Boiler Overheat Lockout, Bypass Valves and Experts

Hot Water TankThere is an apocryphal tale that goes something like: A man has a problem with his boiler that continues for months and causes him no end of bother. Finally, an expert comes to see the boiler, and the expert taps the case a few times, tweaks a fitting… and then charges the man a large sum of money. The customer is surprised and exclaims: “But you only did 2 minutes of work!”, to which the expert replies: “You are not just paying me for my time today, but the ten yeas of experience that I have!”

Intermittent Problem

And now to the point of my post. We moved a few months ago, and have found that the boiler stops working intermittently. It’s easy enough to reset: there is a little button under the boiler that pops out, and can be pressed back in to fire it back to life… but needless to say if the problem occurs when we are out, we could come home to a very cold house. We had one guy come to look at our heating system in general, and he saw the problem occur, and suggested that the pump may be on its way out due to its age. Honestly, I was surprised at that assessment as there was no dust or obvious signs of age on the pump, but as he was not there specifically to look at the issue, there was no need for me to question it in particular.

As time passed, I was able to understand the problem a little bit more. The boiler-cutout was switching the boiler off after it had already been turned off! That is to say, I might see the boiler flames running, then hear the room thermostat turn off; the boiler flames switching off immediately. Then, after a couple of minutes of quiet, I would hear a pronounced ‘click’ and find that the cutout had operated.

Making the Problem Reproducible

I could now use what I had learnt to make the problem reproducible; a key step in IT support to ensure you understand the problem. Steps are as follows (I shall only discuss Central Heating controls and ignore options for Water heating, assuming that option is off):

  • At a point in time where the boiler has been on (with the flames burning and the pump running), turn the room thermostat down until it clicks, or simply set the programmer to turn off the heating and hot water. The flames on the boiler should switch off almost immediately;
  • Wait a couple of minutes next to the boiler. Generally, after a couple of minutes, I would hear the click of the cut-out operating.
  • …therefore it is safe to say that the general problem with my system is that it has overheated. I know the pump works ok normally, so it is not the pump. I guess it could be the over-run thermostat that has failed, but in fact I know from the wiring that the pump has not been wired correctly for the boiler to operate it.

Problem Analysis

What would cause the boiler to overheat after it had turned off? It turns out that when a boiler is running, it requires the pump to be running to move the water around the system, and even when it switches off, lack of movement of the water can allow heat to ‘sit’ in the boiler instead of being carried away. Hence, triggering of the cutout a couple of minutes after the flame has been turned off suggests an issue with the pump, perhaps. Except the pump seems very reliable at other times, and if it was so poor at shifting the water round the system, why would it not switch off at random times when the flame was on, simply because not enough heat was being taken away from the boiler at the time?

Fortunately, I had just replaced the old and broken boiler programmer (being very careful to replace wires in the equivalent locations that they had been before) and I had come to the conclusion that the pump was being switched on and off by the controller. If the boiler was switched off, then the pump was switched off too. So… there was a problem with the wiring, perhaps, rather than the pump? Well yes; it turns out that the pump should not have been wired directly to the controller; it should have been wired to be driven from the boiler – a Potterton Profile 80e which shows in its installation instructions that the pump should be wired into it. This would mean that when the boiler was switched off, it could ‘choose’ to continue to run the pump if it wanted; this is pump ‘over-run’ and is quite frequently required for boiler installations, it seems. So the solution is to rewire the boiler correctly; give it power at all times to the appropriate points, and give it ‘switched power’ when the controller wants to turn the boiler on… and then wire the pump into the boiler pump connections; the boiler then controls how long it needs the pump on for.

Heating Pipework Patterns

However, there is a problem. It turns out that the heating system in our new home has pipework in an arrangement known as ‘S Plan’; which means that there are two 2-port valves; one for each of the central-heating and the hot-water#. When the boiler is turned off by the controller, both of these valves will be closed; so if the pump were to continue running, the water would have nowhere to flow, and thus would indeed cause problems with the pump or other pipework possibly. There are two options:

  • At least one radiator is plumbed in such a way that it is always having hot water from the boiler piped through it if the boiler is on; this would typically be a bathroom radiator, and would usually be apparent from the radiator having two ‘lock-shield’ valves. A lock-shield valve can only be controlled with a spanner or similar, where a normal radiator would have one lock-shield valve and one ‘hand-controllable’ valve. (The point is that the radiator should not be able to be turned off manually);
  • A second option is something called an automatic bypass valve, such as this Honeywell model. This valve needs to be placed as a ‘shortcut’ between the boiler flow and return, and only gets opened by a pressure differential between the input and output. In normal operation with the boiler on, we would epxect the valve to be fully-closed so that water fed through the boiler runs through the heating and / or water circuits. But as the flow through those circuits reduces or is stopped by closure of the hot water / central heating valves, the automatic bypass valve will open enough to let the water to continue to flow through the boiler, carrying enough heat away from the boiler to prevent it overheating.

