Vaillant Ecomax Pro or Myson MPE222 problem?

[Ancient Geek]

I've recently noticed a problem with my CH. The sytem is as follows:

1. Owl Intuition control system.
2. Two Myson MPE222 zone valves, one for CH and one for HW
3. Grunfoss pump.
4. Vaillant Ecomax Pro boiler.

The boiler, pump and zone valves are all about 10 or 11 years old.

The Owl Intuition boxes actuate the zone valves and the auxiliary switches in the zone valves turn on boiler demand. The pump is controlled by the boiler.

Sometimes during a heating cycle the following happens:

1. Thermostat cuts in
2. Zone valve actuates
3. Boiler starts.So far so good.
4. At some point before the thermostat setpoint is reached, typically just as the boiler is reaching its max temp set point, the boiler switches off. When this happens there is no error recorded on the boiler. The light on the zone valve is still illuminated, the valve is still open and the pump is still running. If sufficient time passes before the boiler restarts, then the pump shuts down, presumably at the end of overrun. At some point, the boiler starts up again and normally the demand cycle is completed.

My thoughts are that it's either a fault on the boiler or a fault in the zone valve. Since no error is recorded or displayed on the boiler, I'm inclined to point the finger at the zone valve microswitch.

It's a bit tricky to narrow down as it's not a hard fault. I'm going to try to measure the AC on the switched output from the zone valve tomorrow if I can catch it playing up, but in the meantime, what's the general consensus?

Thanks in advance for your thoughts.

 

[John D v2.0]

Are you sure it's not just normal, if the radiator heat demand is lower than the minimum output of the boiler, it will have to switch on and off to avoid overshooting the selected water temp.

 

[Ancient Geek]

Thanks for the reply, John. I've been observing the boiler (bl**dy cold standing in the garage) and think you're partly right. Towards the end of a heating period, when a number of radiator TRVs have closed, the boiler has modulated down and a pretty stable boiler temperature is achieved. However, I've noticed that when the boiler is trying to work harder there is an oscillation in the burner level with a period of a few seconds, and I think that this is resulting in an overshoot. It's like a control feedback loop without sufficient damping.

So it's not when the demand is lower, but when it's higher that I have the problem.

 

[Jajodu]

Press the ibutton on the boiler when it happens, whats the s code shown?

 

[Ancient Geek]

Press the ibutton on the boiler when it happens, whats the s code shown?

Nothing is shown other than the mode and temperature setting, just as in normal operation.

I can now say that it seems like an intermittent boiler issue. I just observed a cycle where the problem did *not* occur.

The displayed temperature started at 49C and the boiler commenced at about half power as it maintained the maximum differential between flow and return. As the temperature increased towards the maximum power increased slowly. There were slight variations in power, but nothing significant. The indicated temperature reached the maximum, and as TRVs started to close off the power level decreased as expected.

This is in marked contrast to anomalous operation, when as the setpoint and maximum power are approached, the power output and indicated temperature oscillate with a period of about 5 to 8 seconds and an overshoot results, although not a sufficiently large overshoot as to trigger a fault code.

The only noticeable difference between now and when the problem was happening is that the ambient temperature in the garage where the boiler is located is higher now: 9C versus 4C yesterday.

 

[Ancient Geek]

Update:

Well, that's odd. The CH came on again and I noticed that after about 8 minutes the pump stopped. I just got to the boiler in time to see that the front panel showed CH demand, the fan was running, but the burner was not lit. There was no error shown, and the boiler temperature was about 40C. After a few seconds, the fan stopped. About 10 seconds later, the boiler started up again. Unfortunately I was not standing by the boiler to see what the temp was at the time it shut off.

 

[John D v2.0]

Without a lot of test equipment, the other approach is to replace components after a guess, starting with the cheapest. I think you're ahead of anyone here in diagnosing it

 

[Ancient Geek]

If I identify anything I'll post back here. I hate swapping stuff out willy nilly.

Edit: I decided to brave the cold in the garage and observe the last cycle.

Boiler started normally, it got up to 52C and 6 out of 8 bars on the output indicator when it went off with a soft pop. Demand indicator still on. Fan speed increased to maximum. Pump went off. Fan went off after 20-39 seconds. After a further 20-30 seconds the system started up again. I've downloaded the service and installation manual and I'll see if I can monitor what's going on on the main board. I have, and know how to use, quite a bit of electronic test and measurement kit...