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  A central heating system consists of all the pipe work and radiators that are connected to the boiler. The boiler provides the heat but, it's the pump (Circulator) that moves the heated water from the boiler through the pipe work to the radiators, and back to the boiler for re-heating. There are many types of system that can be installed, and which may be tailored to your own preferences. But a carefully designed and installed system will give many years of trouble free running and will not waste heat, therefore keeping fuel costs low

Open Vented 

(used on older systems and cast iron boilers) This means that there is an expansion tank fitted high up, (Usually fitted in the loft next to the header tank) which feeds the water down the “Cold Feed” into the central heating system. The water level in the tank should be very low  when the system is cold (the water level is maintained by a ballcock ), but as the system warms up the water expands and its level in the tank rises. There is also usually a safety pipe, fitted just above the tank and shaped like a hook pointing into the tank. This is called an “Expansion Pipe” or “Vent Pipe”.

1 Header tank for the hot water

2 Expansion tank for the Central heating

3 Air separator (if fitted)

4 Pump

5 Motorised valve

6 Indirect Cylinder

   7 Boiler

                                                                                8 & 9 Heating Flow & Return

Diagram( supplied by Steve Houghton ) is of a fully pumped open vented  "Y" Plan system 

A conventional system with two water tanks in the loft a large cold water storage tank and small feed and expansion tank

Diagram of a fully pumped open vented "S" plan system

Sometimes the ball-cock sticks so the expansion tank is empty, this will cause the highest radiators to go cold or more commonly the cylinder to go cold, especially on gravity hot water systems.   



Sealed System (recommended) Instead of using a tank and an expansion pipe, sealed systems use an “Expansion Vessel”, a “Safety Blow Off Valve” and a “Filling Loop”. Most modern systems use this method because it is easier to install and has less tendency of problems developing in the system at a later date. A lot of manufactures include an expansion vessel, pressure gauge and Safety Valve in the boiler case (System Boiler), but all the components can be obtained separately. Most boilers will work on a sealed system. Note. Not all boilers can be fitted to a sealed system. (consult the boiler manufacture’s instructions)

These systems should be pressurised at 1 Bar pressure when cold, if not some or all of the system will not work.

One Pipe System Hot water is pumped through a single pipe underneath all the radiators, then back to the boiler for re-heating. Hot water (lighter) rises into each radiator at one side of the radiator and the cooler (heavier) water falls back into the single pipe on the opposite side of the radiator. The last radiator is always cooler as the water has already given out most of its heat in the rest of the radiators

These systems were popular 30 years ago and a lot of them are still working. There is no restriction to the number of radiators, so they are ideal for industrial use, but all modern domestic systems use two pipes



Two Pipe System  (Small Bore) Hot water is pumped from the boiler (usually 22mm) through one pipe (Flow) to each radiator, which then gives up its heat to the air and the cooled water is pumped back to the boiler via a second pipe (Return). Each radiator has one pipe (15mm) connected into the flow and another into the return, so each radiator has it own circuit. Not on this system 15mm pipe should only be used were it is only feeding a maximum of 3 radiators or a maximum of 20,000 Btu's of heat output, most modern domestic boilers do have 22mm, connections from the boiler


The Expansion pipe must be one size lager then the cold feed pipe from the expansion tank and connected into the syatem as close as posible to each other and on the primary flow pipe between the boiler and the pump

A standard domestic circulating pump can only satisfactorily feed water to a maximum of twelve radiators because each radiator is fitted in parallel. So if there are twelve radiators fitted, each radiator will only receive approximately 8% of the available water flow. All modern installations use the two pipe system, as all the radiators warm up at the same time and all get to the same temperature

Micro Bore System Uses very small pipe (usually 8mm), so holds less water, but the pipes can easily block up especially in hard water areas. The high restriction of these systems can also cause more wear and tear on the pump and boiler

If fitting TRV's to this system then at least 3 radiator should not have them or and automatic by pass must be fitted

Note above is a twin entry valve which is showing the cap removed that covers the lock shield valve

Below is a twin entry valve showing the small pipe that feeds the water to the opposite end of the radiator

If removing a radiator for decorating both the valves need turning off and don't forget to refit the small pipe when refitting the radiator


A large flow and return pipe (usually 22mm) is taken from the boiler to a manifold on each level of the property, then the micro bore is taken from the manifold to each radiator. (to a maximum of 5 meters each pipe)


Pump Positions   The sighting of the pump on open vented systems is crucial, get it wrong and it could cause a lot of damage to the system through corrosion. The symptoms are leaking radiators and sludge in the system causing cold spots on radiators

O       P

O       P

If the system is “pumping over” then this will waste heat and draw air into the system, causing the radiators to corrode from the inside. The resulting rust will end up causing a blockage, usually at the point where the cold feed enters back into the system

This blockage can be tested for by using a magnet, normally a magnet is not attracted to the copper piping but if the pipes have magnetite inside then the magnet will be attracted to the pipe indicating the pipes have some restriction at he very least

On the examples above I would expect that both the LH drawings will have a blockage and a sludged up system and they may have had several radiators replaced due to corrosion, the bottom RH drawing is only for old cast iron boiler,

 The top RH drawing is correct for all new open vented systems but the cold feed & open vent must be as close as possible to each other


most modern systems should be piped similar to  this which is the same as the top right above

The boiler is on the RH side, the flow is from the 28mm compression elbow, the first connection is the 22mm expansion which rises vertically, then is the 15mm cold feed which enters from the underside, next is the pump with its isolating valve, on the LH side is the Honeywell "Y" plan valve and in-between the valve & the pump is the By-Pass which is this case is in 28mm which is far too big

it is very important to get the cold feed and expansion as close together as possible


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