Lynx cutting out on HW - please help!

[johnbrist]

Hoping some one can help me with this:

I've a Lynx Electronic which now cuts out when drawing hot water, say for the bath, for more than approx 4 minutes. This does not happen when HW is being drawn for the shower, and the CH appears to be working normally.

It seems that it is the CH overheat stat that is cutting out (checked the voltage across it). The other thing is that, just before the boiler cuts out on HW, if I dribble a small amount of water on to the diverter valve it boils away quite nicely, so the temperature of the CH at the diverter valve must be at least 100C.

The Lynx has only one heat exchanger, combining HW and CH. It seems as though when HW is being drawn, the heat going into the CH isn't being dissipated. Presumably it doesn't happen with the shower, as the boiler then modulates down, so there is a less heat being generated in the boiler.

The only options seem to be the diverter valve or the heat exchanger.

Any help would be very gratefully received. (The corgi registered chap I've used has replaced the overheat stat, flame sensor, CH thermistor, but hasn't properly checked what is actually going on.)

 

[kevplumb]

run the hot water and check the c/h flow pipe while the tap is running
best if c/h is off for a while and is cool

 

[DP]

If I may take the liberty to add to Kev's post- take temperature readings on both flow and return pipes in the boiler.

 

[kevplumb]

feel free dp
we are all rowing the same boat

 

[johnbrist]

kevplumb & DP,

Many thanks for the pointers.

Will do some checks & measurements and get back with the results.

 

[johnbrist]

I checked the temperatures of the CH flow and return within the boiler (flow: combustion chamber to diverter valve, return: diverter valve to combustion chamber) - The temperatures when the boiler cut out were: Flow ~102C, Return ~92C.

The flow outside the boiler got a little warm over approx 15 minutes and got up to 27C, but it was a very slow and gradual increase to that.

(I used an Infra Red thermometer, with patches of black insulation tape on the pipes, to measure the temperatures.)

One thing - the voltage across the CH overheat stat seemed to go from zero to whatever (as the stat opened) very slightly after the boiler cut out. Though that might be due to the reaction time of the meter mechanism. - Will recheck that.

 

[kevplumb]

if the stat opened the boiler is overheating
hard water ?

 

[johnbrist]

kev,

Yes it is a hard water area. The CW supply is fed via a brass 'bottle' containing the treatment 'balls' (Siliphos), although I know they haven't been replaced as often as they should have been.

 

[kevplumb]

could well be scaled up then dont live in ne by any chance?

 

[johnbrist]

kev,

Many thanks for the diagnosis. Do you mean it's the heat exchanger that is scaled up (something I had wondered about myself)?

Afraid I'm in Bristol - bit of a hike from the NE!

Is there a way of cleaning out the heat exchanger (once removed from the boiler), say dunking in chemicals, or are there companies who do that sort of cleaning, bearing in mind that it incorporates both CH and HW? (There is no way that I'm going to dismantle the boiler myself, but it is useful to know what the options are, just in case!)

 

[kevplumb]

the water in bristol is as hard as ours
yes there are firms that will do it for you formic acid works well
depends how bad it is
bear in mind its a copper heat exchanger tad delicate once the lime has eaten it

 

[johnbrist]

kev,

Thanks enormously for the help

Would it be fair to say that the best thing to do is see if descaling works works?

If that fixes the problem - fine. But if the heat exchanger is weakened and plays up, then, as the cost of replacement heat exchangers is quite steep, it might be worth thinking about a replacement boiler.