Vaillant boiler, no hot water but heating ok!

11 Oct 2019
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United Kingdom

I have a Vaillant EcoTec Plus 630 boiler with a Heatrae Sadia CL250S28 Megaflo.

The boiler is working for the underfloor heating and the central heating, but is not heating the hot water.

The motorised valve for the hot water on the megaflo is open, and the overheat reset on the tank is ok, but there doesn’t appear to be a demand for the boiler to fire up coming from the hot water system!

Adjusting the thermostat on the megaflo opens and closes the motorised valve.

The pressure in the system is fine and the programmer is set correctly and working ok.

Any ideas please?
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The motorised valve has a small microswitch inside it that is responsible for operating the boiler when the valve is open.

However, in your case I would still expect the hot water to heat up if the valve is open, because the boiler will be operating because of a demand from the heating. Being part of an unvented system, you would probably be best advised to get a professional (Someone with a G3 certificate) in for fault finding.
So, replacing the Honeywell twin port motorised valve head is the most likely cure?
The hot water is flowing through the valve when the heating demand is there.
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In that case, a faulty switch inside the head would cause the symptoms you describe.
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So I fiddled around and swapped the programmer for a spare one and then back to the original, still didn’t work. Then I reset the air gap on the megaflo and it now works!!
Don’t understand it it, but as long as it works now!!
Rather strange indeed :confused:

If the "air gap" was the problem, how would the water heat up OK when the motorised valve was open and the central heating was switching the boiler on.

The valve opening, and the boiler not firing up all points to a control issue. It wouldn't be the programmer, if that was faulty, the motorised valve wouldn't open.

Maybe there's a loose wire somewhere and you disturbed it at some point and so it's working again.

As you say, at least it's working for now.

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