Heat Pump Cascades and buffers

[Jon Glanfield]

We are designing a system to include 2 heat pumps both feeding into 1 buffer. 1 will act as the lead unit, having hot water responsibility along with space heating. They will both tee together on the flow and return to the buffer. The second unit will only have an influence on space heating.

We usually assume 30 litres per kw from advice on here earlier, but do we size on the combined output of the 2 units or just on the lead unit given that this will have responsibility for initially heating the buffer?

The units are both 16kw air source.

Any advice would be greatly appreciated.

Cheers

Jon

[Bigfreeze]

You'll be fine on sizing for one unit John, as if the second unit is required to heat it means there is already a large draw off the buffer. The buffer will allow enough flow (once its a direct cylinder) for both units to operate at once and defrost correctly

[Jon Glanfield]

Thanks BF we thought this was the case but just wanted to be sure before we specced.

Cheers

Jon