Dishwasher Installation in London: What the Plumbing Involves

What a Dishwasher Connection Involves

A dishwasher installation in a London kitchen involves three separate connections: the cold water supply, the waste drain, and the electrical power supply. The plumbing element of the job is the cold supply and waste drain. For most London kitchens, these are already in place from a previous dishwasher or have been roughed in during the kitchen installation -- in this case the plumbing connection is a straightforward job that can be completed in under an hour. Where the connections are not in place, additional pipework is required and the job becomes more involved.

The Cold Water Supply Connection

A dishwasher requires only a cold water supply -- it heats the water internally using a built-in element. The cold supply is connected via a 15 mm washing machine valve, which is a quarter-turn isolation valve with a male threaded outlet to accept the machine inlet hose. The most common position is under the kitchen sink, tee-ing off the existing cold supply pipe that feeds the sink cold tap. The tee is made using a compression or push-fit fitting, and the valve is attached directly. A hose connector kit (typically supplied with the dishwasher) connects the appliance inlet hose to the valve outlet.

In London kitchens with older lead or iron pipework, the plumber will recommend replacing the section of pipework involved in the connection rather than disturbing the aged pipe. Compression fittings on old iron or lead pipe can be unreliable and a short replacement with copper is the correct approach.

The Waste Standpipe Connection

The dishwasher drain hose connects to a waste standpipe or to a dedicated inlet on the sink trap. The standpipe option -- a 40 mm upstand of approximately 600 mm height with a U-bend at the base -- is the most reliable. The height of the standpipe prevents siphonage of water back out of the dishwasher during a drain cycle. If the drain hose is to loop over the sink cabinet instead, the hose must be secured at its highest point at or above the level of the water in the machine when full -- typically 400 to 600 mm above the floor. Many modern kitchen sink waste traps include a 40 mm spigot specifically for dishwasher or washing machine drain hose connections.

In London flats with a soil stack at some distance from the kitchen, the waste run from the kitchen may need to be extended to reach the appliance position. This is more common in open-plan kitchens where the appliance is not adjacent to the sink. The waste must maintain a minimum gradient of 45 mm per metre fall towards the soil stack or drain connection.

Electrical Supply

A dishwasher requires a 13A switched socket at the appliance position. In a new kitchen installation, a dedicated socket behind the dishwasher position is typically included by the kitchen electrician. If it is not present, an electrician must add a socket -- this work is notifiable under Part P if a new circuit or spur is run in the kitchen zone. Where a suitable socket is already in place behind the appliance position, no electrical work is required and the plumber can complete the full installation independently.

Installing a Dishwasher in a Space Without Existing Connections

In some London kitchens -- older properties, kitchen extensions, or conversions where the original kitchen did not have a dishwasher -- the supply and waste connections do not exist at the required position. The plumber must run a new 15 mm cold supply from the nearest accessible supply point (typically the kitchen cold supply under the sink), and route a new 40 mm waste pipe to connect to the existing kitchen waste run. Where the appliance is being installed away from the sink -- in an island unit, for example -- this work can be more extensive and will require the plumber to plan the routing of both pipes through the floor void or ceiling before the kitchen units are installed.

Integrated vs Freestanding Dishwashers

The plumbing connection for an integrated (built-under) dishwasher and a freestanding dishwasher is identical -- both use the same 15 mm inlet hose connection and 22 mm drain hose. The difference is in the fitting of the door panel and the positioning within the unit run. For an integrated dishwasher, the door panel is fitted by the kitchen installer after the appliance is positioned and connected. The plumber completes the supply and waste connections before the door panel is attached.

Pricing

Dishwasher connection to an existing supply and waste point: £80 to £150. New 15 mm cold supply run and waste standpipe installation where connections are not in place: £150 to £300 additional depending on the length of the runs and accessibility. Where an electrician is also required for a new socket, budget an additional £100 to £200 for the electrical element.