I have spent a lot of time trying to use the WaterHeater:HeatPump in OS but I always have a problem with heat loads. First I used a standard boiler (which works perfectly) and then I followed @Kyle Benne instructions to use this component instead.
However, this post (see @Archmage's answer) shows there are significant differences between the two systems. My problem is similar but I can't figure exactly what I have to add and where. I tried to add at least a HeatExchanger:FluidToFluid but It caused node problems... I think I need some help to understand this component before doing more tests.