I tried replying to your message directly but I can't as I am not on your friend list. Hi, thank for replying. Glad you got it sorted out in the end, I think the issue with flatter curves and low flow temperatures is that the boiler can reduce the power enough to simply run nicely without cycling at that temperature, this isn't too much of an issue for me anyway in all but the mildest weather as our house is not very well insulated so usually flow temperatures of >45ºc are needed. I have just got a VRC430f but the outside sensor is not currently outside but from the little testing I have done, it seems to work pretty well. Though I am still unsure what the difference is between "Thermostat' and 'Modulating' Did you manage to find out?