How to Refill or Re-Pressurise a Combi Boiler

How to Refill or Re-Pressurise a Combi Boiler

In this article, we go beyond the basics of pressurizing or re-pressurizing your boiler and take a comprehensive exploration into Combi boilers. We delve into understanding their practicalities—what they are, the ideal operating pressure, and how to address issues if the pressure is too high or low. Additionally, we touch on the sizing considerations, providing insights into the available options as we approach the impending gas boiler ban in 2025.

What is a Combi Boiler?

A 'combi' boiler is a single unit that provides a combination of two important functions.

  1. Provides the hot water required for your central heating system.
  2. Acts as a water heater for providing hot water to your taps and shower

They also come with several benefits:

Save space

The main benefit of a combi boiler is that they save a great deal of space because a separate water tank is not required to heat the water. Traditionally, an old tank-fed system would require a separate tank in the loft or in a cupboard (a.k.a. the airing cupboard).

Instead of needing a large, cumbersome water tank with an indirect coil or heat element running through it, a combi boiler heats up water directly using a plate heat exchanger.

Constant hot water supply

The plate heat exchanger is a very important component of a combi boiler because it has the ability to transfer heat from one body of water to another very quickly - almost in real time.

In basic terms, this means that cold water can be made hot very quickly and created on demand, without the need for it to be heated separately in a large tank.

Not only does this make combi boilers highly efficient, it also avoids that classic problem of busy households running out of hot water in which to shower.

Separate water for heating & washing

In order to combine its two important functions, a combi boiler allows water to flow through it in two different routes, depending on what the water will be used for. Neither flow of water will come into contact with the other.

  • Washing water - The cold water enters the boiler, through an isolating valve and flows across the pressure switch paddle which tells the boiler it needs to turn on. The cold water then enters the heat exchanger where it is heated up. The newly-formed hot water then leaves the heat exchanger, goes through a mixer and is distributed out to your taps.
  • Central heating water - This comes out of the top of the boiler, and can either go off to the heating system or back to the heat exchanger. It then flows down into the pump, through the heat exchanger and back out of the boiler into the return.

What does a 'pressurised system' mean?

If you look at the front of your combi boiler, you should see a gauge that denotes the pressure in your system. An ideal reading with everything off should be about 1 - 1.5 bar - bar being the metric units of pressure.

When cold water gets hot inside your heating system, the molecules expand. When the water expands, it needs somewhere to expand (move) into. In a pressurised system, this place is known as the expansion vessel.

Inside the expansion vessel is a rubber diaphragm. On one side of the diaphragm is theheating system water, on the other side is some compressed air.

As the heating water gets hot and expands, it can expand into the compressed air but without allowing air into the heating system due to the rubber diaphragm in the way.

For your heating system to operate correctly, when your heating is off and you are not running any water, this compressed air should be pressurised to 1 - 1.5 bar. If your reading is considerably higher than this, it quite possible that you need to bleed some air out of your radiators in order to bring the pressure reading down.

What should be boiler pressure?

The water inside central heating systems needs to be able to travel around the network of pipes and radiators in a home. Often this means going against the force of gravity, such as in a two-story home, so there needs to be a certain pressure inside your system to ensure the hot water gets where it needs to. If the boiler pressure is too high, leaks could form in your central heating. If the pressure is too low, water won't flow correctly around your system. The right boiler pressure will allow your system to be running effectively and efficiently.

What should boiler pressure be when the heating is on and off?

Boiler pressure is measured in bars and a healthy boiler pressure should be somewhere between 1 and 2 bars. You can check your boiler pressure by looking at the gauge on your boiler.

  • When the heating is on, you should see your boiler pressure rise to between 1½ and 2.
  • When the heating is off, or cooling down, you should see your boiler pressure lower to between 1 and 1½.

What if my boiler pressure is too low?

If you've bled air out of your radiators but no water has come out and the pressure gauge on your boiler has dropped below half a bar, it is likely there is a another problem that is causing the pressure to be too low.

