descaling a vokera 80 excell 80 sp plate heat exchanger

[idealmaint]

Hi, I have a friend that has the above boiler. The problem arises when hot water is used. Hot water gets hot then goes cold. Called round and was asked to look at it.


Diverter valve isnt divirting to rads (i isolated the flow and return) which made no difference to the hot water temp.

The gas pressure was at full and wasnt modulating down until it switched off. The boiler switches off then after some time switches back on again. As a temp test diconnected the primary flow thermistor to see if the boiler stayed on which it did (didnt do this for long)


Im pretty sure that the boiler primary flow thermistor is sensing that the flow is getting too hot and switching the boiler off until some heat is dissapated in the plate heat exchanger....It also occured to me that maybe because the valve isnt modulating down then this could be a problem.

Im going back tomorrow armed with more tools such as flow cup and clamp on digi thermometer. (it was a flying visit cause i had my daughter)

Im pretty sure that the plate heat ex is scaled up and this is not allowing the heat to transfer to the hot water side then switching off cause of the primary flow thermistor


Also If the minimum isnt set up right on the gas valve would the gas valve just hit maximum then switch off when it should modulate down from say 11 mBar to say 10mBar or would it modulate down lower then switch off.

Is it possible to descale a plate heat exchanger with flow and return valves isolated and just run a tap for 10 minutes and fill boiler with descaler....the heat exchanger looks really awkward to replace


I have my full corgi in domestic and commercial heating and catering appliances, but i only just got my domestic and so am learning this side of things from scratch


Any help really appreciated

David

 

[scatmanjohn]

idealmaint...When was the last time it was serviced properly. Always suspect airflow problems with these boilers.
Remove the fan and clean the fan blades, take out the fan venturi and clean with a small file or similar tool.
Lubricate the fan
Wash the heat exchanger as well.
If these tasks have not been carried out on a regular basis it will be caked up to hell.

Check the gas pressures as you suggested as the gas valve can stick on high.

Good boiler these if looked after.

 

[ACOperson]

If you're cleaning the heat exchanger use some DS40, also DS-3 and boiling kettle & flush it loads.

Be aware that some boilers modulate according to the flow of water going through the hot tap, not the temperature of the primary NTC (eg Worcester 28i junior RSF), not sure about this Vokera though. Ask DP.

I once had a similar problem with a boiler with a 4 pipe h/e, the primary pipes in the h/e were badly scaled and so it overheated every time a high flow of hot water was drawn - went to lockout. Same could well be happening here, just on the plate h/e.

Does it have an NTC on the hot water pipework?

 

[idealmaint]

hi folks thanks for the assistance. I serviced the boiler today and set up the gas valve as per Manufacturer


We now have the desired temp increase at 9.5 litres/min..



Thanks again

David