Ideal Logic combi cycling when CH hits 50

[fronty]

Hi, my Mum's Ideal Logic combi keeps cycling when the temp reaches 50 degrees despite the CH control set to max. The manual says the flow temperature should reach 80 when set to max but I can actually hold the CH feed pipe with my hand and it never gets hot enough to cause me to let go before the boiler cuts the burner, then you see the temp drop on the front panel. The boiler will then fire again after a few mins, the temp readout increases until it hits 50, then it cuts out again. The radiators and pipework only ever get slightly warm. The boiler is 8 years old so was wondering if it might be a dodgy thermistor. Anyone got any ideas?

 

[fronty]

Ok so I've been looking into this more, it seems like the boiler is trying to do something clever here.

The roomstat is a wireless "PRT3" unit, and what I have found is that when the heating comes on in the morning the rads get nice and hot and the house warms up nicely, all indications on the boiler are good with the temperature readout on the boiler LCD showing the CH rad temp at between 70-80 degrees.

The problem seems to happen later on in the day after the house has reached temperature. When the ambient temp falls to 0.5 degree below the roomstat setting, it calls for heat and this is when the rads don't get warm enough and so it takes hours for the ambient temp to get back to what is set on the room stat (and the CH radiator temp on the boiler LCD fluctuates between 45-55 degrees) with the burner constantly cycling and the pump running constantly (with associated humming going on for hours).

So it seems that if the temperature differential is quite small between the ambient temp and what is set on the roomstat then the boiler must be "trying" to run efficiently by only putting a little bit of heat into the rads, but if the differential is bigger then it'll put more heat into the rads.

What my Mum has started doing is raising the setting on the room stat to get some heat into the house, then she has to turn it back down once she is comfy. This seems completely counterintuitive to me. Does anyone know if it's possible to override this behaviour? I want the boiler to stop playing silly buggers, but some heat into the house then switch off when the room stat has got up to temp.

I have tried setting the CH temp dial on the boiler to max but this doesn't seem to make any difference. I even filmed a little video of the boiler display which you can watch here:

 

[John.g]

Are you saying that with a 0.5C differential that the boiler flow temperature SP (setpoint) is ~ 45C but if you increase the differential that the SP increases or is this reading just the flow temperature read out?.

 

[fronty]

Yes, 0.5 differential and you get a message on the boiler saying the set point 45 degrees is controlled by room stat (note there are no settings I can find on the room stat that let you adjust that) and you can hold onto the CH feed pipe without it getting too hot to touch. If the differential is bigger you don't get any message on the boiler about the set point and CH feed pipe gets too hot to touch.

Would be nice to find a setting to turn off this set point "feature" but I've been through the boiler manual and can't find anything.

 

[John.g]

All gas fired boilers fire up at 70/80% of their rated output and then modulate to the required temperature, its more than likely that the flow temp is exceeding the SP by 5C during this period and causing all the cycling especially if the boiler gets away if you increase the differential by raising the roomstat setting which in turn raises the boiler flow temp SP? If this Opentherm was cute enough it should temporarily increase the requested SP and then reduce it gradually to the SP. There was a thread something like this and the boiler was doing something similar but by changing to some other form of control, the above was done as far as I can recall.
If you can't change the 0.5C differential then suggest increasing the anti cycle time which may have the same effect of increasing the differential but of course won't give as tight room control.

E. OPENTHERM F. BUILT IN FROST PROTECTION G. LOCKOUT RESET ADVANCED FEATURES The unit uses the OpenTherm protocol to control the boiler flow temperature set point in central heating operation. This means that the boiler will run in its most efficient manner and offer significant savings in energy over standard room thermostats. It will also give more accurate and comfortable temperature control in the room where it is installed. If the room temperature is close to its setpoint then the pump may run for extended periods without switching the burner on. This is normal operation which reduces gas consumption whilst maintaining the desired room temperature.

 

[fronty]

Ah, opentherm, that sounds like what's happening here. I've done a quick google and can't find any adjustments that can be made, will keep searching.

 

[John.g]

The boiler menu should tell you the pump overrun time and the anti cycle time. What roomstat differential do you have to have to set to allow the boiler to fire up successfully and did you notice the controlled SP then and the actual flow temperatures?.
I would think that you would have to get the return temp down to practically the room temperature to get the boiler to stay on with a controlled SP of 45C.
You video also show various CH temperatures whatever they mean.

 

[John.g]

This is the link to that post but if no time to read it basically the OTC (outside temperature compensation) was removed and the roomstat was then calling for a 50C SP on start up which this boiler could handle apparently alright.

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