Evohome Opentherm Boiler decision and set up

[HenGus]

The upper limit is set via the installer settings. When CH is selected, it is not possible to turn the flow temperature down using the manual controls. That said, my system is currently working at a flow of 52 and return of 42.

[1animal1]

The upper limit is set via the installer settings. When CH is selected, it is not possible to turn the flow temperature down using the manual controls. That said, my system is currently working at a flow of 52 and return of 42.

Ahh ok, that makes sense.

Interesting you say about opentherm. I get the feeling from speaking to merchants and other plumbers (also on forums)... That because of all the manufacturers coming up with their own controls and not subscribing to opentherm - there seems a reluctance to learn about it. As an industry the UK seems to be in the dark ages regards this stuff and we don't seem to want to do anything about it. Recent innovations seem to be pushed by the advancement of smart tech

[mylesm]

I Have Intergas Boiler and opentherm bridge with Evohome and it seems to work very well

[HenGus]

Ahh ok, that makes sense.

Interesting you say about opentherm. I get the feeling from speaking to merchants and other plumbers (also on forums)... That because of all the manufacturers coming up with their own controls and not subscribing to opentherm - there seems a reluctance to learn about it. As an industry the UK seems to be in the dark ages regards this stuff and we don't seem to want to do anything about it. Recent innovations seem to be pushed by the advancement of smart tech

Atag, like most manufacturers, has their own bespoke smart controller - the Atag One - which connects to the Ebus/Opentherm terminal in the boiler. According to the installation manual it also uses weather compensation via an external box connected to the boiler.

[1animal1]

I Have Intergas Boiler and opentherm bridge with Evohome and it seems to work very well

Can I ask how long it has been installed? System combi etc?

Just wondering actually. With the wiring how is everyone wired on an S plan with HW zone valve and open CH circuit. Presume wire OT then wire the zone valve to BDR91 via junction box, including switched live from the boiler and fused mains also into the BDR? And of course fused mains to the boiler.

[mylesm]

Can I ask how long it has been installed? System combi etc?

Just wondering actually. With the wiring how is everyone wired on an S plan with HW zone valve and open CH circuit. Presume wire OT then wire the zone valve to BDR91 via junction box, including switched live from the boiler and fused mains also into the BDR? And of course fused mains to the boiler.

This is First Winter with Intergas Boiler I had a worcester was about 10 years old so changed it for Intergas As you are probably Aware Intergas Boilers can be setup as Combi,System,Open Vent,simply by changing a parameter in the Menu Mine is currently open vent Heating Only no Stored Hot Water All Rads have HR92 Valves except Bathroom which acts as bypass rad I intend to connect the Combi Side for Hot Water in Kitchen later this year so far Opentherm is working well I see flow Temps between 30 - 65c I have 65c set as Max Ch Temp

TO be honest in any well insulated house the savings using opentherm over BDR TPI is minimal

[StephenC]

It was the 100w I am considering even with the issues. My current boiler spits out similar temps of 80 odd degrees without any issues of scalding via the closest rads. Other issue is the 2 or 3 degree over spikes which I could live with as the rooms are heated in prep for using them. So not the most efficient but once passed start up phase and using OT. I'd be good to go no?

Very interesting with the ATAG. My chap thought you had to be an approved installer before you could fit or indeed source... I'll have a look at this option. Are these very much plug and play like the IG?

How did the cost of the is24 compare to other brands?

Hi,

I would from experience, and with owning a Viessmann Vitodens 111-W storage Combi (with radiators sized for ~75 degrees at peak heating load), avoid the Viessmann 1xx range for 3 reasons:

1. As is well known now, *FULL* OpenTherm support is only for their own controller. With Honeywell OpenTherm control, Overshoots and oscillation was really bad from my experience, and subtly it’s not the example you mentioned of morning warm up phase that’s the only issue with overshoots, but also during the day/night, should any zone fall (or be scheduled to come on) 1.0 degree C below the set point.
EvoHome via OpenTherm will demand a max flow temp of 90 degrees C, most of the time clipped at 81/82 on the Viessmann boilers only due to internal temp limiters.
I did see 88 degrees C a couple times.....and if other HR92 valves are still open and rooms happily at set point, then the rooms get a shock burst of unexpected heat and those rooms also oscillate and overshoot.
In practice I saw this max temp request behaviour also at 0.5 degrees below the set point when making manual increase in zone temps. Consistently. I know 1.0 degrees C has been stated as the threshold, but I saw 0.5 degree on a manual uplift in setpoint doing the exact same thing.
This as you can imagine leads for a very unstable system.....

