides how much of that you get. Humidity limits every brand the same way.
Review date: 1 April 2027. Figures in this area change without much notice and this page carries some that will go stale. Every number above is dated and sourced so you can check it yourself. If you find something out of date here, tell us and we will fix it.