OpenTherm Boilers and Controls
Shown below are a list of OpenTherm capable / compatible manufacturers and boilers. These OpenTherm Boilers / OpenTherm capable boilers have integrated OpenTherm unless specified otherwise in brackets. For more info on “What is OpenTherm?” at the OpenTherm Association. You can also see our OpenTherm Capable Controls shown further down the page.
For homeowners: what OpenTherm means see the following page: Modulating ‘A Rated’ Boilers with Smart Modulating Thermostats
What is OpenTherm?
Wikipedia: OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between a central heating boiler and a thermostatic controller. A two way communications interface, usually in the form of a slave and master device.
As a standard, OpenTherm is independent of any single manufacturer. A controller from manufacturer A can in principle be used to control a boiler from manufacturer B. However, OpenTherm controllers and boilers do not in fact always work properly together. The OpenTherm standard comprises a number of optional features and some devices may include manufacturer-specific features. The presence or absence of such features may impair compatibility with other OpenTherm devices.
OpenTherm History?
OpenTherm was originally created by Honeywell and was sold to the OpenTherm Association on November 1996 for one British Pound. This was no doubt to make the standard independent of any manufacturer.
Modulating Control
Unlike room thermostats that simply tell the boiler when to turn on and off (non modulating graph below) , a modulating control (see modulating graph below) tells the boiler the exact target / set point temperature that is required to be achieved. So as the current temperature starts getting closer to the set point temperature, the boiler starts reducing the flow temperature to match the required demand.
Image Above: Standard On-Off type boiler / controls
Image Above: OpenTherm / Modulating boiler with modulating controls.
OpenTherm Advanced setups
OpenTherm is easier to use on combi boilers. The reason for this is that there is no hot water cylinder to worry about, heating to the correct temperatures as required by legionalla prevention(recommended 60 degrees), as OpenTherm can reduce flow temperatures to below 60 degress. So, on system and regular boilers that have opentherm, in order to also work with hot water cylinders, cylinder temperature reporting must be present in some form. This allows the OpenTherm to fire at high temperature just for the hot water. Note, is is advised that in this setup there is priority to the hot water system, otherwise higher temperatures may reach the radiators when you don’t want them to.
OpenTherm Compatible Boilers
For Manufacturers: If your boiler model is not in this list, please contact us via our contact form or via twitterAdditional Information
Vaillant Ecotec 4XX Series requires additional steps. Please see following thread: https://www.automatedhome.co.uk/vbulletin/showthread.php?4715-Evohome-Opentherm-v-Evohome-Boiler-Outside-Weather-Compensation&p=28823&viewfull=1#post28823
| Brand | Model | Additional Info |
|---|---|---|
 | Alpha Heating Innovation https://www.alpha-innovation.co.uk |
|
| Brand | Models | Additional Info |
| Alpha Heating | E-Tec S | |
| Alpha Heating | E-tec Plus | |
| Alpha Heating | E-tec | |
| Alpha Heating | Evoke | |
| Alpha Heating | Intec GS | |
 | Atag Heating Technology http://atagheating.co.uk |
|
| Brand | Models | Additional Info |
| Atag | iC | |
| Atag | iC Economiser | |
| Atag | iS | |
 | Baxi http://www.baxi.co.uk/ |
|
| Brand | Models | Additional Info |
| Baxi | 100 Combi | OpenTherm is Terminals 1 and 2 on the M2 Low Voltage Terminal Block |
| Baxi | 200 Combi | Open Therm connecter is labelled 'Bus' |
| Baxi | 400 Combi | Open Therm connecter is labelled 'Bus' |
| Baxi | 600 Combi | Open Therm connecter is labelled 'Bus' |
| Baxi | 600 System | Open Therm connecter is labelled 'Low Voltage Controls' |
| Baxi | 800 Combi - 825 830 836 | |
 | Daikin https://www.daikin.eu/ |
|
| Brand | Models | Additional Info |
