Ariston 27MFFI - Central Heating on when not required

[helpmeout]

Recently my Ariston 27MFFI has started to supply heating to the radiators when not required, even when the programmer is completely removed!
The boiler periodically fires and heats up the water in the heating outlet pipe to a very high temperature. Some of this heat is reaching the radiators (even though the routing valves have not been opened by the controller). The boiler does not go out on overheat, the flame just shuts down and refires again later. Meanwhile the circulation pumps runs continuously. The only way I can stop this happening is to turn the boiler off completely. If heating is requested it works normally as does hot water. Can anyone please suggest why this might be happening and how to fix it

 

[Rob]

I dont know what routing valves opened by a controller means but its possible the HW microswitch is in NO position?

 

[Agile]

It sounds as if the DHW demand switch is stuck on for some reason.

If this is the case it would not work properly on CH if you requested it.

Tony

 

[helpmeout]

Not sure if this will clarify further but as I understand the boiler was installed (professionally for the previous owner about 6 years ago) to supply two radiator circuits controlled by a stand alone Drayton programmable controller (the timer on the boiler itself is not being used). One channel on the programmer (labelled heating) controls on/off times for the main heating circuit, the other channel (usually used to control hot water and labelled as such) controls on/off times to a second circuit (radiators in airing cupboard and two bathrooms). Each channel operates a routing valve at the point where the heat flow out of the boiler and splits to the two circuits, and presumably each channel sends a signal to the boiler to request heating during the 'on' cycle. To eliminate a permanent 'on' signal I completely removed the controller from its backing plate, but still the boiler fires constantly. Presumably hot water supply is triggered purely by a pressure drop in the hot water outlet circuit and is in permanent stand by mode. I had considered a sticking DHW demand probe/switch but am having trouble identifying it. Can anyone clarify exactly where it is located and if possible how it functions?

 

[Gasguru]

Look behind the gas valve - you will see a plastic switch assembly. with a small mirror you may find the switch is closed despite no hot water demand. If so the diverter is seized. Unhook the wiring to this switch to confirm.

 

[dal5band]

I think the trade calls them "zone" valves. So you have two zone valves controlled by a programmer. As you say each zone valve sends a "live" to the boiler. Normally these valves are wired with a permanent direct live connection and when the programmer turns the valve on, the valve operates a microswitch which connects the permanent live through to the boiler. The valve acts as a relay.

So it may be that one of the valves is stuck open and hence is connecting its permanent live to the boiler even if you remove the programmer. Check the lever on the side of each valve and make sure it is not locked in the on position.

PS No sure if a combi boiler would need a by-pass to get rid of excess heat called an overun.

 

[gas4you]

PS No sure if a combi boiler would need a by-pass to get rid of excess heat called an overun.

All modern boilers need a bypass of some sort!

 

[Agile]

This one does!

The OP seems to have his own ideas of what he thinks is wrong rather than listening to the experts.

He would be better off looking for the DHW demand switch!

As a clue he could look just left of centre.

But even if he finds it will he be able to know if its working or stuck ?

Tony

 

[helpmeout]

Thanks for the help so far. Latest update, zone valves seem to be fine, are not sticking and test meter checks electrics out OK, so as advised am focusing back on DHW switch/probe. Just left of centre there is a microswitch in a black plastic box on the end of the diverter valve, and below it a two pin rubber covered probe in the hot water outlet pipe. If I disconnect the latter probe connection the boiler fires immediately. On reconnecting it shuts down immediately. I presume this is the DHW flow sensor which on the basis of this simple test seems to be working. Returning to the microswitch, when removed I can confirm that the switch itself functions using a continuity tester (depressing the microswitch directly or gently inserting a matchstick through the spindle gland to operate the plastic rocker completes the circuit). I am concerned however that when in situ the switch does not seem to be functioning (completing the electrical circuit) perhaps suggesting that the diverter valve spindle is not moving as far as it should. I'm not sure I am personally any nearer to understand why the boiler is still continuing to fire randomly, but perhaps this extra info. may enable one of you to pinpoint the problem.

 

[Gasguru]

You appear to have the version with a hot water sensor fitted.

The circular plug you disconnected is the hot water sensor.

The rectangular box containing the microswitch is the hot water flow switch. This is arrowed in the image. From your tests the switch appears to be OK. With the switch unclipped from the diverter check the pin fully extends and retracts as the hot water tap is opened/closed.

You may find the pin is sticking and not fully retracting. With the switch assembly disconnected from the diverter the boiler should not run.

 

[DP]

Guys, have been following this thread with interest.

Boiler can run in CH mode when HW demand switch is clear. Await feedback to see where this post is headed before I put my little bit in.

 

[helpmeout]

Whilst investigating the dhw switch on the end of the diverter valve again I think I may have inadvertantly stumbled across the original problem and simultaneously (possibly) discovered another. As I was removing the dhw switch I could here a relay clicking in and out as I disturbed the adjacent wiring loom. Further investigation pinpointed the other black rubber covered probe on the front of the diverter valve which I assume is the heating sensor (it can be seen on the left of the image posted - thanks Gasguru). It became apparent that the terminal pins were heavily corroded and any slight movement broke the contact. Having cleaned the pins and socket it now seems electrically sound and yippee the boiler has (at the time of writing) stopped randomly firing. However, now the bad news, the DHW switch is no longer making contact so no hot water. On measuring the pin it projects approximately 6mm when there is no HW demand and about 11mm when HW is requested. This is not quite enough however to close the DHW switch (takes about 12mm to do this). A small 1mm thick piece of plastic glued to the face of the plastic rocker that the pin contacts has (temporarily) solved the problem but do 6mm and 11mm pin measurements suggest a diverter valve fault or are they normal?