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Help Needed With Explaining Central Heating System

Postby greengrass » Mon Dec 31, 2018 1:37 pm

While I have the chance, Happy new year to all at DIY doctor & Forum members

For the sake of retaining my sanity, explain as though addressing my wife, how central heating works and why rads are hot even when boiler is off after room temp reached in accordance with set target temperature. I know that radiators will remain hot when boiler is off so they give out 'radiant heat 'hence name "radiator" . 'Er indoors' knows it all, and the inevitable word emits from their gob 'WRONG'!!!!!! And turns off rads that were hot. Each rads has TRV the room stat is wireless user controlled mobile unit from British Gas.

Many thanks
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Postby ericmark » Fri Jan 04, 2019 1:19 am

The boiler heats water which is pumped to radiators connected parallel, each radiator has two control devices, the lock shield valve is set so each radiator has the same temperature drop input to output, which in turn causes the water to be equally shared between all radiators.

As the room warms up the TRV starts to close this forces more water into radiators in cold rooms and less into radiators in hot room, and the temperature of the return water starts to rise as the flow through open TRV's increases, then as the pressure increases further the by-pass valve starts to open causing the return water to get even hotter.

The boiler monitors the temperature of the return water and to start with modulates (turns down) the flame, once it hits a minimum valve then the boiler starts to cycle off/on changing the mark/space ratio to reduce output, the boiler will never switch off completely using the return water to control it.

So either it is switched off manually, or you need some thermostat or hub to switch it off.

So a wall thermostat off/on in a room normally kept cool so as summer approaches it switches the boiler off, or using a single most used room a modulating thermostat which slowly turns the boiler down/up, or a hub which collects the info from the electronic TRV heads, and it uses that to modulate the boiler.

Although this is OK in theory in practice it does not always work, it's OK saying wall thermostat in the system should be in a room kept cool on ground floor with no outside door or alternative form of heating, but often there is no such room. And when there is then either no TRV or TRV wide open and the lock shield valve in that room is closed to ensure other rooms heat up first, so the room with off/on thermostat is set differently to rest.

Although the modern TRV can be fitted either on flow or return, if fitted on return then the radiator can get hot before the the valve has chance to adjust, the electronic TRV head measures both air and water temperature so it should be fitted on the supply side.

Where the valve is fitted on the wrong side, then reducing the speed at which it warms up, can assist with control, so again the lock shield valve may need to be reduced further to trim the radiator output.

What often seems strange is where a radiator fails to work, it is often down to the radiators that are working having the lock shield valve open too far.

If the lock shield valves are left wide open, then the water will pass through the easiest radiator first, and the hot return water will turn down the boiler, as that room gets warm it will close the TRV and then start heating next room, and so on. Once the room with a wall thermostat is heated then it will turn off/down boiler so rest of rooms never heat up.
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Postby greengrass » Sat Jan 05, 2019 9:12 am

Hi ericmark,
Very well explained I will now get it printed out , give it to ' The boss lady' then wait to here if she says ' He's got that wrong'.

Just like she did when clutch wore out on our car because of constant 'riding' clutch. The garage showed her what happpens to clutch plate if you don't keep foot off pedal once completeded gear changes and cruising.
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