Domestic Plumbing Systems

I went through most of my life utterly mystified by the workings of a domestic water and heating system, largely because I had never managed to get a "big picture" overview of how everything fit together. Happily, I now have a better idea, after my father-in-law did some explaining and scribbling on the back of an envelope while helping us out with a small plumbing problem in our newly-acquired house. The system he sketched is a common layout and I've made my own version of it, as below.

schematic diagram of a domestic plumbing system

Arrows show the direction of flow of water. Brief explanations of the points marked on the diagram follow. I probably don't have all the right terminology, but this is only intended to give a rough idea of how things work.

(a) Main header tank. Filled by the incoming watermain, controlled by a ballcock valve. Two outlets from the tank supply the hot and cold water systems Not shown is an overflow outlet from near the top, which typically simply runs out of the side of the building.

(b) Heating header tank. The water that runs through the boiler and radiators has its own supply which is isolated from the main system and reuses its water (which, apart from other benefits, means that there is a limited amount of limescale deposit). Not shown is an overflow outlet, as with the main tank, and a ballcock controlled inlet which may be present to ensure the system does not run dry (as in our house).

(c) Hot water tank. The hot water supplied to taps is heated in a hot water tank, with the heat coming from water piped through from the boiler system.

(d) Boiler. A gas or electric boiler heats water from the closed heating subsystem. The heat may be enough to drive the flow of water (and thus heat) around the system, or there may be a pump for this purpose. Activity of the boiler and the pump may be controlled by a timer or thermostat.

(e) Radiators. Only one radiator is shown, but there will probably be several. Hot water goes in, the room is warmed, and not-so-hot water comes out again.

(f) Bathroom (and other) taps. Hot water is provided by the hot water tank and cold water from the main header tank -- and is thus generally not suitable for drinking. Toilet cisterns are also filled from the main header tank.

(g) Kitchen taps. Usually only the kitchen tap is fed directly from the rising main and is suitable for drinking. If the kitchen tap is running at full flow, it is unlikely that the mains water will have sufficient pressure to reach the main header tank.

(h) Overflows/steam outlets. Both parts of the water system include a pipe to allow steam to be vented and prevent a dangerous buildup of pressure. Typically these vents run up to the header tanks and allow any condensed steam to drip back into the system.

This page was last modified on 2005-10-30