Replacing Gas Hot Water and Heating

Having high ceilings always makes heating/cooling more difficult as you have to deal with a greater volume of air for the same floor space. I don’t see what that has to do with putting the ducts in the ceiling.

Unless you stick to a radiant heater like a bar or oil heater (which are way less energy efficient than aircon) all your heating is going to involve moving warm air around in some way. I think you are likely to find your answer in some kind of aircon or heat pump (the principle is the same the name different). If this is beyond the ken of one installer I would be asking for another opinion; for an explanation of how the high ceilings will affect the outcome and how much.

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We use our ceiling fans plus RC split aircons for heating in winter. It does improve how the warm air is circulated. Typically we leave them blowing down as in summer on the lowest setting. It appears to warm the room faster than switching them to blow towards the ceiling. Our ceilings are insulated - wool batts and the ceilings are 3.2+m. We purchased extension rods for the ceiling fans which hang approx 1m off the ceiling. It works well for us.

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It does sound possible, although somewhat painful since I already have ducts in the floor throughout the house. Would a main split system sit outside where the current gas heater is and pipe up the wall into the roof? Then I would need to create ceiling vents into every room (not an open plan house). Would probably need quite a big system to generate the same amount of heat (I think this is what he was saying it would take a long time to heat to the same temperature compared to the gas heater).

There is a degree of flexibility where the outside equipment can go, it should not be too far from the internal heat exchanger and fan, that goes in the ceiling, which is generally near the centre of the house.

It is standard for ducted aircon to have an outlet in all major rooms, this is part of the price. You can turn off some rooms if not in use or control the temperature of rooms separately in some installations. Also you can alter the direction of the outflow from louvres in the internal vents to some extent which may assist with getting hot air where you want it.

Possibly but the fact is aircon (unlike gas and resistive electric heaters) produces much more heating than the amount of energy it consumes by a factor of 2 1/2 to 3. Aside from the relative price of gas and electricity per megajoule this is the major difference in running cost.

In my mind there is no doubt that for running costs and flexibility aircon beats gas hands down. Whether the upfront cost of the change is worth it and when you will get your money back is something you need to consider.

Should you make the change and discover that the hot air sits too high (which I think unlikely unless you live in a cathedral) you can always put in fans in the bigger rooms as Mark suggests. How high are your ceilings?

I am not an aircon expert, I think you need to talk to some more people who are and who can explain to you how their system would work in your particular setting. Ask pointed questions!


The option of an electric heat pump replacement for the gas air heater of a ducted gas heating system does not come up in any of the searches I’ve made. It seems illogical not being able to reuse what one has if all you require is to heat in winter. It appears at least in Australia suppliers are focused on installing new ducted or multiple split system air conditioning (electric heat pumps) to provide heating in winter and cooling in summer.

Possibly it’s that there is much greater value to a supplier/installer to offer a total replacement. It may also be what the majority of their customers are asking for, looking to the following example of relative running costs.

Worth a read if you are looking at the options,

Note the gas heater for your ducted system will have a nameplate or manual and a specification of the maximum rated heat output. (MJoules if newer, or likely BTU if quite old). Homes with large ducted gas heating systems may need an upgraded household electrical supply. Worth ensuring you know the answer to when looking at the options. An 8kW heat output split system delivers approx 29MJ of heat energy while consuming approx 2kW of electricity every hour. For a ducted gas heater the same output requires approx 40MJ (70% nominal efficiency) or greater consumption of gas.

Note: Common models of flued gas heaters suitable for ducted systems include models with heat outputs from 20-30kW (72MJ -108MJ equivalent).


Thank you both, I can ask the right questions when I get quotes.

Yes @syncretic - I agree such a system in the roof should work fine in terms of heating efficiency. I already have fans installed. Not sure where the outside system would go though to reach the roof but that shouldn’t be a problem.

I agree @mark_m - I still don’t really understand why I can’t just switch a heat pump for the gas heater that sits outside and ducts under my floor. Thats a question I will ask.


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We recently (12 months ago) replaced gas ducted heating with reverse cycle ducted heating and cooling in Canberra. The ducting for the gas heating was under-floor, with one or more vents in the floor of each room.

We could have retained under-floor ducting and positioned the aircon under the floor, roughly in the centre of the house, but decided we preferred in-ceiling.

But we had enough space in both places to have a real choice.

