hot water kit, opentherm and combi boiler.

[Bart]

Hi all,

I need some advice on an installation that is a bit different
The boiler is a viesmann vitodens 100-w combi boiler, with a separate circuit for hot water.
Heating is controlled by Evohome through an opentherm bridge. The combi boiler gives priority to DHW when it detects a demand for hot water.
Now, to overcome some of the issues I've had with DHW, I added a hot water tank that I want to use as a buffer - meaning that I want to use the DHW of the combi boiler to heat up the water in the tank.
The set-up in essence is simple, I connected the tank to the combi boiler, with a pump in the circuit. If the pump runs, the boiler notices water flow, and it will start to heat water.

The next step was to control the water temperature in the tank using evohome with it's hot water kit. I've connected the pump to a BDR91, connected the temperature sensor, and configured hot water control to the evohome controller.

The issue I'm facing now is that when the water in the tank needs to be heated, the evohome controller will do 2 things:
- switch on the BDR91. This makes the pump run, which is fine.
- request heating through opentherm. This is not needed as the combi boiler will fire up as a result of the pump running.

Is there a way of setting up the evohome to not request heat through opentherm for DHW?

If I would have been using a second BDR91 instead of the opentherm bridge, I could prevent it to start the boiler by adding a relay or so. But I can't just cut the connection between the combi boiler and the opentherm bridge.

Thanks.

[Richard]

No, there is no way of stopping the bridge calling for 'demand' as it thinks you have a system boiler application on OpenTherm... Why was this not just setup as an external cylinder on a combi boiler with hot water priority?

[Bart]

No, there is no way of stopping the bridge calling for 'demand' as it thinks you have a system boiler application on OpenTherm...

I was afraid of that.

Why was this not just setup as an external cylinder on a combi boiler with hot water priority?

Can you explain in more detial what that setup would be like?

As far as I can see now, I have 3 options.
- leave it as it is, meaning that the combi boiler will get demands both for hot water and heating, resulting in some heating pump overruns and sometimes confusing behaviour of the boiler (maybe it does not get the priorities straight..)
- remove the evohome hot water kit and use a different solution that allows only the DHW pump to start, not interfering with the opentherm bridge
- change the piping to use only the heating circuit of the combi boiler, make it an Y-plan, using the serpentine / coil of the hot water cylinder that I was planning to use for a solar hot water solution in the future.

[Richard]

Can you explain in more detial what that setup would be like?

So I am assuming you understand that an OpenTherm system works on the principle that the heating load is varied depending on the demand of the evohome Controller (or as referred to in OpenTherm speak the 'Master')?

But because the 'Master' controls the heating flow temperature at a much reduced flow temperature for the majority of the time when heating set-point demand is reached, the last thing you want when the heating water temperature is maybe 40*C is then 75-80*C of central heating water dumped down the heating circuit when you have a hot water demand from a hot water cylinder.

Therefore, a modern condensing boiler with OpenTherm control on an S-Plan or Y-Plan system if definitely a big fat NO... OpenTherm 'system' boilers with OpenTherm controls need to be fitted on a Hot Water Priority system and we normally refer to this as 'PDHW' (Priority Domestic Hot Water).

There are a few ways to achieve this, but the main way I teach the guys who come on my training courses is with a new heating schematic I helped design called 'X-Plan'... In a nutshell, this is very similar to an S-Plan system, however the motorised zone valve on the heating flow pipe is a Normally Open (NO) motorised zone valve, instead of a Normally Closed (NC) motorised zone valve.

The NO valve on the heating and the NC valve on the hot water are both wired to the hot water demand controller and in the case of evohome this would be the DHW BDR91. Then whenever there is a demand for the hot water cylinder, the heating is zone valve is shut, the hot water zone valve opens and the OpenTherm Bridge tells the boiler to go to maximum output. This also has the big advantage that the hot water load no longer needs to be accommodated in the boiler output and the boiler can be simply sized for the space heating load.

I am unsure why you have gone to the complexity of introducing a semi-instantaneous hot water system the way you have? Maybe I have missed something? In the right place using the right controls, they work very well but add a certain degree of complexity that for most is certainly not required.

[Bart]

Well, there are some choices made based on available components and the existing installation. The choice was whether to use the hydraulic separation in the hot water cylinder, or the one in the combi boiler - at the point not realizing the conflict between opentherm bridge and the BDR91 hot water controller.
Would it have been a new installation, then I would not have gone this way.

I definitely want to avoid unsolicited hot water circulation through the heating circuit, so if I can't find a solution for the current set-up I'll change the piping in what you call an X-plan, or change the hot water kit with something independent.

Thank you for the information

[Richard]

I definitely want to avoid unsolicited hot water circulation through the heating circuit, so if I can't find a solution for the current set-up I'll change the piping in what you call an X-plan, or change the hot water kit with something independent.

A combi should be turning the heating off when it calls for DHW, irrespective of what the OpenTherm Bridge does?

[Bart]

It does, most of the time.. but for some reason the combi boiler sometimes commutes to heating while DHW demand is on. I'm still trying to figure out why.
In the end it also depends on Viesmann's implementation which as far as opentherm is concerned is not flawless, judging by existing topics on this forum.

[Richard]

It does, most of the time.. but for some reason the combi boiler sometimes commutes to heating while DHW demand is on. I'm still trying to figure out why.
In the end it also depends on Viesmann's implementation which as far as opentherm is concerned is not flawless, judging by existing topics on this forum.

OK, well maybe a fault with the boiler then? Certainly don't have this issue with Intergas boilers on OpenTherm? Maybe a call to Viessmann technical to see what they say?

[Bart]

The installation has been running a couple of days now, and it's working as expected now, meaning that the combi boiler gives priority for DHW over (the redundant in my case) opentherm heat request.
Maybe some left over air could have confused the flow sensor in the beginning?

[Bart]

It does, most of the time.. but for some reason the combi boiler sometimes commutes to heating while DHW demand is on. I'm still trying to figure out why.

Well, I've found the bug in my system, so to say
2018-08-23_08-50-03
Pictured is the flow restrictor and an uninvited guest that was stuck to it.