Boilers on Opentherm

[JohnMo]

Trying to learn something new.

 

I know - Opentherm is a communication protocol between controlling thermostat and boiler and is a two way thing. So thermostat is say set at 21 and the control mechanism is to drip feed energy into house to keep house temp pretty constant.

 

Also heard it mentioned that not all Opentherm control algorithms are equal, some manufacturers run boiler harder than others.

 

Does Opentherm work in reality?

 

Do you get lots of boiler cycles?

 

Do you have cost savings in gas?

 

Does it work for radiators and UFH?

 

Assume UFH would need to be directly connected and no mixer valve at UFH manifold to get best gains?

[MikeGrahamT21]

OpenTherm is just one of many Digital Connectivity methods, this is from the Tado installer manual:

 

 

All of the digital methods allow two way comms, and the setting of things like flow temperature on the fly from the controller, Tado for example now has flow temperature optimisation which changes on an hourly basis depending on outdoor temperature. It also has the ability to request a specific amount of heat, as opposed to analog which is a simple on or off, and the boiler would modulate down based upon the temperature of the water, this in theory stops huge overshoots of temperature from happening, and therefore wasted energy.

 

An UFH system loop would always require a mixer, due to the low temperature requirements, I suspect this could be omitted if it was the only heat source, with no wall hung rads.

 

Does it save on Gas, it should do in theory, but I have no data to back that claim up, as I've always ran this particular boiler in Opentherm mode, so have no comparison. And boiler cycles, i've never really taken note of how many times it fires per hour so couldn't say for sure

[JohnMo]

  On 06/01/2025 at 11:38, MikeGrahamT21 said:

An UFH system loop would always require a mixer, due to the low temperature requirements, I suspect this could be omitted if it was the only heat source, with no wall hung rads

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That is how mine is connected - no mixer or pump, also I have no radiators to demand a higher flow temp.

 

Running fixed flow temp, although boiler thinks it's running WC, via a resistor, and a 0.1 hysterisis on/off thermostat switching between a normal and setback.

[SimonD]

  On 06/01/2025 at 09:25, JohnMo said:

Trying to learn something new.

 

I know - Opentherm is a communication protocol between controlling thermostat and boiler and is a two way thing. So thermostat is say set at 21 and the control mechanism is to drip feed energy into house to keep house temp pretty constant.

 

Also heard it mentioned that not all Opentherm control algorithms are equal, some manufacturers run boiler harder than others.

 

Does Opentherm work in reality?

 

Do you get lots of boiler cycles?

 

Do you have cost savings in gas?

 

Does it work for radiators and UFH?

 

Assume UFH would need to be directly connected and no mixer valve at UFH manifold to get best gains?

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yes, Opentherm does work in reality, but like all things heating, it depends on the boiler and or controller implementation as well as the heating system overall. The Opentherm protocol specifies a certain amount of information to be available but quite a bit of it is optional.

 

Boiler cycling again depends on the setup. I'just installed Opentherm on an Ideal Vogue Max 18kW system boiler. House has a calculated heat loss of 7.35kW and the size of boiler was selected for DHW reheat times. The controller has a default minimum flow temperature of 45 C on opentherm. At this temp, even on a fairly cold day like today, the boiler will short cycle, and even worse with minimum flow temp set to 40C, which is what I set it to first. As soon as I increased this to 50, with a 55C max flow temp, the boiler ran constantly for well over an hour, with an almost constant return temp of 39.5C, increasing and then maintaining room temp nicely.

 

On another recent install with a Viessmann 100-W into a very large, very old place with a very old heating system, I had to set the max flow temp to 80C and minimum at 45C. With Opentherm the boiler would run at 80 to get towards target but then ramp down as it got closer, so when I checked, it had dialled back to 53C to maintain temp at setpoint.

 

It certainly can be used with UFH and rads. 

 

I work mainly with gas systems so always use mixers with ufh.

 

In terms of controls, I like EPH because I have access to set max/min flow temps together with dhw temperature and its also possible to run pdhw on an s-plan system with these controls. I've even done this succesfully with Worcester boilers using the Nefit opentherm ems converter.

 

Savings wise, research suggests between 10-15% although Tado in Germany has at some point suggested 25% iirc.

 

Whilst it is two way, when I've had discussions with Viessmann technical when fault finding control problems, they confirm that the boiler becomes slave to opentherm and just passes info to the controller. However, the boiler doesn't necessarily pass all info across and will continue to manage some aspects like pump modulation relative to fan speeds etc. These will be managed by the boiler when it is simply responding to Opentherm's instruction for flow temp.

 

HTH.

[JohnMo]

@SimonD

 

Got the boiler running Opentherm and as you found getting the flow temperature wrong and you get plenty of cycling. In the end the Opentherm ran no different to my simulated weather compensation (resister instead of outside sensor). So just reverted back to on/off, this also gave me PDHW which didn't work on Opentherm.

 

So now just operate at a fixed flow temp of 37.8, control via a resistor, WC is on setback when thermostat is satisfied (setback will never allow boiler to refire).  To do PDHW, a relay switches resister out of circuit and the boiler ramps to about 56 degs.

 

This was last night, wobbly bit up to 3am was the heat pump on at the same time, doing defrosting. The peak around 5.30 was cylinder heating.

 

Charging a floor via UFH (no mixer, but boiler is behind a CCT). Basically runs once per day (if outside is below 5 degs for 6 hours) maybe twice if super cold.

 

 

[SimonD]

  On 09/02/2025 at 20:17, JohnMo said:

this also gave me PDHW which didn't work on Opentherm.

