We knew that power in winter was key to sizing our solar system, so heating was a key focus. We wanted to be able to heat the whole house, so wanted something beyond the  wood heater planned for the living room.

One of the reasons that we chose our builder was because they had recently installed a ground source heat pump, and watching too much Grand Design, we thought this might be the solution for low power usage central heating.

Ground source heat pumps act essentially like super efficient reverse cycle air conditioners, reducing the power requirement by 50% or more. They’re expensive, but still use a large amount of power – too much for base load winter heat for an off-grid system, so we dumped this option quickly. The efficiency boost to our air conditioning just wasn’t worth it.

I then looked at hydronic heating, with a woodfired boiler, particularly with in-slab coils (don’t particularly like wall panels). I did say I’d watched too much Grand Design? There are lots of great woodfired boilers, many of which only need to be stoked once a day. (These systems can incorporate supplying hot water, although this adds complexity. And if this is your hot water supply, then you have to keep it running year round, so we would have done it without the water.) A very rough pricing of an installed system showed this might add $40 – $50k to the price of the house, although some people have suggested that this might be an overestimate. I was also worried about the impact on our solid timber floating floor. So given the cost, I kept looking. We didn’t need central heating that badly.

Air transfer kits, where hot air is taken from the ceiling of a room with a wood heater and is blown into other rooms, is a cheap effective option, but only works for small houses,  (6m duct limit?) and didn’t suit our roof line, so not a solution for us.

We thought we’d found the perfect solution when we came across Turboheat – a Victorian  based company. Ducted  central heating, with heat recovery from the flue of a wood heater, distributed around the house. Low power usage, since it only uses fans.

We did have a problem with the roof line – with a raked ceiling in the living room, we would need to install the fans and heat exchanger into a bulkhead in the study and pipe it into the part of the roof that supported the ducted distribution. The Turboheat guys were incredibly helpful, with the system priced at less than $15k with 9 outlets. They suggested that we just use the air conditioning ducting system for distribution – their system could just plug straight in. Perfect, or so we thought.

The builders, and their heating, ventilation and air conditioning contractors had other ideas. Although they would VERY reluctantly install it, it would void their warranty and they certainly wouldn’t guarantee that the system would work.  Separate ducting likely wouldn’t fit in the roof line.

Reluctantly, we dropped Turboheat, but its certainly worth consideration for others.

So what do we actually want/need?

At this point, we took stock of where we were, and took a closer, more nuanced look at what we were actually trying to achieve, rather than just saying we wanted “central heating”.

  1. Take the early morning chill off, particularly in the bedrooms and bathrooms – well the wood fired solutions wouldn’t do this anyway. We’d have to get up to relight the fire. Solution: see below
  2. Take the evening chill off in the bedrooms when we go to bed. Nice to have, but not really necessary. Solution: see below
  3. Keep the main open plan living room warm. No issue  –  the wood heater already does this.
  4. Keep my study/painting room warm during the day. Solution: Easy, add wood heater.  I’ve always wanted a room where I could curl up in front of a fire and read a good book. Win.
  5. Keep Mark’s study warm during the day. Solution: Air transfer duct – it backs onto the living room. Cheap, simple.
  6. Keep the guest living room warm during the day. North facing. Open to main living, so should get some warmth. Infrequent use.  Solution: see how we go. Can always retrofit wood heater, but don’t think we’ll need to.

In reality, our house shouldn’t get so cold that the chill in the early morning is a major problem.  But still, we don’t want to compromise on our creature comforts.   (post decision note: we’ve moved into a 6 star rental, and we hardly ever turn the heater on in the morning – the house doesn’t get cold enough. And our house should be a lot better than this one. So this likely wasn’t a real concern at all. But anyway….)

What if we took the money we were using for the central heating (less one additional heater) and put it into the solar system? Enough to run the reverse cycle for an hour in the morning?

Our 16.5 kW Daiken reverse cycle air conditioner uses 4.85kW power for heating, but because it’s an inverter it won’t run at full power once temperatures have been reached.

Copied from Panasonic website – power usage from an inverter air conditioner.

So the literally many thousand dollar question – how much power would the air conditioner use if we ran it for an hour in the morning? Depends on the starting temperature in the room, our desired temperature and the heat loss from the house – this will dictate how long it needs to run at full power to warm the house.

So a really quick, back on the envelop calculation – to heat all of the air in the house from 10 degrees Celsius to 20 degrees Celsius would take 10 minutes. Of course, fittings, fixtures, walls, floors, ceilings etc would also be taking up heat, and we’d also be losing some externally. But I don’t think it will be running flat out for the hour. For a working assumption, I’m assuming power consumption for the hour is 50% of full load.

Just for the morning, this would add say 2.5 kWhr to our load table. A 20% increase of our current power usage.

1kW solar panels in June in Gippsland puts out about 1.4 kWhr per day, so we’d need 1.5 kW panels to support this load. So that’s less than a couple of thousand dollars for the panels.

But the overall cost is difficult to determine – its a question of whether it tips you into the next size of equipment, or an additional battery.

Worst case, assuming we need an additional battery and inverter, we’re looking at around $15k more (based on BYD Box-B Pro 13.8kWhr battery). So we could be up for something between zero and an extra $15k here, but we’d have SO much more battery storage for bad weather. It’d be a real insurance policy. And if we’re running low on power, we just don’t turn it on – its a nice to have.

Actually, this is a lot better way to spend the money, than on wood fired central heating. Gives us much more flexibility and back up, and the heat available at the click of a button.

And then, once we got used to the idea that solar was cheap these days….

We now have electric blankets, heated towel rails, and an electric heated floor in the ensuite…..

Warning though – some of the offgrid suppliers who quoted put the power load at the rated 4.8 kW per hour. Only one put it at 40%. Makes a huge difference to the cost outcome and system sizing because I included 2 hours use on the load list….

Obviously I agree with the 40% estimate. This was from Rory at Off Grid Energy. He really knows his stuff and offers great advice. If it’s wrong, oh well, it was just a nice to have anyway.

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