Worcester Bosch Highflow 400 RSF - Safety Temperature Limiter Thermostat tripping

[skinflint]

Hello. I have a WB Highflow 400 RSF, which was installed in 1999 and is covered by a maintenance contract with British Gas.

I recently called BG out because the "Safety Temperature Limiter Thermostat" keeps tripping. This occurs when the CH is OFF but the HW is on. We only know about it when we come to turn on the HW tap and find it running cold. It doesn't trip after the HW has been running ror a while.

BG advised that the most likely cause was 'sludge/muck' in the water to water heat exchanger and that I would need to have my system power flushed before they could repair. They also advised that I have a MagnaBooster (magnetic filter) fitted. The cost of this work was quoted at £709.

However, I am not convinced that sludge/muck is the problem. I have 14 radiators, 3 towel rails and a plinth heater on my system. All heat up evenly with no cold spots whatsoever. Water runs perfectly clear from the bleed valves of the radiators and also the drain valve of one downstairs radiator that I tested. There is no sign of sludge in the system! If it weren't for the presence of inhibitor in the water I'd be happy to drink it!

So, what else could be the cause of the thermostat tripping? Limescale? I live in Macclesfield which has medium to moderately hard water but we don't suffer too much from scale. We have also had a magnetic descaler fitted to the system since new. Are there any other potential causes and what can be done to find the real cause of the overheating? Is it possible the thermostat itself is faulty?


Any advice you can give would be gratefully received. Thanks

 

[SimonJohns]

Ditch the contract and get a local gas engineer around.

But on a serious note bg shouldn't be saying that system needs flushing if they have installed the boiler and maintained it from installation

 

[skinflint]

BG didn't install, but they've been providing my 'breakdown' cover for a number of years. Up to now I've been happy with their service' but the engineer's diagnosis on this occasion appears to be based on speculation rather than evidence. I suspect they're on commission for every power flush and 'upgrade' they flog. He didn't bother to examine the radiators (for cold spots) or the water in the system for contaminants. He did get me to listen to the water to water heat exchanger and claimed the faint 'hissing' noise it was making was symptom of dirt blocking the passage of water. For all I know it could have been making that noise since day one.

I am not even sure the thermostat relates to the 'water to water' heat exchanger as the installation guide http://www.acwilgar.co.uk/Boiler-Manual-PDF/Worcester%2520Bosch/HIGHFLOW%2520400%2520RSF%2520GCNo.47-311-18.pdf&sa=U&ei=SkXmT-f5HISa0QXn18SpCQ&ved=0CB4QFjAD&usg=AFQjCNH-KTlpt3bLt4D8csSvQ9udRAQwPw is contradictory. There is a diagram (Fig. 36) showing the thermostat PHIAL located in a pocket on that HE but the instructions for replacing (page 37) it say to remove the PHIAL from the gas to water HE.

So, looking for advice from anyone experienced with working on these boilers. I want some ammunition to go back to BG with to say their diagnosis was wrong. I pay good money to BG so don't want to have to pay someone else to fix problems BG should be fixing under the terms of insurance policy.