Vaillant Ecotec 418 regular boiler; which hot water cylinder to use?

[pbanks]

All works great, except that now it cannot run HW and CH at the same time - if there is a demand for HW the CH has to wait until it is satisfied. The d.70 setting on the later 418 does not exist.

So the newer 418 boiler doesn't have a D70 setting to set the position of the 3 port valve to the centre position??. So it can't be set to try and heat the water and the rads at the same time?. Sounds strange.
You're are in the same situation as myself though as I have to set D70 to 0, (water priority) so it heats the tank water to temp before it will move the valve to do the rads. If I set D70 to 1 (centre position) it will try and heat both but because D77 has to be set to only 5kw max boiler output (so that when it is ONLY heating the water it doesn't go up to full 18kw & cycle all the time). The problem seems to be that the boiler doesn't recognise that when the valve is in the centre position it should ignore the D77 (5KW max) & ramp up until it is ONLY do the water. I have tried setting D0 (heating part load) to limit the max KW down from 18kw to find a happy medium where it would heat both water & Rads OK, & do only the water or only rads but haven't succeeded.
Overall I can live with it as it is now and it's only a problem if hot water is run off during the day(bath etc.) and you want to turn the heating on at the same time. Just means I either manually override it from my phone app to temporarily delay the water heating or wait for the cylinder to heat up to temp.

 

[Harry Bloomfield]

So the newer 418 boiler doesn't have a D70 setting to set the position of the 3 port valve to the centre position??. So it can't be set to try and heat the water and the rads at the same time?. Sounds strange.

I thought it strange too and I do not understand why the system does not allow it. My 3-port is able to go to mid-position and went to mid with my old control system. No, there is not a d.70 in the manual or in the boiler, but the big advantage is separating the flow max temperatures, much more refined CH temperatures and the modulation - so overall worth my loosing the mid position. It is only noticeable when there is a demand for the full tank to be reheated, when it is really cold outside.