Hot water TOO HOT !!

You need to do two things:

- provide temperature information to the DCD - this is NOT a thermostat, it's a thermister (or similar) that should give an actual temperature VALUE to the DCD controller, not an on/off signal.

- the safety rules for an unvented cylinder REQUIRE 'direct' control of input heat via a spring-return zone valve so that if the water in the cylinder is overheated for any reason, the thermostat (or in extreme case, the associated cutout) will open and the zone valve will close because of its spring-return.

SO: you MUST keep the zone valve on the flow to the cylinder operational and connected through the thermostat with its cutout;
AND to make the DCD box work properly you need to find away of getting a good 'temperature signal' off the cylinder.

From the DCD website:

With Megaflow systems, sealed hot water cylinders and thermal storage systems, the sensor is normally fitted within the covered space provided for a thermostat on the side of the unit. Where a thermostat is already installed, this should be left in place wherever possible and wired in series with the DCD provided sensor. Further information available by email.

Click to expand...

I think you need to talk to DCD: wiring the sensor in series with the 'stat doesn't sound right.

If you'd mentioned the Megaflo in the first place, you'd have had the answer in one hit

Hello CroydonCorgi

Why is it that the key piece of info is the last bit I always mention!!

Oh well........

I've had (limited) contact with DCD today and hoping for more (soon).

I am trying to fully digest (understand) what you mean by this: "SO: you MUST keep the zone valve on the flow to the cylinder operational and connected through the thermostat with its cutout; "

can you expand?

The connection between the thermister and the tank is certainly Mickey-Mouse, so getting that good temp signal is, I agree, missing.

Finally, the correct settin gfor the spring-return zone valve on the HW system: This device has Manual and Auto positions. Would I be correct that the spring lever should reside (be locked up) in the Auto position?


That leaves the question about how to wire up the two sensors. I wonder what DCD will have to say?


Paul

I've already emailed DCD pointing out the mistake in the DCD controller installation notes.

The 'interlock' provided by the motorised valve on the feed to the Megaflo is NOT optional. Whatever other controls are involved, there MUST be a connection from the cylinder thermostat to the valve so that if the thermostat opens the valve closes and prevents any more heat getting into the hot water. The valve must be left in the Auto position and should stay shut until the thermostat calls for heat.

Since the thermostat should be carrying mains to the motorised valve, to operate it, connecting it in series with the sensor is definitely wrong too. The sensor is essentially a variable resistor 'read' by the DCD controller and MUST NOT have mains voltage on it.

I'm not giving more definite wiring information because there may in fact be TWO motorised valves involved with control of the hot water.

CroydonCorgi.........

Thanks for your reply. I would very much like to know DCD'd response as I am going to have to get this looked at over the next day or two.

The main query I have now, is do I call a Plumber or an electrician?

Any suggestions and what are the key points I would need to ensure are done to ensure safe and proper running?

Many thanks

Neither.
Call DCD first.
Then a Heating Engineer that understands optimisers. Most electricians won't want to know! Most plumbers just won't know.

I've given you the 'key points' already.