| Daikin | D2CND 24Kw | Combi Boiler |
| Daikin | D2CND 28kw | Combi Boiler |
| Daikin | D2CND 35kw | Combi Boiler |
 | Ferroli http://ferroli.co.uk |
|
| Models | Additional Info | |
| Ferroli | i25, i29 Condensing Combination Boiler | |
| Ferroli | Bluehelix Tech RRT 24C, 28C, 34C Condensing Combi Boiler | |
| Ferroli | Bluehelix Tech RRT 18S, 28S, 30S Condensing Combi Boiler | |
| http://ferroli.co.uk | ||
 | Ideal Boilers http://idealboilers.com/ |
|
| Brand | Models | Additional Info |
| Ideal | Independent + Combi | Option Kit may be required |
| Ideal | Independent Combi | Option Kit may be required |
| Ideal | Independent System | Option Kit may be required |
| Ideal | Logic Combi+ Combi C | |
| Ideal | Logic Combi C24, C30, C35 | Requires seperate harness kit |
| Ideal | Logic Code Combi | Requires seperate harness kit |
| Ideal | I-mini C24, c30 | Requires seperate harness kit |
| Ideal | Vogue Combi C26,C32, C40 | |
| Ideal | Logic + System | (Works with Evohome) |
| Ideal | Logic + Heat | (Works with Evohome) |
| Ideal | Logic Combi ESP1 24, 30 , 35 | |
 | Ideal Commercial https://idealcommercialboilers.com/ |
|
| Brand | Models | Additional Info |
| Ideal Commercial | Evomax | |
| Ideal Commercial | Imax | |
 | Intergas Boilers http://www.intergasheating.co.uk/ |
|
| Brand | Models | Additional Info |
| Intergas | Intergas Rapid | |
| Intergas | Intergas Rapid Plus | |
| Intergas | Combi Compact | |
| Intergas | Compact Range | |
| Intergas | ECO RF | |
| Intergas | Xtreme | Dual OpenTherm |
| Intergas | Xclusive | Dual OpenTherm |
 | Keston http://keston.co.uk/ |
|
| Brand | Models | Additional Info |
| Keston | Combi C30 & C35 | Requires seperate harness kit |
| Keston | System S30 | |
| Keston | Heat 45 and 55 | |
 | Main https://www.mainheating.co.uk |
|
| Brand | Models | Additional Info |
| Main | Eco Compact Combi 25-30 | Same accessories as Baxi. |
 | KD Navien http://www.navienuk.com/ |
|
| Brand | Models | Additional Info |
| Navien NCB | ||
| Navien LCB700 Oil | ||
 | Ravenheat | |
| Brand | Models | Additional Info |
| Ravenheat | HE30S Compact | |
| Ravenheat | HE80 | |
| Ravenheat | HE98S Combi | |
 | Remeha Boilers | |
| Brand | Models | Additional Info |
| Remeha | Avanta | |
| Remeha | Quinta Ace | |
| Remeha | Quinta Pro | |
| Remeha | Gas 110 Eco | Commercial |
 | Viessmann https://www.viessmann.co.uk |
|
| Brand | Models | Additional Info |
| Viessmann | Vitodens 100W Type WB1A (Connection: X3.3 and X3.4) | |
| Viessmann | Vitodens 100W Type WB1B (Connection: X21.1 and X21.2) | |
| Viessmann | Vitodens 100W Type WB1C (Connection: X21.1 and X21.2) | |
| Viessmann | Vitodens 100W Type B1HA, B1KA (Connection: X21.1 and X21.2) | |
| Viessmann | Vitodens 200-W WB2B 26+ 35 kW (via Expansion Module OT and OT-A8 + _Terminal -10 and +10 are on expansion A8 boiler ) | |
| Viessmann | Vitodens 200-W WB2C, B2HA,B2JA, B2LA (via icm Expander OpenTherm) | |
 | Vokera https://www.vokera.co.uk |
|
| Brand | Models | Additional Info |
| Vokera | Evolve C | |
| Vokera | Evolve S | |
| Vokera | Linea One (via OpenTherm Kit Part_1221179) | |
| Vokera | Vision Combi (requires control interface 29450143) | |
| Vokera | Compact A (requires control interface 29450143) | |
| Vokera | Verve_(heating only mode) | |
| Vokera | Mynute i (heating only mode) | |
| Vokera | Vision System (heating only mode) | |
| Vokera | Unica i | |
| Vokera | Vibe | System boiler |
| Vokera | Vision C | |
| Vokera | Linea HE | |
| Vokera | Mynute A | System boiler |
| Vokera | Mynute HE | |
| Vokera | Unica HE | |
| Opentherm Compatible Boilers List Last Updated: | 02/09/2019 |
OpenTherm Compatible Boilers – Unofficially Compatible
The following boiler models, mostly Vaillant and Worcester Bosch are capable unofficially. Using the mentioned adaptors may invalidate your warranty. However boilers that are out of warranty will have no known problems, as these adaptors are purpose made by the companies themselves for some European countries, where governments have made it compulsory to have OpenTherm.
