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Pump at slow or fast speed?

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Rad1o

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Pump at slow or fast speed?
« on: May 05, 2021, 04:18:25 PM »
Sorry, a bit of a story.
We have a solar thermal system from 2003, moved once following an extension.
Last year a few tyles were peeled off by a storm and probably damaged the panel on the way down. I am saying probably because the pressure was dropping despite several top ups and I could not see any obvious damage. As the pipes are all in the ceilings we would have noticed a leak inside.
So I used a leak sealer which has done the job. After the winter when it was getting sunnier I noticed that, even though the system had "switched on" the pump, the water was not getting hotter in the cylinder. So I thought the pump had failed and replaced it. I noticed quite a bit of viscous, brown goo in the old pump when I looked at it. The new pump did not help. The water was still not getting hotter despite the system engaging the pump.
My next thought was that the goo has actually blocked the panel. So I plumbed another access port in between the pump and the panel, so that I could flush the panel backwards and forwards. Eventually ..... there was flow. The pressure is still holding and the solar thermal working again.
I probably have an old but functioning pump spare now.

But back to my question. While the old pump was pretty much on / off, the new pump has lots of "modes" and speed settings. It is set in a standard mode at 3/4 maxspeed. With the temperature differential between panel and cylinder often quite high I was wondering whether a slower speed would allow a better heat transfer into the cylinder than a high flow.

Any thoughts or experience with this?

 

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