Green Oversite



How I Use My Air Conditioning System over Winter

By Bruce Barbour - September 2020

Firstly usually I do not heat the bedrooms.

For the living rooms, the air conditioner is off overnight. If necessary I will turn on the air conditioner when I get up. If the room temperature is 17 or above and it is sunny outside I may not turn it on at all as I know the room will naturally warm as the sun comes in through the northern windows.

The air conditioner is set to turn off at 11am though I may turn it off earlier if the sun is shining through the northern windows. Eleven am is the time that the heat pump hot water system comes on. As I have a photovoltaic (PV) system on the roof if possible I do not want to exceed the PV output in order to get most of the electricity for heating from the PV system. If the house cools down below 19 or 20 I can always manually turn the air conditioner on.

The air conditioner is also set to come on at 3pm and to heat to 23 degrees! This will occur even if the living rooms are sitting comfortably at 20 degrees or above, with sun streaming through the windows. It is also set to turn off at 4:30pm. Why have I done this? It is an attempt to heat the thermal mass in the living rooms using heat from the air conditioner while it can still draw electricity from the PV system. I will also turn on the ceiling fan to push the heated air over the tiles on the concrete slab to endeavour to get more heat transfer to the concrete slab. (The slab floor does not have carpet in the living rooms). I do this manually as there is no timer on the ceiling fan. The cost of the electricity from the PV system is either zero or if you consider that I would have made 10.2 cents per kWh from selling the PV electricity back to the grid then the effective cost to me is 10.2 cents per kWh. If I am heating the living rooms after the sun has gone down I will be drawing the electricity from the grid at a cost of 30 cents per kWh. Even if I use a bit more electricity I should be out in front financially. It is also better for the grid as I am using PV rather than relying on grid electricity after the sun goes down which at this stage will still be coming primarily from fossil fuel sources (in Melbourne Victoria).

(That I do this shows that my Northern windows could have been larger and the shading structure better designed to allow in more sunlight in winter to heat the house to a higher level. There are risks associated with increasing the window size - if it goes too far you can get over heating and "sun drenching". The approach with preheating using the  air conditioner largely powered by the PV system works.)

With the preheating of the thermal mass it delays when I have to turn on the air conditioner at night. I might turn the air conditioner on at about 7 pm, maybe earlier if it cold outside. Sometimes 8 pm. Sometimes I don't need to run air conditioner of an evening at all. I will turn it on when the room temperature drops below, say, 19.5. So even if I only shifted a kWh of heating electricity from later, non PV electricity, to a time when PV is still available it could save 20c. Not much but as I said it is also benefiting the grid. I imagine I will stop doing this sometime in September as no heating will be required for the house at from that time on.

The last air conditioner setting I have is for the the unit to turn off at 10 pm. This is because I typically go to bed at 10:30 or 11pm and the house will remain acceptably warm until that time. If not I can always manually turn the air conditioner back on if I stay up later.

For summer operations I will have to see how the house operates. Last year (2019/20) I only used the air conditioner for 2 hours for the entire summer period. So I probably won't need to have it coming on and off on an automated cycle.

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