I'm looking into a new heating/hot water installation based upon a heat pump (~14kW) and ancillary input from a woodburner backboiler (~5kW). Heating will be underfloor throughout. I'll be pumping the woodburner circuit (vented with dump rad) since the cylinder/buffer will be on the ground floor. There seem to be a few common configurations up for consideration;
A) Heat pump and woodburner both direct to a sealed buffer vessel (~150l) connected to the UFH. A separate cylinder (~250l) for DHW which is fed indirect from only the heat pump (if unvented) or also the woodburner (if vented?). Diverter valves to switch heat input from buffer to cylinder.
B) Heat pump and woodburner direct to a vented store (~600l). UFH connected to lower half of store. Mains pressure DHW from large bottom to top internal coil (~9sqm). Diverter valves used to direct the heat pump and woodburner circuits to the top or bottom of the store as required for DHW or UFH.
C) Like B but with pumped loop through plate heat exchanger for DHW.
D) Combination of A and B. The DHW unvented cylinder is a tank (~300l) inside the top half of the sealed 'buffer' (~450l) and is heated by the buffer. The buffer is fed direct by both the heat pump and woodburner and their inputs can be diverted as for B. The UFH is connected direct to the buffer.
E) woodburner connected direct to its own vented store (~200l). Heat pump connected indirect to DHW cylinder (~300l) and also direct to UFH with diverter valves to switch input. The store is connected to the heat pump return via a plate heat exchanger.
My initial assessement is the two tank solution probably requires more utility room space, unless I can stack the pair. I'm attracted to the single tank solutions because I can get mains pressure hot water and still easily have the woodburner input to the DHW. Solution E) satisfies this requirement but I have reservations about passing hot (70degC) water through the heat pump, perhaps this is not an issue? Also there is no UFH buffer in this configuration. I'd rather a vented storage system, which B) satisfies, but if the tank-in-tank solution is technicaly superior then I'd be prepared to forgo this for D). I would appreciate any comments on the relative merits of these configurations to help make up my mind
Martin