How to understand a plumbing & heating system

Introduction

Your home's cold and hot water supply systems will largely be out of sight, with the pipes concealed in walls and under floors. But knowing what type of system you have and where you can find everything is very important for maintenance work, making improvements and, most of all, dealing with emergencies.


Your local water company will provide you with clean, drinkable water, remove waste water and sewage. From their main supply pipe, under the road outside your home, a branch pipe will run underground to a stop valve beneath a metal plate, which is set in the pavement or somewhere in your front garden. This valve allows the company to control your water supply.

From the valve water flows along what is called the service pipe, passing through your home's foundations before rising up through the floor - usually in the kitchen, but sometimes under the stairs or even in your garage. The rising main, as it's known, is fitted with your main stop valve at this point.

From the water company's underground stop valve onwards, the entire system is your responsibility, and it's up to you to make sure this is properly maintained. It's actually an offence to allow water to be wasted through leaks.


Some homes have a direct cold water system. Branch pipes lead directly from the rising main to feed all the cold taps and WC cisterns in the house at mains (high) pressure. The rising main will usually also continue up to a storage cistern in the roof, which provides a supply of water at gravity pressure to a hot water cylinder.

One advantage of a direct system is that you can get drinking water at all the cold taps (you shouldn't drink water from a storage tank as it could be old and contaminated). There's also less pipework in the roof to worry about.

Direct systems can expose the mains water supply to a greater risk of contamination through back-siphonage of dirty water from baths and wash basins. However, you can prevent this by installing a non-return valve above the stop valve that supplies water to your house.


Most houses have an indirect cold water supply system. A branch runs from the rising main after your main stop valve to the cold tap at the kitchen sink, to supply clean drinking water. Other branches might come off at this point to supply your washing machine, dishwasher or garden tap.

The rising main runs up to the roof, where it ends at a cold water storage cistern. This supplies all the remaining cold taps in your house, the WC cisterns and the hot water cylinder.

There are two main advantages of an indirect system. Firstly, the storage cistern holds a reserve of water for washing and flushing WCs if the mains supply is interrupted. Also, the lower pressure of the gravity supply to most of the system causes less wear and tear, and less noise as the water flows through the pipes.


If you have an indirect cold water system, you'll have a storage cistern in your loft that's fed from the rising main through a ball valve. All the pipes that run from this cistern should be controlled by nearby stop valves, which let you turn off individual pipe circuits to be drained for repair or maintenance work without having to empty the entire system.

If you don't have any valves, you'll need to isolate the cistern (either by turning off a stop valve in the supply pipe or tying up the arm of the ball valve) and then drain it.

The cistern has an overflow pipe that runs to the outside if the water level is too high. It must have a lid to stop anything falling into the water and should be insulated to prevent freezing in winter.

A direct cold water system might have a storage cistern to provide top-up water for a hot water cylinder, or a feed-and-expansion cistern to supply a wet central heating system. You isolate this in the same way as an indirect storage system.


Always drain your system if you're going away for a long time in winter. You might also need to drain the system when it needs to be repaired or extended.

stop valve

Stop valves

Fitted to the rising main pipe close to where it comes into the house, your stop valve lets you shut off your entire water supply.

drain cocks

Drain cocks

A drain cock is a small tap fitted at a low point on your plumbing system that allows you to empty a pipe run.

Step 1

Shut down the heating system or turn off your immersion heater.

Step 2

Close the main stop valve on the rising main, then open all your taps. When they stop running, flush all your toilets.

Step 3

Connect a hose to each drain cock on the system in turn (probably above your main stop valve and at the foot of your hot water cylinder) and run it to an outside drain. Then open the valve, using a spanner or small wrench, to empty any remaining water from the system, and close the drain cock before removing the hose. Be careful not to confuse drain cocks on the heating system with those on the water supply system - there's no need to drain the former.

Step 4

Pour a little salt down all your plug-holes to prevent water in the waste traps from freezing.

Step 5

Before you refill the system by opening the main stop valve, make sure that all your taps and drain cocks are closed. It's also worth taking a good look at your storage and WC cisterns to check their ball valves haven't become stuck open. Air can also become trapped in the system when it's filling, causing taps to splutter and the water flow to be sluggish. If the problem continues, clear the air locks with the mains pressure.


The water level in your storage and toilet cisterns is regulated by ball valves. These incorporate a long lever arm with a metal or plastic float (often ball-shaped) at the end. When the cistern is full, the float holds the lever up, which keeps the valve closed. And when the water level drops, both float and lever fall, opening the valve.

By altering the position of the float in relation to the valve, you can change the 'full' water level in the cistern by adjusting the point at which the valve opens and closes. The most important thing to remember about ball valves is that there are high and low pressure versions.

