Types of radiator

Types of radiators

A wet central heating system delivers its heat by radiation and convection. You can feel radiation as heat emitted from a hot surface, but convection warms the air and, in doing so, sets up a gentle circulation centred on the heat source - the warm air rises and cooler air flows in to take its place; in this way, the temperature of the whole room rises gradually.

The simplest form of heat emitter is the panel radiator. Although called a radiator. In fact over half of its heat outputs occur through convection. It has a corrugated body to increase its surface area and ensure maximum heat output. Where space is limited, double panel radiators are ideal. Another means of improving output is to use a convector radiator, which has a number of fins on the back to increase convection.

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Radiator valves

Radiators normally have a threaded hole, or tapping, at each corner. The bottom two are for the valves - a manual or control valve to adjust the incoming flow of water (this is used to turn the radiator on and off), and a lockshield valve to control the outgoing flow of water (this is normally left fully open). One of the top tapings will be fitted with a bleed valve, a simple tapered screw that allows you to remove air from the system; the other has a blanking plug.

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Radiator design

Radiators and towel warmers needn't just be functional heat providers. These days there are lots of stylish options to complement every interior.

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Heating Controls


Many systems incorporate a room thermostat, or a roomstat, which switches the heating on and off in accordance with the temperature you have selected. It is normally sited in the living room or hall, and it operates on the assumption that a rise or drop in temperature in its vicinity will be matched by a corresponding rise or drop throughout the house. A programmable roomstat gives you greater control, enabling you to set different temperatures for different times of the day. These are available in wired or wireless versions. The problem with a roomstat is that it doesn't accommodate local temperature variations - caused perhaps by the sun shining through windows or an open fire in a living room.

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Thermostatic radiator valves

You can reduce the heat output of an individual radiator by closing a manual radiator valve, but much more efficient is to install thermostatic valves, which will do this automatically for you and then open again when the room temperature of individual radiators at set times of the day.

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Time switches and programmers

A simple time switch will turn the system on and off at pre-set times, but a programmers allows independent operation of this heating and hot water circuits as different times of the day. Even greater flexibility is provided by a controller, which operates the system at set times also responds to temperature fluctuations, while a boiler manager ensures that the boiler only runs when necessary by sensing a variety of conditions, including outdoor temperature.

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Cylinder thermostats

The temperature of water required to heat radiators is much higher than that needed at the taps (82°c as opposed to 60°c) and a cylinder thermostat will prevent overheating of the hot water supply. When the temperature in the cylinder reaches the set limit, the thermostat operates a motorised valve, shutting off the flow to its heat exchanger. When the water temperature drops, the valve opens again.

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Zone valves

Motorised valves known as zone or mid-position valves, can also be used to shut off parts of the system at specific times of the day when heating is not required in certain rooms. This can be useful in a large house, where you may not want to heat all the bedrooms on the upper floors during the day, and is a requirement in systems heating more than 150m².

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Wet radiators/pipework - How it works

Wet central heating

In a wet central heating system, a boiler heats water, which is usually pumped around a circuit or pipes and radiators fitted throughout the house. Each radiator has valves that control the rate at which water flows through it, thus controlling the amount of time the water spends in the radiator and the amount of heat that is given off. When the water leaves the radiator, it is piped back to the boiler for reheating. Although some heat is lost from the pipes as the water flows around the system, their narrow diameter and the speed of the flow keeps this to a minimum. A well designed system may have several short circuits radiating form the pump, rather than one large one, ensuring that the last radiator on each circuit heats up just as efficiently as the first.

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The radiator circuits in most modern wet central heating systems are run in a standard 15mm copper pipe, while pipes linking the boiler and pump to the points where the circuits split off will be 22mm or 28mm in diameter. Soldered capillary joints are best (apart from connections to the boiler, pump and valves), as these are least likely to leak. Plastic pipes have been developed for use in wet central heating systems, but they can't be connected directly to a boiler; a short length of copper pipe must be used instead.