If we had kept it under-floor, it would’ve been new ducting replacing all of the old. The aircon cannot be positioned outdoors where the gas furnace had been, so the ducting would have had to be re-routed to the aircon’s central location under the floor. That probably would’ve involved replacing a good deal of the ducting, and because of the age of the existing ducting, it made sense to get all-new (with better insulation) anyway.

To answer your question about the location of the aircon: going by our experience, it’s very unlikely that the main split system could be installed outdoors. Maybe commercial ones can, but not domestic units. It would either go under the floor, with underfloor ducting, or in the ceiling, with in-ceiling ducting.

If you have enough space under the floor for the aircon unit, stay with under-floor ducting, but be prepared to replace all the actual duct to get the best results.

If there isn’t a lot of space in your ceiling, you don’t really have the ceiling option anyway. Those well-insulated ducts take up a lot of room.

With high ceilings, I think you’d find ceiling fans a worthwhile investment. They’ll distribute the warm (or cool) air more efficiently than the vents can, regardless of whether those are in the floor or the ceiling, and fans cost very little to run.

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Thanks @isopeda . Interesting to hear your experience. Also interesting to hear that your external unit needed to be under the floor (which is a problem for me as there is not enough space). The latest guy I spoke to thought I could potentially use the same spot/pad outside that the gas furnace is on. The previous company said no. Perhaps it depends on the type of heat pump. I got a quote for an entire system plus installation that was around $10k. He said maybe wait for the new round of Victorian incentives.

The latest guy explained that the gas ducts are too big to be reused for electric. It would be good to be honest to get the ducts out from under floor as they seriously restrict air circulation and I have damp problems. Switching to ceiling ducts would have the bonus of improving ventilation below.

Typically new/modern homes with pre-installed ducted reverse cycle air conditioning utilise a low profile module for the indoor unit. They mount inline with the ducting, require water drains and proximity to the outdoor unit. Larger homes may require more than one module. Homes with ducting from an older gas heated system will find the ducts are of a different profile. Older ducting may not be insulated or suited to also handle cooling with a RC heat pump upgrade. Duct insulation and condensation is for the installer to assess. Each will make their recommendations based on delivering the best system. The location of the cassette/indoor unit will vary to suit the design/features of a property.

There are also home owners with older flued outdoor gas heating with ducted systems who live further south. It may be uncommon for those wanting to upgrade to a heat pump to not want or need cooling in summer as an added bonus. It may be that one needs to look beyond the everyday installer who only knows one way? There are options for packaged outdoor units. They are not necessarily promoted whether it’s the appearance, relative size, cost, or ……?

One product - premium Daikin brand and an Aussie installer with indicative pricing. The heating capacity of the units available cover the range typical of external gas heaters. From less than 20kW to greater than 40kW. The footprint is similar in outline to some modern flued gas heaters. The packages are fully weather proof with adjacent connection flanges for outlet and return air. Important questions might include the noise level of the unit relative to local requirements and frost protection suitability for areas with below freezing outdoor temperatures. Most well known manufacturers offer models suited to more temperate climates as well as versions subject to snow and frost. Electric resistive boosting is the least efficient. The larger diameter air circulation fans in these outdoor units will be similar to those provided with older gas heaters.

Ignore the reference to rooftop. They can be mounted at ground level. Rooftop is saying they are weatherproof for the typical Aussie outdoors. Common for small businesses where outdoor space may be at a premium. The smaller 16/18kW nominal cooling/heating capacity external packaged unit is equivalent to a mid range ducted RC system suitable for a smaller home.

Hopefully sufficient background to compare further the options. Noted @acreed2002 comments on underfloor air circulation and dampness. The installer who commented on the existing ducts being too large does not make sense without further context. Possibly in comparison to how a modern ducted system is sized.

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Have you considered hydronic heating with a heat pump as the ‘boiler’? Because it’s pumping water, not air, the ducts should be much thinner, so should fit under your floor. See, eg, How Does Hydronic Heating Work? - Hunt Heat](How Does Hydronic Heating Work? - Hunt Heat)

Combine that with ceiling fans, and it’d probably be a very comfortable solution. And I think the radiators do or could have built-in fans to help circulate the air.

Note that with this type of system it’s best to keep it running at a low level all the time rather than turning it off completely overnight, because it will take quite a while to heat the house from scratch in the morning.

That is not typical, the external unit for aircon is generally weatherproof, designed to go out in the weather, unlike the internal unit.