It seems likely that there is no radiator in the house that has been plumbed-in as an always-on radiator, and there is no automatic bypass valve apparent in the pipework*. As a result, it would seem likely that the fix should include some changes to pipework; and that change is likely to be fitting an automatic bypass valve, and then rewiring the boiler and pump. Plumbing in a bypass valve is more costly in parts, probably, but should be easier inasmuch as the changes are all possible in the current airing cupboard; whereas replumbing a radiator would likely involve extensive work under expensive fixed floors.

I’m not sure if we will do this work as it is likely that we will be doing a major overhaul of our central heating an water system in the next year; but I am certainly tempted to try just to see if it does indeed solve the problem. Not doing it may cause problems, as the overheat situations are probably causing some degradation to the boiler itself over time. We’re not planning to keep it long-term, but do not want major repairs before then.

And so, finally, we can return to the question of the expert. I do not deny that an expert could probably have identified the underlying causes of the issue in minutes had they been present. They may have seen warning signs; like the pump wiring running from the heating programmer and not the pump (though not all boilers require pump overruns, it seems); perhaps too they would have noticed the lack of an auto-bypass. Nevertheless; which expert you get through your door is open to some degree of variance, and perhaps an expert in a different kind of boiler would not spot the potential issues with the configuration of this one. And as for the experts who plumbed-in the boiler and controller originally, some 15 or so years ago? Well, I doubt that they were experts at all.

Clearing the Over-heating Lockout

Before signing-off from this long post, I have come up with a way to clear the lockout quickly if and when I find it has done so. When the system has just been locked-out, you will find it difficult or impossible to push the button back in. This is because there is still too much heat in the boiler. How can we cool the boiler down? Well, as we saw above, the problem is caused by the room thermostat or timer / programmer switching in such a way as to cause the boiler to switch off at a point in time when it was burning gas. It seems that for me, with my boiler and plumbing and wiring, the solution is as follows:

  • Make sure the programmer and room thermostat are ON: essentially requiring demand from the boiler. With the way things are connected, this causes the pump to run, pumping water through the radiators at least. In theory, the boiler should be firing, but in practice it does not, because it is ‘locked out’;
  • Leave the pump running for a couple of minutes; this is allowing heat to dissipate through the radiators;
  • If you press the button under the boiler now, you will clear the lockout easily, and the boiler will fire into life. However, you may not want that to happen, if for example you are about to go out. In that case, turn off the programmer to remove ‘demand’ for heating. Only then press the button under the boiler to clear the problem. Change the settings on your room thermostat and programmer as you desire, and now the boiler will be ready and working in advance of your return home, or whatever.


# The alternative is ‘Y-plan’ where there is one three-port motorised valve, which has the pipes arrange in a ‘T’ shape. This system does not present the same problems as ‘S-plan’, as one of the outputs is always open.

* There is a non-automatic bypass; a gate-valve that I can open and close; but there is a problem here too. If I open it, it seems that the boiler it less likely to have this thermal-shutdown… but there is a reason for this and a large down-side. As a short-cut between the flow and return, when the bypass valve is open it offers the easiest path for the water to flow. This means that water is slightly more likely to flow through that pipe than the radiators (or hot water tank); it will therefore return to the boiler hotter, and so the boiler will switch off sooner because it wants the water to be cooler before it turns on again. Note that this time, when it is the boiler that decides not to run because of the temperature of the water in the system, the pump etc will still be running (as long as the programmer and room thermostats are telling it to). The net result is that the boiler is far less likely to warm the house, far less likely to bring the house to a point where the room thermostat switches off, and also far less likely to actually be on when the programmer switches off. In summary, opening the bypass reduces the risk of thermal cutout, but also reduces the heating capability of your boiler and central heating considerably – or at least this is the case in my experience.

14 thoughts on “Boiler Overheat Lockout, Bypass Valves and Experts

  1. Excellent write up. Sound like a very similar system to mine, except I have manual valves instead of the 2 port automatic valves. Mine doesn’t lock itself out even thought the pump stops working at the same time but the pump has suffered degradation and makes an awful racket when the heating is on. Perhaps I should investigate wiring the pump to the boiler, if that is indeed possible with my boiler. I need a new pump regardless.

