'X Plan' - What is it and why?

[Richard]

OK, so when we initially started testing evohome and other Honeywell OpenTherm thermostats with the Intergas boiler range (April 2017), it soon became apparent on Intergas open vented or system boiler applications that super high efficiency, low flow temperature, load compensated heating came at the cost of having to dump really high water temperature into the central heating circuit during hot water cylinder recharge periods when using traditional S Plan or Y Plan systems.

I was asked my thoughts by some friends at Honeywell and we eventually came up with the perfect Priority Domestic Hot Water (PDHW) solution which we unofficially defined as 'X Plan'.

X Plan will work with evohome on any heating system if you need hot water priority, but works exceptionally well with Intergas boilers on OpenTherm.

So what is X Plan and how do we fit it?

As the diagram below shows (this diagram is specifically X Plan with evohome), X Plan is very much like a traditional S Plan system. However unlike an S Plan system, there is a Normally Open (NO) 2 Port Motorised Zone Valve fitted on the heating system pipework and a Normally Closed (NC) 2 Port Motorised Zone Valve is fitted on the hot water cylinder pipework. When you have hot water demand to the cylinder, the NO heating zone valve is closed by the NC hot water zone valve on the hot water cylinder to give you a high temperature recharge of the hot water cylinder without the heating circuit being overheated. Think of it in terms of a combi boiler... When you have hot water demand, the central heating is turned off for a short duration.

x-plan-diagram.png (166.7 KiB) Viewed 156 times

The unique thing about X Plan is how simple the wiring is in relation to other systems. In the case of evohome (and just like the evohome Installation Guide Page 44 Figure 4) the DHW BDR91 still powers the NC valve open for hot water demand, but in the case of X Plan the orange wire from the NC valve powers the NO valve closed. So in the case of X Plan, the motorised zone valves are literally only powered during DHW demand meaning the valve motors will last for many years to come.

Below is a wiring diagram of X Plan and the DHW BDR91 used by evohome. The R8810 OpenTherm Bridge is not shown, as this is wired directly to the Intergas boilers OpenTherm terminals (Rapid, HRE & ECO RF X4 Connector Pins 11 & 12).

x-plan-wiring-diagram.jpg (187.66 KiB) Viewed 156 times

If you have an Un-Vented Cylinder you would need to remove the link wire between terminals 7 & 8 and wire these through the Un-Vented Cylinders High Limit Thermostat to comply with G3 regulations, but again very easy to do.

Obviously the above diagrams are evohome specific and only reflect one heating zone, however the principle is the same for Intergas boilers using 'Pin 5' technology and X Plan. Please see this section () for Intergas 'Pin 5' applications with X Plan.

Any questions, please ask below.

Thanks,

Richard