DIY OpenTherm Weather Compensator for Honeywell evohome

[DBMandrake]

This is the wrong conclusion as far as I'm concerned.

As I've pointed out on a few occasions, connecting a more fully featured OpenTherm thermostat to a weather compensated Intergas boiler means that the boiler expects the thermostat to use its own compensation curve as well as its own maximum central heating temperature setting. The boiler just relays the outside temperature reading over to the thermostat so it can calculate a water temperature to send back to the boiler. It's then down to the boiler how it maintains that temperature.

My OpenTherm/weather compensated Intergas boiler & Remeha iSense thermostat combo works perfectly as above, as does the inlaws Intergas/Nest v3 setup configured as an OpenTherm installation.

You seem to be reading and responding to something that I didn't actually say.

I didn't even refer to weather compensation at all. I actually said "is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control". This was in response to Kevin's remark "I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome."

So while Kevin was trying to implement weather compensation, along the way he discovered that the Intergas boiler in Opentherm mode does not send the CH max flow temperature setting from the front panel to the OpenTherm master, (Evohome) however when he intercepted and modified this value from the boiler Evohome did indeed respect this maximum flow temperature requested by the boiler in the flow temperature requests it sent back to the boiler.

For a long time Intergas and Honeywell have pointed fingers at each other but in my opinion this proves that the blame lies with the boiler not sending the CH max temperature setting to the Evohome.

That's all I was trying to say.

[blowlamp]

You seem to be reading and responding to something that I didn't actually say.

I didn't even refer to weather compensation at all. I actually said "is it Evohome or Intergas responsible for the CH flow temperature control not functioning with OpenTherm control". This was in response to Kevin's remark "I learned that the boiler does not report the Max CH water setpoint set by the front panel but overriding this does influence the Control setpoint temperatures sent by Evohome."

So while Kevin was trying to implement weather compensation, along the way he discovered that the Intergas boiler in Opentherm mode does not send the CH max flow temperature setting from the front panel to the OpenTherm master, (Evohome) however when he intercepted and modified this value from the boiler Evohome did indeed respect this maximum flow temperature requested by the boiler in the flow temperature requests it sent back to the boiler.

For a long time Intergas and Honeywell have pointed fingers at each other but in my opinion this proves that the blame lies with the boiler not sending the CH max temperature setting to the Evohome.

That's all I was trying to say.

The article linked to at the start of this thread has some incorrect information in it that I have applied bold text to below.

Evohome Article:

I was frustrated that both the Intergas HRE SB boiler and Evohome controller ignore the outside temperature reported by a connected external sensor when using OpenTherm. The Intergas manual states…

The outside temperature sensor can be used in combination with an on/off room thermostat or an OpenTherm thermostat.

In fact, the boiler only performs weather-compensation adjustment when using an on/off room thermostat. With an OpenTherm thermostat such as Evohome, the thermostat is responsible for any weather compensation. Although the Evohome controller fetches the outside temperature from the Internet it does nothing with it other than displaying it.

I was trying to point out that the Intergas boiler in OpenTherm configuration ignores its own internal max flow setting as it is expecting to get that from the thermostat - in this case Evohome. I can't see what purpose it would serve to send this figure to the thermostat knowing it will be ignored in favour of the one calculated by the thermostat itself, can you?

In my experience, both iSense and Nest V3 OpenTherm thermostats require the installer to enter a max CH flow temperature to be used by the boiler, but I understand Evohome does not, it instead needs this to be set on the boiler itself, which is turning out to be problematic for many as Evohome seems to have no inbuilt limit to max CH flow temperature and many boilers besides Intergas expect an OpenTherm thermostat to decide what the max CH flow temperature should be.

[kevinsmart]

Yes confirmed, no LEDs are lit.

Are you seeing the "Thermostat disconnected" message at any time?

I have seen this happen. There was a period last week when RF interference appeared to affect Evohome and my Smart Meter display. This resulted in my OpenTherm Bridge intermittently disconnecting leading to the OTGW taking over, transmitting messages.

Now if you have used the "CS=temperature" override it would be sent to the boiler.

The other situation is if a line short is detected during this period. A maximum central heating setpoint would then be sent.

The internal maximum central heating setpoint value starts out at 90 degrees. If the boiler has specified different boundaries via MsgID 49, the provided upper boundary will be used.

So I'm wondering if somehow, perhaps due to incompatibility, or a connection issue, the OTGW is changing mode and you are then seeing setpoint override?

[kevinsmart]

The article linked to at the start of this thread has some incorrect information in it that I have applied bold text to below.

I was trying to point out that the Intergas boiler in OpenTherm configuration ignores its own internal max flow setting as it is expecting to get that from the thermostat - in this case Evohome. I can't see what purpose it would serve to send this figure to the thermostat knowing it will be ignored in favour of the one calculated by the thermostat itself, can you?

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.

[blowlamp]

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.

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.

[bruce_miranda]

If the Intergas never gets to these crazy 80+C max because of the Boiler overriding what the Evohome is trying to reach, then what's the problem. Isnt the Intergas then operating like all the other boilers, besides Viesmann. Viesmann is so far the only boiler we know that doesn't limit itself and blindly tries to get to whatever Evohome is telling it.

[bruce_miranda]

Are you seeing the "Thermostat disconnected" message at any time?

I have seen this happen. There was a period last week when RF interference appeared to affect Evohome and my Smart Meter display. This resulted in my OpenTherm Bridge intermittently disconnecting leading to the OTGW taking over, transmitting messages.

Now if you have used the "CS=temperature" override it would be sent to the boiler.

The other situation is if a line short is detected during this period. A maximum central heating setpoint would then be sent.

The internal maximum central heating setpoint value starts out at 90 degrees. If the boiler has specified different boundaries via MsgID 49, the provided upper boundary will be used.

So I'm wondering if somehow, perhaps due to incompatibility, or a connection issue, the OTGW is changing mode and you are then seeing setpoint override?

I am going to try and connect the OTGW up again because you have now cast a doubt about the order in which I connected things.
BTW what colour does the light on your Honeywell OTB blink when you have the OTGW sitting on the bus?

[kevinsmart]

I am going to try and connect the OTGW up again because you have now cast a doubt about the order in which I connected things.
BTW what colour does the light on your Honeywell OTB blink when you have the OTGW sitting on the bus?

There is no difference on the OpenTherm Bridge side, its green LED flashes every 10 seconds or so.

On the OTGW, in Gateway mode, you should see a green LED lit when OpenTherm messages are communicated and a yellow LED lit when there is a CH demand.

No LEDs lit by the OTGW in Monitor mode.

[paulockenden]

Looks like there's two conversations mingled here!

On the OTGW front, I used it (the Russian Orange Pi version) with an Intergas boiler and saw no difference in boiler temps with or without the OTGW connected.

P.

[bruce_miranda]

OK I figured out what might have been my problem. When you start the OTGW the hardware starts up by default in Gateway mode, then when the OTmonitor service starts up, the OTGW is put into Monitor Mode. However by that time, the Gateway has already negotiated parameters with the boiler. To counter this, I started up the hardware with no connections made, made sure the OTGW was in Monitor mode (no blinking lights and log file on OTMonitor showed GW=0) and then connected the wires to the thermostat and boiler. Now the boiler is trying to ramp up to 90C, as though the OTGW is not present.