Scalding hot water + rads on HW circuit?

4 Jul 2011
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United Kingdom
Worcester Bosch HiFlow 400 in a pressurised system. There is no hot water cylinder as this has a "heatbank" in the boiler (I am not very impressed with it but that is another story!).

The HW coming out of the taps is scalding hot regardless of what the HW temp control is set to. Set it to just above zero (the boiler fires up when there is HW demand) but the water is scalding hot.

I assume that the stat is faulty unless anyone has any other ideas? (It does function as I would expect if I set it to zero - then the water is cold (boiler doesn't fire up when you run a tap).

Second thing is that with just the HW on the towel rail in the bathroom and one radiator get hot! What is that about!?
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Without knowing your exact system it's hard to say. Normally when bathroom rads come on with the hot water it's because they were piped up on the hot water circuit so that in the summer months when you have hot water on it will heat the bathroom rads only so that your towels dry.

As for the hot water temp goes its probably a thermistor issue or possibly a potentiometer prob ( knob to set temp).

Get a break down engineer in is your best bet!
Thanks Neil, makes sense.

One thing that puzzles me though: when I have drained down the system in the past the towel rail and rad mentioned drain down. In fact the towel rail air bleed is the highest point in the system so I use that to put the rust inhibitor in.

Surely if they were on the HW side then they wouldn't drain down? Or is the 60 litre "heatbank" in the boiler for the HW actually part of the CH system in terms of the water that is circulated around?
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Unlikely to be plumbed as suggested, unless it has always heated the rads during hot water operation. More likely to be the diverter valve inside the boiler passing to the rad circuit. Temp over shoot can be as suggested. These are fairly complex units and your best bet may be a fixed price repair from WB.

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