Problems 28CDi

[Mr Smyth]

All..... well perhaps not all.
Have been trying to solve a problem on a Worcester Bosch 28CDi for about a week now (thank God I'm not paying for the parts)
Problem - HW goes cold after about 50 seconds and then hot again and continues to cycle like this. Heating all OK rads get hot very quickly.

Day 1 - Check all the pressures, GV modulating and changed the thermistors. Also checked the mixer taps by isolating the cold. Suspect Plate HE
Day 2 - Changed the plate H/E. No change.
Day 3 - Spoke to Worcester advised check op of diverter valve but this was spot on , then HW going down Heating flow but this is cold. Then suspected the GV so measured resistance across GV which was slightly out so changed the GV. (as I had one) no difference. Also had a PCB so tried that..... Still no change. Worcester suspect the main HE so I have put some fast cleanse in the system and am going back on Monday with a HE. I will also change the pump before the HE but I really don't think the pump is failing.

Any thoughts....

 

[Garfeild]

Are you sure diverter valve is ok?i had exactly the same fault on an Alpha boiler,main diaphragm looked ok until i took it out,and held it up to the light,it was nearly transparent,still couldnt see any actual holes.
New one cured the problem completely.i would service the diverter valve before changing anything else.

 

[ChrisR]

Becomes simpler to diagnose if you have a surface temperature probe to see what's going on. Is the main hex getting too hot?

Presumably you looked at the pin on the diverter valve? (mirror needed).

 

[Mr Smyth]

In reply to both.
Had my eye in the diverter valve pin right through one of its cycles. The pin did not move as the flame dropped out.
I think it is the main HE. This will either by solved (for now atleast) by the fast cleanse or I'll change the HE. Either way I'll advise the customer to P/F the system.

Thanks for your contribution both of you.

Any other engineers want to chuck their oar in

 

[jiminblack]

Have they got a mixer shower? Could be knackered and pulling cold water in to the hot water circuit. Or, have they had any other work done elsewhere? Hot/colds could be crossed.

 

[Slugbabydotcom]

Is there any flame lift present? If so it could be a flue problem

 

[ChrisR]

If twer the main hex you should be able to see excess temp at the thermistor, which would tell the pcb to cut the gas ...

 

[Mr Smyth]

Went back to the job today and all was fine. Looks like the fast cleanse did the job. Drained out the system, water quite clear and refilled, with inhibitor. Customer intends to replace 4 out of the 10 rads in the summer so I have advised him to remove and flush out the others with a hose. Have also advised him that if the problem re-occurs then the first job is the HE.

Thanks for your thoughts