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Hot Water Overrun


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Hello, looking for helpful advice please.

I have a conventional fully pumped central heating system with a 3-way zone control valve.  When run on the radiator system, after end of program the radiators dissipate the system heat and the overrun cuts out the boiler as it should.  When run on the hot water system, which heat water fine, at end of program the heat seems trapped in the system and the overrun keeps the pump going for hours.  There is a by-pass gate valve between the outlet and inlet side of the boiler.  There is no heat-sink radiator.  So all water in the hot-water circuit goes through the cylinder element or the by-pass system.

So am I missing something here?  Why is the boiler not cooling of it's own accord and cutting the overrun and hence pump?

Many Thanks

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Are you SURE it is the over run mechanism keeping it going?

 

To me is sounds like the boiler water temperature is set lower than the hot water cylinder thermostat temperature, so the water from the boiler is less hot that the cylinder target temperature so it never gets there.  Check those settings first.

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OK ProDave.  I will try just reducing the cylinder thermostat setting to see if that has any effect.

 

Thanks for the input.

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I’d be inclined to turn boiler temp up as that may give a quicker response? 
also you don’t really want Luke warm water? 

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Yes Sharpend, I've now turned the tank thermostat down from 65C to 60C (my understanding is this is minimum temperature to kill Legionnaires bug etc) and it made no difference as I expected.  The boiler stat is full on as I believe it is supposed to be.  I can override the hot water system overrun by putting the radiator system on boost for a few seconds (so the zone valve moves and diverts the boiler water to the radiators while the room stat calls for heat).  That cools the boiler and the overrun cuts in a couple of seconds.  It doesn't make much sense to me.  It all seems to work ok on the face of it, but it's like the boiler heat exchanger cannot radiate enough heat out of the hot water system alone to kick the overrun system off.

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Perhaps I'm out of date and wrong here, but I think when I had a similar system the over-run was a timed function from the boiler; nothing to do with actual temperature  and just there to keep the pump on a few moments and make sure the water left in the boiler heat exchanger didn't boil with the residual heat after the flame shuts off.

I would be checking the cylinder stat itself and the microswitches in the 3 way valve.

Edited by Geoff1946
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Okay so I think what is happening here is once the tank gets up to temperature, the over run keeps the pump running until the flow temperature from the boiler has cooled down.  The trouble is I think what is happening here is with the water continuing to flow from the boiler to the HW tank, it is then drawing heat out of the HW tank so the pipes never cool down and the boiler over run function never completes.

 

I don't see a simple fix other than disable the over run function if that is possible with this boiler?

 

This is one reason I hate 3 port mid position valves and would never fit one.  I would always use multiple 2 port valves and with those once HW is up to temperature the only place for the over run to circulate is via the automatic bypass valve and that won't suck heat out of the tank.

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Ok thankyou Geoff1946 and ProDave.  Food for thought, and I will be checking out the components and functionality. As big Arnie said.....I'll be back!

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To confirm, or dismiss my theory, when it is stuck over running for ages, feel the flow and return pipes to the HW cylinder and see if they are staying hot (by sucking heat out of the cylinder)

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