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    Default Circulating pump - start capacitor

    Hi - I've got a hydronic heating system and the circulating pump failed recently.

    It's a system that pumps hot water through a towel rail in the bathroom. It "interrupts" the HWS heating loop, which is supplied by boiler in the wood-fired kitchen range. It's switched by thermostat that sits on the boiler output pipe. Temp rises, thermostat switches on, and the pump circulates hot water to the towel rail until the temp drops enough to switch the thermostat off. It's been running fine since 2014, so I'm generally very happy with it.

    The pump uses an electromagnetically-driven impeller, so there's no shaft or seal to wear out and leak, and it's very quiet, but what failed was the motor start capacitor. Not surprising it let go, considering its environment is very close to hot water. I suspected the cap because I could supply power to the pump, and watch the impeller twitch as it tried to start. If I flicked the impeller manually, it would continue to run. The pump is a "Laing ecocirc" model D5Vario-38/700B.

    So I went a bought a replacement cap - 1000uF, 63V, 105C - and *carefully* soldered it in. I didn't try soldering it to the PCB (I'm not that good), but I clipped the connection wires of the previous cap and soldered the new one to those. I was careful to make sure I got the polarity right.

    Now - it works, but not as well as before. It's slower to start, and even won't start sometimes. I switch off the power and come back half an hour later and then it will start. It's taking longer to start up than before, the startup hum is definitely longer and more drawn-out than before.

    Original Cap: "SamWha" brand, 1000uF, 63V, 105C
    New cap: "SunTan" brand, 1000uF, 63V, -40 - +105C - but it's significantly physically smaller than the original - slightly smaller diameter, and about 3/4 cm shorter.

    Could it be the new cap is optimistic about its abilities? If I wanted to replace it with something better, what would I be looking for? I don't want to replace the pump if it's just a starter cap that needs replacing. Also, a new pump is about $400, so a $4 cap is a better option.

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