Winter is coming and the glorified tents that we call houses in Australia are getting cold, which means people with ducted gas heating are basically setting fire to money to keep warm. But ducted gas systems don’t only suck up your cash: they suck at heating efficiently too. Here’s how to replace them.
You Might Convert Ducted Systems
The ducts are already there, so can’t you just change what is pumping heat into them? Unfortunately, it isn’t so simple. Properly designed heating uses ducts sized for the job, but in the past the focus was usually on minimising noise.
- Evaporative cooling calls for changing the air in your house every two minutes1 because the temperature delivered is at best 10°C lower, so the ducts are larger.
- A ducted reverse-cycle air conditioner (RCAC) can deliver twice as much temperature change, so at 20°C, ducts can be roughly 30% smaller in diameter to deliver half the air volume and the same comfort.
- Burning gas delivers air at 30°C above ambient, so even less hot air needs even smaller ducts. They tend to be cheap and poorly insulated because the gas industry doesn’t care if they’re wasteful. They’ve got heat to burn, as well as your money.
Converting ducted gas systems usually means full replacement, but if the original installer used ducts that are too large, because that’s what they had in the truck, you might be in luck.
The other option is a “Dominator” from Australian air conditioning manufacturer Seely, which is designed to convert existing gas ducts to RCAC.
You’re Better To Abandon Ducts
However, my brutally honest advice is simple: don’t do ducts at all. They’re leaky, dirty, failure-prone, horribly inefficient and as an electrician, I hate climbing over and damaging them in the roof space.
Many people don’t believe that ducted air conditioning is just an extension of Australia’s badly built housing, but the fundamental problems are many:
- Ducted systems are working in a cold tin shed. The ducts and air handling machinery are operating outside the insulated envelope of your house.2
- Ducts are huge. They have to be to shift large volumes of air. No matter how well you insulate them, a massive surface area will always leak more energy into the roof or floor space.
- Ducts leak. Whether it’s from clips holding the registers in the ceiling, taped duct joins, or the air handler; ducted systems leak air you’ve paid to heat.
- Ducts make your house leaky. Punching holes in the ceiling not only makes holes in the insulation, it creates air leaks where your warm air bleeds out, or cold drafts can bring dust in.
- Ducts are unreliable. Sometimes joints fail because plumbers, electricians, cable TV fitters or similar rodents damage them. As for zone motors – when these electric powered valves fail, you can no longer control where the air goes.
- Ducts are dirty. Not only are they difficult to clean, in the event of a fire, ducts will carry smoke throughout the house.
Insulation Above All Else
When the owner of the roof in the photo below got an insurance payout to replace his roof, we removed the old solar and I browbeat him into paying the roofers to install insulated foil-faced insulated sarking. It was now or never.
The upshot was maintaining comfortable indoor warmth with the thermostat set at 3 degrees lower temperature than before.
No longer were they paying to heat the space above the ceiling. Without heat leaking from ducts, this roof didn’t boil off the condensation; and trying to climb it to reinstate the solar nearly killed me.
The dry circle around this electrical conduit shows warm air leaking through the roof blanket. It’s a vivid demonstration of how well insulation under the iron works.
Australians Build Crap Houses
We’ve written about it before. But in a properly designed Passivehaus, which will cope with alpine Germany or cold Canberra, they do use ducted systems to introduce fresh air and recover heat in the process. They key difference is the whole system is inside the house.
This Heat Recovery Ventilation unit runs 24/7, introducing filtered fresh air from outside and exhausting stale air from the kitchen/laundry/bathroom. HRV recovers 95% of the heat, as all the ducts are inside the house.
Passivhaus means properly designed, expertly built and tested. Certification requires sealing a huge fan in the doorway, then subjecting the house to a very stiff breeze, enough to move a water level column by 5mm.
This means that when the building is pressurised or depressurised to 50 Pascals, the total air leakage must not exceed 60% of the building volume, known as Air Change per Hour. To reach that level, you even need to stop the air leaking through the slots in your power points.
Whereas an Australian house, theoretically built to 6 or 7 star standards, will leak 1500% to 2000% under the same test.
Sadly, the National Construction Code doesn’t mandate testing or specific airtightness targets, but it does some handwaving about “efficient thermal performance”.
So while the Germans test for 60% leakage, 0.6ACH@50Pascals, Australian houses with good weather sealing and quality construction might get below 1000% or 10ACH. However, real world research on Melbourne homes assumed to meet 6-star NatHERS showed an average airtightness of 1900% or 19ACH.
And that’s if you can even get a blower door test to work. It’s not uncommon for the building to simply fail testing because it leaks like a sieve.
Suck all the air out of the house and measure how much flows in through all the cracks. image credit mincin-insulation.com
Superior Systems Are Split
Instead of wafting energy around outside the house with a fan, it’s vastly more efficient to pump it as a liquid, through far smaller pipes.
With a heat pump, your plumbing is much better insulated. It pipes heat through the building and only converts it to warm air once it’s inside.
Bulkhead systems can be built into walls & behind cupboards.
And when you have multiple split system air conditioners, you can choose which zones to heat, and no single point of failure will leave you without heating.
They don’t have to be ugly either: built into a bulkhead they’re not obtrusive.
Whereas I’ve seen a living room peppered with twenty-five holes in the ceiling for gas heating, evaporative air conditioning and downlights. What a mess.
Bulkhead air con outlets are no different to ducted units.
Why Do Leaks Matter?
Firstly, no matter what system, leaks mean you’re just pissing money into the wind.
Secondly, insulation leaks and cold drafts generally don’t affect comfort levels. You turn the gas burner up and just use sheer combustion power to overcome the losses. It’s as quick and dirty as it is expensive.
Whereas a heat pump RCAC is inherently more efficient, so with fewer kilowatts of outright power, you have to be more careful not to waste the heat you’ve harvested and pumped inside.
And as an added bonus, if you can stop the drafts, control the atmosphere, regulate humidity & CO² then you’ll also exclude allergens and pollutants. Dust, pollen and bushfire smoke no longer trigger asthma or allergies when your house is well sealed.
Floor consoles are popular in Europe and New Zealand. Filters are easy to clean and they heat from the bottom up.
Plan For Success
Sadly it’s not uncommon to hear about RCAC systems that don’t perform as well as the gas-fired money incinerator they replaced. The complaints are commonly “sure it’s cheaper; but it’s not as fast and I’m not as warm”.
Instead of just bolting on an air con, often what’s needed is a whole-of-home energy audit and a rethink of how you heat.
Automated window dressings or cellular blinds, bulk insulation, draft proofing, window and door sealing, skylight replacement, even double glazing can all be great improvements to make your house more comfortable and cheaper to run.
These improvements can make the difference between a system that struggles and one that’s toasty.
And if you have your own solar power to use, program the RCAC to warm the house up when the sun is shining, so temperature will be under control when you get home. Otherwise, hitting the switch at 6pm when the house is frigid means the system will struggle and your retailer will violate you for peak energy rates.
Comfort Comes In Many Forms
While retrofitting a poorly built or badly designed house can be difficult, what I can’t stress enough is the value of comfort.
It can be really rewarding to make material improvements to where you live, but what’s even better is having a home that’s comfortable to live in without having to sit on top of the heater and fret about what it’s costing. Start with efficiency; then get off gas.
For more, read about how much getting off gas can save you, how to disconnect without getting hit by huge fees, how much more efficient electric appliances are compared to gas, and the latest findings on cancer risk to children from gas stoves.
About Anthony Bennett
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