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Thread: Vaillant flow temperature control for HW using Evohome/OT

  1. #1
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    Default Vaillant flow temperature control for HW using Evohome/OT

    I thought I would describe the two methods I have used for having a higher flow temperature for HW when using Evohome.

    I have a Vaillant 438 boiler. I use Evohome and OT. In order to use OT on the Vaillant boiler, I use a VR33 - which is nothing but an OT to eBUS translator.

    The Vaillant 438 is able to have two separate settings for CH and HW. However this is only available when using Vaillant's own eBUS controls and their wiring centre. When you use Evohome, a lot of that becomes redundant. The other issue is that Evohome has no way of telling the boiler that it's doing a CH or HW cycle, even though the OT protocol allows for this distinction, Honeywell/Evohome hasn't implemented that. Instead it simply raises the Target Flow temperature to constant 90C whenever HW is to be heated. The 90C flow requirement is deliberate and is actually relying on the boiler clamping that down to something more sensible. Luckily the Vaillant boiler's Flow temperature knob remains operational even when using OT via the VR33.

    Solution 1:
    My previous solution involved the following components. An HGI80, an eBUS monitor and Domoticz.
    My boiler Flow temperature dial was set on max on the boiler. Using the HGI80 I would detect when Evohome was performing a HW reheat, that would trigger a script in Domoticz. The script used the eBUS monitor to issue a command that basically varied the "Maximum Heating Dial setting". So when Evohome was doing HW, I would set the max of that dial to 70C, but when it was only doing CH, I would reduce that max dial setting. This solution relied on the Vaillant ability to set and clamp the max flow temperature the boiler was heading towards.
    This solution worked quite well for many years, but it relied on too many things.

    Solution 2:
    Using a VR65 wiring centre. The VR65 has many functions and allows for the pumps and valves etc to be all connected through it. But one of the features of the VR65 is that it can be used to detect when HW is required and that in turns makes the boiler behave differently.
    Normally you would use an NTC sensor VR10 to detect the temperature in the HW cylinder. But with the Evohome you don't need that. Not having the NTC connected to the VR65 means that the Hot Water temperature dial on the boiler does nothing. The VR65 operates in a default HW priority mode. This means that while the cylinder thermostat is closed, the VR65 makes the boiler uses it's HW settings. If the cylinder thermostat is open, the VR65 assumes that the HW heat demand is fulfilled and then reverts to its CH heating mode.
    Besides setting a higher target flow temperature (D.78), you can even set a different KW partial load (D.77) to use when in HW mode. This feature is brilliant. One of the issues with the Vaillant 438 and Evohome is that very quickly the boiler becomes over-sized for the house, as zones' heat demand start to get fulfilled. But you still need the higher load for HW. The VR65 allows for that. So for example, you can have a 18Kw boiler load (D.0) @ max 55C (D.71) for CH and then use 28Kw boiler load (D.77) @ max 70C (D.78) for HW. Ofcourse in CH mode the D.71 is actually the clamped maximum, and OT will often use lower target flow temperatures when in CH mode, visible in D.9

    The trick to use the VR65 in this dual heat manner with Evohome is.
    a. You can either send the same Switched Live from the DHW BDR91 to both the DHW zone valve and the Right hand terminal of the CYL terminals in the VR65. So that way, when Evohome calls for HW, it opens the DHW zone valve and makes the VR65/Boiler enter HW mode. The wiring is becomes very simple. You simply send the Brown wire of the DHW zone valve to the right hand CYL terminal as well.
    b. The other alternative is to use the microswitch in the DHW zone valve to bridge the contacts of the CYL terminal. So you connect the Grey and Orange wires of the zone valve to the CYL terminals, as dry contacts, with no Live connected to either Grey or Orange. What happens then is when the Evohome DHW BDR91 makes the DHW valve open, the micro switch inside the DHW valve is closed and that completes the CYL circuit. This makes the VR65/Boiler enter HW mode. The advantage of this method is that unless the DHW valve opens, the boiler is not put into HW mode. Whereas in the previous option, if for any reason the DHW valve was stuck, the boiler is still kicked into HW mode.

    No other connections are required inside the VR65 because you still want the zones valves, HW kit, schedules, weather compensation etc to be controlled by Evohome. You don't use any other thermostats on the eBUS in conjunction with the VR65 because they will then override the VR33 and stop Evohome from being able to control the firing and temperature of the boiler.

    I hope this is useful to any one who has wanted to know how to use Evohome with dual heating mode.

