Vaillant VR66 wiring control centre causing heating modulation woes?

SoftTopGeek said:

That sounds good. In my situation, I would have the VR66 near the valves.

Mine is an open vent boiler.
Do I take it that the switch wiring in the motorised valves stays as it is then, whereby if either is powered up and is then open, there is a 230 V input to the boiler?
I see how if the VR66 has set the motorised valve open, it (VR66) would know what flow temperature it required from the boiler.
Also, if the value failed to open, I guess the boiler wouldn't fire at all.
Does that sound right?

Thanks for all the tips everyone.

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The VR66 does not have any feedback from the valves, it never knows if they are open/closed, it only assumes that they are open when powered.
This is in part why a ABV is a requirement as the pump may start before the valve opens and stays running after its closed.
If the valve fails closed then what should happen is that the boiler gets stuck overheating then running on bypass etc.
If you are using it with a regular programmer then the boiler trigger is always live as long as there is a "demand", the boiler does the rest with VR66.

rbranco said:

The VR66 does not have any feedback from the valves, it never knows if they are open/closed, it only assumes that they are open when powered.
This is in part why a ABV is a requirement as the pump may start before the valve opens and stays running after its closed.
If the valve fails closed then what should happen is that the boiler gets stuck overheating then running on bypass etc.
If you are using it with a regular programmer then the boiler trigger is always live as long as there is a "demand", the boiler does the rest with VR66.

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Does that also apply to the VR65 combined with a VRC 470f?

If I nudge the temperature up on the 470, there is a noticeably long pause, whilst my 3-port turns to the correct position, before the pump runs and the boiler does its initial purger before firing.

Using a VRC is slightly different as you do not toggle the boiler trigger as its always bridged and eBus just tells it to not fire.
Im sure there is some hysteresis on the VRC controller so it takes a few seconds before it applies your change.

The VR66/65 does not know anything about if you are using a 2/3 port valve. The delay will be the same on either setup.
As it does not know anything about the position of the valve it just adds a default delay which assumes the worst case scenario, my CH valve is slower than my HW valve.

rbranco said:

So you keep your HW tank at full temp 24/7 or do you switch the cyl stat wire?

VR66 does not need to inhibit CH as it handles both valves, so it just overrides the valves, its an effective inhibit.
Remember there is cross over of the VR66 and system boilers. The VR66 is invisible to VRC controls, it really only speaks to the boiler and then the boiler pushes out stuff on bus that the VRC needs.
My VRC does not show the VR66 as a component on the network, according to vaillant that is expected.

Ps. Dispite what the manual says on VR66 it is possable to use they cyl stat and NTC at the same time as backup and temp control. Odd but I did it as test and worked as I expected.

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Hi, I'm using the VR66 unit in monozone mode with the NTC. The system works fine, but the boiler continually gives me the f78 fault code when I check for faults - "Interruption to DHW outlet sensor at external controller. UK link box is connected, but the hot water NTC is not bridged" - my understanding is that the VR66 is the UK link box, so my question is how can the NTC be made to be bridged? Thanks in advance for any advice you can give.

Matthew B said:

Hi, yes, that's exactly how the boiler's supposed to work, according to the Vaillant advertising. The boiler is adjusting the flame so that once it reaches the maximum flow temperature we've allowed (65°C), it holds it there until it switches off. However, what's meant to happen next is that the vSmart reduces the flow temperature to maintain the room temperature. It's meant to stop the boiler switching on and off so often, which reduces wear.
When the boiler switches off, the display shows the standard "egg timer" symbol for anti-cycling. I haven't done any digging deeper into the status codes at that point. Is there anything particular I should look for?
The vSmart has been adjusting our heating curve setting. It's currently at 1. In the past we've tried adjusting the heating curve manually but it seems to make very little difference in terms of the eBUS request temperatures.

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@Matthew B - I have pretty much the same set up as you, and the same observations - when heating the room to the setpoint the boiler modulates in the sense that it turns the flame down as necessary to maintain the required flow temp as the temp of the returning water increases, but the requested flow temp itself remains constant during a single boiler firing period. So consequently I have overshoots like you, and I am manually setting the heat curve. Having tried an experiment this morning firstly with the vSMART unit in our snug, asking for 18.5C with a room temp of 17.5C, and then outside with a 'room' temp of 13.5C, I found that the flow temp being requested by vSMART remained the same (this was with completely turning off the boiler between to clear any residual settings) - flow temp requested was 67C using advanced alogrithm with heat curve 1.4 - so as far as I can tell, vSMART only seems to use weather compensation, with no input in terms of load/room compensation. I also find that the flow and return temps in my Ecotec 624 are only about 5C different once the system's been running for a while - the problem with this is that if I adjust the heat curve to ask for a lower flow temp, the boiler almost immediately goes into anticycling as the incoming return temp from the previously hotter setting is too close to the requested flow temp - and therefore I guess the same problem would occur if vSMART lowered the requested flow temp during a boiler firing period - so if vSMART is somehow checking the return temp, perhaps it knows that it can't turn the flow temp down without anti-cycling being triggered, so waits until the set point is reached, switches off, waits for the set anticyling time to finish (egg timer), and then uses a different flow temp for the next period.