I don’t check our whole-home energy monitoring system several-times-a-day like I used to, so it was pure coincidence that I happened to catch this when I was computering the other day.
There’s a lot going on in this graph, but focus on the dark blue shapes. That shows when the water heater came on, how long it stayed on and how much power the water heater is drawing. The water heater cycle that starts around 10:30 pm goes on for 7.5 hours!! What? Too much. The cycle on the left is more normal for this time of year. In the summer when the basement is warmer the water heater cycle is a little shorter. But 7.5 hours to reheat itself after we… what? washed dishes?
Actually, I didn’t really question it much when I first saw that. But a couple of days later I happened to glance at the water heater and saw this
That’s dust on the fins in the heat pump that makes our hot water. For a primer on heat pumps, see this post. The fins are where energy in the air is transferred to the refrigerant. A lot of air has to move through the fins to deliver enough energy to make hot water. There’s just a teensy space between the fins, and as we are discovering dust can get caught on the way through. What’s the problem? As the space through the fins gets clogged up the fan that blows air through there has to work longer to deliver enough energy to the heat pump. At this point, as I’m standing in the basement contemplating the dusty water heater I remember the thing I saw on our whole-home energy monitor: a long, long water heater cycle. This dust is doubling the amount of energy our water heater uses.
Nik got on a stool and vacuumed the fins. If you try this at your house, be careful! The fins are easily damaged. Use the brushy attachment on your vacuum. Here’s a shot after Nik’s work.
What you can’t see is the lower part of the fin-assembly, hidden behind the black box. That part is still gunked up and there is absolutely no way to get at it with the vacuum. I asked the Stiebel Eltron manufacturer’s representative about that a couple of days later and he said, “It’s supposed to be self-cleaning.” I pointed out that all other heat pump water heaters have a filter over the heat pump compartment. A real let-down from a company from whom I expect top-notch engineering. Later an old timer-heating installer told me to use compressed air to blow it out.
Since the cleaning we’ve had a few more very long water heater cycles. Guess the dirty fins weren’t the whole story. Or maybe it’ll right itself once we blow out the rest of the fins.
Today I was in a funk and was casting about for some comfort cleaning. Nik said he hadn’t cleaned the filters on the mini-split heat pump lately and offered to show me how he does it. Just like the heat pump water heater, the mini-split blows air through a fin assembly to move air in and out of the refrigerant. Unlike the the water heater, filters protect fins from getting clogged up with dust. Nik cleans them every month in the winter.
Heat pumps need a little care. Not much, really. The consequences of neglecting them is lower performance and higher energy use. We want the opposite, so we’re happy to keep up with whatever the heat pumps need.