Pump Overruns

My house has gas central heating, with a Myson Economist boiler in the kitchen, and a pump, timer, etc in the airing cupboard upstairs.  It's made a lot more noise than it should have done.  One of the problems I've had is detailed on this page, so you can find out if this is happening to you.  It isn't intended as a guide to fixing it, but rather to give you enough info so you can get the job fixed professionally.

(The other reason these systems make a lot of noise is trapped air - which causes waterfall/roaring sounds in the pipework.  Just keep bleeding the radiators as often as the noise appears.  Apparently it's best to do this when the system is cold, or more air can be sucked in.  There must be a better solution, but I don't know it yet).

The Problem - symptoms

One interesting 'feature' of the system has been annoying me for a year.   When the timer switches the heating off, the noise doesn't stop.  On the contrary, the pump in the airing cupboard runs on... and on... and on.  My system switches off automatically at 9pm.  But I have heard the pump continue until midnight!   After a tiring stressful day, being unable to shut the wretched thing off, when you desperately need peace and quiet, can be the  last thing you need.

The industry 'professionals' I brought in indulged as usual in a great deal of head-scratching and difficulty-making. 

The Problem - explanation

What is happening, is that when the heating switches off, the water is being pumped around the cooling radiators, without being heated.  (You'll see that the flame on the boiler has reduced to a pilot light, so no heating is going on).   The water is being cooled by this process, which is perfectly normal - it is supposed to happen, for a short while, to allow the system to cool down. 

(Note that there is a knob on the boiler - mine is labelled with numbers 1-5 - which determines how strong the heating is, and so how much gas is used. Higher values make for hotter water, which means warmer radiators, but also higher gas bills. Of course if you have the boiler knob turned up full, the water gets hotter, so it takes longer to cool - so always try turning that down first, if you want it to cool quicker).

The process is controlled by a thermostat in the boiler - known as the pump overrun boiler thermostat.  Physically this consists of a control unit, about the size of a matchbox, with a long copper wire leading to a metal cigarette-sized object - the actual thermostat, which is plugged into a hole in the case.

These things go wrong.  Basically they lose their calibration, being heated and cooled, heated and cooled, every day for years. My previous house had exactly the same problem, which I was never able to resolve.  In fact this problem probably affects all systems after a time.


These pump overrun boiler thermostats cannot be repaired - you have to buy a new one.  Mine cost £90, including fitting by a Corgi-registered tradesman, which takes around 10 minutes.  I found that the part was listed in the manual that came with the boiler, but that it was best to get the gas-chappie to read it himself, otherwise he got confused, and hesitant, etc etc.  They can get the part on next day delivery, so there really is no reason not to get it done.  Just tell the guy that this is what you want, to deal with the pump overruns.

That's it.  Simple, really.  Which makes one wonder why the tradesmen make such a mountain out of a simple, easily diagnosable problem.  Perhaps the best thing is to get them automatically replaced after 10 years (the thermostats, not the tradesmen).


Now it may be that you want some quiet, but can't get around to fixing this just now.  What can you do in the mean time?

Firstly, I have an electric switch, in the kitchen next to the boiler, which controls all power to the central heating system, both boiler and pump.   Switching it off silences the lot.  It doesn't interfere with the pilot light - which none of us want to try to relight - but it can be left off perfectly happily.   Of course this is no use at night in winter when you do very much want the system to come on next morning.

Secondly, I found that it was sometimes possible to get the system to switch off, by twiddling dials.  After the system was cool, and the house was cool, and the pump was still going, I went down to the room thermostat (in my living room) and turned that right up.  The timer, of course, had automatically switched off the system, so nothing happened.  Then I went back to the timer, and pressed the manual override, to set the system going.  This is followed as per normal by a massive roar from the boiler, as the system switches on.  I let this run for half a minute, and then pressed the manual override again to switch it off/back to automatic.  This would invariably switch off the pump as well.  Of course one needed to remember to turn the room thermostat back down.  I do not know why this works, but it certainly does.  And of course it doesn't interfere with the system switching on tomorrow.


I hope this page will help you - I'd have been glad of it myself.  As I gradually overhaul my ten-year old system, more pages will probably appear here. 

A general caution:  I do not know if faults in these systems are always this easy to diagnose.  Probably the best course of action - and the one the tradesmen will follow, without telling you - is simply to replace parts of the system for new ones, starting with the most probable, and keep going until your problem is cured.   This seems inevitable, as I have met few tradesmen who can diagnose problems.   Usually if you press them, they just go all vague, and go off without doing anything at all.  If you take responsibility, and order them to replace parts, once they are clear that they are not held responsible they will then do what you want.   Pathetic, but it does seem to be true, and it makes for faster fixes if you realise that this is what is behind all the maddening dithering that goes on when you try to get jobs done.

