Help with Evohome / Opentherm / Intergas

[the crooner]

No its a large 4 bedroom house with two adults 2 growing children and a planned extension including a shower room. Its all about the DHW. But there are 11 radiators/towel rails and I wanted to fit a smart TRV on each one and create a few zones. Looks like I may be in trouble.

The other option is the Vaillant ecoTEC plus combination 938 but the opentherm mod on that is non UK supported.

Buying a new boiler and making the right decision is far more complicated than expected!

[blowlamp]

Hello all, new here.

I am considering the purchase and install of the ECO RF 36 Combi boiler, with opentherm (and price) the main attraction. Is there any follow up to the information in this thread?

I intend to use Evohome to control the whole system.

Thanks

Tony

My feeling is that the problem probably lies with Evohome.

Evohome is at its core a TPI (Time Proportional Integral) controller system, which, at a basic level, means the average circulating water temperature is regulated by heating in bursts of high power with a varying ratio of flame on/off time.

This is a different strategy to that of OpenTherm, which is one of modulating the flame of the boiler in an analogue fashion, in an attempt to arrive at a near constant circulating water temperature - ideally without repeated flame cycling over short time periods.

I'm inclined to think that although Evohome & its OpenTherm bridge do together, under some circumstances, modulate the flame of the boiler, there does appear to be a large in-built bias towards using TPI like control, at least during the heating system warm-up phase to within ~ 1.5 degrees of set temperature. This is what I interpret from reports by users.

In contrast, seeing my Intergas boiler anywhere near its maximum power under OpenTherm control via the connected (single zone) Remeha iSense controller is very rare indeed. Instead, the flame stays low unless the house has become very cold for some unusual reason.


Martin.

[the crooner]

Thanks for this.

Do you have any experience with Tado which also claims to be opentherm capable?

A further issue concerning me now is acieving an adequate delta between flow and return temperature, which when I had my last combi installed, and seeing several online fitters quoting an installed price, seems not to be a consideration.

Is one able to specify a max flow temperature in Evohome in TPI mode, opentherm mode, both or neither.

It would seem that having the boiler operating in condensing mode would help economy, but if that is at the expense of running the boiler for longer to achieve and maintain the desired room temperature.

This is the point at which a little knowledge becomes a dangerous thing.

[HenGus]

Thanks for this.

Do you have any experience with Tado which also claims to be opentherm capable?

A further issue concerning me now is acieving an adequate delta between flow and return temperature, which when I had my last combi installed, and seeing several online fitters quoting an installed price, seems not to be a consideration.

Is one able to specify a max flow temperature in Evohome in TPI mode, opentherm mode, both or neither.

It would seem that having the boiler operating in condensing mode would help economy, but if that is at the expense of running the boiler for longer to achieve and maintain the desired room temperature.

This is the point at which a little knowledge becomes a dangerous thing.

Evohome/Opentherm is essentially closed to the user. It works with the boiler to manage boiler flow and gas valve modulation depending on how it aggregates the various heat demands from the HR92s. The issues to date with Evohome/Opentherm and some boilers (not all) is that (a) when a hot water demand is asked for Opentherm will demand a max flow temperature of 92C (if memory serves) which is supposed to be limited by the max flow temperature set by the user on the boiler itself and (b) some boilers are not coping with the data flow from Evohome whereas they are very happy with single zone Opentherm devices such as Lyric.

So what does all this mean? For a system boiler, failure to control the hot water flow temperature can result in a cylinder temperature overshoot.

Unless you choose the boiler manufacturer's own Opentherm controls, then it would seem that there is no guarantee that Opentherm will work as it should. That said, I had an Atag system boiler installed earlier this year which is connected to a 3 year old Evohome system using the Opentherm bridge. As far as I can tell, it is working as both Honeywell and Atag said it would. When hot water is demanded, then the flow temperature rises to 70C. I have hot water priority so this flow is limited only to the hot water side of the system. Other Opentherm users accept that whenever hot water is demanded the flow temperature around the heating loop will rise leading to potential zone overshoots. My worry would be with a 36kW boiler running at an uncontrolled flow temperature, the risk of zone temperature overshoots is high and radiators may get very hot. This doesn't happen with the Atag. My only slight concern about the Atag is that it takes a while for the boiler to get into the condensing range but as the boiler was only installed in April, it is still early days. I should add that whenever a zone is c.2C below its target temperature, Evohome will demand max flow temperature from the boiler to get the zone up to target temperature ASAP.