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No ducting, it’s all done with purposeful insulated pipe work.
Hunt from prior experience has a good team, although not sure if they service the ACT?

I was using ‘duct’ generically - “a tube or passageway in a building or machine for air, liquid, cables, etc.” :wink:

They do service the ACT, but I have no experience of them. Just picked a site with a description of hydronic heating.

Understood. We’ve a small system with radiators piped to a wetback with a Grundfos hot water circulation pump. Hydronics when used for heating can make use of a variety of energy sources. Using more than one option or converting is relatively straight forward in that the in home elements stay the same.

I love the gentle warmth. They are premium systems even when installed with a new build. To install in an older home, it might be a high cost solution relative to other options. Especially if one is looking to underfloor heating. For cooling I’ve no relatable experience, although some offer hydronics with a cooling option.

Being able to afford the upfront cost of conversion to all electric is one of the reasons change is being held back. It’s also a factor for rental properties, especially those in appartments or on multi unit sites. As an expense for the landlord and often limited by the design and construction decisions made for each project. The article suggests a range of savings, based on where one lives.

Link to the full report for those with an interest in greater detail.

The reality is that many homes are not wired to be all electric and the cost of upgrading them can easily be in the $1,000s just to update the breaker box, plus wiring. Many houses around here still have old wire fuses for perspective. The costs of the appliances can pale to the immediate costs of being able to run them.

15 years ago a commercial upgrade in the CBD required a local grid upgrade, quoted at over $500,000 ‘customer contribution’ for the single feed. There are discussions about the grid being able to absorb solar inputs as well as deliver outputs, but can the ‘local grid’ cope?

The investment to go all electric seems focused rather than all-encompassing. Secondarily many of us do not have 99.99% reliable power. Not all gas appliances will run without electricity for various reasons but many will. Those having ‘friendly’ gas appliances might be able to at least cook a meal or have a cuppa and a few might even have gas fire places for heat. A small consolation when sitting in the dark.

The situation resembles an unrelated topic about the importance of being connected and how it goes when the connection isn’t there.

It is not so simple as just replacing appliances. Many of us realise that yet the sell goes on expecting the grid to magically evolve as well as the economics of using it and for solar owners, feeding into it.

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The ABC article relies on the source Grattan Institute report. There is an appended link to it in my prior post. It’s polite to suggest the ABC has chosen simplicity over detail if comparing the two.

A sample from the report, hopefully encouragement for all to take a more critical look.

A well placed mark, especially for those supply regions where high reliance on gas has a positive benefit in reducing peak demand on the electricity network.

The Grattan report also recognises there are different priorities and outcomes depending on which state one is in. It responds accordingly. The one detail I could not find a quick answer to is whether the analysis accounted for LPG use and numbers of homes using it. More than 2 million potentially not fully considered?


The Victorian Govt (Sustainability Vic) also publishes this guide specific to heating homes. Note it has been updated to 2023.

As well as the above graphic it includes tables with comprehensive comparisons of different heating options. For housing of different sizes as well as rooms of varying sizes. There’s guidance on the impact of location and home energy star rating. The information provided estimates running costs and greenhouse emissions over 10 years.

For those wondering it would be correct to question how the only wood fuel device, a slow combustion stove has been assessed. Wood burning produces slightly less greenhouse gas per kg of fuel compared to natural gas. Wood also produces less than one half the heat energy per kg in a comparison. Hence at the 60% heating efficiency indicated wood will produce approx 2-3 times the emissions of heating based on natural gas. This also excludes any additional particulate or other combustion products. The tables in the guide provide a very low relative emissions result. There’s a foot note suggesting the wood has been sustainably sourced.


Replacing gas heating with reverse-cycle aircon leaves some people feeling cold. Why? And what’s the solution?

I had trouble finding a home for this as it is relevant to replacing gas but also to the need for insulation and the effects of aircon with wall outlets when in heating mode.

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There’s an interesting shorter topic that the Conversation article also responds to.

The OP suggested Choice might consider assessing RC Air Conditioners on how well they warmed one’s feet in heating mode.

The Conversation article offers a scientific view as to ‘why it is so’ or ‘not so’ and how to improve the outcome. It’s due to the home/room design and insulation/heat losses from the room. Useful advice for anyone choosing to convert from gas and considering a high wall mounted split system RC air conditioner.

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