pump overrun

10 May 2006
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United Kingdom
Hi guys. I have a Potterton Prima F50 and i realized that after the call for ch or hw goes off, the pump runs for ever...and i mean it! The pump itself Grundfos 15-50, is scorching hot and so are the primary flow pipe, the by-pass in the airing cupboard fitted with a gate valve and the return pipe back to the boiler, despite not having been a demand for ch or hw for many hours and the boiler has not fired up. Last night, to try and stop the pump running, i called just for a few minutes,for ch which was completely cold and shut off the bi pass valve. Since the system was completely cold, the return pipe went cold and at that moment i switched off the ch and reopened the by pass valve and the pump switched itself off. My question is
1) is the pump overrun timed or connected with the boiler stat so that the pump keeps running until the boiler stat drops under a set temp. I do not know whether that was a coincidence but as i said as the return pipe went cold yesterday and i switched the ch off the pump stopped running .
2)what is causing the flow pipe, by pass, and return pipe to the boiler getting so hot and not cooling down even though there has not been a call for heating and the boiler has remained off for long time? The pump seems to work ok and there does not seem to be any big problem with sludge as whenever there is a call for ch the rads upstairs and downstairs get hot within reasonable amount of time. Thank you for your time.
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BTW i forgot to mention that there are two wires ( red and black) coming out of the pump and connect up into the junction box it the airing cupboard.
the pump is controlled by a stat but because it goes into the sealed casing it's a job for a RGI.
I dont think the overrun stat is the problem as it seems to be doing what its designed to do, the question is where is the heat coming from to keep the pump running, have you had any problems with lack of hot water ?
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Duff microswitch keeps the boiler going with the valves shut; only reason flow pipes stays hot is the boiler firing.
Thank you all for answering my thread. The boiler shuts off perfectly well when there is no call for ch or hw. I have no probes with hw either. Could it be the bypass which is not opened enough or too opened......I mean, how opened should the bypass fitted with gate valve be?
Could it be the bypass which is not opened enough or too opened......I mean, how opened should the bypass fitted with gate valve be?
A bypass should not be a gate valve, it should be a bypass valve.
I know bengasman but it is an old house and that's what I have found.would you know how many turns shall I open it by? Could that cause the pipes go scorching hot?
Do you think that might be the actual problem though?
A faulty bypass will not cause the boiler to come on when the timer is off, if that is what you mean.
Hi bengasman and thank you for replying. I never said the boiler fires up when the timer is off. I just said that the flow pipe from the boiler to the bypass, the bypass itself and the return pipe from the bypass back to the boiler are scorching hot even after 2 hours the boiler has shut off and that in turn make the pump spin forever probably because the pump is trying to dessipate the heat (as picasso said).Now, could a wrongly set bypass ( fitted with gate valve) cause the above mentioned pipes go so hot even after the boiler has gone off for such a long time?.....probably because the bypass valve is set in a way that does not allow enough water flow? Btw, my system is fitted with 2*two port valves.thank you so much for your precious time.
As bengasman says, there must be some heat source involved here. There are two scenarios possible given what you have described:

1. The gate valve in your system (the one being used as a bypass) is working fine (by which I mean it has been left suitably open (not too much, not too little). In this case, the water in the pipes (you didn't say how large a loop this is) will circulate via the bypass valve until it is sufficiently cool. This is done either by a thermostat or a crude timer in some older boilers. In this case the water should cool fairly quickly (with 15 mins or so) and the pipes wil be warm but not hot to touch.

BTW, it would be useful to know how large the bypass loop is (how long is the pipe that includes the boiler, pump and bypass valve). Also, what gauge of pipe is used?

2. The gate valve in your system is either too closed or is somehow blocked. In that case, the water has nowhere to ciculate once the two zone valves (two port valves you have described) are closed. In this case, the water in the pipes will not move and the pump will strain away trying to move it around the system. Even in this casre though, the volume of water in the pipes is fairly low and will cool naturally in a short time. The only reason the pump overrun is used is to pass water through the heat exchanger of the boiler.

So, there MUST be another source of heat in the system. The prime suspect is (of course) the boiler but you seem very sure that it is not firing. I don't mean to ask stupid questiuons but how are you disabling it? Presumably you have set your controller so that neither Central Heating or Water Heating are active?

Even with these controls set to 'off', if the zone valves are defective, it is possible tha tthe boiler may still be running (most likely for the hot water circuit since you can easily tell ius the CH is on but not so easily the HW).

I'm guessing though that you have had a good listen to the boiler throughout the pump overrun time and it is never firing up?

Lastly, and grabbing at straws here, if the bypass valve was almost fully closed and the bypass loop really (really) small, I guess the strain on the pump could cause it to heat up and thus heat the water in the loop. That seems failry fanciful to me though.

First port of call is to adjust a couple of things and see what changes. With the system off, close the bypass fully and counmt how many turns it was open. Write this down (so you can go back to it). Then open it up fully and see if the pump overun still occurs.

Also, can you check the flow and return pipes to your hot water cylinder. During pump overun, are they hot or juts warm?

Hi iep thank you very much for your input. I ll check it up tonight and will let you know. Big thank you to as well bengasman I ll keep you posted. Ta

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