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28mm primaries for the first 5m or so. That's how the original installation was done and I've subsequently been told many times its a good thing and recommended. The pump arrangement is very simple. The primary pump is controlled by the boiler with overrun. The secondary is on the 3 MVs SL.

I had a large system like yours put in about 10 years ago. Vaillant 438 boiler, 3 zones (u/s, d/s, hot water). 28mm primaries from boiler, reducing to 22mm throughout the house and then 15mm to rads. With multiple zones and a large boiler (in particular the vaillant, because of the pump head...

Yes, the VR33. Previous members on here have successfully used it on the Vaillant 4 series boiler. I considered it, however I didn't see any great advantage to the Nest (or any thermostat for that matter) controlling flow temps without weather compensation. The Vaillant weather compensating...

Not completely true as Nest supports OpenTherm, which also controls boiler flow temps in OT compatible boilers. I liked the Nest controls and their thermostat, but wanted to keep the weather comp and PDHW. The solution I have makes the 2 compatible.

No, both my nests are wall mounted and powered directly by the heat links (using my existing thermostat wiring, obviously now only supplying 12V).

I've not heard of 35mm mdpe before. You sure it isn't 32mm? If so, and you're going to 28mm copper, I'd recommend a plasson pushfit reducer (mdpe with insert on one side, and copper in the other).

The heat link can switch 230V zone valves. Take a look at the heat link instructions and specifically the wiring diagram for s plan. Terminal 5 is 230V common and terminal 6 is call for heat going to your cylinder stat, which then supplies 230V to your zone valve. If you measure the voltage on...

There's pictures on that thread I pasted above from Notorious. My setup is somewhat more complex with relays for DHW priority. I can send you pictures later if you need. The nest is wired directly to the zone valves, with HW ON wired to the VR65/66 to instruct it to boost the flow temp to 80...

You can use nest thermostats whilst continuing to benefit from the lower flow temps that ebus and the weather compensator provide. See here for details: https://www.diynot.com/diy/threads/vr65-vr66-for-hw-flow-temp-control.494071/page-2#post-4938942

The only difference between the single channel and dual channel receiver is that the single channel allows volt free / low voltage switching of heating for combi boilers. As you have zone valves, you'll need 230V switching of each valve anyway, so it makes little difference. If you wanted, you...

Oversized boiler, possibly. Has it been range rated? Check the ABV to ensure it is not always bypassing (may need replacing).

To clarify my post above - the MV should be doing the volt free switching. I havent tried to make heads nor tails of the picture of the wiring to ascertain whether it is or not. I agree with posters above that the 12V wiring should be kept completely separate and labelled accordingly.

OP has 2 x 2 port Danfoss valves. The MV will most likely be doing the volt free switching.

My understanding is that a large differential between flow and return could lead to the hex cracking. So there's zero leeway without the risk of damage. Like I say, I don't think this is vaillant specific - every manufacturer must have a similar safety mechanism.

I'm not sure this is a Vaillant specific issue, though. The 30 degree max differential between flow and return is to protect the heat exchanger. An influx of cold water from another zone would almost certainly exceed this if the flow temp is at 70 degrees at the time - this would be a problem...

Similar to mine, documented here: https://www.diynot.com/diy/threads/vr65-vr66-for-hw-flow-temp-control.494071/page-2#post-4938942 You're right that when DHW priority finishes, the boiler turns the flame off and then shortly refires in CH mode.

I didn't see your earlier posts, but I had the exact same issue with my 438. There is no solution to it, afaik. My boiler is correctly sized. The problem occurred with any of my 3 zones (u/s, d/s, DHW) and just as you said, only when one zone was running at very high flow temps and another...

Drayton Wiser uses wireless thermostats. You should locate the wiser hub close to the boiler, wired from a fused spur, and then link out the terminals where your current stat is (insulate the wires sufficiently, as they will be permanently live). You can then mount the wireless thermostat in the...

I agree with muggles completely that there are different standards of "service", as opposed to just a gas safety check. However, I disagree that price has anything to do with it. More expensive does not equate to more thorough. I would ask each GSE to explain what their service entails before...

In that case, it could be unbalanced supplies, as said by a previous poster. Do you know what your incoming cold pressure is? Putting a PRedV on the incoming cold is one solution.