Boiler pressure keeps dropping: how to top it up (and when it is a leak)

Is your boiler pressure dropping? Then top up the system yourself to the pressure given in your boiler’s manual. For many boilers that is 1 to 2 bar when the system is cold. Topping up a few times a year is normal. If you have to top up more often, the system is losing water somewhere and there is probably a leak.

In this article you can read how to top up your central heating boiler step by step, how often that is normal and how to look for the cause yourself: a leaking radiator, the overflow valve, the expansion vessel or a hidden leak. You can also read when to call in a professional for your boiler and heating, and what you do yourself as a tenant.

Why does the pressure of my boiler keep dropping?

Because air has been let out of the system or because water is disappearing. A small drop after bleeding the radiators is normal: the space where the air was has to be filled with water again.

If the pressure keeps dropping after that, water is being lost somewhere. In the user instructions for its HRE boiler, Intergas writes that you should alert your installer if you have to top up more than a few times a year, because there is then probably a leak.

It is often noticed first at the start of the heating season. As soon as the heating comes back on, you see the pressure gauge drop or the boiler shows a warning. Low pressure is usually not dangerous, but many boilers show a fault or shut down if the pressure drops too far.

This article is about homes with their own central heating boiler. Many homes in Almere are connected to Vattenfall’s district heating and do not have their own boiler; these steps do not apply then. If you do have a boiler in Almere or Lelystad, this step-by-step plan will help you.

What is the right pressure for a boiler?

That differs per boiler and per home, so always check the manual of your own boiler first. Many manufacturers give a pressure between 1 and 2 bar when the system is cold.

If there are radiators high up in the house, for example in the attic, the boiler needs a little more pressure. Bosch works with about 0.1 bar extra per metre of height difference between the boiler and the highest radiator. Read the pressure when the heating has been off for a while. Warm water expands, so in a warm system the pressure is higher.

What does the red needle on the boiler pressure gauge mean?

Many pressure gauges have two needles. The black needle moves and shows the pressure in the system right now. The red needle does not move by itself: it is a marker that is set by hand, usually by the installer, at the pressure the system should be filled to.

If, with the system cold, the black needle is clearly below the red one, the pressure has dropped and you can top up to roughly the red needle. If your manual gives a different value, follow the manual. On some gauges part of the scale is coloured red as well: the black needle should not be in that area.

Is the pressure gauge broken?

A pressure gauge can get stuck or show the wrong value. Signs of that:

• The black needle does not move while you top up, although you can hear the water flowing in.
• The gauge stays at 0 while the radiators get warm and the boiler shows no fault.
• The boiler’s display shows a different pressure than the needle.

In that case, do not keep topping up on a guess. If the pressure gets too high, the overflow valve opens and water runs out of the system. If your boiler has a display that shows the water pressure, read the pressure there. A broken pressure gauge is a small part that a heating engineer can replace. Send a photo of the gauge and the display via WhatsApp and we will take a look with you.

How do you top up a boiler? Step by step

You can top up yourself. You need a filling hose (sold at DIY stores), a bucket and a cloth. Check in the manual whether your boiler has its own filling programme, because the steps sometimes differ slightly.

1. Turn the heating off. Turn the thermostat down and let the boiler cool down. If you have an older boiler, unplug it as well.
2. Fill the hose with water. Connect the hose to the tap by the boiler or to the washing machine tap. Over the bucket, let water run through the hose until there is no air left in it, then close the tap again.
3. Connect the hose to the filling valve. It is usually under the boiler. If you are not sure which valve it is, check the manual.
4. Open the water tap first, then the filling valve. Let the water flow in slowly and keep a close eye on the pressure gauge.
5. Stop at the right pressure. Fill to the value in your manual. Do not overfill: if the pressure is too high, the system lets water out through the overflow valve.
6. Close the filling valve first, then the water tap. Disconnect the hose and let the remaining water run into the bucket.
7. Bleed and check. Bleed the radiators, look at the pressure gauge again and top up a little more if needed. Then turn the heating back on.

Is water getting near the plug or the electronics of the boiler, or are you unsure about a step? Then switch the boiler off and leave it to a professional. Water and electrics do not mix.

How often is topping up normal?

A few times a year. Check the pressure at the start of the heating season and after that glance at the pressure gauge regularly, especially in the first weeks after the heating has come back on.

Not sure? Then for a few weeks write down the reading of the pressure gauge every morning, with a cold system. That shows you whether the pressure drops slowly or quickly. That record helps an engineer find the cause faster, which saves time and therefore money.

Boiler pressure dropping: is it a leak?

If you keep having to top up, water is being lost. Sometimes you see it straight away, sometimes it happens in a place you never go. Check these places one by one.

A leaking radiator, radiator valve or coupling

At every radiator, look at the radiator valve, the bleed valve and the couplings at the bottom. Look for drops, limescale marks, rust or a damp spot on the floor. Couplings in the pipes near the boiler can leak too. We replace a leaking valve or radiator through our radiators and underfloor heating service.

The overflow valve keeps running

The overflow valve, also called the pressure relief valve, lets water out when the pressure gets too high. It sits by the boiler and usually drains through a small pipe into a funnel or the drain. If water drips out of it when you have not just topped up, that is how you are losing water.

The expansion vessel is not working properly

The expansion vessel takes up the extra water that appears when the water heats up and expands. If the vessel does not work properly, the pressure shoots up as soon as the boiler gets warm and drops again as the system cools down. Bosch calls large pressure swings a sign of a faulty expansion vessel. Often the overflow valve then runs as well, so you lose water without noticing.

