How does central heating work?

[Pancho]

Prompted by someone's question about their cavitating boiler; how does central heating work?

We've lived here 15 years and I've never really fathomed what's going on. Never had to as it's always just worked.

We have a boiler in a room downstairs. No idea what it is beyond the observations that it takes gas, has a chimney (which heats every room it runs near) and is big.

Upstairs, in a room behind the airing cupboard, there is a hot water water tank (or at least, something that's lagged) and a maze of pipework with loads of big stopcocks. The water tank seems to take electricity?

In the attic is more stuff which I, vaguely, remember the previous owner saying was heating related. But I've never ventured as far as looking at it as it's in an unlit section.

So hows it all work then?

[border-rider]

The Central Heating Gnomes silently scurry from room-to-room carrying invisible scuttles of hot from the boiler to you. 

They keep one eye on the timer clock, so they know when to stop, one eye on the polished mahogany mercury thermometer strapped to their ankle boot, and one eye on the thermostat - so they know how fast to run.

Obvious really.

[Jurek]

I think you should PM Tiger directly with your question.
I know he can explain it all to you in detail.
Let us know what he says.

[rogerzilla]

The boiler heats up water and a pump circulates it round the radiators.  In a conventional indirect system, hot water is heated by a copper coil in the cylinder which has the hot water from the boiler flowing through it.  The electric connection is for the immersion heater, which you only need if the CH is b0rked or you want to heat up a tank really fast.

Unless the system is pressurised, there will be a small header tank with a ballcock in the loft to keep the system full at all times and to make up any losses.

The radiators are plumbed in parallel with each other so that one shutting down doesn't stop the whole flow.  There is often a manifold somewhere under the floor which diverts the main boiler flow to the individual rads and collects it again.

[tonycollinet]

The Central Heating Gnomes silently scurry from room-to-room carrying invisible scuttles of hot from the boiler to you. 

They keep one eye on the timer clock, so they know when to stop, one eye on the polished mahogany mercury thermometer strapped to their ankle boot, and one eye on the thermostat - so they know how fast to run.

Obvious really.

Absolutely correct.

Or...

It could be more like the diagram here. If your system is older, it is likely to have gravity fed heating water from the boiler to the water cyclinder, as shown in the diagram (there is a pump only for the radiators - water for the hot water cylinder flows due to convection)

A more modern system is likely to be fully pumped, where the pumped water from the boiler is put through a motorised valve, which can send it to the water cylinder, or the radiators, or both.

In both cases, the stuff in the loft is header tanks which work like a toilet cistern (with float valve) to keep the whole lot full of water.

The hot water cylinder is normally "indirect" which means the water you use for the bath is heated by a coil inside the cylinder which has the hot "boiler" water flowing through it (like a radiator inside the cylinder).

[gordon taylor]

I installed our oil-fired central heating many years ago, having virtually no plumbing experience before then.

The joy as it fired up and warmed every corner of our cold and damp cottage has never yet been bettered. Central heating is wonderful - technically and psychologically.

[Pancho]

OK.

Where am I likely to find the pump(s) that keep the whole thing circulating?

Also, if it's a closed system, why does it need a header tank? Leaks?

[rogerzilla]

Normally in the airing cupboard, but some modern boilers have it inside the boiler casing.  It's pretty obvious; normally blue or bronze hammered finish and vaguely round in shape.  There should be a big gate valve either side of it.

The header tank is there to initially fill the system and to top up in the case of leaks.  The system is full but not actually closed; the tank also accommodates expansion, so there is some evaporation to be made up.

[Fi]

In addition to boilers and pipes,  sums are involved to ensure that the potential heat output of the boiler is sufficient for the size of building and radiators attached to it. In turn, the radiators have to be the right size to push out enough heat into the room. 

 Just before he retired, my husband went to the library, came back with a pile of books on central heating systems and announced that he was going to get rid of our old and inefficient system and replace it with something that actually kept the house warm.   

