Fault guide
Worcester Bosch H07, why your boiler pressure keeps dropping
H07 is a low pressure warning rather than a broken boiler. The boiler is fine. The system has lost water, and the interesting question is where it went.
What H07 actually means
A sealed heating system holds a fixed volume of water under pressure. The gauge should sit around 1 to 1.5 bar when cold and rise a little when hot. When Worcester Greenstar boilers see pressure fall below roughly 0.7 bar they display H07 and eventually stop, because circulating too little water risks overheating the heat exchanger.
Topping it up, and why that is not the fix
Refilling is easy and safe to do yourself, but treat it as a stopgap.
- Find the filling loop, usually a braided hose or a keyed valve under the boiler.
- Open it slowly with the boiler off and cold, watching the gauge.
- Close it at about 1.2 bar, then reset the boiler.
- Note the date. If you are back within weeks, you have a leak, not a quirk.
Where the water is actually going
A leak on the system is the obvious candidate: a radiator valve weeping, a compression joint under a floor, or a corroded radiator. Small leaks evaporate before they stain, which is why they go unnoticed for months.
A failed expansion vessel is the second cause, and it is the one most often missed. As water heats it expands, and the vessel absorbs that expansion. When the vessel loses its air charge the pressure spikes when hot, pushes water out through the pressure relief valve, and reads low again once cold.
A passing pressure relief valve is the third. Look for the small copper discharge pipe outside; if it drips, water is leaving there.
How we diagnose it
We pressure test the system, check the expansion vessel charge against the manufacturer figure and recharge or replace it, inspect the pressure relief valve, and trace visible or suspected leaks back through the pipework. Only then do we refill and re-inhibit.
That is the difference between fixing H07 and topping it up forever.
Questions we get asked
- What does H07 mean on a Worcester boiler?
- H07 is a low water pressure warning. System pressure has dropped below the level the boiler needs, usually under about 0.7 bar, so it warns you rather than running dry.
- Can I just top the pressure back up?
- Yes, using the filling loop, back to around 1 to 1.5 bar cold. But if you are topping up more than a couple of times a year the water is going somewhere, and that is what needs fixing.
- Why does the pressure keep dropping?
- The three real causes are a leak on the system, a failed expansion vessel, or a pressure relief valve that is passing water out through the discharge pipe outside.