CH Boiler Burner firing intermittently for DHW

23 Mar 2006
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United Kingdom
I've got a Ravenheat RSF 820/20 combi boiler. Recently, on turning the hot water taps on the water often remains cold. Listening to the boiler it sounds as if the burner is only coming on when the tap(s) are turned on full, and even then it is intermittent (previously the burner had fired and the DHW had run hot even on small tap openings). However, if the burner is already on due to the CH, the DHW runs hot even at small tap openings ?

For info the pressure is (and always has been) set at ~1.5 bar, the temp gauge reads 60 degrees, the DWH thermostat is set high and CH thermostat at mid-range. The water pressure out of the taps is - and has always been - good.

What could be causing the burner to only fire with the hot taps wide open ?
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Diaphragm in the diverter valve is holed.

Hole/split is large enough to prevent proper actuation on the microswitches but still moves diverter enough for hot water with heating on.

Either repair diverter with service kit or swap hole valve. Easy repair.

Diverter is at back of boiler in the center

While boiler drained down check expansion vessel pressure (schraider valve just about accessible if room on left side of boiler) and replace auto air vent.
An easy repair?

I have to say that along with the Sime Unfriendly that Ravenheat is one of the most difficult boilers to change the diverter valve on. I would allow about 1.5 hours just in case.

You also usually end up with some scraped knuckles!

Remove control box lid and one other pipe from memory - I must have tough skin on my knuckles.

I do have top quality pump pliers ( ie they are very slim design).

Checking vessel pre-charge is certainly challenging - just enough access with left hand circular access hatch removed.

Agree Tony allow a couple of hours
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Be aware that diverter will be considerably higher price in merchants.

Aav's usually block up or leak so always worth changing as low cost (£5.00).

Make sure expansion vessel pressure checked (set around 0.8 Bar) if you live in a normal house.
An adjustable basin wrench, the type with the spring loaded end will save your knuckles. When you fit the new one put all the nuts on the threads [with the washers in place] THEN tighten them up. Dont tighten as you go or you will end up struggling.

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