Technical Information

Complete Domestic Gas, LPG & Plumbing Service

Serving Runcorn, Widnes, Vale Royal, Warrington, Merseyside & North Cheshire

Mark Williams Heating & Plumbing
  • Boilers installed, repaired & serviced - combi, conventional hot water/heating & system
  • Gas Safety checks & Landlord Safety Certificates
  • Gas fires & room heaters installed, repaired & serviced
  • Cookers & hobs installed & serviced
  • Central heating systems cleaned / powerflushed / upgraded
  • Unvented hot water systems maintained
  • Maintenance of all domestic Natural Gas & LPG appliances
  • General plumbing works
  • Minor electrical works carried out
  • Free estimates
  • Full public liability insurance
Please contact us today and let us know how we can help you. We aim to respond to your enquiry within 24 hours.

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Mark Williams Heating & Plumbing

Technical Information

Combi Boiler

A Combi boiler heats domestic hot water on demand, removing the need for the hot water storage cylinder and cold water storage tanks that are required in a concentrional hot water system.

They usually have a sealed central heating system i.e. the circulating water is topped up by means of a tap, hose and valve known as a filling loop. There is no requirement for a tank to supply the central heating system, and the expansion as the water heats is absorbed by a vessel in the system. Pressure in the system may fluctuate in use, usually between 1 and 2.5 bar. If the pressure exceeds 3 bar, a safety device discharges excess water.

Modern combi boilers can deliver a high flow of hot water if they are sized correctly, but are not always the best solution for hot water, especially in larger properties.

Conventional Hot Water & Central Heating Systems

These systems have a boiler and a pump which circulates hot water around radiators that give out heat to warm your home. Some of the circulating water may be diverted to warm the stored hot water which is kept in a cylinder. The stored hot water may also be heated by an electric immersion heater.

The circulating water is usually topped up from a small storage tank in the loft, which also allows for expansion of the circulating water. This is also known as an open vented system.

Stored hot water is delivered to the taps under pressure which is supplied by a larger storage tank, also usually in the loft. The height of the tank determines the pressure of the water at the tap; sometimes this leads to poor hot water performance especially upstairs, and sometimes it leads to erratic shower performance.


Powerflushing is a method of cleaning existing central heating systems so that they work efficiently. It also ensures that modern boilers are protected from the harm caused by debris in the system.

Powerflushing involves circulating water and chemicals at high velocity around the whole system and then treating each radiator individually. The system is then washed out with clean water and treated with corrosion-inhibiting chemicals.
If you are having this work done, it is a good idea to consider:

upgrading radiator valves;
installing a filter for further protection;
upgrading the central heating controls.
It is always recommended that existing central heating systems are cleaned when a new boiler is installed - some manufacturers'
warranties depend on this work being done.

Unvented Hot Water Systems

Unvented hot water systems can deliver a high flow of hot water, filling baths quickly and giving excellent showering results. These systems rely on stored hot water; the pressure in the cold water mains supply is used to power the delivery of hot water so a large storage tank is unnecessary.

The hot water storage vessel in unvented systems may be heated by a boiler, electric immersion heaters or a combination of both.

Because stored hot water under pressure could be dangerous, several safety features are incorporated into the design of the system. Also, additional qualifications are required to work on these systems.

D M Williams Limited is qualified to work with unvented hot water systems.


LPG (Liquid Petroleum Gas) is supplied from a cylinder, cartridge or a bulk tank. It is usually used in caravans, boats and park homes and sometimes in permanent dwellings - anywhere that mains (natural) gas is not available.

As the testing procedures are different from those used for natural gas, additional qulifications are required to work with LPG appliances.

D M Williams Limited is qualified to work with LPG (although we do not supply LPG cylinders).

Company  Information

The company D M Williams Limited was established in 2003.

Mark, the business owner, is a Gas Safe registered engineer and is qualified to work on both Natural Gas and LPG appliances.

The company provides a complete domestic heating and plumbing service in the north Cheshire area, including the maintenance of unvented hot water systems and powerflushing.

We undertake the installation, servicing and repair of boilers and gas appliances.

We also carry out Landlord Safety Checks and provide full documentation.

Mark is also qualified to carry out minor electrical works.

Our aim is to provide a friendly, efficient and courteous service at fair prices.