Understanding my combi-boiler


A combi boiler is not really a new heating system but has been around for quite some time. However, the older models were known to be unreliable and there weren't many choices available to homeowners. However, because of technological advances there have been highly efficient, newer and better boilers in the market today that are being used by almost 50% of homeowners in UK. The prices of these systems can vary greatly depending on the brand, reputation, power consumption and dimension. These devices are space saving and are very compact. These systems do not need laborious tanks or water cylinders and can heat water instantly and deliver it throughout your home.
How Do They Work?
The combi boiler has two main functions- providing hot water and heating your home. When the temperature in your home goes below the set temperature on the thermostat it will signal the boiler to fire the pump up. Water will flow through a small gap in the pipe to build up pressure. This...

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Conventional boilers

Conventional boilers work by heating hot water and storing in a tank known as a hot water cylinder. You'll often find these in an airing cupboard. One of the big advantages of a conventional boiler is that you can provide hot water to more than one tap or shower at a time. This makes it ideal for larger properties or people who have families using hot water at the same time. Another benefit is that you can fit an electrical immersion heater to the hot water tank giving you backup should there be any issues with your boiler.

However, there are some considerations with conventional boilers. You need space for it in addition to a cold water tank – such as in an airing cupboard or in your loft. You'll also need controls so that the hot water is heated at the right time for your needs.

Finally, when using a hot water cylinder, once the water runs out that's it. So if you particularly like lots of baths, its important that you get a right tank...

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Combi boilers play an important role in the home. These boilers offer high levels of efficiency and take up minimal space in your home. Combi boilers, also commonly referred to as combination of bo...

Combi boilers play an important role in the home. These boilers offer high levels of efficiency and take up minimal space in your home. Combi boilers, also commonly referred to as combination of boilers are now the preferred choice for many home owners who want to have a reliable heating system for their homes. The reason why people get attracted to it is the size and the efficiency levels which it offers.

With a Combi boiler, there is no need for hot water storage hence eliminating the need for a hot water cylinder within the airing cupboard. The boilers have been named Combi because they offer high levels of water heating efficiency while at the same time acting as a central water heater.

Why buy this type of boiler?

First, you are protected against quick...

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A combi boiler (or combination boiler) is a device for the supply of residential spatial heating and domestic hot water demand, usually run on natural gas and designed for a single dwelling. The part of the system that prepares the domestic hot water can have considerable stand-by losses and consequently significant stand-by energy consumption. I have observed that when such a heating device is being installed in new houses, the manual control (if available) of the hot water preparation system often is not connected. This means that the hot water preparation system is always simply 'on', having hot water available instantly, which results in stand-by losses. Heat is leaking away from the hot water vessel continuously and natural gas is being used at intervals during the whole day, even when no hot water is being tapped. Modern technology combi boilers might be equipped with an 'automatic mode', which might be an energy-saving mode or a self-learning timer-mode. However, I think...

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Combi boiler are the ideal answer for those wanting heating and hot water for a sometimes smaller house. The key characteristic is that they don’t store hot... Read more

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Small volume, high efficiency, environmentally friendly energy efficiency class A, modulated structure can adjust itself to suit the changing conditions of... Read more

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Vaillant’s leading brand in the industry, natural gas appliances, the yield is up to 109 percent of the waste heat from gas-fired boilers with condensation... Read more

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35E MCA / MCA 45 / 65 MCA / MCA 95 / 115 MCA CONDENSING COMBI According to the classical boilers more efficient and environmentally friendly products for the... Read...

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Gastec offer a comprehensive range of combination boilers from a wide range of manufacturers. They are all fully functional and maintained in top condition for you to work on during your training. At Gastec we think you will learn faster and gain better experience by training on boilers that respond to your actions. So if it's faulty you can fix it and see your results to completion. You won't get this with dummy units set up purely for display.

The course contains basic Essential Electrics which also covers the understanding of the Multi-meter which is a fundamental for the combination boiler course.

At the end of the session there is a short assessment and on your successful completion will result in a fully certified Achievement Certificate and card from the Construction Skills proving your competence in this area. This 2 day course is designed for Plumbing and Heating Engineers who install and service combination boilers. All operatives will have...

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I have no technical understanding of combi boilers, but mine used to rattle when the water got too hot. Turn the wall dial down AND the one on the front of the boiler, no idea if that should work for you and actually no idea if it really worked for me but the rattling seemed to go.

I've had the same problem - it seems to be the thermostatic radiator valves. These often state on the packing that they work either in the flow or the return, but can you really beleive that?? Probably change ends of the two valves on the guilty radiator (easily found when rattling by "taking its pulse" with a finger, and it will go away. Try just a tiny adjustment of each valve in turn as another search method.

Turn all your rad thermostats up to full. If the noise stops then you know it's the 'stats restricting the flow of water. 'Stats used to be one-way, some are now 2 way, if your 'stats were fitted the wrong way then the noise would be instantaneous. After you've checked your 'stats...

