worcester bosch highflow 400 RSF cutting out

[bobb]

my 7 yr old W-B 400RSF is cutting out on overheat and needs to be reset every now and then. this only happens when on CH, doesn't occur when on HW only. whooshing noise seems to coincide with cutting out. have put in fernox and bled rads.

when it cuts out only the ignition doesn't work, the pump keeps running.

any ideas?

 

[croydoncorgi]

Yes - it's overheating, which kills the burner. Pump keeps on running to cool things down. From your description, there's a problem with circulation through the rads. Are you talking about the non-electronic version of the 400 Highflow storage combi (floor mounted, looks like a washing machine)? When it cuts out, are the rads actually hot? If not, it's probably a diverter valve problem.

 

[bobb]

Thanks for yor reply.

Yes, it's the floor mounted one with the heat bank at the bottom. Not sure what you mean by non-electronic - it's got a couple of circuit boards but the temp controls are knobs rather than digital inputs.

I have got three rads without thermostatic valves which should give enough flow even when the thermostatic valve rads are closed and I have no problem with the amount of heat from the rads which indicates flow in the circuit is OK.

When you say a diverter valve problem, are you saying that it is restricting the flow in the CH circuit? This might explain why the problem doesn't happen when on HW only.

 

[croydoncorgi]

There are 2 models of the Highflow, the early one and the (curent) Electronic one. The early one was also slightly 'electronic', just to confuse things.
My thought is that the diverter is sometimes jamming in the hot water position so that when the call for CH comes, there a lot of heat in the boiler with nowhere to go! Hence the cutout. The diverter is on the LH side of the boiler near the pump. Have a look at the pipes, work out which are the CH Flow and one leading to hot water / heatstore and see which one is hot when the boiler trips. The heatstore makes dianosis more difficult but you should be able to puzzle out what leads to where.

 

[ChrisR]

I think I've read of a filter getting blocked in these. Check the MI.

 

[bobb]

thanks for the replies.

Chris R, what's an MI? Excuse my ignorance.

Croydoncorgi, thanks for that suggestion. It makes sense. What you're saying is that when the diverter changes over from HW to CH it does not happen quickly enough and restricts the primary water flow causing overheating in the gas to water heat exchanger.

That would also explain the whooshing that happens when the boiler cuts out.

By the way, I changed the diverter valve about four years ago after I was getting hot radiators when on HW in the summer. (the man from Worcester Bosch wanted to change my Plate Heat Exchanger to cure this - I sent the bounder packing, as you can imagine)

 

[kevplumb]

Manufactrers Instructions

 

[bobb]

I've checked for a filter and the only one I can see is in the cold feed to the plate heat exchanger. can't find one in the primary circuit.

 

[croydoncorgi]

There's a gauze filter in the 'top manifold assembly' of the 400 Electronic. It might not be there on the older model.

 

[bobb]

There's a gauze filter in the 'top manifold assembly' of the 400 Electronic. It might not be there on the older model.

Thanks again. I've looked again in the manual and the only filter is in the cold feed line. there isn't one in the top manifold. my boiler is 7-8 years old so i guess this was a later modification.

 

[bobb]

My thought is that the diverter is sometimes jamming in the hot water position so that when the call for CH comes, there a lot of heat in the boiler with nowhere to go! Hence the cutout.

I have sat and watched as the cut-out happens and it happens just after the boiler calls for central heating. There is an erratic rise on the pressure gauge in the heat bank, a lot of clunking and whooshing from the boiler and then the temperature cut-out activates stopping the burner.

It seems the changeover of the divert valve is slow and for a while the flow round the primary circuit is obstructed. The valve is changing over eventually as the central heating operates OK after resetting.

What could be causing this slow changeover? I think this valve is actuated by the prmary water itsself. Could the small pipe that carries this water be blocked? Or is there another common cause?

 

[bobb]

My thought is that the diverter is sometimes jamming in the hot water position so that when the call for CH comes, there a lot of heat in the boiler with nowhere to go! Hence the cutout. The diverter is on the LH side of the boiler near the pump.

just to let you know that you were right croydoncorgi. changed the divert valve. boiler running sweet as a nut. thanks mate.

i'm sure the old valve is just gunked up and i'm going to clean it up as a spare. any ideas what to dip it in to remove all the stuff that has built up?

 

[kevplumb]

nice one john a victory