Pipework layouts #gravity, #semi-gravity, #pumped, #fully #pumped, #natural #convection, #thermosyphon


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(Another hurriedly thrown together page I’m afraid. )

People often ask me how how the pipework circuits are arranged in a central heating system.

There are almost infinite variations but there are four main types;

The first two are completely obsolete in domestic heating and only rarely encountered. The other two are commonplace.

Recent changes to the Building Regulations have made semi-gravity non-compliant, so fully pumped is the only layout currently suitable for new installations. The Building Regulations now control boiler replacements too, and effectively require conversion of semi-gravity systems to fully-pumped whenever a boiler is replaced.

I’ll be adding nice tidy diagrams of each type here eventually, but for now I only have a few diagrams (shown below) collected from a variety of sources. Not a finished page once again, but some roughly presented information is better than none I hope you’ll agree 😉

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This is the system layout most commonly installed from the 1960’s through to about 1990. The boiler warms up and water circulates by natural convection (‘gravity’) and heats up the hot water cylinder. The HWC has to be fitted higher than the boiler for this to work. The radiators are controlled by turning the pump on and off, this being done automatically by a room thermostat. As you can imagine, the boiler (and therefore the hot water function) has to be ‘on’ before the heating will work. This is taken into account by the type of programmer fitted to semi-gravity systems – hot water alone can be selected but not central heating alone. Central heating can only be selected when hot water is also selected.

Here, the boiler output goes to a pair of motorised valves (or a single three-port valve), and each valve is controlled by a thermostat. When either the room thermostat or the cylinder thermostat calls for heat, it’s eqivalent motorised valve opens and also turns on the boiler. The advantages of this system are that the boiler remains shut down and cold when neither thermostat is calling for heat (leading to fuel savings and reduced CO2 emissions), and the hot water cylinder no longer needs to be located above the boiler. They can be installed side by side in the same cupboard for example, or a wall-hung boiler fitted in a bungalow with an airing/hot water cylinder cupboard on the same level.

System layout diagram reproduced from the Keston Celsius 25 installation instruction manual. (If anyone from Keston objects to me reproducing it here please contact me and I’ll remove it.)

Note the absence of a pump in this diagram. This is because this particular boiler has an integral pump in the flow pipe. Most boilers require a separate pump to be installed externally just before the motorised valves. The two valves in the flow to the cylinder and radiators in this diagram would be the motorised valves controlled by the cylinder and room thermostats.

Semi Gravity with thermostatic zone control

I’ve robbed this diagram from the Honeywell Sundial C Plan installation instructions. The C Plan is a method of installing thermostatic control on both hot water and room heating zones on a semi-gravity system. Unusual. The main advantage of this is that as on a fully pumped system, the boiler shuts down when both thermostats are satisfied giving improved fuel economy. (Note the feed and expansion tank and pipework connections are not shown in the diagram.)

It is essential to use the 28mm version of the V4043 two port motorised valve because unlike the 22mm version, it has a two-way switch that operates when the valve opens rather than the simple on/off switch of the 22mm valve. The two-way switch is essential for the wiring method which makes this system work. For full information about the C Plan design and wiring you can download the installation instructions PDF from the Honeywell UK website here. You’ll need to register.

This diagram illustrates how simple the heating system connected to a combi boiler is. No external pump, no tanks, no external expansion vessel, no motorised valves and in many cases item 6 is not needed either. (An automatic bypass valve is fitted inside most combi boilers by the manufacturer these days.) No wonder lazy heating engineers push the combi boiler heating system in preference to a proper boiler and hot water cylinder.

This is my own rough sketch of a traditional gravity system. It’s the same as an old coal fired system but with a gas boiler inserted in place of the original coal boiler in the kitchen. There is no pump (obviously), and the whole thing is installed using huge diameter pipes because the only motive force for circulation is natural convection. Hot water is less dense than cold causing it to rise to the top of the system. Water inside the rads cools as it gives up it’s heat to warm the house and falls to the bottom of the system where is is re-heated by the boiler and rises to the top again. Old unmodified gravity systems are usually direct, which means the water from the hot taps and hot water cylinder is the same water that is circulating through the radiators. There is no separate header tank and heating coil inside the HWC as in modern systems.

One pipe system

This is a diagram of the obsolete one-pipe pumped system. There are a few systems like this remaining in use but generally they are approaching 40 years of age or fitted by a DIY installer with a very old book of how to fit central heating.

One pipe systems were originally fitted and add-ons to coal fires with back boilers. A loop of pipe was installed around the house and a pump pushed the hot water around the loop. Some of the hot water passed up into the rads by natural convection or by luck and made the rads warm (but never HOT). When gas boilers began to be installed in ordinary domestic houses the format was copied but was rapidly superceded by the ‘two-pipe’ method as all the rads heated up properly. As you can see from the diagram the cooled water from each radiator dilutes the hot water in the pipe loop so the last rad on the system has no hope of getting properly hot. I know this because my bedroom in the house I grew up in had the last rad on the loop.

On a technical note, anyone sharp-eyed will notice the pump in this diagram is fitted backwards so is pumping the wrong way. It should be pumping from right to left, back into the boiler!

First created 16th january 2004
Last updated 10th November 2016


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