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Combi boilers, sometimes also known as combination boilers, are a specific type of condensing boilers. Combi boilers create hot water on demand for your home’s taps and domestic central heating. As a result of this ‘instant hot water’ Combi boilers are the most popular type of gas boiler and sales equate to more than 50% of all new boiler sales within the UK. Having said this combi boilers have some disadvantages and are not the right boiler type for everyone and it is important to understand whether a combi boiler is the best boiler for you and your family.

Most traditional central heating systems (regular boilers) store the boiler heated water in a copper cylinder in the upstairs airing cupboard to provide domestic hot water. Combi boilers do not require this hot water storage saving you valuable space and reducing heat loss from the previously stored water. As a result combi boilers can be more energy efficient than other boiler types.


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We had a new one last year in our village house - a Baxi - you can see the full range on line. Careful though, a new boiler, unless it is an exact replacement can open up a can of worms. Hungary has strengthened its gas safety regulations. We had to have a new Gas Plan drawn up & approved. We had to get rid of a gas hob unless we opened up a new vent & we had to have some very fancy work done to the chimney so that fresh air was pulled down the chimney, and fumes pushed out of the chimney - but the two must never meet. The fresh air is no longer taken from the house.

I'm sure all the work can be justified - and it all works very well now - but it took along time & was expensive. Next time we will replace with identical boiler. Good luck with your project.

This is correct and the reasoning is that with people putting in new efficient doors and windows houses are no longer...

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Follow our top tips to prevent permanent damage to your property

Christmas is normally a very busy time of year for every family. What with all the preparations to be made, presents to buy and wrap, and cards to write and send. But what would happen to all your plans if your boiler suddenly stops working?

There is never a good time to experience a boiler breakdown, but by following our handy tips you can be prepared if this ever happens to you, and you could help prevent some serious damage happening to your property if you take action quickly.

Calling an emergency plumber is the most urgent thing to do, but be aware that plumbers can be in very high demand during the festive season, especially if weather conditions turn particularly cold, causing a number of boiler breakdowns, leaks, frozen and burst pipes to keep them busy.

To be on the safe side, save yourself a lot of unnecessary trouble frantically phoning around trying to get a plumber that you do not...

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If you needed supplementary bonding, you would want one of these round each copper pipe, linked together. (You can usually get away without doing the safety vent pipe) You don't need to have an electrician do it.


Normally people link them with 4mm earth wire but if the pipes are close enough you can tighten the first one then thread it through the next one and so on, which looks neater.

I thought it was only ever if the boiler was in a bathroom. If the circuit is RCD protected you don't need supplementary bonding at all as of the wiring regs 17th edition. On the one hand doing it would keep your AA man happy, on the other hand sometimes you need to challenge this sort of thing. As swbjackson says it's probably a waste of time, but then a lot of the electrical regs are concerned with adding extra earth connections between things that are already electrically contiguous.

All the above assumes they...

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Snowgirl has a very good but undefined flow. If she has room and funds for a Megaflo or other pressurised cylinder, it will give better HW than a combi, and will be more reliable.

In most houses, the constraining factor will be the amount of water (flow, measured in litres per minute), delivered to the house from the water main. Usually this is limited by the size of the pipe between the pavement and the boiler. Very often you can greatly improve things by digging a trench and laying a new, larger one. This can be more or less difficult or expensive depending on distance and how much concrete is in the way. Older houses traditionally have small pipes and low flow.

Usually, a combi is fine if there is only one person in the home at a time running a shower or bath, and increasingly less fine as the number of users increases.

The traditional cold water tank in the loft has the purpose of allowing the flow of water from all the taps in the house at any one time to...

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Connect pressure gauge to a cold W/M tap or outside tap, and turn that tap on. That will measure the static pressure. Your plumber suggests it should be about 3 Bar

With your gauge still connected, open another tap (say, kitchen tap) and note the new pressure reading with the tap running. That is a dynamic pressure, but it is meaningless without knowing the associated flow rate.

Time how long it takes to fill a bucket of known volume, and convert that time to a flow rate in litres per minute.

The second pressure reading and flow rate together give a meaningful dynamic pressure (eg 1.5 Bar @ 15 Litres per minute)

You can adjust the flow rate at the tap to whatever value you wish to know the dynamic pressure for, and measure the pressure, or repeat the test for a few random (but measured) flow rates, and compile a graph of pressure and flow rates (say 2.5 Bar @ 10 Litres / Minute, 2.8 Bar @ 5 litres / min)

You can repeat the above for the hot water...

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Book a course today!

We have training centres all across the UK, so we are never too far away from you.

As well as free of charge boiler training, we also run unique non-brand specific courses, designed to help you grow your knowledge and make your work even easier. These include Multimeter training, boiler diagnostics, combi boiler appreciation and system wiring and controls. Instruction is very practical, ensuring you leave with a thorough understanding of the subject.

We also run accredited courses so you can gain the qualifications you need for your business.

Water regulations bylaws course Domestic ventilation systems Boiler Diagnostics Combination Boilers Appreciation System Wiring and Controls Basic Electrical Appreciation Unvented Hot Water Storage Systems (G3) Certificate in Energy Efficiency for Domestic Heating Solar Thermal Domestic Hot Water Course ...
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