Below we'll look at the possible reasons for a pressure drop and how you can solve them.

A leak in your system

To check for a leak in your system, it's a good idea to look at all the valves on all the radiators in your house.

Lift up the heads with a pair of grips to check there's no leakage on top of the valves.

You should also ensure that the compression fittings on both side of each valve are fully tighten and that there is no moisture seeping out.

Boiler pump is set too high

Cavitation' is a situation where the pump is working too fast for the water to escape the boiler. The water and air is then separated and hydrogen is created.

The hydrogen will then travel round your system and escape out of the automatic air vent. This can then cause the pressure in your system to drop. This situation can be avoided by reducing the speed on your pump.

Expansion vessel issues

It's possible that the rubber diaphragm in your expansion vessel has perished and is no longer doing the job of keeping the hot water and compressed air separate.

You'll initially see an increase in pressure before the pressure release valve kicks in and drops the pressure very low.

You can check if the rubber diaphragm is the problem by locating the valve (like a valve on a tyre) on your expansion vessel and letting the air out. If water starts coming out, it is likely a problem with your rubber diaphragm and your vessel will probably need replacing.

If water doesn't come out, you can simply pump up the expansion vessel using a bicycle pump until the reading on your boiler is 1 - 1.5 bar again.

Pressure release valve issues

A pressure relief valve is a simple component that contains a rubber valve and a spring. If the water in your system gets up to a certain pressure, the water will force open the valve and the the water will flow through an escape pipe and be released somewhere outside.

Over time, the strength of the spring and the rubber can get less and less causing the valve to provide less resistance to the water flow, dropping the pressure in your system.

If you suspect any of these issues to be affecting your combi boiler and central heating system, then it's best to get the profesionals in to have a look.

How to Pressurise / Re-pressurise your Boiler

Some common signs that you've got low boiler pressure are:

  • Your radiators won't get hot
  • You've got no hot water
  • The gauge on your boiler has dropped below 1

When bleeding radiators or carrying out alterations to a combi boiler central heating system the boiler can switch off due to not enough pressure. Some models have a pressure gauge on the front of the boiler to indicate when the pressure is low or high. When the indicator is below 1 the system may switch off or you may notice the heating not preforming as well as usual

Some boilers have an internal filling loop and others have an external filling hose. For either methods of refilling, you MUST turn the power off to the boiler first.

Combi Boilers with Internal Filling Loops

1. If your appliance has an internal filling loop installed within the boiler you will need to remove the tray at the bottom. Here you will find the filling key attached to the tray.

2. Remove the key from the tray and locate the manifold into which you will need to insert the key. This filling manifold is actually linked from your cold mains to the boiler through to your central heating return.

3. What you will then need to do is fit the key into the manifold with the arrow on the key facing the open padlock position. You then turn it to the locked padlock position.

4. You will then begin to turn the plastic nut located next to where the key was inserted anticlockwise. You should begin to hear the water filling the boiler.

5. Once you have done that you need to concentrate on the pressure gage; you should be able to see it filling up. Once it is up in the green zone to the required 1.5 you will begin to tighten the nut you unscrewed momentarily, until you can actually hear the water stop.

6. Turn the key back to the open padlock position, before removing the key. You can expect a small amount of water to expel from the manifold. Then remove the key.

7. If you do find the water continuing to expel from the hole, this will be because the nut is not tightened enough.

8. If you find you have filled the boiler to the second bar, don't worry. Simply go round your radiators and bleed them. You may find you have to repeat this process again.

Other Boilers with External Filling Hose

If your boiler has an external filling hose or sometimes called a filling loop, then you need to follow the corresponding instructions. You MUST always switch the boiler off before pressurising any system.


1. Firstly you need to locate the filling hose and see if it is attached to both ends of the pipe (mains cold water to boiler pipe). Make sure both ends of the hose are attached to the valves properly. The hose should be located close to the boiler, and in most cases it is under the kitchen sink.

2. Depending on how the installer has chosen to install the filling hose, there may be a valve at either end or a valve at only one end.

