DIY OpenTherm Weather Compensator for Honeywell evohome

[DBMandrake]

That's actually incorrect. If you refer to the "Weather - compensation adjustment" section in the Intergas manual you'll see "The maximum supply temperature (Tmax) is set via the temperature display".

An OpenTherm thermostat can request a higher flow temperature than Tmax (as Evohome does). The boiler will of course limit it to Tmax as set via the temperature display.

The issue is that the Intergas boiler does not report the max CH water setpoint (Msg ID=57) as Tmax, in fact the HRE SB does not support this message at all. Evohome then assumes the max CH water setpoint is 90C and proportionally reduces down from there.

I think it would be useful for a thermostat to know the max CH water setpoint, I think that was DBMandrake's point.

Yes that was my point.

Ultimately the user should be in control of deciding the absolute maximum flow temperature, regardless of whether weather compensation is in use or not. If the OpenTherm standard allows the boiler to communicate the user specified max flow temperature setting on its control panel to the thermostat via Msg ID 57, then it should be doing this.

From the testing you've done Evohome seems to be following the specification appropriately by acting on Msg ID 57 if it is present and using that to scale it's heat demand proportionally, so it seems to be Intergas dropping the ball IMHO by not sending that information.

Although on the other hand it would probably be helpful if the Evotouch allowed for a manual override of this value as well for those boilers that don't send it, and perhaps even built in weather compensation. Perhaps I've read too much into the post in the Evohome 2020 thread but it sounds like some sort of weather compensation might be coming in a future firmware update...

In principle it wouldn't be hard to implement in a software update - the Evotouch already gets the local temperature via online weather report and displays it (but does nothing else with it) and that could be used to modify the heat demand for an Opentherm boiler, and while they're at it they could chuck in a manual max flow temperature setting as well. It would sure solve a LOT of compatibility problems.

Who knows, maybe they could even add back in the ability to bind the (admittedly very over priced) outdoor temperature sensor which had support dropped in the Wifi model... nah, that will never happen....

[DBMandrake]

What you're talking about in the part I've highlighted only applies to systems equiped with on/off thermostats, but not OpenTherm systems, because, an Intergas boiler (and most others) connected to an OpenTherm thermostat, ignores its own internal weather compensation curve and relies on whatever flow temperature the thermostat sends it, weather compensated or not.

If an OpenTherm thermostat's job is to tell the boiler what flow temperature & max flow temperature to run at, then what purpose does it serve for the boiler to inform the thermostat of its (now unused) internal max CH flow temperature?

As you say, Evohome "assumes" a max temperature of 90C because it hasn't a max CH flow temperature parameter to limit itself by.

Opentherm is a two way protocol. The boiler can send a lot of information to the thermostat, it's not just about the thermostat telling the boiler what to do. One of those things is a maximum allowed flow temperature setting as input by the user on the boilers control panel.

This can allow the thermostat to make a more informed decision about what range of flow temperatures to request based on the maximum flow temperature that the boiler says should be used and the current heat demand from the zones. Allowing the thermostat to scale its heat demand instead of just using a full range up to 90C and then having the boiler clip it at a much lower temperature is preferable from a control point of view.

Of course you could add a maximum flow temperature setting in the thermostat too, and just ignore what the boiler does or doesn't send but I would consider that a workaround when the OpenTherm standard clearly allows the boiler to send this information to the thermostat, despite some boilers not bothering to implement it.

For the average user without weather compensation who just wants to adjust their maximum flow temperature with the seasons, turning the traditional knob on the boiler makes so much more sense than burrowing into the installer menu on something like an Evotouch. Why should that knob on their boiler loose its ability to function (or function inappropriately by only applying clipping) just because they use Opentherm instead of a relay ?

Something lost in this discussion too is that OpenTherm was developed by Honeywell in the first place and then released for more general use. So I tend to give Honeywell the benefit of the doubt that they might actually be implementing it properly and that boiler manufacturers often aren't.

Unfortunately it looks like they may eventually have to add a maximum flow temperature setting in the Evotouch to work around boilers that can't be bothered sending the necessary information.

[blowlamp]

I don't think it's boiler manufacturers that can't be bothered to implement a maximum flow temperature message, I think it's just the case that it makes more sense to have the whole heating requirement calculation done in one place, that being in and by the thermostat.

If we're talking about a weather compensated OpenTherm setup, then the thermostat is guided by its heating curve, which for comfort, might require an upper and lower CH flow temperature limit of say 65C & 25C respectively. If the thermostat had to obey the boilers internal max setting of, say 80C rather than its own, then I don't see how a predictable heating curve could be arranged, particularly if that 80C figure could be randomly changed.


Just some loosely connected info about my Intergas/iSense OpenTherm setup:

When first installed, I set the DHW on the boiler to 45C. Running the hot tap some time later I found the water to be much hotter so I rechecked the boiler setting and found it to be 60C. Thinking I'd not applied the setting, I adjusted it back to 45C on the boiler and ran the tap to check the temperature. It was back to 60C again!
Long story short was that the boiler DHW setting was being overridden by the iSense setting.

This at first seemed back to front, but seeing how boilers end up in garages and lofts etc, using the thermostat does make for a very convenient way to tweak boiler settings.