Boosting Pressure and Flows in Gravity Systems with Mains Pressure Cold
If your cold is mains supply and your hot is gravity the normal solution is a single ShowerPowerBooster on the hot. For 80% of people a single pump fitted in minutes – job done and a big smile.
But for perhaps 20% of customers it is not as straightforward.
For a very challenging kitchen tap a SP21S is the only solution. You can double boost the hot as there is no chance that this will imbalance the cold, so go for a SP21S. The extra flow you get adding a second pump on the hot is rarely worth it for an upstairs shower.
The majority of mixers will balance differential pressures of 4 or even 5 to 1 so no problem. For some shower/tap mixers there is a requirement for balanced flow (similar pressures on hot and cold).
If you have a shower mixer which needs balanced flow the high pressure cold restricts the low pressure hot so as soon as you add a single shower power booster to the hot pipe you get very good shower.
Before the pump starts the high cold pressure can hold the hot flow back. If the reduced flow on the hot falls below the 1.5 litres a minute needed to activate the flow switch and you may need to fit a pressure reducing valve on the cold only.