3. Next we need to open the valve(s) to allow water to fill from our mains cold water to our heating system. You will be able to hear the water filling into the boiler. This tells us that the system if filling up.

4. Continue to fill your system until the gauge on the front of the boiler in at the required 1.5 in the green zone.

5. What you then need to do is isolate the valve(s). This ensures there is no more water entering the central heating system. What you don't want to happen is for the needle to keep going up.

6. You then need to release the filling loop from the pipe. Expect a bit of water to come out of the loop, but if the water continues to come out you will then need to tighten the valve. If you exceed the green area on the pressure gauge you can simply bleed the radiators in your home and the pressure will go down.
 

Pressure too high? How to release Pressure

Some common signs that you've got high boiler pressure are

  • The needle on your pressure gauge has increased dramatically or gone into the red
  • Your central heating system has completely shut down

High boiler pressure is not a problem that happens too often, but if it does, it could have been caused by the valves on your filling loop being left open, allowing water to continue to flow in your system. Additionally, high boiler pressure is often a symptom of trying to correct low water pressure, whereby too much water is allowed in at the filling loop in an effort to boost boiler pressure. Whatever the reason, high boiler pressure is rarely cause for concern because modern boilers have a pressure release valve that allows the excess water to escape. Boilers and heating systems will also shut down if they detect high levels of pressure.

In order to reduce your boiler pressure, try the steps laid out below:

1. Switch off your boiler and allow the system to cool down

2. Locate your filling loop and close the valves

3. Take a radiator key and bleed each radiator in your home. You'll hear air escaping as the pressure is relieved but have a cloth on hand to catch and drips.

4. After bleeding each radiator, check your pressure gauge on your boiler to see if it has lowered. If not, repeat the process of bleeding your radiators again until the pressure drops to below 1 and 2 bar, or at least comes out of the red zone. If your boiler pressure won't come down having carried out these steps then call in a professional to diagnose the problem.

What Size Boiler do I need?

Boiler technology has developed in leaps and bounds over recent years.

Modern boilers are considerably more efficient than their predecessors. Additionally they are substantially quieter and can handle both heating (without a hot water tank) and domestic hot water.

Calculating the required size boiler needs to be done carefully, there are many factors and parameters to take into consideration.

These include the layout of the property and number of rooms, bedrooms and bathrooms a property has.

Below we've included some points for you to consider when choosing a boiler

How big is your home and how many radiators do you have?

The average combi boiler is available in one of four sizes 24-25kw, 28-30kw, 33-35kw and 40kw.

As a rough guide most apartments and smaller houses, with up to 10 radiators will require a 24-25kw boiler.

A 28-30kw would be installed for a medium to large 3-4 bed house with up to 15 radiators, and a 33-35kw and a 40kw would be for a large house with anything up to 20 radiators.

For properties with higher demand of hot water (how much hot water you use) the higher kw (kilowatt) boiler would be required, however combi boilers cannot cope with 2 showers running at the same time in the same property.

These figures are not accurate calculations as different house sizes and other factors must be taken into account.

Take these figures into consideration when receiving quotes from installers. They may try and sell you the higher kw boilers to maximise profit for themselves, so go with a trusted installation company, and shop around.

How long can you be without heating and hot water?

The average time to change an existing combi will take around 6-10 hours (a day).

Changing from a system boiler to a combi will take longer; usually 2-3 days.

When undergoing a boiler refit you will be without hot water and heating for hours, maybe even days.

Be prepared so you are able to take showers elsewhere if need be, and you may need to think about alternatives for heating your home.

What is your budget for a new boiler?

Although timescale and boiler size are important, the big question on everyone’s lips is how much will it cost?

If you are simply replacing a combi boiler with another combi boiler then expect the work and parts to cost around £1400 to £1800 including VAT.

For people wanting to implement a boiler conversion from a regular system to a combi boiler, this can cost between £1900 and £2500.

