CH Wiki question

[David WE Roberts]

the diagram a zone valve is shown before the flow trunk splits into branches. Is this intended to show a valve with one input and two output ports, so that the flow can go to either or both branches? This doesn't seem right as the radiator without the TRV should always be on (I think). Otherwise there has to be a seperate return loop (I think). Should the zone valve be the other side of the T junction so it can shut of the branch with all TRVs?

[John Rumm]

>> In the diagram a zone valve is shown before the flow trunk splits into > branches.

No... Its just a normal 2 port zone valve with a 28mm pipe in and a 28mm pipe out. Some time after the valve is am unequal tee, with reduced 22mm legs.

Its really just indicating the way that the pipework does not have to be a linear trek round each of the rads in turn. You can use larger pipes toward the start of the circuit (i.e. where most flow and heat is being shifted) and branch off those in smaller pipes to do batches of rads.

All six rads in that diagram are in effect in parallel. Note however it is possibly only part of a system - there may be additional zones, hot water circuits, or even bypass loops not shown.

[Roger Chapman]

>> In the diagram a zone valve is shown before the flow trunk splits into > branches.

I have missed that when you mentioned this diagram previously. I can't see the reference to a bypass circuit that I expected to see elsewhere in the Wiki.

The manual control valve will provide a circuit even when all the TRVs close down but close the zone valve and the boiler will do what its name implies, boil, if the heat has nowhere else to go which is why a bypass circuit is an important part of most modern central heating systems.

[Roger Mills]

>> In the diagram a zone valve is shown before the flow trunk splits into > branches.

No. This diagram is perfectly correct for illustrating trunk and branch design - i.e. fat pipes feeding multiple branches before they split, and thinner ones feeding each branch.

In this particular example, there is only one heating zone shown with two branches (probably upstairs and downstairs). The radiator with the manual valve would be in the room which has the room stat.

There may be a bypass valve elsewhere (if the boiler needs it) - but it's not shown here because it's not relevant to the particular point being illustrated. equally, the complete system is likely to have a HW zone, and maybe additional heating zones - each with its own zone valve.

[Dave Liquorice]

And the room stat controls the valve for that zone of course.

I think the diagram could be made clearer by not having the flow/return cross each other the boiler and a bit of text saying additional feeds for DHW bypass etc for the flow/return that point to the left.

Trouble is it has a (c) Copyright in it and nothing about rights on the page for it...

[Roger Mills]

A bypass valve *may* be required, depending on the boiler. A setup like this would usually be part of an S-Plan system, so that when the valve closed its auxiliary contacts would switch off the boiler. Many boilers will overheat under these circumstances if there is no flow, due to the residual heat in the metal parts of the heat exchanger - requiring a bypass circuit and pump over-run logic. However, some will not - mainly slightly older boilers which contain more water and which would be happy with a gravity hot water circuit. Provided the water to metal ratio is high enough, the boiler won't overheat even if there is no flow path - as long as it stops firing when the path closes, of course.

[Roger Chapman]

One of the reasons I referred to a 'modern central heating system' and restricted my comments to 'most' was to avoid being dragged into an argument as to whether a 30 year old boiler with a cwt of cast iron as its heat exchanger would or would not boil if there was no pump overrun. Of course a gravity hot water circuit lessens the chance of the boiler boiling to almost zero provided that hasn't been retro fitted with its own zone valve.

[jgharston]

In two of the houses I've lived in over the years, there were no valves, just a pump, and the hot water tank heat exchanger was just another radiator.

JGH

[John Rumm]

There is a certain amount to be said for the simplicity of it... If the cylinder is lagged, then once it is up to the same temperature as the primary water then it won't place any real load on the heating. However it does mean the cylinder will be dangerously hot for direct use, and also prone to rapid scaling in hard water areas.

[Roger Mills]

Either that, or you have to restrict your primary flow (boiler stat) temperature to around 60degC - which means that the radiator output is severely degraded.

[John Rumm]

Indeed, although good for efficiency with a modern boiler.