saunier duval therma dondence

[smiley plumber]

ok here gose ive been called to a four year old "therma dondence " combination boiler no hot water ok so i do all the checks
end up changing thermisters ( but of course hrpc send wrong parts first)
i fit them the water starts off hot then runs cold
but primary heat exchanger red hot so i take off the domestic heat exchanger and hay prestow full of little black bits ( ellectrolite i think its call ? )
put it all back together and still starts off hot and then gose cold ( GGrrrr)
i run central heating to get any possible air out of heat exchanger incase its trapted in domestic mode
then i notice that the boiler starts to bang and kinda kettle but with out any flame or heat sauce ??? like its got stuck residual heat left in the primary heat exchanger ( vailant type stainless steel spiral heat exchanger)

so my question to you all is would a power flush sort it out or should i try heating restora ?? im still waiting for the second thermister to arive but im not sure it will sort it out all though it dose seam to shut off when the primary circuit reaches temp it gose very cold before it starts up agian so comes out tepid i bloody hate condencing boilers
can any one give me an unbias objective view of any thing a may have missed lol ive only been doing it for 24 years now still learning

 

[peteh20]

i ve had a simular problem on a glowworm cxi (same boiler inside) did what you did ended up being the condensate trap blocked causing the boiler to short cycle and kettle explains some of your problems but worth a look,

 

[fireblade]

Last on i did was a vailant smae hydro block no hot water or intermitent ie got hot water but had to turn heating off i change the motor on the diverter that sorted it

 

[smiley plumber]

hmmm changed divertor motor no change there

if the condence pipe was blocked then it would glug and bubble before heating wouldnt it this heats then when hit temp flame gose off after 20 or 30 seconds then it bangs and fizzles

ive have been down and put some X800 in it this morning wich seams to stop the banging noise but still waiting on 2nd thermistor should have this tomoro morning so will see if right

ok feel abit daft now this was one of those simple but easy to over look problems get one every now and then usealy when im under presure to get stuff done wich i have been this week ok . boiler was full or black crud ! divertor motor had gone awol put the thermisters on slight improvement but still not right
then i noticed ( stupid that i am ) the divertor valve was still passing slightly enough to take the heat away from domestice plate heat exchanger !
so i took it apart to find even after the cleaner went through it there was still carbon on the spindles so divertor couldnt move fully thus burning out origanal motor !
striped it cleaned it put back together new motor and hey prestow works like new again one happy customer !! mind you she hasnt had the bill yet
so moral or story is make all the simple checks first and dont asume putting new motors sorts out stifff valves " simples "


oh and thanks for your help guys i did check them wich led me to looking at the valve again