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Two-pipe and single-pipe systems

More modern systems have two-pipe layout, where hot water is carried from the pump to the inlet valve of each radiator by a flow of pipe, which terminates at the last radiator on circuit. A second pipe, the return pipe, collects the cooled water from the radiators and carries it back to the boiler for reheating. In older single pipe layout, the hot water passes through one radiator, then is returned to the flow pipe to be carried on to the next and so on around the circuit, losing some of its heat each time. Consequently, radiators towards the end of the circuit have to be bigger in order to give off the same amount of heat.

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Microbore pipe system

A variation of the two-pipe layout uses small-diameter micro bore pipe, which is available in 8mm and 10mm sizes. It is supplied in coils and is very easy to bend, so you can feed the pipe through cavities and around corners without the need for joints. These narrow pipes only carry enough water to heat one radiator at a time, so each radiator must has its own circuit. These radiate out from fitting known as manifolds. The flow pipes to several radiators are connected to one end of the manifold and returns to the other. Both feed and return microbore pipes are connected to the same end of the radiator.

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Tackling faults cold radiators

Cold radiators

All radiators are cold when the boilers running

Some radiators are cold.

Single radiators cold

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Top of radiator cold

This means there is air trapped. Open the bleed valve so that air can escape; if the problem persists, call an engineer.

Centre and bottom of radiator cold

Corrosion is probably restricting the water flow. Remove the radiator and flush out or replace it as necessary; add corrosion inhibitor to the system.

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Replacing a radiator

You don't have to drain the entire heating system to replace a radiator, just turn off the heating and isolate the radiator by closing off the valve at either end. Turn the manual valve clockwise until it won't turn any further. Pull off the plastic shield of the lockshield valve at the other end and turn the square shaft clockwise with an adjustable spanner. Count the number of turns so that you can reset the new radiator at the same flow rate. If replacing several radiators, you will lose corrosion inhibitor from the system and should drain it down and add more.

Step one

With both valves turned off, use an adjustable spanner to slacken the cap nut holding one of them to the radiator. You may need to hold the valve body with a second spanner or pipe wrench to prevent it turning and buckling in the pipe.

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Step two

Place a tray beneath to catch the water as it drains out. Open the bleed valve at the top of the radiator and loosen the capnut. When the tray is nearly full re-tighten the capnut and empty the tray. Be ready with cloths to mop up any spillage - the water will be filthy. Repeat until all the water has drained out then disconnect the other valve.

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Step three

Lift the radiator from its brackets and tilt to drain any remaining water. Get a helper to stuff tissue into the outlet the other end to stop it leaking. If the existing brackets don't suit the new radiator, unscrew them and fit those supplied with the new radiator. Check for pipes and cables before drilling and fixing holes.

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Step four

Remove the valve connectors from the old radiator, using an adjustable spanner or large allen key. Clean the threads with wire wool and wind PTFE tape around the threads about five times to ensure a good seal. Screw the connectors into the new radiator, making sure they are tightened fully. Hang the radiator. Connect the valves and reset them, allowing the water to enter the radiator. You will need to open and bleed the valve about half a turn so that air can escape; close it when water starts to appear.

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Draining & refilling

Draining and refilling the system
All wet central heating systems will be provided with at least one draincock to allow the system to be emptied for maintenance or repairs. Usually this will be in the return pipe close to the boiler. However, where a solid ground floor pipe prevents from being run below it, the pipes will drop down from the ceiling to void supply ground floor radiators. These sections of pipe work will remain full of water when the system is drained from the boiler draincock and will have their own draincocks, allowing them to be emptied separately. When refilling the system, make sure all draincocks and radiator bleed valves are closed before restoring the water supply to the feed - and - expansion cistern in the loft. As water flows in the system, air will become trapped in the radiators, so bleed each by opening the bleed valve about half a turn, starting at the bottom of the house and working upwards. Before turning the pump back on, bleed this too.

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