Gas combination condensing boilers, better known as combi boilers, are highly efficient water heaters. They are compact fuel misers, designed and installed to extract the maximum heat from their fuel source.
Unlike their open vented counterparts, combi’s are designed to run a pressurised hot water central heating system that eliminates the need for feed and expansion tanks. This attribute is most advantageous in a property where available space is limited.
Like any sophisticated appliance, combi boilers respond well to good and regular attention. Annual maintenance and servicing by suitably qualified engineers will help to keep a combi in good operating condition. This provides peace of mind for a homeowner in knowing that the boiler is performing in line with requirements and unlikely to break down in the depths of winter.
However, combi boilers require another simple check that can be carried out by the homeowner. Checking that the combi boiler operating pressure is correct and undertaking the necessary operations to maintain it is a relatively easy task.
A combi boiler pressure check should be carried out once a month on a correctly functioning boiler.
The pressure within the central heating system is registered on an analogue dial or digital display panel. In most modern combi boilers, the dial or display is located on the boiler. This can be either on the front, sometimes beneath a protective flap, or at the base, but not always immediately visible due to the boiler cover. It is not usually necessary to remove the boiler cover to observe the pressure register.
Very occasionally, the pressure registering display device is located separately from the boiler but is generally in the vicinity of it.
To locate and correctly identify the pressure gauge, the homeowner should refer to the combi boiler instruction manual.
Having located the pressure-registering device the current pressure within the system can be ascertained. A normal pressure range will be between 1 and 2 bar. On an analogue gauge, a black needle will indicate the pressure on the numbered dial face. On some models, the acceptable cold working pressure area will appear as a green coloured fraction on the dial face. Occasionally a further red needle will be present. This is adjustable and can be set to indicate the optimum operating position that conforms to the manufacturer’s recommendations.
As operating pressures may vary between manufacturers and boiler models, the manufacturer’s instruction manual should be consulted to establish the correct pressure ranges.
Where a boiler appears to be operating at a lower pressure than recommended, the system will require pressurising.
To pressurise a combi boiler central heating system, a filling loop is usually provided as part of the installation. The loop consists of a short length of flexible metal or plastic tubing. This will have screw fittings at each end. There should also be valves at either end of the loop. These may be of a lever or screwdriver operated type.
Before commencing to pressurise the system, the gas burner on the boiler should be turned off. It is probably easier to work on a cold boiler and central heating system.
With the loop valves in the closed position, one end of the loop must be screwed onto cold-water input branch feed beneath the boiler. The other end should be attached to the cold-water branch from the mains cold feed. Both these feed points will have flow control valves.
With the loop securely attached, the loop valves can be opened. The mains water branch feed valve can also be opened. To commence pressurisation, the cold mains inlet feed valve should be carefully operated. The sound of water entering the boiler should be heard.
Whilst observing the boiler pressure indicator gauge or digital display, the valve should be kept open until the correct pressure has been achieved and registers on the display.
Once the correct pressure has been reached, the valves on the loop and the two feed pipes should be turned off.
The filling loop can then be disconnected and the boiler operated. It is not good practice to leave a filling loop permanently attached to a combi boiler.
On some combi boilers, a filling loop is not required and the boiler has an internal pressurising system. This is operated by a dedicated key that has to be inserted into the base of the boiler. The key locks into an internal pipework mechanism and turning it operates the pressurising system. The pressure gauge must still be monitored. When the correct pressure is achieved, the key can be unlocked and removed.
On some boilers, instructions on pressurising are displayed on the boiler. However, the best practice is to consult the operator’s manual where detailed instructions for pressurising the specific boiler model will be found.
After pressurising, the central heating radiators may require bleeding. After bleeding the radiators, the pressure gauge should be re-checked, as it is often necessary to add a little more pressure into the system.
If, when attempting to pressurise a combi, the pressure cannot be raised, immediately check the entire system for evidence of a possible leak. Another cause of not being able to pressurise the system is an inadequate mains water pressure. This may be caused by maintenance operations or burst pipes on the mains network. Often, when the mains pressure is low, the boiler will not function by design.
Where a combi boiler loses pressure frequently, a fault may lie within the central heating system or the boiler itself. If, after checking the system for leaks and checking the boiler’s pressure release valve for faulty operation no problems are evident, it may be necessary to employ the services of a qualified engineer.
Significant problems can often occur when a combi boiler is installed to replace a conventional boiler. The pressure produced by a combi boiler may cause problems in an older central heating system. It is important to have the old system professionally pressure checked prior to installing a combi boiler.
When properly maintained and cared for, a combi boiler will continue to work efficiently and reliably for many years.