A hidden leak in the floor, wall or crawl space

You cannot see water anywhere, but the pressure still drops quickly? Then there may be a leak in a pipe in the floor or wall, in the crawl space or in the underfloor heating. If you have a crawl space, check with a torch whether there is standing water or whether pipes are wet. Inside the house, look out for warm, damp spots on the floor and stains on the ceiling. A leak like that is found fastest with targeted leak detection, so that floors or walls do not have to be opened up unnecessarily.

A leak in the boiler itself

Water can also leak inside the boiler. Check whether there are drops hanging under the boiler, but do not open the boiler yourself. Leave work on a boiler to a CO-certified company.

What should I do if my boiler pressure drops quickly?

First look for visible water, and only top up if you see no leak. If the pressure drops again within a few days, call a professional.

1. Is a radiator leaking? Close the radiator valve and put a tray or towel under the leak.
2. Is there water by the boiler? Switch the boiler off and unplug it, especially if water is getting near the electronics.
3. Take photos of the leak and the pressure gauge. That helps the engineer and, if there is water damage, your insurer too.
4. Write down when you last topped up and what the pressure was then.

Has water already got into the house? Then also read our article on a leak in your home and what to do straight away.

Do it yourself or call in a professional?

You can top up and bleed yourself. If a part needs replacing or you cannot find the leak, the work is for a professional.

• Yourself: read the pressure, top up, bleed the radiators and check that the filling valve is properly closed.
• Professional: check and replace the overflow valve or expansion vessel, replace leaking pipes or radiator valves, trace a hidden leak and all work in or on the boiler.

I rent my home: who tops up and who pays for the repair?

As a tenant you top up and bleed yourself. That is set out in the Besluit kleine herstellingen, the Dutch regulation that decides which small jobs are the tenant’s responsibility. According to the Dutch tenants’ union Woonbond, repairs to the central heating system and the periodic check are for the landlord.

If you keep having to top up, report it to your landlord or housing association. Talk to the landlord first before you call in an engineer yourself, so that it is clear in advance who pays the bill.

What does it cost to have pressure loss fixed?

Topping up and bleeding cost you nothing except a filling hose. The cost of a repair depends mainly on how long the search takes and which part needs replacing.

• A visible leak at a radiator valve or coupling is usually found quickly.
• For a new expansion vessel, the Dutch review platform Werkspot gives a guide price of €30 for the vessel itself, plus labour (updated 21 August 2026).
• A hidden leak in a floor or wall takes more time, because it has to be traced first.

Klusbedrijf MSZ works by the hour, without fixed package prices. Send us a photo of the boiler, the pressure gauge or the leak via WhatsApp beforehand, and we will give you a free estimate.

How do you stop the pressure from dropping all the time?

• Bleed the radiators at the start of the heating season and then top up.
• Always close the filling valve properly after topping up.
• Have the boiler serviced in good time. Milieu Centraal, the Dutch information centre for energy and the environment, advises having an older boiler serviced every year and a modern boiler about once every two years, in consultation, by an installer with CO-vrij certification (the Dutch certificate for safe work on gas appliances).
• Ask the engineer to check the expansion vessel and the overflow valve during the service as well.
• Does your boiler break down more often and is it getting on in years? The Dutch consumers’ association (Consumentenbond) gives an average lifespan of about 15 years. Also read what changes with the hybrid heat pump from 2029.

When should you call a professional?

• You have to top up more than a few times a year.
• The pressure drops again within a few days of topping up.
• Water drips from the overflow valve or the pressure shoots up when the boiler gets warm.
• You see water under the boiler or a radiator, or damp patches on the floor or ceiling.
• The boiler keeps showing a fault, even after topping up.

Can you smell gas near the boiler? That has nothing to do with the water pressure. Go outside and call the national gas emergency number: 0800 9009.

Frequently asked questions

How many bar should a boiler have?

That is in your boiler’s manual. Many manufacturers give 1 to 2 bar with a cold system. If there are radiators high up in the house, the boiler needs a little more pressure.

Is it a problem if my boiler pressure drops?

Low pressure is usually not dangerous, but the boiler can show a fault and switch off. A small drop after bleeding is normal. If you often have to top up, the system is losing water.

Why does the pressure rise when the boiler gets warm?

Warm water expands, so a small rise is normal. If the pressure shoots up a lot and then drops again, the expansion vessel is probably not working properly. Have that checked by a professional.

How often can I top up my boiler?

A few times a year is normal. Intergas advises alerting your installer if you have to top up more than a few times a year, because there is then probably a leak.

As a tenant, do I have to top up the boiler myself?

Yes. Under the Besluit kleine herstellingen, topping up and bleeding are tasks for the tenant. If the system leaks or a part is broken, report it to your landlord.

What do I do if I have topped up too much?

Then the overflow valve lets water out to protect the system. If the pressure is too high, Bosch advises letting a little water out through the bleed valve of a radiator, preferably one low down in the house.

Need help with a boiler that keeps losing pressure in Almere?

Is your boiler pressure still dropping, even after topping up? Klusbedrijf MSZ is a family business from Almere and available 24/7. We find the cause of the pressure loss, fix leaks in radiators and pipes and are CO-certified for work on gas-fired appliances such as your boiler. On our page about boiler servicing and faults you can read what we do in Almere and the surrounding area. If you live in Lelystad, see boiler and heating in Lelystad.

The first hour on site costs €181.44 incl. VAT (€149.95 excl. VAT) and is the minimum per visit. After that you pay €108.84 incl. VAT (€89.95 excl. VAT) per hour. In Almere we charge no call-out charge; elsewhere in our service area there is one fixed amount of €42.35 incl. VAT (€35 excl. VAT). For evenings, nights or weekends, we agree the rate with you by phone beforehand. See all our prices or ask for a free quote. Call or WhatsApp 06 19 74 36 78 and feel free to send a photo via WhatsApp of the pressure gauge or the leak.