Honeywell evohome and OpenTherm integration

[The EVOHOME Shop]

Hi,

Thanks for the confirmation of point 2.

I shall look at alternatives to OpenTherm (at reducing gas usage and therefore on-going cost saving and emissions savings) at this point now I know.

Re: theevohomeshop - no I worked with my local installer (and seem to be teaching him on this...). No on site support from HoneyWell, but I did have support over the phone from one of your colleagues and they setup data logging etc.


Stephen

Hi Stephen,

Can you clarify why a Tmax setpoint is required from the controls?

The Intergas boiler I have at our training centre has the ability to set this manually (I designed our system with a 70 degree flow temp, so have limited it to this), but I understand that your Viessmann doesn't?

Most heating systems have a 75 degree flow temperature design point and I would imagine that once evohome is over its initial 'learning' phase, Tmax should certainly be no higher than this? I assume you are not seeing this?

I have a friend with an Intergas ECO RF 24 and a 10 zone evohome Wi-Fi and we are getting set points generated depending on the aggregate demand of the 10 zones (confirmed by an OpenTherm monitor and the boilers diagnostic software).

Capture3.jpg

I was testing several zones at the time, hence the demand was dropping off sometimes suddenly but should give you an idea of what is there and what should happen.

I am more than willing to invite Automated Home members over to our facility in Mid Wales to see this is action and discuss face-to-face? Maybe TB can even meet us there so you can ask questions directly?

Thanks,

Richard

[bruce_miranda]

OT works great in a single zone environment, or multiple zones all following a similar schedule. The moment you add multiple zone, each following their own schedule the usefulness wears off. This is due to the fact that it only takes a single zone that is outside of the proportional band to kick the boiler into full. When you have various zones, this is more than likely to mean that the boiler is running at full too often, simply to satisfy single zones. Other zones, if they are close to the set-point before the boiler full kicks in, will then often over shoot as a result.

[blowlamp]

OT works great in a single zone environment, or multiple zones all following a similar schedule. The moment you add multiple zone, each following their own schedule the usefulness wears off. This is due to the fact that it only takes a single zone that is outside of the proportional band to kick the boiler into full. When you have various zones, this is more than likely to mean that the boiler is running at full too often, simply to satisfy single zones. Other zones, if they are close to the set-point before the boiler full kicks in, will then often over shoot as a result.

This seems to be true judging by the user feedback I've seen here. By choosing to use full boiler output as often as it does, Evohome appears to be breaking the fundamental OpenTherm rule of keeping flow temperature low and the boiler in condensing mode whenever possible.

[StephenC]

OT works great in a single zone environment, or multiple zones all following a similar schedule. The moment you add multiple zone, each following their own schedule the usefulness wears off. This is due to the fact that it only takes a single zone that is outside of the proportional band to kick the boiler into full. When you have various zones, this is more than likely to mean that the boiler is running at full too often, simply to satisfy single zones. Other zones, if they are close to the set-point before the boiler full kicks in, will then often over shoot as a result.

+1 I can vouch for this behaviour. It doesn't work well in practice. Hence a need to have some additional control over flow temps on EvoHome side.

[StephenC]

Hi Stephen,

Can you clarify why a Tmax setpoint is required from the controls?

The Intergas boiler I have at our training centre has the ability to set this manually (I designed our system with a 70 degree flow temp, so have limited it to this), but I understand that your Viessmann doesn't?

Most heating systems have a 75 degree flow temperature design point and I would imagine that once evohome is over its initial 'learning' phase, Tmax should certainly be no higher than this? I assume you are not seeing this?

I have a friend with an Intergas ECO RF 24 and a 10 zone evohome Wi-Fi and we are getting set points generated depending on the aggregate demand of the 10 zones (confirmed by an OpenTherm monitor and the boilers diagnostic software).

Capture3.jpg

I was testing several zones at the time, hence the demand was dropping off sometimes suddenly but should give you an idea of what is there and what should happen.

I am more than willing to invite Automated Home members over to our facility in Mid Wales to see this is action and discuss face-to-face? Maybe TB can even meet us there so you can ask questions directly?

