Quote Originally Posted by kevinsmart View Post
Not quite. You will see this with OpenTherm and no heating valve when the low load algorithm comes into play. (I seem to keep repeating myself on this).

The algorithm sends a >10C flow temperature request (on) followed by a 10C request (off).
Which "low load algorithm" is this ? Is this something new in the Betas ? I can't see any reason why OpenTherm would keep switching the boiler off and on in a partial load situation. The whole point of OpenTherm is to just ask for a specific flow temperature and not cycle on and off at all. The boiler may have to cycle of course if it can't modulate low enough but that's a separate issue.
Typically the boiler in these low load situations will reach 5C above the flow temperature request and cut ignition. It will start to anti-cycle but the 10C off request followed by another flow temp request can interrupt this and result in further heating if the flow temp has dropped.

The OpenTherm Monitor plots clearly show this behaviour.
Are you certain the OpenTherm signal from the Evohome is causing this ? Because it sounds like the boiler is just cycling due to the load being too low. Perhaps the Evohome's OpenTherm implementation monitors the reported flow temperature and if it goes too far above what it is requesting it cuts the request ? Effectively duplicating what the boiler should already be doing itself ?