  2. #2
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    Quote Originally Posted by bruce_miranda View Post
    I thought I would describe the two methods I have used for having a higher flow temperature for HW when using Evohome.

    I have a Vaillant 438 boiler. I use Evohome and OT. In order to use OT on the Vaillant boiler, I use a VR33 - which is nothing but an OT to eBUS translator.

    The Vaillant 438 is able to have two separate settings for CH and HW. However this is only available when using Vaillant's own eBUS controls and their wiring centre. When you use Evohome, a lot of that becomes redundant. The other issue is that Evohome has no way of telling the boiler that it's doing a CH or HW cycle, even though the OT protocol allows for this distinction, Honeywell/Evohome hasn't implemented that. Instead it simply raises the Target Flow temperature to constant 90C whenever HW is to be heated. The 90C flow requirement is deliberate and is actually relying on the boiler clamping that down to something more sensible. Luckily the Vaillant boiler's Flow temperature knob remains operational even when using OT via the VR33.

    Solution 1:
    My previous solution involved the following components. An HGI80, an eBUS monitor and Domoticz.
    My boiler Flow temperature dial was set on max on the boiler. Using the HGI80 I would detect when Evohome was performing a HW reheat, that would trigger a script in Domoticz. The script used the eBUS monitor to issue a command that basically varied the "Maximum Heating Dial setting". So when Evohome was doing HW, I would set the max of that dial to 70C, but when it was only doing CH, I would reduce that max dial setting. This solution relied on the Vaillant ability to set and clamp the max flow temperature the boiler was heading towards.
    This solution worked quite well for many years, but it relied on too many things.

    Solution 2:
    Using a VR65 wiring centre. The VR65 has many functions and allows for the pumps and valves etc to be all connected through it. But one of the features of the VR65 is that it can be used to detect when HW is required and that in turns makes the boiler behave differently.
    Normally you would use an NTC sensor VR10 to detect the temperature in the HW cylinder. But with the Evohome you don't need that. Not having the NTC connected to the VR65 means that the Hot Water temperature dial on the boiler does nothing. The VR65 operates in a default HW priority mode. This means that while the cylinder thermostat is closed, the VR65 makes the boiler uses it's HW settings. If the cylinder thermostat is open, the VR65 assumes that the HW heat demand is fulfilled and then reverts to its CH heating mode.
    Besides setting a higher target flow temperature (D.78), you can even set a different KW partial load (D.77) to use when in HW mode. This feature is brilliant. One of the issues with the Vaillant 438 and Evohome is that very quickly the boiler becomes over-sized for the house, as zones' heat demand start to get fulfilled. But you still need the higher load for HW. The VR65 allows for that. So for example, you can have a 18Kw boiler load (D.0) @ max 55C (D.71) for CH and then use 28Kw boiler load (D.77) @ max 70C (D.78) for HW. Ofcourse in CH mode the D.71 is actually the clamped maximum, and OT will often use lower target flow temperatures when in CH mode, visible in D.9

    The trick to use the VR65 in this dual heat manner with Evohome is.
    a. You can either send the same Switched Live from the DHW BDR91 to both the DHW zone valve and the Right hand terminal of the CYL terminals in the VR65. So that way, when Evohome calls for HW, it opens the DHW zone valve and makes the VR65/Boiler enter HW mode. The wiring is becomes very simple. You simply send the Brown wire of the DHW zone valve to the right hand CYL terminal as well.
    b. The other alternative is to use the microswitch in the DHW zone valve to bridge the contacts of the CYL terminal. So you connect the Grey and Orange wires of the zone valve to the CYL terminals, as dry contacts, with no Live connected to either Grey or Orange. What happens then is when the Evohome DHW BDR91 makes the DHW valve open, the micro switch inside the DHW valve is closed and that completes the CYL circuit. This makes the VR65/Boiler enter HW mode. The advantage of this method is that unless the DHW valve opens, the boiler is not put into HW mode. Whereas in the previous option, if for any reason the DHW valve was stuck, the boiler is still kicked into HW mode.

    No other connections are required inside the VR65 because you still want the zones valves, HW kit, schedules, weather compensation etc to be controlled by Evohome. You don't use any other thermostats on the eBUS in conjunction with the VR65 because they will then override the VR33 and stop Evohome from being able to control the firing and temperature of the boiler.

    I hope this is useful to any one who has wanted to know how to use Evohome with dual heating mode.
    Thanks bruce_miranda. Even though I'm a 'MasterTech' Vaillant installer I get very few people requesting either Evohome or Vaillant Controls so haven't managed to get stuck into a project where I can use both. However it was something I was looking into when I come to replace my old Glow Worm boiler eventually. Having the Evohome System I was wanting to tap into the CH and HW demands on the Vaillant boiler, I haven't decided whether I want to go down the route of the VR33 yet.