  2. Please help! I have a Suprima 60 boiler, which I believe is about 14 years old and a relatively new pump. Recently the pump has starting making a “banging” noise and then the boiler cuts out. British Gas says its sludge, but I could not afford their power flush, so I called in a reputable plumber to carry out this work who also told me that there is a lot of air in the system and offered to fit an AAV (for just over £100), which I told him I would think about. I called British Gas back for the second time (as I have Homecare 400 with them, but this does not cover blockages or sludge) a couple of days later, and was told that the cold water feed was blocked. Again I called out the plumber who took off the cold water feed and showed me that it wasn’t blocked and there was no sludge. For the third time in about five days I called out BG again and told him what the previous engineer had said and also what the plumber said. He insisted there is a blockage, but it “could be anywhere”.

    Please could someone help – I am at the end of my tether, as I am a single mum and so far it has cost me over £400 for the plumber and endless engineers from British Gas. It has absolutely ruined my bank holiday.

  3. Just how the title regular boilers came about is a little uncertain, but it doesn’t reflect that well on a major part of the UK industry which is worth millions and supplies nearly every home in the UK with the heart of its central heating system.

  4. Brilliant posting! Like many people, I’ve had loads of plumbers round my house, most of then useless. Like you, I’ve reluctantly discovered the only way to fix my heating and hot water problems is to research and understand how my system works and then fix it myself.

  5. marie benn,,,,,make sure your pump is not set too slow 3 being the best setting open all radiator valves or
    thermostat valves fully,bleed all rads, last resort is to flush heating system thro lowest drain plug, let water run
    through a hose pipe till clear lock off lock off drain plug then rebleed rads,,,that should clear your problem

  6. The problem with the boiler overheat stat is a result of no pump overrun. The pump should continue to run for a short while after the boiler burner has shutdown. The overrun is designed to dissipate the heat safely and prevent the overheat stat from tripping. It should be possible to wire the pump through the boiler to support the overrun requirement.

  7. I,m told a WM Baxi solo 50/4 RS s s can be used SAFELY without the over
    heat thermostat being operational, is this correct. Look forward to your reply Ali Clark PS My system has a header tank in loft and gravity fed hot water.

  8. Hi Ali,

    This blog is not the place to get that sort of advice I am afraid. If I have understood your question correctly I don’t think anyone of any appropriate authority (e.g. a Trained & Certified Gas tradesman e.g. on the Gas Safe Register if in the UK) would recommend running a system without the safety features on and working! And if I was your neighbour I would not be happy about it either!

    The main conclusion of this post was a procedure for re-enabling the boiler after it had overheated (in a sense; how to manually fudge the controls so that the boiler was not too hot any more)… which is to say that the Overheat sensor and system are still very much intact on my boiler!

    With Best Wishes,
    Nij / KSB

  9. Hi KSB thanks for your reply, when my baxi was fitted about 25 years ago was told that the overheat stat can be a nuisance sometimes but can be bypassed if it becomes a problem as there is another safety stat in the boiler. This has not been a problem as only happened very rarely, seemed to be when setting was above 3 setting which is rarely used, house is kept warm enough on 2 to 2!/2 in winter. Thanks again Ali c

  10. The basic premis of understand the problem is very true. The other is it used to work (obviously not applicable to a new install).

    So my question would by why is the lack of bypass a problem now after changing the controller when it wasn’t before?

  11. Reading this as am trying to understand why our Vokera combo keeps switching itself off with error code A07.New thermistor has not solved problem.Differential has been mentioned by Vokera engineer.Should we have this done?Does it involve a lot of work?

  12. Hi Alison,
    I’m afraid that I am literally unable to comment on your situation as I am not an expert nor do I know the details of your situation! That said, my advice is to try and make the problem reproducible (assuming that it is easy to clear the error you mention and does not have other concerning symptoms). If you can do that, and then demonstrate that to an engineer then I imagine that will help them analyse the correct fix for you.
    Best of luck in getting the problem solved!

  13. My Aniston boiler of 3 years old cuts out before all the radiators are
    warm and the return pipe to the boiler is not really that warm.

  14. Hi Jack, I am not an expert. That said, I do wonder if the issue might be your pump (or general lack of flow of water around the system) considering your comments about the rads and return pipe not being warm.
    Best of luck in resolving your problem,

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