Thanks,

Richard

Hi Richard,

It appears that Viessmann have setup their boilers so that when OT controlled, it hands all control over to the controller. Their own controller for example has the option to set a max CH flow temp. When I challenged them on this, they told me Honeywell were at fault for not providing the capability in the EvoHome controller. It looks like the Nest v3 (from reading) also has the ability to set a max CH flow temp.

The only thing that limits how hot the water gets when the CH comes on (@ 100% demand - so any zone under the set point by 1.5 degrees C - although I saw it at the set point too) is the boiler hard limiter to avoid the water boiling!! That is around 82 degrees C, but I have seen 89 degrees C a few times.

I've done everything I can to try and work with this, but it's so hot I got overshoots of 2-4 degrees in zones as the HR92 closed too late (latent heat in radiator).

As for learning, 100% demand is 100% demand. That translates on OT bridge to boiler max CH flow temp value. Nothing to learn there. I had this in for 5 weeks and had to go back to BDR91 as it is the HR92 that learn (not EvoHome controller according to HW), and they didn't get any better at controlling (i.e. demanding less). I was even sent a new replacement OT bridge, but behaviour was the same.

Hope that explains things from a Viessmann owners perspective. I can't talk about behaviour for other boilers as I only have the one. ;-)

[HenGus]

OT works great in a single zone environment, or multiple zones all following a similar schedule. The moment you add multiple zone, each following their own schedule the usefulness wears off. This is due to the fact that it only takes a single zone that is outside of the proportional band to kick the boiler into full. When you have various zones, this is more than likely to mean that the boiler is running at full too often, simply to satisfy single zones. Other zones, if they are close to the set-point before the boiler full kicks in, will then often over shoot as a result.

I concur with this view, and I suspect it is the reason behind my previous comments that the Atag is24 is slow to get into full condensing mode. It would seem to me that this rather undermines the savings that zoning - with a modern condensing boiler - should be able to achieve. As far as zone temperature overshoots are concerned, I haven't seen anything that would cause me concern but my boiler profile P0 has been set at P2 with a CH Tmax: 70C (which confusingly the actual boiler display calls T - set):

P0 1 1. CH Tmax: 80°C; Gradient: 5; Heating line 24
2. CH Tmax: 70°C; Gradient: 5; Heating line 19
3. CH Tmax: 60°C; Gradient: 4; Heating line 15
4. CH Tmax: 50°C; Gradient: 3; Heating line 11

[paulockenden]

I come back to what I said right at the start of this thread - when under OT control (and maybe in some instances with cycling BDR control) things would work much better if the proportional band was wider than 1.5 degrees. Much wider.

However, because the TPI demand comes from the individual sensors used for each zone, I'm not sure that was the current EvoHome system this would be possible.

Also, I think we're in danger of confusing the two issues that have been reported with OT:

1 - The fact that with some boilers an OT connection disables the front panel set CH Tmax, allowing the output to get too hot (possibly dangerously hot?) for radiator use.

2 - The issue reported with Intergas, which the manufacturer said would need new controller boards but which Richard believes is simply installer error.

I suspect these are two very separate issues.

P.

[blowlamp]

Would I be right in thinking that under OpenTherm control, Evohome ignores heating flow temperature until within the proportional band and only measures air temperature up to that point?

[paulockenden]

Evohome doesn't measure flow temperatures. Ever.

[blowlamp]

Evohome doesn't measure flow temperatures. Ever.

This is from Honeywell's OpenTherm factsheet

How does OpenTherm work?

The standard way to control a heating system is to switch the boiler on and off at intervals with On/Off controls. OpenTherm provides more precise control on the boiler through the control of the gas valve.
It allows the amount of heat provided by the boiler to be controlled to match the varying demand signal. By reducing the flow temperature to a minimum as it leaves the boiler, OpenTherm modulation helps the boiler efficiency by running at lower flow temperatures for longer periods; this ensures more of the heat exchanger is at condensing temperatures.

Identifying the products

Every OpenTherm product can be recognised by the OpenTherm Logo being displayed on the product or the packaging. The logo’s presence guarantees a minimum level of interaction between appliance and room thermostat.


So Evohome/OpenTherm controls flow temperature via measured air temperature rather than a direct reading of the boiler water even in the proportional band? That sounds like a crude way of doing things when the boiler & OpenTherm bridge can provide access to all the data Evohome needs for good, stable control.