| Brand | Model | Additional Info |
|---|---|---|
 | Vaillant https://www.vaillant.co.uk |
|
| Brand | Models | Additional Info |
| Vaillant | Ecotec Pro [1] | |
| Vaillant | Ecotec Plus_[1][2] | |
| Vaillant | Ecotec Exclusive[1] | |
| Vaillant | Ecofit Pure_[1] | |
| [1] Using un-official VR33 Module | Vaillant OpenTherm and VR33 | |
| [2] Ecotec 4XX Series requires additional steps. | ||
| Glow Worm https://www.glow-worm.co.uk |
||
| Brand | Models | Additional Info |
| Glow-Worm | Betacom 1,2,3,4 | Requires Adaptor |
| Glow-Worm | Easicom 1, 2, 3 | Requires Adaptor |
| Glow-Worm | Energy 35 Store | Requires Adaptor |
| Glow-Worm | Energy Combi | Requires Adaptor |
| Glow-Worm | Essential | Requires Adaptor |
| Glow-Worm | Flexicom | Requires Adaptor |
| Glow-Worm | Utracom | Requires Adaptor |
| Glow-Worm | Ultimate3 | Requires Adaptor |
 | Worcester Bosch | |
| Brand | Models | Additional Info |
| Worcester Bosch | EMS capable boilers [1] | Worcester Bosch and OpenTherm Page |
| Worcester Bosch | Greenstar i [1] | Requires Adaptor |
| Worcester Bosch | Greenstar i Junior [1][**] | Requires Adaptor |
| Worcester Bosch | Greenstar Si Compact [1] | Requires Adaptor |
| Worcester Bosch | Greenstar CDi Compact [1] | Requires Adaptor |
| Worcester Bosch | Greenstar CDi Classic [1]í_í_] | Requires Adaptor |
| Worcester Bosch | Greenstar Highflow CDi [1] | Requires Adaptor |
| Worcester Bosch | Greenstar 12i System - 24i System [1][*^] | Requires Adaptor |
| Worcester Bosch | Greenstar 27i System - 30i System [1][*] | Requires Adaptor |
| Worcester Bosch | Greenstar CDi Classic System [1][*í_í_] | Requires Adaptor |
| Worcester Bosch | Note: Worcester Bosch OpenTherm information is very limited, please accept this list as capable, but not yet confirmed. There is also no confirmation of hot water working with OpenTherm | |
| [1] Using un-official Nefit Adaptor | ||
| [*] Provided optional integral diverter valve is installed | ||
| [**] Manufactured after July 2013 | ||
| [í_í_] Manufactured after 16th January 2007 with software version CF12.10 onwards. | ||
| [^] Manufactured after February 2011. | ||
| Opentherm Compatible Boilers List Last Updated: 24/11/2018 |
OpenTherm Compatible Controls
Following are a list of OpenTherm compatible controls. These controls are compatible with Open Therm Capable boilers shown above. Smart Thermostats are listed first. Where an addition tranceiver is required it will be specificied in the brackets. See more info on OpenTherm at the OpenTherm Association.For homeowners: what OpenTherm means see the following post: What is OpenTherm?
