vokera maxin 24e BIG problem

[luke1612]

VOKERA MAXIN 24E
background- my central heating for some time has not been maintaining the 1.5 bar pressure. i checked high and low and could not figure where the leak was coming from, and would top up pressure at least once a day. But the boiler itself worked fine. (this been going on for 3 months)
eventually i found that the main heat exchanger was leaking, so i removed it, had it repaired and refitted it along with new insulation pads.
now when the ch is on, the boiler will fire up and stay on for about 3 min and then will start to go on and off every 30-40 sec.
i have noticed that if i disconnect the temp thermistor (for test purpose) the boiler will stay on, (if that helps).

few other things i notice which my be of help.

-the DV pin is still making contact with the micro switch when this happens.
-the grundfos pump is running
-although the hot water works i have noticed that since replacing the main heat exchanger it has not been as hot as it usually would be.

ANY advice on possible causes will be greatly appreciated...

 

[Agile]

How did you get the HE repaired and how did they do that?

Tony

 

[luke1612]

had the leak silver soldered by a specialist,
and after refitting there were no leaks

luke

 

[Agile]

Generally if its leaking in one spot another is not far away.

Most failures are caused by electrolytic corrosion from iron bits in a copper HE.

I will be interested to see how long it lasts for.

Tony

 

[luke1612]

the HE is not made from copper, and a full pressure test was done, but i suppose there will always be a risk. in this case the risk of another leak was weighed up against the cost of a new HE...... only time can tell

we live and learn
luke

 

[nickso]

the HE is not made from copper

yes it is....and you are talking about the main one above the burner.

the main heat exchanger

 

[luke1612]

yes, its the main one above the burner. when i took it in to be repaired the chaps first reaction was that it wasn't made of copper. but said that they would be able to sort it for me. they found two leaks on it and silver soldered them. Since fitting it i haven't lost any pressure so the actual HE
seems to be ok.

ANY thoughts on possible causes would be greatly appreciated.

im not a heating engineer, but i wandered if it was possible that some debris may have got into the domestic HE causing the hot water to drop in temp but still cant understand why the CH will keep knocking off..??

luke

 

[nickso]

hmm...and what did he say it was when he scraped the paint off it?

 

[luke1612]

he didnt say...copper it is then,

ANY thoughts on the faults

luke

 

[scatmanjohn]

When its running look through the sight glass at the flames.

If you turn the temp control knobs on the front do the flame alter (get bigger and smaller)?

 

[luke1612]

scatmanjohn-

right, first i put the radiators on and altered the temp control- flame stayed the same.

next, tried with rads off and hot water on- flame stayed the same But went out when got to the min range of the temp controller and back on when turned back up (but flames same size)

next, both CH & HW- flame stayes the same.


luke.

 

[DP]

I suspect the repair is the cause of boiler behaving as it does.

Initial leak would most likely (as Tony suggests) be due to installation of boiler onto a dirty system. Then to get the heat exchanger repaired, how do you know the silver solder (if in FACT it IS silver solder) has not dripped down into the cavity to effect primary water flow?

Removing the lead from the sensor will not help boiler diagnosis (in case of this model) as boiler has belt and braces controls to keep it working when sensors fail but still maintain safety. The fact that the boiler is cycling like this would suggest heat is not being removed at the desired rate (if the fault is indeed due to water circulation and not another component malfunction).

 

[luke1612]

ok thanks DP, so would you suggest a new HE and system flush.

just as a matter of interest is there a method what could verify this theory before i went ahead and bought a new HE

thanks
luke

 

[DP]

I as a heating engineer would alway replace a leaking heat exchanger. I would not trust anyone to repair it. For you it is OK to fit a repaired component but for me it is a time waster should it start leaking again.

 

[scatmanjohn]

I'd be looking at the sensor. (after checking the boiler temperatures of course).