OPENTHERM Compatible Combi Boilers?

I got a Viessmann Vitodens 100-W installed last month. It's coupled with the EvoHome Opentherm module and controls 9 radiators and 1 DHW cyclinder.
Hope this helps.

Have a look at Intergas. When my current boiler packs up (it is 19 years old) Intergas is probably what I will go for. Meant to be more economical than most others, a little dearer but worth it.

Sounds like the plumber doesn't understand what Opentherm does...

The boiler is no doubt a "modulating" boiler, but all that means is it adjusts its heat output by varying the gas flow to the burner. This allows it to maintain a set flow temperature more precisely in the face of varying loads from radiators by adjusting the flame size up and down smoothly rather than turning the flame right off when too hot and then back on full blast when too low, as old fashioned boilers (like my ancient one) do. This is more efficient and prevents the flow temperature from constantly see-sawing up and down.

Any modern boiler made in the last 10+ years will have a modulating burner. However the "modulation" that OpenTherm can do is something else, and is to vary the requested flow temperature based on the demands that each radiator zone wirelessly calls for. So if only a small amount of heat is required the Evohome can ask the boiler to run at 45 degrees for example, then crank it up to 70+ at a time when demands are high. All automatically.

Without this the boiler is only ever aiming for one manually set flow temperature. (Unless you have weather compensation varying it based on outside temperature)

If you just use a boiler relay with TPI (such as a BDR91) with a modern "modulating" boiler then you lose the modulation ability of the boiler under low load conditions. For example -

Say the flow temperature is set manually to 75 degrees. If the flow temperature is much lower than this the boiler will run the burner full tilt to try to get there as fast as possible. When it approaches the desired temperature, say 70 degrees it will start ramping back the flame so that it smoothly reaches 75 degrees then make small adjustments to keep the desired 75 degree flow temperature - it is "modulating" the burner.

However if total system demand is low and a flow temperature of only about 45 degrees is required the Honeywell system will cycle the boiler relay on and off with a low duty cycle (say 2 minutes on out of 10) to cause the flow temperature to drop to an average of 45 degrees. However because this is way below what the boiler is set to (75) every time the relay comes on the burner will go full tilt for 2 minutes trying in vain to reach 75 degrees only to be shut off again after 2 minutes - it will never get anywhere near the set temperature so it will never "modulate".

It will just go on full blast for a short time then off again for a longer time, just like an old non-modulating boiler. Clearly this is not as efficient as being in modulating mode where the boiler itself is being directly told by opentherm to modulate to 45 degrees and run the burner continuously but at a reduced gas flow and flame size to do so. (One reason is that you get incomplete combustion for a short while every time a boiler starts up)

So yes, OpenTherm has got to be more efficient than TPI modulation of a boiler relay. I know if I was in the market for a new boiler I would definitely be trying to get one with Opentherm support.

Don't get me wrong, TPI modulation of a boiler relay is a big improvement over a non-TPI wall stat, but it is still only making the best of a bad situation, direct control of the flow temperature is much preferred. Not just for efficiency reasons but also control reasons - anecdotally Opentherm helps prevent temperature overshoots and maintains room temperatures much more precisely as TPI modulation to control flow temperature is very inexact and subject to a lot of variables that are out of the control of the Evohome system. (The average flow temperature attained from a given duty cycle varies wildly depending on the temperature of radiators around the house and how open their TRV's are, among other things...)

Intergas - no brainer! Very well engineered and fully OpenTherm compliant. We are just about to start selling them too - https://theintergasshop.co.uk/!

Anyone who says WB is better than Ideal is getting £50 a boiler off WB... The Logic+ and the Vogue are great boilers, but Intergas is the one for me!

My CH service man tells me of big savings with Intergas, fewer call outs (which is not good for him!) and even the energy company inspecting a commercial customer because they could not believe the drop in consumption and thought the meter may have been interred with!

Intergas have been on my radar for a long time and if I was back on the tools fitting boilers this would be my boiler of choice without doubt (and Ideal second) as a perfect evohome partnership.

Richard - good morning. I thought that you were a committed WB fan? Seriously, is the added advantage of Opentherm a key consideration in the choice of a new boiler? Surely, the most important - and most expensive - factor is the build/reliability of the boiler itself. What has persuaded you to make Intergas your boiler of choice?

Many moons ago... The difference is technology has moved on and WB hasn't.

I suggest having a read of the Intergas website - quality and reliability is not an issue (they are far better built than WB, hence they are used widely in social housing)! Intergas also won product of the year for their HRE this year! As for OpenTherm, if you drove your car by turning the engine 'on and off' every time you wanted to accelerate or decelerate, would this an efficient way of doing things? However, this is effectively what you are doing when not using OpenTherm and using 230v switching with a heating controller instead.

Thanks - that's helpful.

I found their website a bit confusing, if I'm honest.

And "less parts" set my teeth on edge - FEWER!!!!

They might be good at making boilers, but they need some serious marketing / copywriting help.

P.

Is the ATAG IC economiser compatible with Opentherm as can be used by Evohome or is it another locked down control version only allowing use of their One system for non on/off control.

yes all ATAG are OpenTherm ready, the evohome is fully compatible

Hi, I have set up an Open Therm capable boiler list. OpenTherm Capable Boiler List . If any boilers are missing, please feel free to contact me via the site form, twitter.com/myboiler_com or post a message to this thread.

Warm Regards, Jay.

Hi. Vaillant EcoFit Pure works with VR33.