I was a little sceptical, but as Gordy says, when you turn the boiler on, it fires and the house warms up, it's wonderful.  We sold the old coked up cylinder and the not needed pipework which helped to defray the cost of the new system

[tonycollinet]

Normally in the airing cupboard, but some modern boilers have it inside the boiler casing.  It's pretty obvious; normally blue or bronze hammered finish and vaguely round in shape.  There should be a big gate valve either side of it.

Could also be in the pipework close to the boiler.

[Martin]

 There is often a manifold somewhere under the floor which diverts the main boiler flow to the individual rads and collects it again.

only with 10mm pipe; a norman rad circuit starts as 22mm teeing off each rad at 15mm until it's fed about half the system then goes down to 15 for the most remote ones and then goes back up to 22 on the return (to accomodate the increased water flow the more rads are fed)

at least that was how it worked when I got to Texas to design my system sell me a load of bits and draw a diagram on the back of a fag packet

[Fab Foodie]

Well, my system (I asked the previous question) seemed screwed...
Last night it seemed to be working OK ish, it heated the water, and the rads started to heat.
This morning the boiler tripped on overheat.

Must be airlocked or blocked somewhere.  The pump runs OK (yes it's spinning), the 3-way valve's operating according to demand, yet the heat from the boiler is not removed at any significant rate now.  I'm minded to drain the system, flush and fill with Fernox cleany-stuff... but... I can't find a suitable drain point low-down in the system.  I can find one on top of the boilet on the return side, but thats 8' above the ground.

How do you drain the low-down stuff?
 

[rogerzilla]

You disconnect a radiator.

Top tip: if your boiler trips out because the pump's not running, remove the little screw cap in the centre of the pump (no need to drain anything), stick a big screwdriver in there and work the pump spindle until it comes free.  Often a piece of grit or scale jams the impeller.  A plumber will try the same thing first, and charge you £100.

[Martin]

You disconnect a radiator.

in a modern concrete floored house the rads will be looped down from the first floor; each should have a drain off cock fitted. If there is a ground floor rad circuit  the single cock will be more or less at floor level but may be elsewhere; have a look around.

 If you are having to disconnect a rad to drain the system it's worth fitting a drain off cock at the lowest point before you refill it (it can all be done with spanner tightening fittings for those who speak neither Yorkshire or EndFeed)

Airlocks should clear themselves in a properly designed system so if it's suddenly happened suspect either the pump or 3 way valve or programmer.

Question; in an old direct cyclinder setup what happened to rads; did they use the same water as the taps?

[citoyen]

Question; in an old direct cyclinder setup what happened to rads; did they use the same water as the taps?

Not unless you want to get legionnaires. IIRC the cylinder acts as a thermal store and has a built-in header/expansion tank. Hot tap water is direct from the mains, via a heat exchanger. The biggest advantage of this system is drinkable hot water supplied at mains pressure. The heating is on a separate loop, as with an indirect vented system.

Or do you mean an unvented cylinder? These work roughly the same as a vented system except that the cylinder is supplied directly from the mains rather than a header tank, and has no outlet for expansion (there's a sealed bladder in the cylinder to cope with expansion). You need very good mains pressure to run an unvented system safely, especially if you want to run a bath while also using the washing machine. I'm not sure how you heat your radiators with an unvented system.

d.

[bomber]

Well i've learnt something today.

Better than wikipedia this is

[Martin]

Question; in an old direct cyclinder setup what happened to rads; did they use the same water as the taps?

Not unless you want to get legionnaires. IIRC the cylinder acts as a thermal store and has a built-in header/expansion tank.

nooh that's a much more recent development; I'm talking about the old direct cylinders where the water from the boiler just circulated around and the hot water came out of the taps. This is probably a throwback to when hot water was got by a simple convection pipe from a water matrix (ie a metal box) at the back of an old coal fire in the kitchen and the hot water tank was a square galvanised affair. In those days of course radiators were the coal fire and we all had to sit on an electric storage heater on the landing to get a warm bum (pre 1976 in my case). Even in the 80s you could get conversion kits to make direct copper cylinders into indirect.

I presume old Victorian rads like in schools and workhouses hospitals were also convection and not pumped.