Combi water diverter valve - schematic?

[David]

I'm having trouble finding a decent diagram of how the diverter valve works in a combi boiler.

I am assuming that there are two completely separate water flows through the heat exchanger, only one of which is active.

The heating circuit water must be kept completely separate from the hot water taps otherwise the chemicals in the central heating circuit would leak out of the hot water taps over time.

However I can't find a diagram to show the complete flow.

I am mainly hitting descriptions of problems when the valve is faulty.

Cheers

Dave R

[newshound]

Have you tried Googling for a manual for your particular model? It will probably be easier to identify the parts and the flow route from this.

[newshound]

This is the first hit that I get on a generic search

[ARW]

Try YouTube.

[rmlaws54]

There are typically two heat exchangers. One (CH) for gas/circuitwater and another (DHW) or circuitwater/domesticwater. THey have different designs. The diverter valve switches the circuitwater flow between the DHW or the radiators.

RObert

[Roger Hayter]

Some combis have a separate heat exchanger for hot water which has pumped hot water from the main heat exchanger (the same water as can be used for circulating to the radiators) on one side, and the mains water supply being heated on the other side. So the main heat exchanger heated by the boilder flame has only one water flow.

[John Rumm]

I take it you mean a flow / hydraulic diagram rather than circuit?

Not usually. (there are a few boilers that use the main HE to heat the water for the taps as well - however they are rare)

That's right

There are typically two separate heat exchangers. The primary HE, has the CH water flow through one side, and has the other side heated directly by the gas flame.

There is a secondary (smaller) water to water cross flow heat exchanger that is used to heat the hot water for the taps.

When heating the rads, the primary water (plus inhibitor etc) flows through the main HE, the pump, the rads, then back again.

When there is a demand for hot water (usually sensed by a flow switch), this activates the diversion valve. It the switches the output from the main HE so that it now flows through the secondary HE rather than the rads. Thus creating a short loop for the primary water that runs through the main HE, pump, and the secondary HE, and then back to the main HE.

So the main HE is ramped up to full power, that heats the primary water, which in turn heats the mains cold water flow through the "other" side of the secondary HE.

Typically if it fails to operate, turning on a tap will cause the boiler to fire, but instead of the water getting hot, it will just start heating the rads instead.

[David]

Thanks.

My GoogleFu is, as usual, sadly lacking.

So, if I am understanding this correctly, the primary heat exchanger either heats the water for the central heating or heats the secondary heat exchanger which in turn heats the hot water.

A bit like heating a hot water tank but much more concentrated and instantaneous.

That makes sense (I think).

Cheers

Dave R

[newshound]

Correct. One weakness of Combis, particularly if there is corrosion in the intermediate circuit, is that the DHW heat exchanger may become blocked, and then you get inadequate temperature and/or flow in your DHW.