Of course Father’s Day is a commercialised rort and an excuse for selling more stuff. Nonetheless, siblings, and anyone without a dad handy, will probably be feeling a pang today. I still have one of Dad’s old shirts, and his dressing gown. More practically  I have been honouring our Dad in a practical way, by doing a spreadsheet about energy use.

(To bring non-family members up to speed, our Dad, David, was an engineer. Toward the end of his career, he developed a particular interest in energy conservation. Unfortunately, as in other things, he was ahead of his time. In the eighties, most people thought fossil fuel energy was cheap, and would last forever. Peak oil and global warming were unheard of out here.)

Dad therefore would have been totally across my spreadsheet, which bears the catchy title “Cooling & heating costs with split system versus separate evaporative cooling + gas ducted heating” (Aron, 2018, unpublished.) You may not find this magnum opus in a bookshop near you, if there still is one. So I can upload it to Google Drive, if anyone is interested. (No? Fair enough.)

I will prevail on your patience with a précis. We have to replace our 15 year old evaporative cooler with a newer system. This has opened a bit of a Pandora’s box. Should we replace it with another evaporative system? Or go reverse cycle ducted? This would use more power, but wouldn’t use any extra water. What are the comparative costs of these systems?

This begs another interesting question. How much water do evaporative systems use, anyway? The answers are complicated. Modern evaporative coolers recycle their water. They can’t do this indefinitely, however, as the salts in the water become concentrated. This shortens the life of the cooling pads. The point at which a cooler will dump its water varies between brands, the salinity of the water, etc., and is just about impossible to find out. Different manufacturers’ web sites mostly say: it depends.

I consulted Dr Google on this matter and found two figures for water consumption of evaporative coolers: 58.6  and 113 litres per hour. The first is contained in a paper written under the auspices of the Federal Government’s Water Efficiency Labelling and Standards Scheme. (The link points to the .pdf format.) The second is derived from post to the Whirlpool discussion list. I included both figures in my spreadsheet, as Scenarios 1 and 2. (As you do.)

How much energy do different types of coolers and heaters use? This is much easier to find out. Sustainability Victoria has useful summaries for cooling and heating .

Those still awake will be on the edge of their seats, I know, so here is the executive summary of what I found:

1. Reverse cycle would be cheaper for us, by between $239-$891 dollars per year.
2. Reverse cycle systems are more expensive to purchase than evaporative. What would the payback period be? I did two calculations; one guesstimated the extra cost of a ducted system at $4,000, the second at $8,000. Accounting for multiple variables, the payback period varies from 4 to 33 years!

Anyway, we have pretty much decided to go with another evaporative unit. This is for non-financial reasons. My beloved has sinus problems, and the extra humidity of the evaporative is better for her. We can also use our existing ceiling ducts. Refrigerant system ducts are smaller, so we would have to have some new holes cut in our ceilings, and the old ducts covered over.

This is what we were leaning toward in the first place, but we now know why this is the right answer – for us. I hope Dad would approve.