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Description
If your boiler runs boiling hot or shuts off during use — firing up and then cutting out while you’re mid-shower — then low incoming mains pressure is almost always the cause, and the SP21S is likely the best pump for you.
Combi boilers shut themselves down when incoming pressure falls below 1 bar (10m head), and in many UK homes the mains sits right around that line — especially at busy times of day when the whole street is drawing water. The whole house feels it, because every hot tap depends on the same boiler.
Whilst you may have been told your mains pressure is out of your hands, there is now a pump that fixes it.
The SP21S for combi boilers is two automatic booster pumps fitted in series on the cold mains, before your boiler. Together they add 0.4 to 0.6 bar of pressure — enough to keep the boiler active throughout the home, . This is the whole-house solution.
The pumps detect flow and start by themselves — no switches, nothing to turn on. The SP21S is WRAS approved and fully compliant with UK Water Byelaws, which makes it legal to fit to the incoming water supply. WRAS approval means the pump is safe to connect to your home’s drinking water.
Q: How do I know whether I need the SP21S or the SP2B?
A: If the boiler itself is cutting out, or the whole house is affected, it’s the SP21S fitted before the boiler. If the boiler is fine and one shower suffers when other taps run, it’s the SP2B fitted on the hot pipe after the boiler.
Q: Why two pumps?
A: A single pump adds a single boost. The SP21S runs two pumps in series on the same pipe, doubling the pressure gain — that’s what lifts a struggling mains supply above the boiler’s cut-out threshold.
Q: My mains pressure is very low. Will the SP21S be enough?
A: The SP21S adds 0.4–0.6 bar, which solves most cut-out problems. If your mains pressure is unusually low, call the team on 01928 620 099 before you buy and they’ll talk you through it.