Boiler Pressure at Zero: Can You Still Use It?

Why a boiler will not run at zero pressure

A sealed heating system needs water pushed round it under a little pressure. Without that, the pump can draw air, water can boil in the heat exchanger and parts can overheat quickly. To stop that happening, every modern sealed boiler has a water pressure sensor or switch that shuts it down when the pressure falls too low, which on an Ideal shows as the F1 low water pressure fault. The cut-off varies by make and model, but most lock out somewhere below about 0.3 to 0.5 bar, and some show a warning before that.

That lockout is the boiler protecting itself, not a failure. It is also why you cannot get round the problem by resetting over and over. Until there is water pressure again, the boiler will keep stopping.

On a combi the same circuit heats your tap water through a small plate heat exchanger, so zero pressure means no hot water either. If you have a system or regular boiler with a hot water cylinder, an immersion heater, if fitted, will still give you hot water while you sort the boiler out.

Is the gauge really at zero?

Check this before you open anything. Mechanical dial gauges do stick, and a needle resting on zero while the radiators are still hot and the boiler shows no fault is a gauge problem, not a pressure problem. Tap the glass gently. If the needle does not move while you add water, and the boiler is running normally, the gauge itself may have failed.

The other thing to confirm is that you have a sealed system at all. If there is a small tank in the loft feeding the heating, you have an open vented system. It has no pressure gauge and the pressure advice here does not apply.

How to refill a boiler from zero

Refilling from zero takes longer than a normal top-up, because the pipes and radiators may have lost more water than usual and air will have got in. Go slowly.

• Turn the heating and hot water off and let the boiler cool for about an hour
• Find the filling loop under the boiler: a braided hose with two valves, a built-in key point or a lever, depending on the model
• Open the valves slowly, a quarter turn at a time, and watch the gauge climb
• Stop at 1 to 1.5 bar and close both valves fully, then disconnect the hose if it is the removable type
• Reset the boiler once using its reset button, then turn the heating on
• Bleed the radiators if they gurgle or are cold at the top, and top up again afterwards if the pressure dips

If you smell gas at any point, stop topping up. Open windows, turn the gas off at the meter if it is safe to, leave switches alone, then get out and call the National Gas Emergency Service on 0800 111 999. Never open the boiler casing.

What if the pressure will not go up?

Sometimes you open the filling loop and nothing happens. Work through these before assuming the boiler has failed.

• The main stopcock is closed, or partly closed after other work. Check the kitchen cold tap runs at full flow
• The mains water pressure is low in the street. If every tap is weak, the problem is the supply, not the boiler, and our low water pressure repair page covers that
• One filling loop valve is still shut. Braided loops have a valve at each end and both must be open
• The filling loop is blocked with scale or the check valve inside it has stuck

Our guide to a filling loop that is not working goes through each of these in more detail.

How did it get to zero, and can it wait?

Pressure rarely drops from 1 bar to nothing without a reason. How quickly it happened tells you how urgent it is.

If it drifted to zero over weeks or months, you have a slow leak, and the article on pressure dropping every day sets out where to look. Refill it, keep the heating on and book a repair.

If someone has just bled every radiator, had a radiator taken off to decorate or had the system drained, zero is expected. Refill and the job is done.

If it went from normal to zero overnight or within a few hours, a lot of water has gone somewhere. Look at the relief valve pipe outside, which may have been discharging, then check ceilings, floors and the area under the boiler. If water is coming through a ceiling near a light fitting, do not use the light switch. Turn the power off at the consumer unit if you can reach it without touching water. Refilling the boiler now will only feed the leak. Leave it off and call for help that night.

What an engineer does when pressure keeps going to zero

An engineer will refill the system and watch what happens to the gauge with the boiler cold and then hot. A pressure that races up to 3 bar as the water heats and then lets go through the relief valve points to a flat expansion vessel. A pressure that falls with the boiler off points to a leak on the pipework, radiators or inside the boiler.

Inside the casing, the usual leak points are the pump seals, the relief valve itself, the heat exchanger and the plastic or brass hydraulic block on combis. An engineer can see these once the case is off and will tell you what has failed before any work starts. That is the core of a boiler pressure repair. Gas work is carried out by Gas Safe registered engineers.

The £60 call-out applies at any hour, and the repair price is agreed with you on site before any work starts. Typical UK market prices for the common fixes are around £90 to £200 for a relief valve and about £180 to £450 for an expansion vessel. A leaking heat exchanger is the expensive one and can make replacing an older boiler the better choice.