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What we're you using for Opentherm control? This is entirely possible within the protocol - the master simply asks for relevant flow temp from boiler when the dhw cylinder calls for heat and you wire in a relay for any external valves.

[JohnMo]

I have a Polypipe UFH controller (rebranded Tech Controller/Roth controller) which is Opentherm and or on/off.

 

My basic issue is the boiler is a combi, but I use it as a system boiler. The Opentherm knows it's a combi, so the control is tailored to the combi.

[joth]

Nothing specific to add here other than thank you for a very useful thread.

I'm in the planning stage for some friend's victorian town house refit which will be system boiler + PDHW + UFH+Rads. (Plus a2a multisplit aircon) I've only done ASHP installs thus far, so this is all useful info

Looking at an ATAG boiler controlled from Loxone via MB-OTG modbus to Opentherm adapter.

But if ATAG boiler doesn't happen, then good to know Worcester maybe an option via the Nefit EMS-OT, although I'm not sure I have the appetite for going to inception level 3 and speaking EMS via Opentherm via Modbus, given on/off control Just Works.

 

 

[JohnMo]

  On 10/02/2025 at 11:06, joth said:

inception level 3 and speaking EMS via Opentherm via Modbus, given on/off control Just Works.

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Just sounds complex.

 

My on/off isn't really on/off, as far the boiler knows, it's weather compensation and setback weather compensation. There is always an outside temperature which the boiler controls against, but the temperature is basically fixed. On/off thermostat moving between the two set points of normal WC and setback. PDHW just interrupting the outside weather temperature.

[SimonD]

  On 10/02/2025 at 11:06, joth said:

good to know Worcester maybe an option

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I can't speak for ATAG but you'd be far better off with a Viessmann 100-W or 200-W than a Worcester. 4 pipe out of the box PDHW with directly connected cylinder stat. then you just have to worry about heating control. They're also 12 year warranty out of the box if registered through the Viessmann installer portal.

 

Too many fudges and system compromises (as well as unnecessary costs) doing it with a Worcester, whose system control implementation strategy is terrible IMHO.

 

Edit: BTW poor system control, heating not working very well in new and newish builds with Worcesters installed with UVC and combined rads/ufh is one of my  common jobs to sort out!

Edited February 10 by SimonD

[joth]

  On 10/02/2025 at 11:42, SimonD said:

 

I can't speak for ATAG but you'd be far better off with a Viessmann 100-W or 200-W than a Worcester. 4 pipe out of the box PDHW with directly connected cylinder stat. then you just have to worry about heating control. They're also 12 year warranty out of the box if registered through the Viessmann installer portal.

 

Too many fudges and system compromises (as well as unnecessary costs) doing it with a Worcester, whose system control implementation strategy is terrible IMHO.

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Thanks for this. Viessmann was my first thought, but so far been a dead end with their UK installers claiming Opentherm is not supported/possible

 

 

[marshian]

  On 10/02/2025 at 11:50, joth said:

Thanks for this. Viessmann was my first thought, but so far been a dead end with their UK installers claiming Opentherm is not supported/possible

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Sorry I must have missed it but why would you specifically want opentherm over std weather compensation using an external temp sensor?

[SimonD]

  On 10/02/2025 at 11:50, joth said:

Thanks for this. Viessmann was my first thought, but so far been a dead end with their UK installers claiming Opentherm is not supported/possible

 

 

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Doesn't instill much confidence, does it? But yes, Opentherm is supported with the 050-W and 100-W which are plusbus, weather comp or opentherm, or relay. With the 200-W it's plusbus or canbus and you'd probably go weather comp.

 

You can even add opentherm to many older 100-W with an X21 wiring harness for less than £10.

[joth]

  On 10/02/2025 at 12:04, marshian said:

 

Sorry I must have missed it but why would you specifically want opentherm over std weather compensation using an external temp sensor?

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For load compensation: if the legacy radiators are open, or the house is very cold run higher flow temperatures; if just using ufh and house nearish to target, use lower flow temperature

 

An approximation can be made using two setpoints and that's probably almost as good, but as Loxone already calculates a dynamic numeric target temperature  and opentherm is the ""standard"" it seemed at face value that would be the simpler/cleaner approach.

 

(Also we're already putting in an external weather station for Loxone, installing another proprietary sensor for the boiler seems unnecessary but this is less of a deciding factor. )

Edited February 10 by joth

[MikeGrahamT21]

  On 10/02/2025 at 11:06, joth said:

Nothing specific to add here other than thank you for a very useful thread.

I'm in the planning stage for some friend's victorian town house refit which will be system boiler + PDHW + UFH+Rads. (Plus a2a multisplit aircon) I've only done ASHP installs thus far, so this is all useful info

Looking at an ATAG boiler controlled from Loxone via MB-OTG modbus to Opentherm adapter.

But if ATAG boiler doesn't happen, then good to know Worcester maybe an option via the Nefit EMS-OT, although I'm not sure I have the appetite for going to inception level 3 and speaking EMS via Opentherm via Modbus, given on/off control Just Works.

 

 

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+1 for ATAG, mine has been flawless since install

[joth]

  On 10/02/2025 at 12:20, SimonD said:

With the 200-W it's plusbus or canbus and you'd probably go weather comp.

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200-W is discussed a lot on here; interesting they dropped Opentherm support in it.

But looks like it maybe possible to get fine grained control of it by installing the appropriate LON comms module (MW2 Vitotronic 300K here?) and talk to that via a Vitogate 300 gateway

https://library.loxone.com/detail/viessmann-1437/overview

 

It's a lot of vito-soup; I'm starting to feel like a heat pump would be simpler...