COMBI BOILER GUIDE

1) What is a combi boiler, exactly?

In a single unit, a combination boiler provides both heating and hot water. Combi boilers, as opposed to a normal or system boiler, warm water straight from the cold mains.

2) When it comes to a combi boiler, how does it work?

An inbuilt heat exchanger heats the fresh drinking and washing water (known in the trade as potable water) from the water mains before sending it to the taps and bathrooms. In other words, instead of keeping hot water in a cylinder, this system instantly delivers it. Freshwater is on one side of the heat exchanger, while "system" water is on the other. Freshwater is heated by the boiler's system water, which is transferred to it through metal plates via the heat exchanger.

3) Benefits and drawbacks

Your home's heating system and the benefits and drawbacks of a combi boiler are intertwined. A combi may not be the best option if you have many bathrooms with low incoming pressure.

Combi boilers have several advantages.

• Removes a hot water cylinder to make room
• Cost savings since the boiler manufacturer's warranty and guarantee cover the integrated heating and hot water operations.
• There are no lengthy waits for hot water anymore.

Combi boiler drawbacks

• They fail miserably in homes with low incoming water pressure or flow rates.
• Providing hot water to many outlets at once reduces flow rates, although this is mainly an issue in homes with multiple showers.
• Combustion boilers with considerable hot water capacity might be inefficient for heating systems 

4) What are the pros and cons of using a combi boiler?

There are a number of considerations to consider when deciding whether or not to maintain your current boiler or to convert to a combi. Water pressure, internal flow rates, and hot water needs all have a role. It's more costly to go from a heat-only or system boiler to a combi boiler than one is to just replace it. Combi's benefits, such as improved warranty coverage and extra room, must be weighed against the increased installation costs. It is a good time to examine your heating and hot water needs if you have a combi boiler in order to ensure that you have the proper boiler to maximise efficiency and the boiler's lifetime.

• Combi boilers might leak if they are installed over existing radiators or pipes buried in screed. A'sealed system' is required for open vented systems with a Feed and Expansion (F&E) tank in the loft, putting the system under pressure and increasing the risk of leakage. You may want to keep your open vented heat-only boiler.
• Flow Rates/Pressure: Combi boilers need high incoming water pressure and high internal flow rates in order to function. A combi boiler will not work in certain parts of the UK because of low incoming water pressure. It's possible that the boiler won't be able to function properly due to complicated plumbing in certain houses. At least 10 litres of water per minute is required to run a condensing boiler. A pressure accumulator tank or a mains pump may be used to raise the flow rate to 12 litres per minute if you have less. Find out if anybody in your neighbourhood has a combination boiler by asking around. Your water flow rates will also be tested by the surveying engineer to verify whether they are enough.
• In most houses with one or two bathrooms, a combi boiler can provide flow rates of between 10 and 25 litres per minute. Even the largest home condensing boiler may not be able to handle a large property, many bathrooms, or a large family showering at the same time. Even the largest boilers' hot water flow rate is half when several hot water outputs are in use at the same time, such as two showers. A system boiler with an unvented hot water cylinder might be an option to investigate (if the water pressure is good). One can only go so far with even the most powerful combi boilers. A system boiler with a high-performance hot water cylinder may be preferable option for simultaneous hot water flow rates.

5) How much hot water do you need?

It's important to know how quickly the boiler can heat water for the hot water outputs by measuring the flow rate in litres per minute (LPM) (taps, showers etc). Combi boilers have hot water flow rates ranging from 9.8 litres per minute (LPM) up to 25 LPM. Your hot water needs may be determined by the number of bathrooms in your home and the frequency with which they are used at the same time.

If the home just has one bathroom, a tiny 10LPM combi is all that's needed. If there are two or more bathrooms in a home, the hot water flow rate will be half if, for example, two showers are running at the same time. System boilers, on the other hand, are ideal since they may provide superior flow rates even with a smaller boiler.

6) What kind of boiler do I need for my house?

When it comes to heating, all combi boilers are designed for hot water, which means that they are almost always overly large. A chilly day in the UK only requires 6-8kW of heating in the typical house. Combi boiler outputs range from around 18kW to more than 30kW. Whatever combi boiler you choose, it's essential that the installation adjust the heating output to meet your heat need; otherwise, the system won't work properly.

Decisions on whether or not to move to a combi boiler are influenced by a variety of variables. Water pressure, internal flow rates, and hot water needs all have a role. It's more costly to go from a heat-only or system boiler to a combi boiler. Combi's benefits, such as improved warranty coverage and extra room, must be weighed against the increased installation costs.

7) Choosing a condensing boiler

Over a hundred different combi boiler types are available in the UK, each with its own set of features and specifications.

8) Bigger residences may benefit from combis.

A compact combi boiler can heat most bigger houses, but it may not provide enough hot water for your needs. The issue is not which boiler is best for a huge house, but which boiler is best for my hot water needs. If the incoming mains pressure is sufficient, the largest combi boilers can provide roughly 20 litres of hot water per minute.

9) Drawbacks of a larger combi Boilers

Since the introduction of combination boilers, manufacturers have been compelled to build equipment that can handle larger amounts of hot water. However, large combis with high hot water flow rates might actually reduce the efficiency of the system's heating. Combi boilers can provide 25 litres of hot water per minute with a power input of 50 kW. As a result, the boiler, its parts, and the gas line all need to be larger and more costly.

The heating system only requires 6-10kW for the majority of UK residences. Combi boilers with tiny cylinders that store water ready to be drawn off at a high flow rate are more efficient than larger boilers with large cylinders. So that it doesn't run out of water while being drained, a combi boiler warms new water as it enters the cylinder.

Buying a smaller boiler not only saves money, but many of the parts and pipes will be smaller as well. Since a result, the boiler may spend more time in 'condensing mode', which improves efficiency, as the heating output is much more precisely suited to the heat needs of the residence.

10) The efficiency of a condensing boiler (combination boiler)

Boilers used to be 60-80% efficient, which meant that 20-40% of the heat they generated was lost down the flue. The efficiency of condensing boilers is at least 90%. By reclaiming heat that would otherwise be lost via the chimney and repurposing it as water, condensing boilers are more energy-efficient than their non-condensing counterparts.

The return temperatures from the heating system to the boiler must be about 54 degrees or below for the boiler to function in "condensing mode." As a result, many heating systems have return temperatures over 54 degrees Fahrenheit, which lowers the amount of time the boiler can spend condensing, resulting in a significant decrease in efficiency.

Boiler output must equal, not exceed, the property's heat needs if it is to condense. Because of this, boiler sizing is very critical. See our guide on boiler sizing for more information on how to estimate your heating needs. As temperatures rise and fall, a modulating control must be used to adjust output accordingly.

11) The most effective combination boilers

Despite the abundance of best boiler guides, only a handful are unbiased or extend their scope beyond a small number of boiler manufacturers. To find the greatest value for money, warranty duration, and efficiency in a combi boiler, we conduct an independent analysis of 11 boilers from 11 different manufacturers.

12) Combi vs system boilers

A system boiler is preferable to a combi boiler for houses with high hot water demand. 

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