However this price guide can vary regionally, don’t expect to pay the costs shown in the centre of London as they will be much higher.

Do you need to upgrade your radiators?

One of the main reasons people upgrade or update their boiler system is to save money in the long run.

The best way to make the most of a new boiler is to update the rest of your heating system at the same time.

Installing new radiators makes the most of the heat provided; old radiators don’t radiate heat as well as new ECO friendly units, which provide an ultra-efficient heat output.

The range of different radiators is vast and can accommodate homes of all different styles and sizes.

Gas boiler ban: what will replace boilers in 2025?

There has been much talk and speculation about the UK government phasing out gas boilers in the UK. As almost 80% of the energy used for heating UK buildings comes from gas, this could potentially have huge impact on many households up and down the country. In this article, we'll look at the what's changing in terms of gas boilers and make some alternative replacement heating suggestions.

What is the gas boiler ban?

In a massive effort to slow and deal with climate change, the UK government has set a target of being 'net-zero' for greenhouse gas emissions by 2050.

Part of this ambitious programme is to ban gas boilers from being installed in new-build homes. There has been some confusion about the date this ban will take effect from, with the government originally setting the date at 2025, then later bringing it forward to 2023, and then removing a specific date from their documentation altogether. At the time of writing, the 'Future Homes Standard' bill is going through Parliament and one would hope that the dates will become clear.

Will it affect you?

Regardless of the exact date, a important point to keep in mind is that the boiler ban applies to new homes only. There are currently no plans to force people with existing gas-fired boilers into an alternative method of heating by 2023 or 2025, which will leave millions of people breathing a huge sigh of relief.

As changing the type of heating system we have in our homes is quite an expensive process, we will likely see drives to upgrade the efficiency of the existing housing stock, such as through better insulation. However, we can all probably expect to see Government policies over the coming years to encourage and incentivise us into upgrading the way we heat our homes to a more renewable source.

What are the heating alternatives to boilers?

So if we were thinking of changing our heating systems or considering buying one of the new-builds after the boiler ban comes into effect, what are the most common types of alternative heating systems that could become more popular?

Heat pumps

A heat pump is powered by electricity and works by taking energy from outside, transfers that energy into heat and distributes it internally around a heating and hot water system in the home. This is quite new technology in the UK but has been popular in the Scandinavian countries for several years due to their efficiency and low-carbon footprint.

There are two main types of heat pump suitable for domestic heating

Ground source heat pumps

This type of heat pump collects heat from the ground through a collection pipe that is filled with a special fluid that transfers the heat into the property. Ground source heat pumps can be installed into the ground horizontally or vertically, depending on the space available.

In order to be able to provide enough heat to comfortably warm a property, a horizontal ground source heat pump would need space the size of a tennis court, while a vertically installed one would need holes digging down to around 150m in depth. This is of course a very large amount of space and so is one of the main drawbacks of this type of heat pump.

Air source heat pumps

Perhaps the more practical of the two heat pumps, air source heat pumps work a bit like a refrigerator in reverse. Outside air is drawn into the system using a fan. The air is then compressed heated to a very high temperature, passes through a heat exchanger and is transferred into the water that circulates around the house. Because this system relies on the air outside, there is no need to have a large system of pipes installed into the ground.

District heating/ heat network systems

District heating (a.k.a. heat networks) is essentially a way of heating lots of different homes or buildings from a central source, such as a heat and power plant. This centralised energy facility would generate the heat and transfer out hot water (or steam) to homes connected to the network via a series of insulated pipes.

A district heating system was opened in Glasgow in 2011 and provides heating and hot water to around 2,000 people, removing the need for each dwelling to have their own individual boilers and providing low-cost heating with much lower carbon emissions than other, traditionally heated neighbourhoods.

Hydrogen boilers

While not yet available for purchase, once available, hydrogen boilers will be able to be installed in much the same way as gas-fired boilers currently are. They would be connected to the mains gas supply and would be capable of powering a home heating system on gas or pure hydrogen.

