Furnace: July 2010 Archives

Can I improve my air conditioner's efficiency?

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Summer is finally here. Temperatures in Winnipeg are getting up to 30C and with that, the air conditioners are powering up. I've been tracking my A/C usage along with my furnace since January 2010. I'm actually surprised how little electricity the air conditioner uses compared to the electric furnace. In fact, to make the daily & monthly graphs 'interesting', I've had to multiple the calculated power usage by 10.

Now, 30C is not that hot compared to other places in the world, and in Manitoba our power is reliable, plentiful and inexpensive. After returning from a business trip to Southern Ontario during a very hot spell (highs of 35C) I began to appreciate how the air conditioners are able to strain the grid.

In a conversation back home the question came up - can  an A/C unit be made more efficient by pouring water on the condenser coils outside? (We had been noticing water droplets coming off our office building during hot weather - and are assuming that the building is using water to cool the outside coils).

So, yesterday while the A/C was running at home I tried some experiments. The temperature was 27C outside, and because I have my thermostat cool the house down for night, the unit was going to run for a couple hours non-stop.

I measured the current draw on the compressor at 7.5A. This was my baseline. (And is what I have been using for my power usage graphs.)

Next I set up the garden hose and a hand held nozzle to direct a gentle mist or spray of water onto the condensing coils. I was thinking that a gentle mist would provide extra cooling by evaporating, and I also didn't want to create a flood beside my house from the water.

Back inside I took a few measurements of the current draw. I also timed how long 10 liters of water took to be sprayed onto the A/C unit. I wanted to compare any money saved by lower power with the extra cost of consuming water.

To my pleasant surprise, the current draw dropped by nearly 10%! After a few minutes of spraying water, the new reading was 6.85A (a drop of 0.65A).

10 liters of water was being used every 4 minutes and 12 seconds, or about 143 liters / hour.

Before pulling out the calculator, I did one last test. The coils outside pick up dust, seeds, etc and should be cleaned every year - I opened the nozzle up and gave the coils a good cleaning (600 liters / hour). The current dropped another 0.65A down to 6.2A!

Without a doubt, cooling the coils saves power. This is because the coolant condenses much easier when the coils are cooled down, the condensing lowers the pressure on the high side making the load on the compressor lower and hence the lower power consumption.

So, how much money do I save? Well, at $0.065 / kWh plus taxes, it works out to about $1.30 / month. Not much, but every dollar saved is one dollar earned, or something like that.

But while running water on the coils lowers power consumption and is good for the electrical grid, and lowers my power bill by a small amount - am I actually saving money?

Turns out, no. Water in Winnipeg costs $3.20 / 1000 liters. At 143 liters / hour, in one month (6 hours / day, 20 days / month), my water bill will increase by $54 - easily negating any electrical savings.

Anyway, for those who want to see the number crunching details, I've created a google spreadsheet where I ran the numbers.

Enjoy the summer!