You need a high-pressure type where the supply is at mains pressure (on a cold-water storage cistern or toilet cistern on a direct system). While low-pressure valves must be fitted to cisterns supplied under gravity pressure (toilet cisterns on an indirect system).

ball valve

Adjusting your ball valve

You can adjust some ball valves simply by bending the metal float arm, while others have an adjuster screw and locknut. Some also have floats that you can reposition vertically. Try to set your float so the water level is about 25mm below the overflow outlet.


If your water is heated by a boiler and stored in a cylinder, chances are your water is heated indirectly. This means you have two hot water systems - a primary circuit and a secondary circuit. In the former, a pipe runs from the boiler to a heat exchanger (usually a coil of copper pipe) inside the hot water cylinder and then back to the boiler.

When the boiler is running, hot water flows continuously around the primary circuit. As the pipes of the heat exchanger warm up, so does the water in the cylinder. This can then be drawn off from the top of the cylinder to supply the hot taps around your house (the secondary circuit).

The hot water cylinder (usually found in an airing cupboard) is kept topped up with water from the cold water storage cistern in the roof. The primary circuit has its own top-up supply of water from a small tank in the roof (known as the feed-and-expansion cistern).

The cylinder's hot water draw-off pipe continues to the roof and terminates over the cold water storage cistern while the boiler's hot feed pipe to the heat exchanger ends over the feed-and-expansion cistern in the roof. This lets any air or steam in either circuit escape safely.

A major advantage of the indirect system is that the water that passes through the boiler doesn't mix with the water in the cylinder and, effectively, is recycled time and again. As a result, less scale builds up in the boiler, and corrosion inhibitor can be added to the water, which can cut your maintenance costs.

Safety first

If you're having a new boiler or heating system installed, it's very important that you get a qualified heating engineer to fit it. They'll work to industry standards and advise you on the correct and safe siting of your boiler and flue. If you're using gas, the firm must be Gas Safe registered. For other types of fuel, the firm should be registered with the Heating and Ventilating Contractors' Association (HVCA).

immersion heaters

Immersion heaters

An electric immersion heater, fitted into the top of the hot water cylinder, lets you heat water in the summer without having to run your boiler. It has a long, thermostatically controlled heating element that extends down to the inside of the cylinder. Some have two elements (a short one and a long one) that allow just the top portion of the cylinder to be heated when you only need a small amount of hot water. Or the entire cylinder when you want more (to run a bath, for example). You can also have two immersion heaters with short elements fitted in the cylinder, one above the other. An immersion heater has a thermostat to control the maximum water temperature, and you can adjust it by turning an adjuster screw with a screwdriver. 60 degrees C is a comfortable level for most homes.


These days, people are increasingly going for combination (or combi) boilers, which do away with the need for a hot water cylinder by heating water on demand. This can be more economical and energy-efficient as the water is only heated when it's actually needed and there's no energy drain through heat loss from a cylinder.

In small properties, particularly flats, where there's no room for a hot water storage cylinder, an instantaneous water heater (usually gas-fired) can heat the water just before it is drawn from the tap. You can also get electric versions for showers.

What sort of fuel?

If you have mains gas, this is the cheapest fuel for your boiler to burn. If not, the second-cheapest option is oil, followed by solid fuel (such as anthracite), and then LPG (liquefied petroleum gas). You can store oil and LPG in bulk tanks, and LPG can also be supplied in large cylinders. But this is the least convenient and most expensive way of using the fuel.

You can also have solid fuel delivered loose or in pre-packed bags but it's dirty, and you have to stoke the boiler regularly to keep it burning. That said, some solid-fuel boilers have hopper feeds that cut down on the amount of stoking required. You also need some means of disposing of the ash after burning. In a standard boiler, the fuel is burned beneath a chamber (known as a heat exchanger), through which the water passes to be warmed. After that, it's pumped around the heating circuit or circulates by gravity. This basic design can operate on gas, oil or solid fuel.

Where should you put your boiler?

Standard, combination and condensing boilers can be free-standing or wall-mounted, although solid-fuel types are free-standing only. They must be connected to a flue so the gases produced by burning the fuel can be discharged to the atmosphere. In many cases, this can be an existing chimney, although it must be lined correctly as the gases can corrode the structure. For safety, it's also important that the room where the boiler is situated is well ventilated. That way, there's enough air for combustion and poisonous fumes won't build up.

Fumes aren't a problem if the boiler has a balanced, (or 'room-sealed') flue. This is basically a divided horizontal duct that passes through an outside wall; air is drawn in along one passage and flue gases are discharged through the other. There are special regulations about balanced flues - your heating engineer can advise you about them.