Open Therm fact sheet from Honeywell the original creators of OpenTherm: OpenTherm Fact Sheet
| Brand | Model | Review Page / Info page. | Wired / Wireless | Radiator Valves | Internet Connected | Homekit | Other |
|---|---|---|---|---|---|---|---|
| Baxi | uSense smart room thermostat | N | Y | ||||
| Danfoss | RET2000-OT | Wired | N | N | N | ||
| Delta Dore | Tybox Bus OT - 6053056 | Wired | N | N | N | ||
| Drayton | Wiser Kit 1 | Wireless | Y | Y | N | Heating Only | |
| Drayton | Wiser Kit 2 | Wireless | Y | Y | N | Heating and Hot Water | |
| Drayton | Wiser Kit 3 | Wireless | Y | Y | N | 3 Zone Support | |
| Drayton | Digistat 2290M | Wired | N | N | N | (Bluetooth App) | |
| Drayton | Digistat RF901 | Wired | N | N | N | (Bluetooth App) | |
| EPH Controls | CP4B | Wired (Batteries) | N | N | |||
| EPH Controls | CP4M | Wired(No Batteries Required) | N | ||||
| EPH Controls | RFRP-OT & RF1A-OT (Combi Pack 4) | Wireless | N | 1 Zone | |||
| EPH Controls | CP4-HW-OT | Wireless | N | ||||
| EPH Controls | CP4i | Wireless | Y | ||||
| Esi | ESRTP4OT Programmable Room Thermostat | Wired | N | N | N | ||
| Esi | ESRTP4RF+ Programmable Room Thermostat | Wireless | N | N | N | ||
| Esi | ESRTP6WHW | Wireless | Y | N | |||
| Heatmiser | neoHub Mini with NeoAir | Wireless | N | Y(with NeoHub) | 2 Zones with OpenTherm capable | ||
| Honeywell | T87M2018 | Wired | N | N | N | ||
| Honeywell | T3M | Wired | N | N | N | ||
| Honeywell | T4M | Wired | N | N | N | optional WC curve setting and internal reference - no support for DHW setpoint. | |
| Honeywell | T4R | Wireless | N | N | N | no support for DHW setpoint. | |
| Honeywell | Lyric T6 | Wired | Y | Y | Y | support for DHW setpoint and overnight preheat disable for combi boiler applications. | |
| Honeywell | Lyric T6R | Wireless | Y | Y | Y | Support for DHW setpoint and overnight preheat disable for combi boiler applications. | |
| Honeywell | T6R-HW | Wireless | Y | Y | Y | Support for DHW setpoint and overnight preheat disable, plus timed hot water control for storage combis and system boilers. | |
| Honeywell | Evohome | Wireless | Y | Y | N | Requires R8810 | |
| Honeywell | CM727 | Wireless | N | N | N | Required R8810 | |
| Honeywell | CMS927B1049 Room Unit | Wireless | N | N | N | Requires R8810 | |
| Honeywell | CMT937A1011 | Wired | N | N | N | ||
| Ideal | Logic RF7 | Wireless | N | N | N | ||
| Ideal | Touch Connect Wifi Combi Control 214217 | Wireless | N | N | N | ||
| Nest | Smart Thermostat 3rd Generation | Wireless | N | Y | N | ||
| Nest | Smart Thermostat E 3rd Generation | Product Info | Wireless | N | Y | N | |
| Remeha | iSense | Wired / Wireless | N | N | N | ||
| Salus | RT520 | Wired | N | N | N | ||
| Salus | RT520 | Wired | N | N | N | ||
| Salus | RT520RF | Wireless | N | N | N | ||
| Tado | Tado Smart Thermostat | Wired / Wireless | Y | Y | Y | Requires Extension Kit | |
| Vokera | BeSmart | Wireless | Y | ||||
| Vokera | 20050690 Open Therm 7 Day Programmable RF Room Thermostat | Wired | N | N | N | ||
| Vokera | OpenTherm RF Programmer (20050690) | Wireless | N | N | N | ||
| Last Updated: 27/05/21 |
Credits/ Sources / Public Contributors:
Richard Huish
Automatedhome; HenGus, Bruce_Miranda
Steve Todd
John Lockwood
Patrick McMahon
Information Sources:
https://www.vokera.co.uk/wp-content/uploads/2016/02/BeSMART_Installer_Brochure1.pdf
My boiler is a Vitodens 100-W Type B1HA, B1KA, 6.5 to 35Kw Wall mounted condensing boiler.
It is not on your list of OT compliant boilers.
The installation instructions suggest that it is compliant and I am hoping you can confirm one way of the other.
Thanks
Paul Cracknell
Hi Paul,
We have checked the instruction manual, it clearly shows on page 22, that it supports opentherm. We have updated the list to reflect this. Please note, we cannot state the effectiveness as manufacturer implementations of OpenTherm vary between boiler brands, as has been reported by various specialists, so the performance of the OpenTherm setup may also vary. Please feel free to report your results
Jayesh.
Excellent article! In your ”unofficially compatible” list sits my Glow-worm Ultracom, which you mention will require an adaptor. Apologies if I’ve missed it within the above, but do you have any more info/links to such adaptors? The Climapro2 controller we currently use is starting to play up so I’m looking at alternatives. Thanks, Andy.
