Vaillant ecotec 418 wont heat both water and rads succesfully

9 Mar 2011
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United Kingdom
I have a Vaillant 418 on a Y plan. I'll try to explain problem briefly -- I can't get it to do both water and rads at the same time. With the 3 port valve in mid position (d70=1), the boiler only uses the D77 setting of 7KW and so it doesn't have a high enough KW output to do both rads and water. . D77 has to be set to 7kw to limit the heat output for when it is only doing water otherwise it goes to anti cycling mode (S28) all the time.
Why doesn't the boiler use the D0 setting of 18kw when the 3 port valve is in the mid position?.

I have the Vaillant VR65 , with Tado extension kit, and use the eBUS connection from the VR65 to boiler for control signals. The boiler recognizes when it is receiving signals for water and\or heating OK as the radiator\tap symbols come on as the boiler panel as appropriate. I just can't get it to use the heating KW D0 setting of 18kw max when the 3port valve is in the mid position
The only way I can run the system successfully is in water priority mode (D70=0), it's not ideal because you can't have the rads on until the water is heated up.

Any help \suggestion appreciated.
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What are you using to control the hot water temp ? vr10 or cylinder stat ?
I've tried both the cyl stat and the VR10 (not both connected at the same time!) . using the cyl stat the valve goes to the mid postion but with the VR10 it never seems to go to the mid position even though D35 on the boiler does change to "40" (centre position) when both heat and water are on.
Can't make sense of it :) .
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Yes, I've done all that and set D70 to "1" , but when using the VR10 the valve doesn't go to the mid position, even though the boiler D35 settings shows it as 40 which should be mid position. When using the cyl stat it does go to the mid position. Problem then is the boiler doesn't ramp up the KW and uses the D77 water limiting KW setting of 6KW (or whatever it's set at) and hasn;t enough power to heat both water and rads. If D77 is changed to allow to go to18kw it then can;t heat just the water without going into anti cycling mode. (which is where I started in my first post) .
I think there is a special plug to connect the vr10 directly to the boiler, the boiler then ramps up when a call for hot water is made.
The Vr10 currently connects into the VR65 and I think that tells the boiler via the eBus what the state of play is. The boiler gets the message somehow because its D35 setting shows the changes and shows "40" which is supposed to be mid position, but the actual valve doesn;t go to mis postion when the VR10 is used??. It does go to mid position if the cyl stat is plugged into the VR65 instead but still have the problem of the boiler using the hot water limiting KW setting on D77 of 6kw. It's how to get it to use the heating KW max setting in D0 of 18kw when the valve is in the mid position that's the problem.
As I said there is a plug that plugs directly into the boiler, the 400 series of boilers use this connection to directly control the heat input to the cylinder, do a search on here for vr10 to vaillant 400 series.
Don't think the boiler should need the vr10 directly connected if the vr65 and eBus is used?? . The vr10 came with the vr65 and has the plug and connector to plug sraight into the vr65 . It would be difficult to run yet another wire from the hot water tank back to the boiler for just the vr10.

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