Boiler Control and Adjustment

Good adjustment is more than turning a dial to a different temperature. The engineer checks the whole system first and then fine-tunes the controls. A typical visit runs like this:

1. The engineer asks how you use the home: when you are in, which rooms you want warmer and what bothers you about the heating.
2. They check the water pressure in the system and the expansion vessel. The correct pressure comes from the manufacturer. In a house it is usually around 1 to 2 bar when the system is cold.
3. They inspect the gas connection and the flue, and check that the safety devices work.
4. They set the boiler's maximum heating output to match the heat loss of the building. A boiler with too much output switches on and off far too often.
5. They measure the flue gases with an analyser and fine-tune combustion following the manufacturer's instructions.
6. They set the heating water temperature, or the weather compensation curve if the boiler has an outdoor sensor.
7. They connect and set up the room thermostat and the time programmes for heating and hot water.
8. If needed, they recommend balancing the radiators or underfloor heating circuits, or do it on the spot.

Work on the gas side and combustion settings must only be done by a qualified, registered gas engineer. Often this is the manufacturer's authorised service partner. With a new boiler, professional commissioning is usually also a condition of the warranty.

Every manufacturer gives recommended ranges for output and temperatures, and says how the boiler should be controlled. Wrong values show up as higher gas use, more noise or more frequent faults.

With a condensing boiler, the key is a low temperature in the water coming back from the radiators. The boiler can only recover heat from the water vapour in the flue gases when this return water is cool enough. Manufacturers usually put this below roughly 55 °C. If the boiler always runs flat out with very hot water, it hardly condenses and loses its main advantage. That is why the engineer focuses on:

• The lowest water temperature that still keeps the home warm
• An output that suits the house, so the burner runs steadily
• Good interaction between weather compensation and the thermostat
• As few unnecessary burner starts as possible

An older non-condensing boiler works differently, and water that is too cool does it no good. The engineer therefore follows the instructions for that specific model.

Weather compensation is a simple idea. An outdoor sensor measures the temperature outside, and the boiler chooses the water temperature on its own. When it is freezing, it sends hotter water to the radiators. On mild days it sends cooler water. You do not have to switch anything, and the indoor temperature stays more even.

The trick is getting the curve right. If it is too steep, the house overheats. If it is too flat, it gets chilly during cold spells. Fine-tuning often takes a few days, so the engineer will show you how to shift the curve slightly yourself.

A room thermostat adds information from inside the home, for example when the sun shines into the living room or someone is cooking. Thermostatic radiator valves then look after the temperature in individual rooms. So why does one room never warm up while another is too hot? Often the system has never been balanced. Balancing means setting the flow on each radiator valve so the water does not just rush through the radiators nearest the boiler.

The amount of work can vary a lot. It tends to be quicker and simpler when:

• The boiler is new and has a single heating circuit
• The manual is available and the engineer knows the model well
• The radiators have presettable valves and thermostatic heads
• The boiler is easy to reach

It takes more time and work when there is:

• An older system with no documentation
• More than one circuit, for example radiators and underfloor heating together
• An outdoor sensor, thermostat or zone controls to add
• Balancing of every radiator in the house
• Sludge in the system, trapped air or a faulty expansion vessel that must be sorted out first

In a flat with its own boiler, any work on the flue or a shared chimney may need approval from the building management or owners' association. If the boiler is being moved or gas pipework changed, check with your local building authority what permission or notification is needed. Do not assume a simple notice will be enough.

In practice it is common to find a boiler that has run on factory settings for years without ever being matched to the house. Other frequent mistakes include:

• Water temperature set too high "just to be safe", especially on a condensing boiler
• A thermostat fitted next to a radiator, in the kitchen or in direct sun
• Thermostatic valves turned down in the room where the room thermostat is
• Radiators full of air and low system pressure
• Boiler output set far too high, leading to constant on-off cycling
• Amateur changes in the service menu or to the gas valve

If you smell gas, turn off the gas at the meter, do not use electrical switches or lights, open the windows and leave. From a safe place, call your gas supplier's emergency line or the local emergency number, 112.

When the engineer leaves, you should know what has been set and how to use your heating. At handover, check that:

• The pressure gauge shows the value the engineer gave you
• You have a service record with the measured flue gas values
• You know the set water temperature or weather compensation curve
• The heating and hot water programmes fit your daily routine
• The thermostat is in a sensible spot and responds
• The radiators warm up evenly and without gurgling
• The boiler does not keep switching on and off in short bursts once it reaches temperature
• The work was done by a service provider the manufacturer accepts, so the warranty stays valid