Hi Andy,
Have a look at the following article Vaillant Opentherm/, as the Vaillant and Glowworm share the same PCB they use the same adapters. Most likely for the Ultracom is the VR33 0020017895 / equivalent AWB 0020092426 or the AWB OPENTHERM MODULE 0020036768 for certain Glowworm Boilers. Search these parts and look at the inside of your PCB housing to determine which is the correct one. Hope that helps.
Jayesh Kanbi
Hi i have recently installed a Baxi Platinum combi which does not appear on any of your lists? I know this is a modulating boiler but i’m not sure if this means its opentherm compatible.
Can you please confirm if this boiler is able to use opentherm?
Hi, the Baxi Platinum boiler is not OpenTherm compatible. Most modern condensing boilers are internally modulating ie room controls ask a boiler to come on off, the boiler determines it’s own power level. Where as an opentherm boiler is told by opentherm controls what powerlevel it should be at. Hope that answers your question.
Hi,
Do you know whether the Ideal Combi ESP1 35 supports Opentherm ?
Regards,
Patrick.
Hi, Thank you Patrick.
Boiler added to list, credit given to you for help.
Regards,
Jayesh Kanbi
MyBoiler.com
George Arnautu,
Hi, I have a Viessmann boiler, Vitodens 111 – W, type B1LA-26 , and I don’t know whether tthe SALUS thermostat RT520 RF is Opentherm compatible,
Thanks,
George Arnautu
Hi George,
This control definetely has OpenTherm, please see following link. https://salus-controls.com/uk/product/rt520/#downloads , select the quick guide to show you the connection diagram.
Regards,
Jayesh Kanbi
MyBoiler.com
Veissmann vitodens 050 w supports opentherm.
What would be more useful is to know what commands and data boilers support as it seems the spec allows for a lot of data which not all boilers support.
Hi there. Just moved in to my new flat. How can I find out if worcester greenstar 28i junior is opentherm or not ???
Hi Mickey, it’s not OpenTherm out of the box. There does appear to be a work around , however not widely tested. See the following article https://myboiler.com/opentherm/worcester-bosch-opentherm/ If that seems complicated, I would suggest contacting Tado as it is able to work with a wide range of boilers in modulating control mode, as well as having an OpenTherm interface. The other alternative is to use Worcester Wave to allow app control as well as modulating the boiler similar to OpenTherm. If you need more info, fill in the contact form and send a message. Hope that helps.
Hi is the Viessman Vitodens 111 35kw OpenTherm Compatible?
Hi yes it is. See page 15 of the Installation and Servicing manual.
Hi
I had a new boiler installed last year without any smart thermostat installation as I did not like any of their offerings.
1. I have a Worcester Bosch Greenstar 27Ri Compact ERP. Do you know if this is Opentherm compatible?
2. If it is, do you know of a smart thermostat that is uses Opentherm within a Zwave network as I am thinking to invest in Zwave network to reduce the load on my every increasing Wifi network?
Hi Kai,
1. The boiler you have is not known to be OpenTherm Compatible.
2. I would suggest to go to https://www.automatedhome.co.uk/vbulletin/ Forum to see of any Z Wave devices that may work for you. Also I found another one that is https://www.uk-automation.co.uk
Hope that helps.
Jayesh, MyBoiler Ltd
Hi,
I have a Glow Worm Ultimate 35C, it’s getting on a bit now but still going strong, I used to have a climapro2 thermostat connected to it via the bus connection. It worked very well until it got dropped and screen cracked so I decided to upgrade to a nest 3rd generation. My question is would I be able to use the nest with OpenTherm or would I only be able to use it to start/stop the boiler? If OpenTherm is possible what extra adapter would I need if any?
Thanks in advance
Kind regards
Jonny
Hi Jonny,
Please read the following article for your answer: https://myboiler.com/opentherm/vaillant-opentherm/ As mentioned in the article OpenTherm protocols are implemented differently by manufacturers, so the level of OpenTherm control may vary. Hope that answer’s your question.
Jayesh, MyBoiler Ltd
Hi , I am guessing that Opentherm only works on gas boilers…Is that correct? I have an Firebird Enviromax condensing oil boiler. Regards Carmel
Hi Carmel,
Mostly on gas boilers at the moment, there are one or two oil boiler companies with it in the works although nothing concrete as such. Here’s one such example – https://www.geniushub.co.uk/blog/genius-hub-eogb/.
