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It’s still not clear whether the boiler cycling is the result of the return feed being somehow at a high temperature, which seems unlikely unless there is a short circuit bypass we don’t know about, or for some reason the flow through the boiler is actually reduced when both UFH and Radiators...

At full system startup when the boiler quickly brings the flow temp over 65C and is about to cut out what is the return temp? I’m assuming it must be fairly high and if so what path is this hot water taking through the system? If on the other hand the return temp is low there isn’t enough flow...

When you say the heating is not getting up to temp do you mean the radiators are not getting as hot as they used to?

Can you post some pictures to show the layout?

Does the pump have a number of speed settings and if so which is it set to. I have to run my own Vaillant on the highest setting in order to avoid similar problems.

I had to do this several years ago and it was fine except my Vaillant wasn’t happy if the water level in the bucket rose above the bottom of the pipe. Still can’t get my head around exactly why, but it resulted in a small amount of condensate flowing over from the trap into the boiler casing.

Apologies, I'm still struggling to follow the red wire from Heatlink terminal 3. It should be connected to the 3 port valve white wire but I can't see where it goes.

Can you post a photo taken from the left side of the wiring centre. It's hard to see exactly what's connected to the LHS of the terminals.

Not clear in the photo where in the Heatlink the first brown Live link is connected. Is it from L to terminal 2 as it should be?

The 450 can automatically alter the heating flow temperature setting depending on the outside temperature. Is there a connection to an exterior thermostat? I don't have any experience of using or setting up one of these units so hopefully someone else here will be able to advise on your options.

Hi, do you have any Vaillant controls connected which may be overriding the target temperature you are manually setting?

My 428 is over 15 years old now and I’ve no idea what I might replace it with when it packs in. If there are boilers which have separate connections so they can easily be fired up in either Heating or DHW mode, with associated temperature and power settings, I’d be interested in finding out.

It would have made things a lot easier if my 428 had the same connections for triggering DHW. Just had another close look at the manual in case I’ve missed it; unfortunately I don’t think so.

I was thinking that it simply wasn’t good to have 65C water going to the UFH but I now understand that isn’t a problem for your system when both heating and HW are required together, so apologies for the diversion and no need for a relay.

I definitely had to use a relay because without it the 3 port valve would have been in the mid position for both heating and DHW when both were calling for heat, probably with the boiler delivering 70C flow temps to both cylinder and radiators. If I had used the VR65 as it is designed with the...

I would think it is pretty much irrelevant which method you use for triggering heating, whether 24V or the mains voltage alternative. I doubt the boiler would operate differently. Unfortunately I can’t absolutely guarantee that the ebus DHW signal will take priority over the heating signal as...

Hi I’m using a VR65 on my 428 and you should hopefully find enough info on the thread below to get started. https://www.diynot.com/diy/threads/vr65-vr66-for-hw-flow-temp-control.494071/

Can you post photos of the thermostat and the wall box near the boiler?

That’s useful information. When I started using a Vaillant ebus controller to handle the DHW side of things it did get rid of the startup burn which used to be at about two thirds of full power and sometimes caused the boiler to trip. I hadn’t realised that using the official Vaillant room...