Worcester Bosch H07 fault — low pressure, and how to fix it

What does the Worcester Bosch H07 fault mean?

H07 on a Worcester Bosch boiler means your central heating water pressure has dropped too low for the boiler to run properly. It's a low-pressure warning, not a breakdown — and most of the time you can fix it yourself in a few minutes by topping the system back up.

Worcester Bosch's own Greenstar manuals describe this as "Fault 1017 — system pressure too low", shown when the pressure falls into the red/yellow zone below about 0.8 bar. The boiler limits its performance in both heating and hot water until you put the pressure back.

One quick note before you start. H07 is a newer Greenstar display code (Greenstar i, 2000, 4000, 8000 Life/Style). A few websites confuse it with a software fault or low gas pressure — that's a misread. On Worcester's own boilers, H07 means low system water pressure. Codes do vary a little between Greenstar models, so check your boiler's manual for the exact wording.

It's also easy to muddle H07 with HO7, H7 or a different H-code — they're not the same. If your display clearly shows H07, low pressure is your culprit.

H07 at a glance — the key facts

What's the right Worcester Bosch boiler pressure?

What causes the H07 fault?

Most of the time H07 is harmless — your sealed system has slowly lost a tiny bit of water over the months, or someone bled the radiators and forgot to top the pressure back up afterwards. A quick refill sorts it.

But if H07 keeps coming back, the boiler is trying to tell you something. The table below runs through the usual causes, from the simple to the engineer-only.

Why does the pressure keep dropping?

A one-off top-up is normal maintenance. Pressure that falls again within a day or two — or that needs topping up more than two or three times a year — almost always means one of three things: a small leak, a tired expansion vessel, or a weeping pressure-relief valve.

A failed or flat expansion vessel is the most common hidden cause when there's no visible leak. It can't soak up the expansion as the water heats, so the pressure swings up when hot and drops when cold. None of these is something you should chase yourself — but they're all straightforward for a Gas Safe registered engineer.

H07 causes and symptoms

How do I fix the H07 fault? Repressurising your Worcester boiler

Topping your pressure back up is a safe homeowner job — there's no gas work involved. The trick is knowing which filling system your boiler has, then opening the valves slowly so you don't overshoot.

First, work out which one you've got by looking underneath your boiler:

1. External filling loop

A silver braided hose with a tap or valve at each end, linking two pipes under the boiler. The most common on older Greenstars.

2. Internal keyed filling key

A plastic key (often clipped under a flap beneath the boiler) that slots into a black port. You turn it to the open-padlock symbol to let water in.

3. Keyless internal filling link

A blue lever at the bottom of the boiler that you pull down to let water in, then release.

Whichever you have, the method is the same idea: boiler off and cold, open slowly, watch the gauge, stop in the green band, close up firmly.

How to repressurise a Worcester Bosch boiler — step by step

Do this with the boiler switched off and the system cold. Take it slowly.

• Switch the boiler off at the fuse spur or controls and let it cool. Find the pressure gauge or pressure menu so you can watch the reading.
• Locate your filling device underneath the boiler — the braided loop, the keyed port, or the blue keyless lever.
• Open the first valve (or insert the key / pull the lever) slowly. On an external loop, open the mains side first, then ease the second tap open.
• Watch the gauge climb. The moment it reaches about 1.0–1.5 bar, stop. Open the valves slowly so you don't shoot past 1.5 bar.
• Close the valves firmly (heating side first on a loop, then the mains side), or return the key to the open-padlock position and remove it, or release the blue lever.
• If you used an external filling loop, detach it once you're done — leaving it connected risks back-contamination into the mains.
• Turn the boiler back on. The H07 code should clear once pressure is back in range. Reset only once if it lingers — and never keep resetting if it returns.

Is the H07 fault dangerous?

No. H07 is a low-pressure warning, not a safety lockout. It isn't a gas leak, it isn't a carbon monoxide risk, and it isn't a 999 emergency. The boiler is simply protecting itself until the pressure is back.

Because of that, the National Gas Emergency line — 0800 111 999 — is not the number for H07. That line is only for a smell of gas or a carbon monoxide alarm. If you ever smell gas, leave that to one side, call 0800 111 999 straight away, open the windows, turn the gas off at the meter, don't touch any electrical switches, and leave the house.

For H07 itself, the worst that happens is no heating or hot water until you top up. It's safe to repressurise and carry on. If it won't hold pressure, that's an engineer job, not an evacuation.

What can I do safely, and when must I call a Gas Safe registered engineer?

Here's the honest line between the two. Repressurising is yours to do. Anything that involves finding or fixing the reason the pressure won't hold is an engineer's job.

Safe to do yourself

Topping the pressure up via the filling loop or key. Having a look at radiator valves, visible joints and pipework for damp or drips. Checking whether the small overflow pipe on your outside wall is dripping — a sign the pressure-relief valve is weeping.

Call a Gas Safe registered engineer when

Pressure drops again within 24–48 hours, you're topping up more than a few times a year, you've found a leak, the overflow pipe outside is dripping, or you suspect the expansion vessel. Recharging or replacing the expansion vessel, fixing a leak, swapping the PRV and any work on the gas side all need a registered engineer — never attempt gas work yourself.

You can check any engineer is properly registered on the Gas Safe Register. Worcester Bosch also runs its own customer line on 0330 123 9559 if you want to talk to the manufacturer.

How much does it cost to repair an H07 fault?

If it's just a top-up, it costs you nothing — it's normal maintenance you do yourself. The cost only comes in when H07 keeps returning and an engineer needs to find the cause.

As a rough guide using typical UK industry ranges (not our prices, and worth confirming with a local quote): a gas engineer's call-out and diagnosis is often around £50–£60 an hour plus parts. An expansion vessel recharge or replacement is commonly in the £120–£350 region fitted, and can be more on hard-to-reach boilers; a pressure-relief valve swap is often £100–£250 fitted. A pressure sensor is usually a low-cost part, so the bill is mostly labour, broadly £120–£200 fitted. Leak detection varies depending on where it's hiding.

Repairs add up, especially on an older boiler where one failed part is often followed by another. That's where ongoing cover earns its keep — more on that next.

Pressure keeps dropping? Here's how Smart Plan helps

A one-off top-up is fine. But if you're reaching for the filling loop every few weeks, your boiler is telling you there's a real fault behind it — a leak, a tired expansion vessel or a weeping valve — and that's not something to keep papering over.

Smart Plan is a service plan, not insurance. When the simple fix runs out, you've got two easy options.

Need it fixed now?

Book a one-off repair and we'll come and fix it for you. A Gas Safe registered engineer traces why the pressure won't hold — expansion vessel, leak or valve — and gets your heating back on.

Want cover for next time?

Take out a Smart Plan boiler module and you're sorted when the next fault appears. Parts and labour are included up to your cover limit, and an annual service helps catch a tired expansion vessel before it turns into a no-heat breakdown.

Cover is modular, so you pick only what you want — don't pay for what you don't use. Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies. We've looked after over 15,000 customers, we've been trading since 2014, and we're rated on Trustpilot. When the pressure won't hold, we'll come and fix it for you.

Worcester Bosch H07 fault FAQs

Pressure won't hold? We'll come and fix it for you.

Book a one-off repair or set up an ongoing Smart Plan boiler module. A service plan, not insurance — parts and labour included up to your cover limit.

Helpful links

• All Worcester Bosch fault codes
• No heating? Book a repair
• Boiler & central heating cover
• More boiler guides on the blog
• Check an engineer on the Gas Safe Register