DIY heating of a granny flat with wood heater in the shed

Have a large shed with a granny flat down one end.
Daughter has moved in to save some money for a house and the winter electricity bills are extremely large at $700 a month which is effectively the equiv of running 2 winter households.

Will downgrade the electric water heater element from 3.6Kw or 4.2Kw (cant remember which one it was) to 3Kw or 2.5K with longer heating times so as not to exceed the large reduced winter 12Kw solar system output to minimize cost of HW heating.

If she is still here next winter, need to look at more efficient heating options particularly at night as there is no solar power contributing when the sun goes down. Not concerned with any issues such as councils or insurance etc and would like some ideas to put in a secondhand wood heater of some sort in the shed and duct the fan forced heat from the wood heater into the unit...plenty of wood. I may then also plumb water heating coils on the outside...but primarily looking at space heating

$700 a month. What is on? Multiple bar heaters 24/7?

What is the main heating?

Downgrading a water heating element unlikely to help much, will just take longer to heat.
Heat pump water heater much more efficient, though up front $$

You have 12kW of solar panels and still spend $700 a month on electricity? Holy sh**!

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Originally Posted by jack620

You have 12kW of solar panels and still spend $700 a month on electricity? Holy sh**!

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...next to nothing in the spring, summer and early autumn. Summer its lucky to be $30 a month. Winter peak (later june/july) its $300 due to the lack of sunshine, shorter days.

So with the daughter moved into the granny flat its essentially a second residence...so ad the additional $300/400 for the winter peak
- there is no heating, so she has an electric heater which cost a fortune to run ....this is the main costs. See pic below
- and is running the Hot Water service which is normally turned off in the winter due to lower solar output and therefore no free hot water on poor solar output days
....hence the doubling of the winter electricity bill. warmer weather/longer sunny days/Summer is forecast to be back to $30 a month with heaps of surplus being generated.

Pippen88:
The lower KW water heater does make a difference as its designed to use less than what is being generated by the solar panels...and therefore free to heat the hot water. Done the same thing on the house and works very well in keeping overall staggered peak loads down to a minimum.

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I take it that you don't want to put the wood heater in the granny flat but can put it in the adjacent part of the shed.

If you fully enclose the heater flue from the top of the heater ('cos you still need to open the door to put wood in it) to the shed roof with insulated fire rated material - fibro is usually okay - at the necessary distance from the flue and heater, and insulate under the shed roof to increase efficiency, you could get a reasonable unpowered convection system with an intake vent for cold air from the granny flat just above the top of the heater and an outlet vent close to the ceiling of the granny flat. There's a lot of heat coming off the flue and, depending upon the type of wood heater you're using, the top of an uninsulated heater top, like the ones designed to allow cooking on top. Efficiency could be increased with a low wattage fan on the top outlet vent. These can also be used to spread heat from room to room.

Flue attachments like this are supposed to increase heat output significantly. https://www.timelessclassicwoodstove...ng-heat-output

You could also heat water with either or both a pipe system like a big S on the back of the firebox and a pipe system coiled around the full length of the flue. You could put water heating pipes in the firebox, but the heat there is much more aggressive on the pipes and requires better engineering that the external system, as shown by how quickly fire grates degrade. Or you might pick up a heater with an already integrated water heating system.

It'd still work way better and be much simpler with the wood heater in the granny flat, and probably retain the heat much better as your daughter wouldn't be opening a door to go into the shed to feed the heater.

Don't underrate the power of convection. Many moons ago I installed in my house a double skin open fire firebox with an intake at the bottom and an outlet at the top. It wasn't as efficient as a well designed wood heater, but it was outstanding compared with any type of open fireplace.

Depending on the size and layout of the granny flat, insulation is the best bang for buck you can do. Putting it in the walls may be problematic, but if you can it would be well worth it.

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Originally Posted by 419

I take it that you don't want to put the wood heater in the granny flat but can put it in the adjacent part of the shed.

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Correct

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Originally Posted by 419

You could also heat water with either or both a pipe system like a big S on the back of the firebox and a pipe system coiled around the full length of the flue.

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Yes, this is what I was thinking. Looking at some Utube projects that have successfully done the same thing and much lower risk that having a pipe burst within the firebox.

Tkx for the other ideas on convection heating

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Originally Posted by cava

Depending on the size and layout of the granny flat, insulation is the best bang for buck you can do. Putting it in the walls may be problematic, but if you can it would be well worth it.

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Its very well insulated with the exception of the 2 single glazed windows and sliding glass door but they are under a wide verandah.
Its only since its been lived in full time, the 3 months of winter with the lack of solar generation and coldness becomes an issue resulting in a significant increase in winter electricity usage costs.

Due to it being well insulated, in the summer on a run of 36 degree days, its still 20 to 23 degrees in the granny flat that has no cooling whilst its a high 20's/low 30's in the 4.5m high shed at ground level (hotter on the open fronted mezzanine). The walls dont hold the summer heat as it's an insulated stud wall (Sarking and batts) inside the shed wall (which has left over roof blanket lining the colorbond) with any heat generated in the cavity simply rises to the top and out above the insulated granny flat/mezzanine floor into the 4.5m high shed roof.

....which is why I'm only looking for a winter heating solution (space heating and possibly hot water) using a wood fire as I've plenty of wood. The hot water service is inside the shed so I could locate the wood heater next to the HWS which is also next the back wall of the granny flat

Reverse cycle in the granny flat makes most sense

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Originally Posted by Bart1080

- there is no heating, so she has an electric heater which cost a fortune to run ....this is the main costs. See pic below
- and is running the Hot Water service which is normally turned off in the winter due to lower solar output and therefore no free hot water on poor solar output days
....hence the doubling of the winter electricity bill. warmer weather/longer sunny days/Summer is forecast to be back to $30 a month with heaps of surplus being generated.

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hi Bart,
I understand that you wish to help your daughter save money for a house deposit, but in terms of the additional electricity bill have you

considered sharing the increase in electricity costs with your daughter?
which might encourage her to put an extra donna on the bed and wear a puffy coat indoors.

Just the suggestion to take shorter showers and wear additional clothes indoors might be enough to modify your daughter's living by herself habits without needing to share the increased electricity bill with her.


would be even cheaper than a reverse cycle air conditioning

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Originally Posted by ian

hi Bart, but in terms of the additional electricity bill have you considered sharing the increase in electricity costs with your daughter?

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Yep I hear ya. I've educated her on the showers, washing etc with her partner a lot more frugal...and doesnt feel the cold.
They live here rent free to build up their savings for a house with electricity the only thing they pay for. I was expecting this when the mother inlaw stayed for a week last winter and could see in the app the massive increase in electricity with her heater usage. Maybe a split system would be a lot cheaper to run?? I may be able to get a perfectly fine system off a friend down the road for free who pulled one out from a recent reno.

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Originally Posted by 419

ou could get a reasonable unpowered convection system with an intake vent for cold air from the granny flat just above the top of the heater and an outlet vent close to the ceiling of the granny flat.

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This.

Another option is to build a brick alcove in the dividing wall for the heater to sit in, recessed into the granny flat. This would act as a thermal mass which could continue to radiate heat into the granny flat for quite a few hours after the fire has extinguished. Perfect for the wee hours of the morning.

Rural Canadians are masters of this sort of thing (unsurprisingly. :roll: ) and I've seen quite a few nifty ideas that I'd like to implement should I ever be in a situation where I have carte blanche on a wood heater installation.