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Building an OpenTherm weather Compensator for Honeywell evohome

We fitted Honeywell evohome in our last house and it remains one of our much-loved smart home heating systems. now Automated home reader Kevin smart has created his own diy weather Compensator for evohome…

This project adds weather compensation support to an Evohome OpenTherm Intergas setup by modifying the OpenTherm Gateway photo firmware.

I was frustrated that both the Intergas HRE SB boiler and Evohome controller neglect 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.

The Hardware

I bought an OpenTherm Gateway, Soldering Service, Enclosure and FTDI cord from nodo-shop.nl out of curiosity, to monitor communication between the Evohome R8810A OpenTherm bridge and boiler. It turns out that the bridge does read the outside temperature from the boiler. Also, 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.


Basically, the Evohome OpenTherm bridge calculates the control setpoint temperature from the aggregated height percentage demand from all zones, and this is ranged within any received Max CH water setpoint. By default, with my boiler, the Max is 90C despite the maximum CH temperature being set to 60C at the boiler front panel. The boiler will clamp the maximum to the boiler setting though e.g. 60C. For domestic hot Water heating I have a DHW priority setup which makes sure the boiler fires at 70C overriding the control setpoint, so this will be unaffected by weather compensation.

The consideration was which weather compensation method to use:

1. get the OpenTherm Gateway to modify the Max CH water setpoint based on a heat curve calculation, or 2. Cap the control setpoint based on a heat curve calculation

The difference will be that for 2) a % demand will be mapped to a consistent temperature if that temperature is below a weather compensated maximum. but for 1) the % demand will map to a different temperature based on the outside temperature, lowering as the outside temperature increases.

As an experiment I used the Opentherm monitor application to modify the Max CH setpoint. immediately the control setpoint was seen to minimize proportionally, but I found that some of the temperatures were likely to be too low for my system, so I selected method 2) Cap the control setpoint.

Now in terms of weather compensation calculation I made a decision to carry out the default heat curve A in the Intergas manual.


Since the photo microcontroller does not support division or multiplication, only bit shifting, I simplified this equation Y = 25+(25-X)(80-25)/(25+7) = 67.97-X1.72 to 68-X*1.75 = 68-(X+X/2+X/4), which is only two ideal rotations, two additions and a subtraction.

We need to manage below zero outside temperatures (as two’s complement) adding a little a lot more complexity to the photo assembly code, the first time I had written assembly code in lots of years!

movfw byte3 ;Outside temperature integer
movwf TMax ;TMax = outside
clrc ;Support -ve temperatures by shifting in 1 rather than 0
btfsc byte3,7
rrf TMax,F ;TMax = outside/2
addwf TMax,W ;W=outside+outside/2
clrc ;Support -ve temperatures
btfsc byte3,7
rrf TMax,F ;TMax = outside/4
addwf TMax,W ;W=outside+outside/2+outside/4
sublw 68 ;W=68-(outside+outside/2+outside/4) based on Intergas default heat curve
movwf TMax ;Calculated max setpoint

This code is called around once per minute when the gateway fetches the outside temperature (MessageID27).

Then all there is left to do is to make sure that a control setpoint temperature write does not exceed the calculated max setpoint temperature, by adding some code to the MessageID1 handler, which already supports overriding the control setpoint:

btfss byte1,4 ;WriteData request?
clrf controlsetpt1 ;Clear any previous override
clrf controlsetpt2
movfw byte3
subwf TMax,w ;If setpoint is higher than TMax, clamp it at TMax
skpnc ;In photo for sub, C flag is opposite to what I would usually expect!
movfw TMax
movwf controlsetpt1


For efficiency I have set my boiler’s maximum central Heating temperature to 60C, by means of its front panel, so this will be the maximum flow temperature even if a higher setpoint is requested. With this setup, the max flowTemperatura zostanie zminimalizowana tylko poniżej 60 ° C, gdy temperatura zewnętrzna jest powyżej 5C. Evohome oczywiście dodatkowo zminimalizuje temperaturę, gdy procentowe zapotrzebowanie ze stref zmniejszy się, a kocioł odpowiednio się moduluje.

W ciągu najbliższych kilku dni monitorowałem obsługę przy użyciu oprogramowania diagnostycznego monitora opentherm i intergas. Nastąpiła duża huśtawka w temperaturze zewnętrznej między 9c (maksymalny punkt zadany 53C) i -1C (maksymalny punkt zadany 72C, ograniczony przez kocioł przy 60 ° C), a opcja wydawała się działać dobrze. Większy test będzie, gdy zmierzymy się na wiosnę, a średnie temperatury wzrastają.

Temperatura zewnętrzna -1C, zastąpienie zadań kontrolnych 72C, kotła 60C

Temperatura zewnętrzna 5C, Zastąpienie zadań kontrolnych 60C

Więc masz to, co było dla mnie najlepszym projektem świątecznym!

Kevin można się skontaktować pod adresem – Kevin Smart INMAME com

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Operacja pojedynczej strefy Honeywell Evohome

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Ostatnia aktualizacja 2021-10-04 / Linki afiliacyjne / obrazy z interfejsu API reklamowego produktu Amazon

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