What it means
What A1 is telling you
A1 tells you the boiler isn't seeing enough water flow through it. The burner will not fire properly without circulation, so the appliance protects itself.
On Greenstar boilers this is most often a pump that has seized after a summer of standing idle, which is exactly why so many A1 faults appear on the first cold morning of the year.
It can also be caused by sludge restricting the heat exchanger, a blocked filter, or air trapped in the pump head.
Before you call
Safe checks you can do yourself
- Check the pressure gauge and top up to between 1 and 1.5 bar if it's low
- Listen at the boiler — silence where you'd expect a gentle hum often means a seized pump
- Check the magnetic filter, if you have one, isn't completely clogged
- Reset once, then call us if it returns
Never remove the boiler casing or work on the sealed combustion chamber yourself — by law that’s Gas Safe registered engineers only. If you smell gas, call 0800 111 999 immediately.
How we fix it
What we do on the visit
- Pump freed, tested under load, or replaced with a like-for-like unit
- Air bled from the pump head and the system re-pressurised
- Magnetic filter cleaned out and inhibitor levels restored
- Heat exchanger flushed where restriction is the real cause
How urgent is it?
No heating until it's resolved. Report it in the morning and we aim to attend the same day.
Costs
What a repair costs
Our diagnostic visit is £99 inc VAT and it comes off the bill if we carry out the repair. We give you a fixed price before touching anything — no hourly rates, no surprises on the invoice.
We’re Gas Safe registered, Which? Trusted Trader endorsed and rated 4.9 across 206+ reviews. We carry the common Worcester Bosch and Viessmann spares on the van, so most faults are fixed on the first visit rather than booked back in.
Common questions
How much is a boiler pump replacement?
Typically £220–£380 inc VAT fitted, depending on the pump. We'll confirm the exact price before doing the work — never after.
Why do pumps fail at the start of winter?
They sit still all summer and the impeller sticks. Running your heating for ten minutes once a month through the summer genuinely prevents a lot of A1 callouts.
Do you carry pumps on the van?
We carry the common Worcester and Viessmann spares including pumps, so most A1 faults are fixed on the first visit rather than booked back in.
Book it in
Get an engineer out
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60 second boiler quote
No callbacks unless you want one
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