    Do you just use the Vaillant VR65 wiring centre without any Vaillant controls? I was unaware a Vaillant wiring centre would work without any of their own controls. Obviously Vaillant never tell you this as they want to sell you the whole lot.

  3. #3
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    Thanks Bruce for collating this info in one place.

    mtmcgavock, I don't have evohome, but I do want to use weather comp with my on/off controls (nest multi zone). The VR65 will allow this (by boosting flow temps for DHW) combined with a VRC controller (thermostat function turned off).

  4. #4
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    You don't need to use any Vaillant controls besides the VR65 wiring centre. I don't.

    Look in the VR65/VR66 manual and you will notice that everything on the eBUS side is marked as optional. Including the NTC sensor. And with Evohome the NTC is unnecessary too, because Evohome is controlling the HW temperature cut. You just need to set the HW max flow to be about 10C than you will ever need via Evohome.

    If you wanted Vaillant's weather compensation, you can fit the VRC into the boiler front panel and it becomes a scheduler and weather compensation controller, and looses its ability to act as a remote Thermostat.
    With Evohome, I see no point in that either because schedules are managed by Evohome. I just wanted the ability to use a higher flow temperature for HW and a much lower flow for CH.

    Just the VR65/66 allows that, with the hack to use the CYL Thermostat input on the VR65/66.

    Some of the other Vaillant boilers don't even need the VR65 because they have the CYL inputs on the boiler PCB itself. Close that, and the boiler enters HW heating mode. It's magic.

  5. #5
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    This is what my VR65 connections look like. Notice there is no valves, or NTC connections.

    The eBUS is connected directly to the eBUS connections in the boiler. As my VR33 has its own slot. But even without a VR33, the connections would have been the same. You use whatever you want to trigger Terminal 4 on the boiler.

    The Grey and Orange are just a loop round my DHW zone valve microswitch with no Live connection made to either of them inside my traditional wiring box. Ofcourse Grey is Live 230V because the Left hand CYL terminal is Live and when the DHW valve opens, the Orange goes Live and the boiler is fired up using the HW settings in D.77 and D.78.
    Attached Images Attached Images

  6. #6
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    What is the purpose of d.77? I understand it allows you to lower the kW output for HW, but I've always found my boiler automatically modulates down when heating my megaflo (which I believe has a 22kw coil). So not sure if there's any advantage in lowering it?

    On a separate note, do you have priority hot water? As it stands, because I won't be using the zone valve connections on the Vr66, its possible my central heating zone valves will be open during HW heating cycle. I'm contemplating whether to do anything about it or just leave as is.

  7. #7
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    I believe you have a LLH, which is probably why you have never seen the need for the partial load settings. I don't, so if the load is too high, my boiler will go into an S.53 because my pump isn't able to shift the hot water away from the boiler fast enough. Also I have Evohome and I am rarely heating the entire house. Since the boiler was specced the entire downstairs is now UFH and that has lowered the requirements even further.

    In terms of HW priority, by the time my HW schedule comes on, most of my rooms are already at temperature and so the valves are shut. The other way to deal with the HW boost is to time your HW reheat to when the radiators are the coldest.
    Otherwise you could always use the HW priority features of the VR66 itself. But then you need to rewire your valves etc.

  8. #8
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    Another advantage of Evohome and the VR65 is that you can set the HW differential on Evohome.

    In the VR65, the differential is always 5C with the NTC and almost 0C if using a standard cylinder Thermostat.

    For anyone with a Vaillant and Evohome, having a VR65/66 is almost a no brainer, just to get the dual heat modes.

  9. #9
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    Bruce, speaking to notorious on diynot, he said d.78 sets the HW flow temp *regardless of what the CH knob on front of the boiler is set to*.

    This would mean a vr65 alone would allow setting HW temps higher than CH, without the need for any other ebus controller.

  10. #10
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    D.78 is effective only while the CYL terminals are closed (looped). And yes then, any boiler knob position is ignored and the flow temperature ramps up to what D.78 is set to.

    When the CYL terminals are open the, boiler front knob limits the temperature of the Flow, up to a maximum set in D.71.

    And like I said, all eBUS controls are optional any way, so none are needed to manage this dual heating mode.
    Last edited by bruce_miranda; 7th April 2021 at 01:32 PM.

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