Should the gas network be converted to hydrogen, there would be a (relatively) seamless transition for householders with minimal need for new infrastructure or lengthy disruption. A new hydrogen boiler would be much easier to install than having to switch a home over to a district heating system or installing a heat pump. While more eco-friendly than gas to heat homes, the major downside is that hydrogen is still very expensive to produce, especially in the quantities needed to adequately heat millions of homes across the UK.

Electric radiators

Electric radiators heat homes by using electricity converted into heat by an element contained in the radiator. This heat is then circulated around your room, usually with the added benefit of timers and smart controls to allow you to maximise control over your heating. They are a low-maintenance way of heating your home and are a convenient way of heating rooms not connected to the mains gas supply, like attics, garages or garden offices.

If you are concerned about the carbon footprint created by your gas boiler, then electric radiators are a fantastic alternative. There's a huge range of them available right now and they are very easy for a qualified electrician to install. They are wall mounted and connected up to your home's electrical system without the need for a network of pipes, outside heat pumps or relying on not-yet-available technology.

At Trade Radiators, we have a really big variety of electric radiators in a massive range of sizes and styles, all at brilliant prices. Our electric radiator range provides you with the choice of superb heaters that will match the décor and look stylish in any room. With excellent efficiency as well as high BTU outputs, our electric radiators allow you to get the best of both worlds. 

When bleeding radiators or carrying out alterations to a combi boiler central heating system the boiler can switch off due to not enough pressure. Some models have a pressure gauge on the front of the boiler to indicate when the pressure is low or high. When the indicator is below 1 the system may switch off or you may notice the heating not preforming as well as usual.‹¯¨

Some boilers have an internal filling loop and others have an external filling hose. For either methods of refilling, you MUST turn the power off to the boiler first.

1. If your appliance has an internal filling loop installed within the boiler you will need to remove the tray at the bottom. Here you will find the filling key attached to the tray.
2. Remove the key from the tray and locate the manifold into which you will need to insert the key. This filling manifold is actually linked from your cold mains to the boiler through to your central heating return.
3. What you will then need to do is fit the key into the manifold with the arrow on the key facing the open padlock position. You then turn it to the locked padlock position.
4. You will then begin to turn the plastic nut located next to where the key was inserted anticlockwise. You should begin to hear the water filling the boiler.
5. Once you have done that you need to concentrate on the pressure gage; you should be able to see it filling up. Once it is up in the green zone to the required 1.5 you will begin to tighten the nut you unscrewed momentarily, until you can actually hear the water stop.
6. Turn the key back to the open padlock position, before removing the key. You can expect a small amount of water to expel from the manifold. Then remove the key.
7. If you do find the water continuing to expel from the hole, this will be because the nut is not tightened enough.
8. If you find you have filled the boiler to the second bar, don’t worry. Simply go round your radiators and bleed them. You may find you have to repeat this process again.

If your boiler has an external filling hose or sometimes called a filling loop, then you need to follow the corresponding instructions. You MUST always switch the boiler off before pressurising any system.

1. Firstly you need to locate the filling hose and see if it is attached to both ends of the pipe (mains cold water to boiler pipe). Make sure both ends of the hose are attached to the valves properly. The hose should be located close to the boiler, and in most cases it is under the kitchen sink.
2. Depending on how the installer has chosen to install the filling hose, there may be a valve at either end or a valve at only one end.
3. Next we need to open the valve(s) to allow water to fill from our mains cold water to our heating system. You will be able to hear the water filling into the boiler. This tells us that the system if filling up.
4. Continue to fill your system until the gauge on the front of the boiler in at the required 1.5 in the green zone.
5. What you then need to do is isolate the valve(s). This ensures there is no more water entering the central heating system. What you don’t want to happen is for the needle to keep going up.
6. You then need to release the filling loop from the pipe. Expect a bit of water to come out of the loop, but if the water continues to come out you will then need to tighten the valve. If you exceed the green area on the pressure gauge you can simply bleed the radiators in your home and the pressure will go down.

‹¯¨

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