Cylinder thermostat query

[Will]

If a cylinder thermostat is attached to the hot water indirect cylinder will it compete against the room thermostat (or even boiler thermostat) in a gas centrally heated system. I presume the lowest set thermostat will "win" every time and make the other thermostats redundant. If I set the room stat low I presume that will also mean the temp of the hot water will be low.

[cynic]

You need to read up on central heating control systems.

[Andy Burns]

Depends whether your system is plumbed/wired with a single zone, or multiple zones, if the latter you'll have one or more automatic valves, exact configuration depending on which "plan" it's wired/plumbed for.

with a with a single zone you can have some control, rather than wiring the room stat and tank stat in series which "ANDs" them so they both need to be calling for heat before the boiler will fire, you can use a relay within the wiring centre to "OR" them together instead ...

[Andy Champ]

There should be a valve (or valves) which route the boiler water to the cylinder or the radiators as needed.

Have a look on

the bit on central heating controls and zoning.

Andy

[John Rumm]

Room stat no, boiler stat maybe...

Either room or cylinder stat can "call for heat" - assuming the programmer also has them turned "on". Depending on how your system is zoned and what arrangement and type of valves it uses:

to how concurrent calls for heat are handled. Generally, either calling for heat will fire the boiler, and allow water to flow through the rads or the cylinder coil as appropriate. Some systems may permit both to be heated at the same time, others may assign priority to one (usually the hot water) such that it has to reach its set point temperature first.

If the boiler stat is set lower than the cylinder stat, then you may get a situation where the cylinder stat is never "satisfied" since the boiler is limiting the temperature of the primary heating water. (its unlikely to have a room stat set higher than the flow temperature of the boiler)

[dennis]

It depends on who installed the system.

Most heating engineers, DIY people and some plumbers know how to install controls so they work correctly. OTOH most plumbers and some heating engineers don't have a clue.

If the system has been done properly the room stat and the cylinder stat will be independent of each other so the water will get hot even if the room stat is off. Likewise the other way around.

[Harry Bloomfield]

Will was thinking very hard :

That is not the way it works, put simply....

The cylinder thermostat and the room stats set completely separate demands on the boiler. The heat from the boiler is directed to whichever system has a demand for heat and will supply heat until the stat indicates the demands are satisfied. Usually the bias is to satisfy the cylinders heating demands first, before the space heating demands.

The boiler has its own separate stat which limits the temperature of the water exiting the boiler. obviously this has to be set higher than the cylinder stat, or the cylinder stat will never be satisfied.

[Steve Walker]

>> Even with a with a single zone you can have some control, rather than

If you're using two stats, but no valves (and assuming there isn't a gravity loop for the hot tank and a pumped loop for the radiators), then you should be able to just connect them in parallel at the wiring centre (a so-called "wire-or").

SteveW

[Andy Burns]

Yes, well that's yet another thing it depends on. Mine *is* pumped C/H and gravity H/W so I have a relay which allows the H/W call for heat to fire the boiler without running the pump, while the C/H call for heat runs both ...

[The Natural Philosopher]

Rubbish. On my system both (or all three - I have UFH on a different zone) motorised valves go on, and its anybodies guess which one gets the most heat. PROBABLY DHW since its a shorter run of pipework. But its not 'designed in' that way

[ARWadsworth]

But there are systems where the DHW does get priority.

[Mark]

Mine works this way. It's quite common.

[ARWadsworth]

I was going to say that I very rarely see them:-)))

[The Natural Philosopher]

Its standard on a combi, sure. But no way on a system boiler/tank. If you have a three way valve possibly.

[John Rumm]

There was a time that three port diversion valves were common. The problem used to be that the cylinder could only take a fraction of the boiler's output, and so it would have to cycle. Hence mid position values became more common. Now we have modulating boilers and fast(er) recovery cylinders, the diversion valve can actually be an attractive option again.

[Harry Bloomfield]

The Natural Philosopher was thinking very hard :

I used the word 'usually'. My present condensing system does the DHW as a priority and its predecessor a none condensing boiler also treated DHW as a priority.