Main Combi 24 HE

[nihonsuki]

Hi!

Main Combi 24 HE

I had one of these fitted around five years ago, and it's been fine until recently when the DHW became erratic; CH is still fine. Initial symptoms were that hot water was only available when the CH was actually firing. I opened the case and saw that the pushrod on the DHW Pressure Differential Valve was not engaging the microswitch on full extension (i.e. tap on). I did the obvious thing and bent the tang on the switch so that it would engage on extension and all was well for a day. However, now we have not DHW at all as the pushrod does not extend at all.

Two questions:

1) Am I correct to conclude that either the diaphragm in the valve is torn or the pushrod is jammed? (see p.s.)

2) What does it mean to drain the primary circuit, can I isolate the boiler and simply drain that or must I drain the whole heating system?

TIA for your advice

p.s. Quick test this morning (like you do) and the pushrod extended fine under DHW demand. However, a later trial failed, with the pushrod again not moving. I guess that means the diaphragm's probably OK. So, jamming pushrod or something further upstream (i.e. blockage in the pipes to the diff)?

 

[HITGAS]

You are correct the diaphragm is split I used to repair them years ago with a condom and some super glue but I wouldn't advise that. The top of the divertor valve can be changed without draining the heating system. Shut off the cold inlet. Be warned the screws holding the divertor top are not easy to undo.

 

[misterdubya]

1. Turn off mains and open a hot tap.
2. Undo the screw securing the microswitch bracket to the valve.
3. Disconnect the front sensing pipe.
4. To replace the diaphragm, carefully remove the six screws of the diaphragm housing. The diaphragm spring will force apart the two halves of the valve - so be ready!
6. Remove the plastic disc and pushrod assembly. Carefully remove & replace the diaphragm.
7. Reassemble in reverse order.
8. Replace stuffing box on the front of the brass diaphragm housing (thats probably the bit thats leaking). Replace washer & 'o' ring that are supplied in the repair kit.
9. Close the hot tap, turn on the mains & check for leaks.

 

[dcawkwell]

You are correct the diaphragm is split I used to repair them years ago with a condom and some super glue but I wouldn't advise that. The top of the divertor valve can be changed without draining the heating system. Shut off the cold inlet. Be warned the screws holding the divertor top are not easy to undo.

I've heard a few excuses to wives and girlfriends of why you were carrying condoms but I think that is the best one.

 

[nihonsuki]

Much thanks to HITGAS and misterdubya for clear instructions. Twenty minute job this morning (including the replacement of the condensate trap, disconnected last year because of a rust short across the contacts). The diaphragm was perished and slimy so I'm guessing it had become porous or slack around the centre hole.

Everything working fine now. Thanks again!

Tony

 

[cabinbuilder]

Thank you for the simple instructions. Worked a treat on my daughter's boiler.