2. Weather Compensation as an alternative configuration to control flow temps with standard BDR91 control technically works, but on the Viessmann 1xx series is very crude to the point of frustrating.
There is gain control (relative temps can be increased/decreased), however the temperature gradient/heating curve is fixed.
What this means for me is for example at a given gain setting (analogue dial on front of boiler) when it’s 0 degrees outside the flow temp sits at approx 70 degrees.
At 10 degrees outside the flow temp is auto adjusted down to 35 degrees or so.
35 degrees is too cool for this old 1880’s farmhouse at 10 degrees, so I end up have to increase the gain (dial) just as I used to have to adjust before weather compensation!!!!
It’s really very crude. Viessmann Vitodens 2xx series Weather Compensation looks to be digital and has loads of configuration and adjustment control (importantly curve control) to match the house, so in theory could be very good.
Weather Compensation when used with intermittent heating and without room based load control (ie OpenTherm) also causes issues with high thermal mass (eg solid brick walls) providing high inertia to change when moving from a very cold few days to suddenly a warm spell. Suddenly there isn’t enough heat in the flow to compensate for the internal walls that are colder than the outside temperature being sensed. A manual gain control is then needed....OpenTherm working as intended with decent configurable Weather Compensation is the ideal setup as Martin and Simon have described in another thread/post however.

3. Viessmann technical support were terrible for both me and my heating engineer (Viessmann trained), and didn’t know what they were talking about (I showed them evidence to counter their incorrect statements).
They were also extremely arrogant overall. I would avoid this company.


I know you didn’t mention Weather Compensation - but I thought I’d arm you with additional info.

It was a relief going back to BDR91 is all I can say. The Opentherm theory driven by Honeywell components just doesn’t work in practice *with Viessmann 1xx boilers*

I’m now looking at oversized (calculated via the BRE low temp heating calculator) radiators to drive down permanently the needed flow temp for our house.
This is benificial for the gas boiler to be permanently in the condensing temp zone in the short term, as well as setting me up for moving to a heat pump of some description in the medium term which really requires lower temp based heat emitters.

I’ve come to the conclusion on my journey that I want to move away from being reliant on gas that could be unstable in price and/or supply, and also move away from fossil fuels to renewables.....but that’s a different topic entirely...

[bruce_miranda]

I am still in disbelief that a company like Viessmann have allowed their brand to be so badly tarnished due to the way you have been treated. Do they know this forum exists and the damage their incompatibility has caused to future sales. Boilers, like cars are long term decisions and negative publicity sticks.

[HenGus]

I am not sure that I would go to the expense of changing radiators in my home just to force a boiler into condensing mode. As others have posted, the issue here is Honeywell's 'fuzzy logic' and Evohome's demand for max flow temperature when any zone is outwith TPI range. For the first hour or so, my boiler is definitely not in a condensing mode; however, once all the zones are up to heat, the flow temperature drops markedly and the boiler condenses. It will stay in this mode throughout the day provided no one fiddles with any of the HR92s.

Has a new boiler with Opentherm produced any savings? After 10 months, my rolling 12 months usage has fallen from 11175 to 8794 kWhs - a 21% decrease. I would suggest that this is mostly down to installing a much more efficient boiler rather than Opentherm. Do I get overshoots? The answer is 'yes' but that was also the case with the old boiler and BDR control. I now have HW priority and when the hot water reaches temperature, the flow has to go somewhere. I cannot say that a 1.5C overshoot in a zone demanding some heat really bothers me.

My wife and I were looking around a new home over the weekend which has a Valliant 18 system boiler. What struck me was the minute size of all the radiators in the property. They were all single panel. I came away thinking what, if any, thought has been given to boiler condensing efficiency? Not a lot, I suspect.

[StephenC]

To be fair, the radiator point I was making is part of my own longer term strategy towards a heat pump based heat source due to not being satisfied with gas/fossil fuel usage, and will be taking place when other large works will occur.
I wasn’t suggesting it was an answer to anyone, merely my thoughts are now thinking beyond gas and OpenTherm as the “destination” solution as that is clearly out of my control at this point with the current kit/investment.

The selection of EvoHome and afterwards after looking at it OpenTherm, are part of peak load shifting, and a reduction in gas/fossil fuel usage (that’s important to me) as well as the usual cost and comfort reasons to be clear. Maybe I hadn’t stated that as a driver.