Vaillant 242 hot water ok but no central heating.

[heeley]

Hello people,

I've recently encountered a puzzling problem with a Vaillant GB 242 . Hot water works fine, but on calling for central heating the boiler appears to be dead. There is no roomstat and on on operating the external programmer I can hear a relay click inside the boiler control box. On switching the boiler off at the spur and on again the boiler will fire up and I then get central heating for about a minute, the boiler then shuts down again.

I'm inclined to think its an electrical fault maybe the heating thermistor being out of range, but since I don't know what the correct resistance of it should be I can't draw any conclusions. I assume the overheat stat will prevent the boiler firing up in both hot water and heating modes? so I can't imagine it is that. Any experts around ? I deserve an easy boiler after my last (successful ) escapade with an Isar.

Perplexed

Heeley

[Ed Sirett]

I presume this is the Vaillant VCW 242 GB which is a combi with two white metal doors and a grey metal fascia?

This problem could stem from a number of sources. The fact that the boiler works for DHW means a lot of stuff must but OK. So we know the pump, fan, gas valve, ignition, DHW flow detector, DHW microswitch, diverter servo valve are good. Also the servo valve must be able to go to the right from neutral to DHW mode.

No CH? Could be servo valve not returning under spring pressure to the CH position. That's the triangular valve front mid bottom. Or the diverter valve (back right bottom) can't go right. Or there is some problem with the control boards, or maybe even external controls. Possible with the temperature sensor. Servo pipe blocked.

Good chance of it being the servo valve not being returned to CH, because of general crud/corrosion in the DHW flow detector.

HTH if the above leaves you bewildered get in a pro who knows VCWs

[heeley]

Thank you for your rapid response. I was hoping you'd read my post. I have always noted how well informed you are re: boilers and gaswork.

I've actually got another 242 which was replaced though working , from which I can cannibalise parts with a view to perhaps discovering what the issue can be. Can you acquaint me with the sequence of operation w.r.t the heating operation of a (grey front panel) 242?

I'd really like to be able to sort this one myself.

Regards

Heeley

[Ed Sirett]

1) There is a demand caused by applying 230V ac at terminal 4 (or by joining terminals 3 & $ together which is the same difference). 2) The circulation pump starts. 3) The servo valve (the triangular one on top of the DHW flow detector) is in the CH position (normal position). 4) The pump forces water through those tiny 4mm tubes to the diverter valve. 5) The DV moves to the left from its normal neutral mid position. 6) One (or more?) micro switches are activated by the DV (in a box on the RHS of the DV) 7) The boiler begins the ignition sequence. Which should be OK as DHW is OK.

[heeley]

Regards

Heeley

[100Packets]

replying to Ed Sirett, 100Packets wrote: COmbi boilers do not have diverter values. Hot water and radiator systems are completely separate

[Graham.]

On Mon, 23 Oct 2017 11:44:02 GMT, 100Packets coalesced the vapors of human experience into a viable and meaningful comprehension...

What Ed Sirett doesn't know about plumbing, is not worth knowing so piss off.

Why are you responding to a 2009 post anyway?

[Fredxxx]

Some do, especially old ones, some have two pumps for water to either flow to the radiators or the HW exchanger.

There is a heat exchanger so hot water is not mixed with primary circuit water.

This might assist you with posting to a newsgroup, albeit through a website:

- If you are sending a reply to a message or a posting be sure you summarize the original at the top of the message, or include just enough text of the original to give a context.

[John Rumm]

Strike one!

Erm, yes they do. Almost all of them.

The Domestic Hot Water is heated by a secondary water to water crossflow heat exchanger. That in turn is heated by a flow of primary system water that is heated by the main gas fired heat exchanger. That's the same water that also flows through the rads. The way the boiler gets it to flow through the DHW heat exchanger is via an internal diversion valve.

Strike two!

No they are not. They both depend on the same primary circuit to provide a pumped source of hot water, which can be use directly in the rads, and indirectly for heating the DHW.

And you are out.