Tado uses its own implementation of the Opentherm protocol as does Atag One. Both connect via the low voltage Opentherm bus on the boiler (with or without a converter)

[dty]

when a hot water demand is asked for Opentherm will demand a max flow temperature of 92C (if memory serves) which is supposed to be limited by the max flow temperature set by the user on the boiler itself

90, not 92. This also happens for heating if there's a big demand - at least it does on my Vaillant. The dury is out as to whether the boiler should still be respecting its configured maximums (I believe it should) or whether Evohome should have a configurable maximum (which would solve the problem regardless of fault).

[the crooner]

Thank you.

There is real knowledge here and I appreciate the sharing of it.

Tony

[blowlamp]

With the iSense controller/thermostat there is an RT Effect (Room Temperature Effect) setting, which, depending on how far away the current room temperature is from its set temperature, determines the flame modulation level required to achieve set temperature in a time that suits the occupants. In short, the boiler uses its maximum output if the RT Effect is set to maximum, but at other settings a lower flow temperature is used.


Martin.

[HenGus]

90, not 92. This also happens for heating if there's a big demand - at least it does on my Vaillant. The dury is out as to whether the boiler should still be respecting its configured maximums (I believe it should) or whether Evohome should have a configurable maximum (which would solve the problem regardless of fault).

Thanks for the clarification: the benefits of having monitoring in place. As I think I said in my post above, Evohome will normally get a zone up to temperature ASAP by going for a high boiler flow temperature. This is why I think that my Atag boiler is taking time to get into condensing mode. When Winter returns, I will have to give more thought than I did to zone target temps and times. My installer was adamant that maximum efficiency with Evohome/Opentherm is achieved by having the zones set to a constant target temperature rather than having massive swings in zone temperatures throughout the day.

[the crooner]

So it seems that I will need some 31000 btu for heating assuming a 20 degree delta from 75 to 55 degrees necessitating larger radiators to cope with reduced flow temperature. As I said earlier the DHW is really important and I am taken more and more with the Vaillant ecotec plus 938 which has an inbuilt water store.

I am also starting to doubt the wisdom of adopting opentherm so I suspect I will have Evohome run it via a standard boiler relay band set the flow temperature manually to 75 degrees. With the DHW set to 55 degrees I'm hoping not to have dramatic increaes in radiator temperature mentioned earlier. I then have the option to convert to opentherm if and when its officially supported in the UK.

If the installed price of the Vaillant is too eye watering I will revert to the Atag iC Economiser 35 but will still run it under TPI mode unless anyone can give me a convincing argument otherwise.

[HenGus]

So it seems that I will need some 31000 btu for heating assuming a 20 degree delta from 75 to 55 degrees necessitating larger radiators to cope with reduced flow temperature. As I said earlier the DHW is really important and I am taken more and more with the Vaillant ecotec plus 938 which has an inbuilt water store.

I am also starting to doubt the wisdom of adopting opentherm so I suspect I will have Evohome run it via a standard boiler relay band set the flow temperature manually to 75 degrees. With the DHW set to 55 degrees I'm hoping not to have dramatic increaes in radiator temperature mentioned earlier. I then have the option to convert to opentherm if and when its officially supported in the UK.

If the installed price of the Vaillant is too eye watering I will revert to the Atag iC Economiser 35 but will still run it under TPI mode unless anyone can give me a convincing argument otherwise.

I would argue that Opentherm is officially supported by companies such as Honeywell, Nest, Tado etc. The problem is that boiler manufacturers see that there is money to be made by promoting their own control systems which may connect to the boiler via an EBus/Opentherm connection. Sadly, I cannot see this changing anytime soon as installers tend to want to install and run. They know that if they agree to install something like Evohome it may well require a number of return visits as the siting of BDRs (for example) are critical to error free Evohome operation. FWiW, when I raised the question of an Intergas RF Eco with Evohome with one of Intergas' main installers he told me that Evohome with an Intergas RF Eco along with weather compensation was a particularly good match using standard BDRs. This is what he had in his own house.