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03/01/2013

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Bob Lemaire

After two years of monitoring a mini-split, I discovered that there can be a significant increase in performance and efficiency when the outdoor unit is cleaned. My unit looked fine, but the output was dropping because it was defrosting more. Typically 14-16 times in a 12 hour period in typical seasonal conditions. After back blowing it with a leaf blower, the frequency dropped to about 10 and the output was noticeably warmer. There was no visual indication that the unit needed cleaning. Similar issues with HRV filters not being cleaned will drive up static pressures and power use in addition to reducing capacity. So in addition to setting expectations of variability, there should be an expectation that these houses need to be monitored and maintained to achieve peak performance.

Marc

Bob, when you used the leaf blower, did you see stuff coming out of the fins? Would a vacuum work better?

Bob Lemaire

It's located under a porch so I would have needed another person to see the discharge. The installer actually recommended hitting it with a hose, but after three freezing month I couldn't see adding to the glacier that it has created. I don't think a vac would do a better job than a leaf blower. When I do the math, the thing processes about 50-100 million cubic feet of unfiltered outdoor air during a winter season. Makes sense that it would need cleaning.

Bob Lemaire

To the vacuum question, when I jam a hose in the end of my leaf blower, it will support 24" of water - about 1 psi. When I put a vacuum gauge on my shop-vac, it will pull 5 psi. So with respect to putting more force on a clogged heat exchanger channel, the shop-vac appears to have greater potential. Better than either, I think I'll carefully hit it with compressed air this summer.

Ben Hemberger

Marc, did you analyze the uptick in heating load vs. heating degree days? Is that something that might explain the differences in the two years? I came across a neat web site the other day that monitored changes in winter temperatures by asking builders of back yard ice rinks to check in as often as they could with a simple question: is your ice rink skateable today? Smart and simple!

Marc

Ben, I think there are two components - lower efficiency at lower outdoor temperatures, and higher time spent at 70F setpoint instead of 66F setpoint

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