Hello,
if you are using Homeamtic valves HM-CC-RT-DN to control the heater in your rooms and you have some bigger rooms, it’s a good idea to use external temperature sensors. In most cases you’ll get a non linear offset between wanted, at the heater measured and real temperature. Therefore Homematic sells additonal temp sensors (which are quite expensive).
As workaround I’m using some cheap one-wire sensors in FHEM (I’m playing with OH2 and think of migration from FHEM to OH2) to have those temp values forwarded to the Homematic valve ones.
In Homegear it was declined to build in some direct support of using non Homematic devices, cmp. to Homegear future request -> it got the state won’t fix.
In FHEM you could use some virtual devices peer temp sensor with valve to exactly achieve this. So you could use very simple and cheap window contacts, temp sensors and so on.
I already thought about creating some own very simple PID rule for this. Disadvantage would be the valve would show not the desired temp and desired temp could be only set by OH2 (via some virtual temp item).
Is there any workaround for this problem known in OH2? I couldn’t find any up to now 
Robert