Richard McGuire
Hi Alison. It's possible to reduce the flow rate from a combi, but the boiler will only reduce the hot. In order to balance the supply you'd have to recuce It as The cold main enters the building. Ultimately however, it sounds very much like your issue is with the shower drainage. Maybe it flows slightly uphill when exiting the trap? This is what I would focus on first. I hope this helps. Rich