Do not just rely on this page - I am an amateur like you!  You should always get the advice of a trained professional, carrying the relevant insurance, who you can sue (or try to) if it all goes bad.  Note however that some of them will really be plumbers, while others will be useless at plumbing and good on boilers.  Try to find out which is which, and get the former lot to do the radiators, and the latter to swap thermostats.  If you can find one who is both, you're laughing.

Always remember - don't mess about with gas systems; it's a quick way to end up dead.  Get a Corgi-registered tradesman to do the gas bits.  And get your boiler serviced every year.

Last updated 8th April 1999.

Steve Wain sent me the following interesting email, which I reproduce with permission as it may be of use to people:

I was quite interested in your problems with the overrun  on your boiler. 

Spookily enough I had exactly the opposite problem, i.e. overrun timer not operating.. This meant that when the boiler shut down because the boiler stat was satisfied the residual heat in the cast iron heat exchanger had nowhere to go ,(because the pump had stopped) so built up until the boiler safety stat operated and shut down the whole boiler, including the pilot light, which as you know can be a pain to relight.

A new timer cost £35 and I fitted it myself, it being an electronic item and not  a gas item. It then worked perfectly.

I think you should be careful suggesting that the boiler thermostat is faulty, this would normally cause the boiler to overheat or most likely not work at all. In fact the overrun timers I have come across, in my limited experience, simply maintain the power to the pump for 5 mins. after the boiler thermostat has switched the mains to the gas valve off, they have no connection to the thermostat, they detect when the mains is no longer on the gas valve. 

Another thing that can cause the boiler to run on for ages when you don't want it to is the frost thermostat. These all activate at about 4 deg. C which isn't that cold, and as they override the house stat, I've found myself waking up at 2am sweating buckets, the only way to reduce this effect is to turn the boiler stat. right down. The trouble is you might wake up feeling cold. 

If you have any other gas problems try heating@gasman.fsbusiness.co.uk

Thank you Steve for those interesting ideas.  

Updated 23rd May 2002.

Brian sent me this interesting note:

Hi, I am a corgi registered engineer and you have listed what can only be described as nonsense. 

Your heating system is not as you describe, first of all the stat on the boiler has no effect on the temperature of your hot water. It dictates the maximum and minimum temp of the water in the heat exchanger before the thermostat shuts off or enables the main burner, turning the knob up or down will either make the boiler stay on for longer periods or shorter periods of time. If you want hot water/heating quickly then the stat should be high, if you are not in a rush then turn it down. 

Your water temperature is dictated by a cylinder stat on the side of the cylinder which should be low on the cylinder ie positioned third of the way up. It becomes more complex now depending on what system you have installed ie: S plan Y plan etc If you have hot water priority - mid position valve - zone valves etc etc. 

If you get an engineer that knows what he is doing he would be able to diagnose the faults and reinstall the system properly if needed. Please understand I know there are alot of engineers that work on systems that to be honest ought to go back to pulling scrap cars apart because they have not got a clue. They think because they have corgi they are heating engineers but they are not. I spent a long time working with older engineers on a council contract and the diversity of problems we attended was so educational for me I really could not do this job without that experience.

Thank you Brian for these notes!

Updated 23rd July 2004.

Ken James sent me another note on this:

Re: Boiler Overrun problems.

The problems you describe could have been caused when an older balanced flue boiler was replaced with a modern fan assisted boiler. 

The newer boilers have smaller heat exchangers and cannot dissipate heat as easily as their older cousins. Hence the need for a pump overrun. Without this the heat in the boiler can continue to rise tripping the boiler overheat thermostat.

The residual hot water in the boiler needs to be pumped around the system, with the boiler off, for a while to cool it. On a system using a three way motorised valve this will not usually present a problem as the valve is usually open when the room and cylinder thermostats have both turned off.

However, if using a Honeywell S Plan system, which utilises two motorised valves, one for heating and one for hot water, this is not the case. This time when the room and cylinder thermostats both turn off, so do the two motorised valves. Hence when the boiler overrun circuitry kicks in to cool down the water in the heat exchanger, there is no where for the water to flow, unless a bypass circuit has been fitted. You can test for this by feeling the pipes before and after the pump - they should be the same temperature if water is being circulated by the pump. Measure this a few feet away from the pump.

