I have a fully pumped CH/HW system with a room thermostat, a cylinder thermostat (HTS3) and a mid-position actuator (Drayton MA1). Recently I noticed that the boiler kept cutting out even though the pump was still on and, after further investigation, it appears that the hot water is never satisfied and that the boiler was just cutting out as it was getting too hot. The CH on its own is working OK, the actuator appears from the outside to be working OK, i.e. the levers are moving to W, M, H as appropriate. I tried to turn the cylinder stat 'off' but the dial wouldn't turn so I assumed the stat was faulty and replaced it. This has made no difference. On the new stat, the dial can be turned to off and this will switch off the boiler and pump. However, even with it set at its lowest temp (50C), and after the HW has been on for 50 mins and is piping hot, turning the programmer on to HW always calls for it and it is never satisfied. I am trying to eliminate the actuator as a problem so does anyone know if the 'off' position on the stat dial is the same as 'satisfied' or does it work differently? The actuator is only 15 months old. Any ideas on what might be wrong? This set-up has worked perfectly well until now so it is unlikely to be wrongly wired, for example.
"Recently I noticed that the boiler kept cutting out even though the pump was still on"
That's how it should work, just because the boiler is at temperature doesn't mean the hot water is, the pump will continue to run until the cylinder stat is satisfied, it'll be the boiler stat cutting the burner off.
"However, even with it set at its lowest temp (50C), and after the HW has been on for 50 mins and is piping hot, turning the programmer on to HW always calls for it and it is never satisfied."
Is the water at 50C ? use a thermometer if it's at 50 or above it could be another faulty cylinder stat, if it's not at 50C then it's just doing it's job.
Thanks for replying - after watching exactly what the boiler is doing when it stops, I now think this may be an ignition problem. When the boiler failed to re-light previously it was quite hot but I have now watched it cut out when the CH thermostat has been satisifed (as expected) and fail to re-light once the temperature dropped and the thermostat called again. The boiler was quite cool and repeatedly failed to light. Turning the electricity supply off and on seemed to clear the problem (temporarily). Does this sound like an ignition problem?
Since if you turn power off and on again it works OK I will guess on micro switches in motorized valve. As when power switched it will default to domestic hot water only. If htg engineer answers go by what he says but I have had problems with the mid position valves in the past the plan only shows one switch but when you open one you find three.
If it continuously sparking and failing to light, then yes this would cause the boiler to go to lockout.
There could be a number of faults,
If the boiler lights a pilot, the pilot could be partially blocked - with the fan running the gas that is getting through would be quickly drawn away failing to ignite. Clean the pilot injector and burner should solve this problem.
If the spark electrode lights the burner, the spark gap may need adjusting. The spark may not be strong enough, the ignition lead may have a break in it or the electrode may be cracked. Also the gas pressure may need adjusting, if the minimum gas rate is too low it would light but the electrode may not sense the flame..
When it sparks, does the pilot/burner light ? there will be a flame superision device, if the flame is not detected the boiler again will go to lockout. Could again be gas pressure, faulty electrode or break in HT lead.
If you give me the make and model of the boiler, it should narrow down what the problem could be depending on if it has a pilot etc etc
Thanks for the replies - the boiler is a Myson Orion Fanfare approx 19 years old. It has been regularly serviced. The problem seems to be intermittent - it always fires when the heating and/or hot water first calls during an 'on' period, but then may fail to relight once it has gone off, either because the thermostat has been satisfied or the boiler stat cuts in. When it's working OK, you can see a spark before the boiler fires but there seems to be no spark when it's having a problem. (Difficult to see clearly). The problem does seem to occur more often when the hot water is involved but it has also occured when the heating is on and the water is off. Some days it works OK!
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!