I have the Ecotec plus 637 system boiler too. You’re getting the overshoot because with Vaillant eBus controls (vrt 350, vrc 470, with a vr65 or vr66 wiring center (for your 2 or 3 port valves), the boiler knows via ebus what the current room temperature reported by your controller is and the current temperature is in your hot water cylinder (via the NTC module for the Vaillant cylinder or for a normal non-Vaillant pressurised cylinder there is a cylinder thermostat that ‘clicks on’) and automatically adjusts the flow temperature to suit the temperature being reported so that it only burns the minimal amount of gas that’s needed.
With just a relay, it ramps up to 100% of the flow temperature set in the boiler because it’s not being told what the current temperature is which it normally can use to decide what to do (by default this is 75 degrees and recommended you stick to it). With eBus it’s more efficient and burns less gas and saves your bills, if it knows what the current temp is it will automatically reduce the target flow temperature.
For central heating 75 degrees is fine, for water you probably want it quite a bit less as target water temp in the cylinder tends to be ~60 degrees (to stop the perfect conditions for legionella bacteria and legionnaires disease growing inside your system and causing you to get sick), any more is too hot for most people as you will probably waste it by mixing it with cold water at your tap and shower anyway.
Now with ebusd and openhab, you could build a controller via openhab to handle it all, openhab 2 allows you to take action based on the information, i.e if cylinder temp drops a certain threshold, tell the boiler to switch on and give it a flow temp to hit, if the room temperature drops by a certain threshold to set the boiler flow temp etc.
However, you are better off with Vaillant controls. They’re not hard to wire up and the instructions are included (I did it all by myself). This is because the boiler has a heat curve already set inside by Vaillant engineers to allow it to be most efficient, which is better than trying to tell the boiler what to do when it already knows how, it just needs to know target temps and when you want it to come on.
What I use (or trying to use again as I haven’t touched it in a year) openhab for is to simply monitor what is going on via my phone or web browser and I can get email alerts for things such as error codes (when it gets a fault) or if the pressure goes up to the red (My heat exchanger failed due to overpressure recently so I had to have the boiler replaced 
)
You can’t use both however, there is a bridged ebus terminal behind the boiler PCB that you remove if you use ebus or the 24v bridge for 24v controllers, or 240v controllers, you can only run one or the other otherwise the boiler won’t respond.
Do you have the wiring center connected (vr65/vr66) or is it just a normal 240v junction box that wires the port valve(s) at your cylinder to the boiler? you would probably want a vr66 (vr65 is now replaced by a vr66) to control the valves, then via ebus you can tell the boiler what you want and it will tell the wiring center to open the valves. Having a controller too is much better as a fall back option if your openhab setup fails.
I’ll try porting my old config to openhab 2, I started writing a blog about it so I’m trying to set up a full guide.
