Water flow sensor

This morning I was woken at 4.30 by a tremendous banging coming from the boiler. As it turned out the c/h pump had stuck and the banging was presumably the water boiling in the heat exchanger. The pumps now free but after installing a new one I want to avoid this again by putting a flow sensor in line with the pump so that if the pump's not pumping then the boiler wont fire. Does anyone know where I can get such a thing.
TIA
Andy R
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but
Hi
Why not look at the obvious things first. What was the banging as yes your boiler would cycle as it heated and then cooled but not banged and definately not boiled. it sounds like you have another problem that needs looking at before putting in "bodges" to overcome a rare occurance that showed up another problem.
Ian
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The boiler is a long way from the cylinder with lots of bends in the pipework and I think that convection alone is insufficient to send the hot water round the system. As soon as I gave the pump a whack and it started up the banging stopped and the pipe to the pump got ferociously hot as the water from the boiler started circulating. I expect you're right that the water in the boiler shouldn't have boiled but if the flow is non existent then I suppose hot spots can occur and the water in places will boil before the boiler stat detects the heat. The idea of a flow switch I don't see as a bodge, more a way of protecting the boiler if the new pump fails some time in the future. Just as a matter of interest the boiler is a Halstead Balmoral 55 about 7 years old. The house is also about 7 years old and I've no intention of re-routing any pipework.
Tks & rgds
Andy R
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but
They are available from many places. http://www.heatweb.com sell them. The DPS version has to be vertical. Best put it on the cooler boiler return pipe if possible. Or more comprehensive: http://www.axdistribution.com/FluidManagement/flow_switchs.htm
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I'm trying to get my head round how this will work!
With most modern boilers, the pump is controlled by the boiler - and is powered whenever the boiler receives a switched live signal from the CH/HW control system plus whenever the boiler temperature is above that of the pump over-run stat setting.
Presumably your flow sensor would interrupt the switched live feed to the boiler if there wasn't any flow? In this case, the system would never start up from cold - because you would have a deadly embrace between the boiler and pump!
Of course, if the pump is powered independently, and is not required to over-run when the boiler stops, your solution should be ok.
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Maybe a thermostat on the flow close to the boiler (i.e. where the pipe comes out of the top - if it does) and set to open when the temperature reaches nearly boiling might solve his problem
Ideally this should be wired to stop the boiler but not the pump.
Michael Chare
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CH/HW
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start
boiler
comes
his
Better to have the one boiler stat as the high limit (90C) and have a pipe stat on the flow as the run stat. These would switch out the burner at run temp (pipe stat) and high limit (boiler stat).
If the boiler doe not control the pump then a flow switch to switch in the burner is the best solution. Positive in that: no flow, no burner is on.
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In an earlier contribution to this discussion,

You seem to be overlooking the fact that ther boiler stat will have turned off the burner *long* before the water gets to boiling point. The water only boils because the residual heat in the metal parts of the heat exchanger goes on heating it after the burner has stopped. That's why many boilers need to control the pump - so it continues to run long enough to carry the residual heat away and prevent boiling.
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wrote:

So if the pump stops the water inevitably boils because by the time the temperature rises enough for the boiler stat to stop the boiler more heat will have gone in than can be absorbed by the water without boiling.
Does the water also boil if there is a power cut when pump and boiler would stop at the same time?
Going back to the idea of using a flow sensor in the scenario where the boiler supplies power to the pump, you could avoid the deadly embrace by using an 'On pulse' relay i.e. a realy where the contacts close for a short period when power is supplied and then re-open until tghe power is swithed off and on again.
Michael Chare
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In an earlier contribution to this discussion,

Yes, unless the water is replaced by fresh.

Potentially. My boiler user manual warns of this possibility - but I can't remember it ever happening in practice. [That's a dangerous thing to say! <g>]

I guess so. I've been thinking about it in the meantime, and had more or less come to the conclusion that you would need something which by-passed the pressure switch for a short finite time whenever the external boiler demand is turned on - which is what you're suggesting.
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I'm taking a close interest in this subject as I am contemplating replacing my floor standing boiler + natural H/W circulation with a fully pumped wall mounted one.
What I know is
a) I get all to frequent power cuts.
b) My Grundfos central heating pump quite often fails to start after a period of summer idleness. It actually has a large screw in the front which can be removed to allow a small screw driver to be used to give the motor a tweak.
Michael Chare
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In an earlier contribution to this discussion,

So your current boiler has a large enough water chamber so as not to overheat when there is little or no flow.
Assuming that you use the boiler (rather than an immersion heater) for hot water during the summer, you'll need to run the pump all year round with the new boiler - so it will have less chance to seize.
Whether or not the boiler actually trips in the event of a power cut depends on a number of factors including: * internal water capacity versus thermal capacity of the metal bits * main boiler stat setting [if the flow temperature is considerably lower than the trip point, there is a greater margin for the water temperature to rise without tripping - but that would also reduce the heat output from your radiators, and may thus be undesirable] * gravity circulation [some water may continue to circulate by gravity even without the pump running - presumably the zone valves or diverter valve will still be open]
FWIW, I can only remember my boiler tripping when I had a genuine flow problem - I had partually draining the system to move some pipework, and had succeeded in getting a major airlock in the boiler loop - so the boiler was running full chat, but the water wasn't going anywhere!
It's still desirable (maybe even essential) to make use of the pump over-run facity though (accepting that this doesn't do anything in the event of a power cut!). Under normal operating conditions, I am often aware of the following scenario: The room stat clicks off, and the boiler and pump immediately stop. After a few seconds, the pump starts up again (triggered by the over-run stat) and then stops after maybe 20 seconds or so (I've never timed it).
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I reckon you have a problem with the boiler, it should not boil the water. It should only fire when the water needs heat.
Rick

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In an earlier contribution to this discussion,

Not necessarily! Many boilers need the pump to keep going for a bit after they stop firing - otherwise the residual heat in the heat exchanger heats the static water up to boiling point.
If the OP's boiler is like this, his flow switch solution won't work anyway (see my earlier post) - although it may be possible to devise something which *would* work by using an additional relay.
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