Jayesh
Thank you, for your speedy reply.
Evening guys.
I have recently fitted a Worcester greenstar 30i erp combi.
Using combipack 4 open therm wireless thermostat.
I have wired into live rf of boiler.
Is this correct?
Stat responds works fine.
Or do I have to fit adapter to ems.
Which I presume then fits into ot connector of receiver?
Thanks Ian carmichael.
If this is so.
What is the cheapest or best option of other
Controls?
Hi Ian,
Yes that is correct. You would have to obtain an Opentherm to EMS adaptor for the boiler. Then connect OpenTherm Thermostat terminal to this adaptor. Please note as information is limited, the results would not be known of the performance of this setup until it is completed and tested.
If you wish to obtain modulation without the adaptor, you would have to use Worcester Bosch’s own controls.
Hope that helps.
Jayesh Kanbi on Behalf of MyBoiler.com
Hello, is Viessmann Vitodens 050W supported? I saw that is in comments, but I just wanted to make sure, also maybe there are some guides how to connect to it ? I was not able to find details in my manual.
Hi Arminas, It is opentherm enabled. It’s shown on page 21 of the manual under electrical connections. OT- and OT+, the connector is under the boiler. Hope that helps. Jayesh, MyBoiler
Hi, I am considering installing a new Main Combi Compact or Baxi 600 same boiler I know. I would like to link it to a Honeywell T6R smart stat but read something about some stats not being compatible because of the baxi protocol is this correct
Dsve
Hi Dave,
Baxi doesn’t implement OpenTherm it to it’s full capability and generally leaves everything to the controller. There are some things the boiler should still have control of or at least set a maximum limit. The most compatible OpenTherm boilers are generally Viessmann, Intergas, Atag. They have a very good implementation of OpenTherm. If you are set on Baxi of which is generally a good boiler you should perhaps consider their own controller for maximum compatibility.
Jayesh, MyBoiler Ltd
The Ideal Combi pre 2016 (like mine) supports OpenTherm with a harness kit, but not OpenTherm SmartPower, so you will need either a powered thermostat or a gateway. The newer boilers support SmartPower and work with the Ideal Halo Lite wired OpenTherm thermostat. The Ideal Touch thermostat seems to have been discontinued.
Good afternoon! I use a protherm bear KLOM 17 gas boiler. These adapters 0020017895 VR33 OpenTherm-Modul NL Vaillant or AWB OPENTHERM MODULE EBUS 0020092426 are compatible with my boiler for use with Nest Thermostat 3rd generation. Thanks.
I have an Atag iS32 boiler which is two years old.I also have a Honeywell EVOhome system. The Atag boiler says it has an OpenTherm bus which is compatible with sone Atag products BUT I cant make it talk to my Honeywell EVO home. with the RF bridge.Atag and Honeywell have not been terribly helpful.
Hi David,
This combination has been reported working by a few people on some forums. Provided you have gone through the correct pairing sequences and still it is not working, I would suggest possibly getting another Honeywell R8810A1018 OpenTherm bridge to compare if your current bridge is faulty. Additionally you may want to get a low cost OpenTherm programmer just to confirm the OpenTherm on the iS32 is working, although I see no reason why it shouldn’t be.
Jayesh.
Thank you for your reply.I am on my second R8810A1018 and it is the same as the first one. What low cost OT programmer would you suggest?
Hey guys,
I’m wondering if you might shed some light for me. I can’t wrap my head around with how the OT protocol can work with Hot Water tank. Any chance you can dumb it down for me to understand how the controls work with such a setup?
Hi, as an overview on HW OpenTherm, the cylinder must be able to report it’s temperature to the OpenTherm controller and zone valves / diverting valves must be present. When hot water is being requested by the OpenTherm controller, usually it will prevent the heating zone from activating and activate the HW zone valve and send high temperature ie 80 degrees etc (unless set differently) flow water to the cylinder until it has reached it’s desired temperature. Once HW cylinder is satisfied, Heating Zone valve is free to open up again at desired flow temperatures dependent on room temp or aggregrate temps. This setup is known as priority hot water. There are other setups possible whereby the boiler is directly able to accept cylinder sensors, in this setup it’s possible to obtain a hybrid OpenTherm setup. Hope that answers your question.
JK, MyBoiler Ltd