We have a Vaillant Ecotec combi boiler in the loft. When showering, hot water pressure is fine. But if someone opens any other hot tap in the house, it massively reduces the hot water coming into the shower (obviously because now two taps are sharing the same hot water). Is there a cost effective way to allow both taps to run at the same time but the pressure to remain constant so it doesn't interrupt someone's shower? It's very annoying if someone else in the house decides to use the hot kitchen tap while someone is showering.
There are several ways to fix this problem...
Increase the supply pressure of the incoming water feed.
Increase the diameter of the piping feeding to the shower.
Install flow restrictors in the shower head and the kitchen faucet so that they share the available pressure and flow at lower levels and thus less interaction for the existing piping and delivery pressure.
Your boiler and probably your piping also do not have sufficient flow rate to allow hot water distribution from two taps. A possible fix is to split the cold water supply in the kitchen and then install a separate compact kitchen boiler for hot water in stead of your current boiler in the loft.