Combi Boilers - Users Instructions

Exclusively written by Herts Boilers (2025)

Combi boilers are a good engineering product. Nowadays they are installed in small to medium houses and about 60% new boilers sold in this country are combi boilers.

• Compact size: With a combi boiler, there is no need for additional water tanks or cylinders. Therefore, these units can be installed in kitchens, utility rooms, or inside cupboards.
  
• On-demand hot water: Hot water is provided directly from the mains whenever required.
  
• Easy to install: These systems are simple to install with less pipework compared to other systems. They are also faster and cheaper to install because they do not require additional tanks or plumbing.
  

Combi boiler diagram

The left is a diagram of a classic combi boiler. A combi boiler combines a water heater and central heating boiler in one compact casing. If the room thermostat sends a signal to the boiler to fire up because the temperature has fallen below the set point, the boiler will fire up so that heat can be pumped into the central heating system, radiators or under floor heating. As the thermostat set point is reached, the boiler will modulate or turn itself down, keeping the heating at the right temperature and saving energy.

When someone turns on a hot water tap, a valve inside the boiler diverts the heat into the domestic hot water supply, providing hot water until the tap is turned off, when the valve returns to the ‘central heating’ position. The heat can be directed to either the central heating system or the hot water taps but never at the same time.

Hot water preheat

Combi boiler hot water preheat is a feature that keeps a small amount of water warm in the boiler, ready to be used instantly when you turn on the hot tap. While this provides quicker hot water, it can also increase gas consumption as the boiler cycles on and off to maintain the temperature. Turning off the preheat can save energy and money, but it may increase the time it takes for hot water to reach the tap.

Programmer and Programming

Central heating programmers allow you to set specific times for your heating to turn on and off, optimising energy usage and comfort. They can be simple timers that switch heating on and off at set times, or more advanced digital programmers that offer 2-6 heating periods per day with different temperature settings. This helps ensure your home is warm when you need it, and not when you're away or asleep.

System pressure

A combi boiler typically runs at a pressure between 1.2 and 2.5 bars. With expansion vessel being good by a compression air charge, when heating is on and your radiators are getting hot, this system pressure can go up a bit for example over 2 bar but it should still be less than 3 bar.

It is a resident's responsibility to make an occasional system topping up. In heating industry it is commonly accepted that one can make a few times of top ups each year. You can follow these steps for a system pressure topping up.

The system pressure should be topped up to 1.2 bar to 1.6 bar when system water is cold. When heating is fully on in the winter, the pressure is rising somehow, for example to 1.8 bar or even over 2.2 bar, but nowhere near 3.0 bar. When you use hot water from a combi boiler, the pressure is supposed to maintain the same.

Boiler output temperatures

Both hot water and heating should be at about 80-90% of the full scales.

It should be 100% or at least about 70% to 80% of the full scale. Hot water on its own will give a burnt hot temperature which one can’t take shower directly, and a portion of cold water is also added at the tap or shower to give a suitable and stable mixing for showering or washing.

Particularly on old combi boilers, it isn’t always stable if you set hot water temperature at the boiler to about 30% or so. It often gives a hot and cold temperature cycling.

So the correct way is to set hot water at combi boilers to its full or high, and get hot water mixed with cold water at the tap, for a stable hot water output.

Radiators

Leave them all on at radiator valves, it is suggested to control your heating by a programmer and a room stat.

Room stat

One centrallised one is surfficent for most of households, plus TRV on each radiator to balance each room temperatures.

There on and off concept anymore, just at what time and you set what temperature.

6pm to 10pm, family time 20

12am to 6am, sleep time, set to 15

9am to 4pm, away from home, set to 10

Holiday away

When you are going away for a holiday, always leave your combi boiler on. Image if you leave your car not run for a few weeks, can you re-start it once you come back? It is the same.

Modern boilers have a mechanism to run pump and fan on a regular basis to prevent them being seized.

Put heating temperature to a lower point, and leave hot water (pre heat) on, these will make boiler and pump run briefly everyday.

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