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Thread: Short Cycle on MZ25s

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    DIY Junior Member HappySoul's Avatar
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    Nov 2011

    Question Short Cycle on MZ25s

    After many years of excellent service the boiler started short cycling. The DHW is still hot and the house warm. But, the boiler is starting every thirty or so seconds and running for only part of the purge cycle or a few seconds of burn unless we need heat. When I disconnected the DHW thermostat it stopped short cycling. Do DHW thermostats go bad?

    I doubt that the DHW piping is dumping enough heat into the room to make it cycle. I did add a little very well insulated DHW buffer tank this summer and the boiler was recently switched over to the winter setting. It does the same thing on the summer setting now anyway so that is not the problem.

    The DHW has been great, but could the tank siphon fast enough to defeat the delta? No, that makes no sense since it worked OK all summer long.

    The thermostat part number in the manual is not a part manufacturer's number so I cannot find a replacement easily. Monitor wants to hook me up with a dealer instead of taking my money. Very professional of them.

    I pulled the thermostat and there are tiny little markings but nothing like a manufacturer's name that is visible. BTW, the bottom few inches of the probe were coated with a black wet powder. Is that significant?

    Well, it turns out that the DHW tank was partially full of air. The "Automatic air vent" is no longer automatic. The system still short cycles after manually bleeding the air out of the tank. This tank is full of water from the boiler and the DHW is heated by a finned coil that passes through the tank. The tank was only partially full of hot water but the DHW was still hot enough since we have a tempering tank.
    Last edited by HappySoul; 11-05-2011 at 06:35 PM. Reason: additional information...

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