This bypass circuit should not be confused with a bypass radiator that you would normally have on an older balanced flue system to provide a water flow when all the other radiators fitted with thermostatic valves have shut down. This is not made very clear in the installation manuals supplied by some boiler manufacturers, including Halstead, supplied by Wickes. It could also be clearer in the Central Heating specifications in the current Building Regulations. 

Basically a bypass needs to be fitted immediately after the pump and before the motorised valves. This should be run in 22mm piping using an automatic bypass valve connected back to the return pipe work. The bypass loop must have a minimum capacity of 7 litres.

A Honeywell DU145 Automatic Bypass Valve is designed for this purpose, and costs under £25 from Plumbase. This bypass also helps to reduce pump noise when the system is running, and most thermostatic valves have closed. It will open at a pre-set adjustable pressure to ensure an adequate water flow is maintained.

For a better description of the circumstances when a bypass should be fitted visit the Baxi website.

I have recently fitted this bypass circuit and my pump no longer groans when the pump overrun starts, and the pump soon turns off - before it ran until I turned it off manually or until the water eventually cooled.

He also added:

Originally, in trying to locate the cause of this problem I had suspected an air lock in the pump. I later realised that the problem was the motorised valves shutting when the pump was still running, but suspected a fault with the pump control pcb or one of the motorised valves. Consequently, unnecessarily replacing the pump control pcb and a motorized valve.

The pump running on with both motorized valves closed caused the pump bearings to wear necessitating a new pump. It also caused water to be pushed up the vent pipe and the cold feed pipe into the header tank. The feed pipe eventually becoming blocked by calcium deposits caused by the resultant introduction of air bubbles. This had to be unblocked manually and the whole system flushed out twice with Fernox DS40.

A costly learning experience which could have been avoided if the boiler manufacturer had clearly explained the need for, and purpose of a bypass circuit. The pump overrun function was not explained either.

Wickes are likewise guilty of selling fan assisted boilers while showing no bypass circuit in the pipe work drawings of their Installing a Gas Fired Central Heating System - Good Idea Leaflet. Perhaps it is because they do not stock bypass valves ?

Thank you Ken for sharing this with us. While in fact my own boiler was original, this may well  help someone else.

Updated 28th October 2004.

One further note of my own experience: if the pilot light goes out and refuses to relight, I have been told that this is always caused by the thermocouple giving up.  This is a twisty bit of metal which is poked into the flame, and regulates the temperature.  

Yalcin Murad emailed with this comment:

I was doing a search on domestic heating boilers boilers and stumbled across this site.  I didn't understand the comments by Brian.

The part about setting the boilers built in thermostat, and the suggestion that this knob does not dictate how hot the water gets  --- but surely that's why they fitted it?! I.e. It dictates the heat available to the entire system, not just how long the burner runs. The general rule is "set high during the winter and low during the summer". Setting this low "if you are not in a a rush" also sounds a bit strange.

Obviously setting too high will be a waste of gas and money. Set too low will cost you money also, just think of a kettle fitted with an element incapable of heating the water to boiling point; it would heat the water all day and never switch off. The same goes for central heating - the room thermostat would never shut the boiler off because of the lack of heat.

Cheer up my boiler was made by Potterton and celebrates its 30th birthday next year. Modern boilers are not as efficient as they are cracked up to be, most "engineers" don't want to repair because they want to sell you a new one.

Thank you Yalcin for your thoughts!  

Updated 30th March 2006.

I have discovered that British Gas do not now service boilers every year, if you have one of their contracts; they just "inspect" them.  But they charge you the same.  It leads one to wonder how useful their service contract really is.  However if your system breaks down on Saturday night, they will send you someone to try to fix it on Sunday, if you're under contract.  I can't stand them, and I have found their customer service call centre is often a terribly bad experience, but it's worth considering. 

Updated 30/3/7.

I see that they've managed to 'forget' to even do my inspection this year (2007)!  Last one was summer 2006, some 15 months ago.

Updated 8th November 2007.

Caron French emailed this comment:

Just wanted to say that you saved my sanity with the Central Heating pump overrun solution. I have absolutely NO plumbing experience & followed your advice re turning the system on full, running it for a few minutes, then turning off, as I was desperate & getting no sleep due to the racket all night from the pump clattering away. So to all the doubting 'techies' out  there...."IT WORKED FOR ME & HAS DONE EVERY TIME ! " My engineer will be calling very soon to fix it, but thank you soooooo much Roger!

Thank you Caron for letting us all know!

Updated 8th November 2007.

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