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PostPosted: Sun Jan 20, 2019 10:05 pm 
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Hello Troops,

Looking for a little advise with an issue i am having not getting hot water from my boiler.

CH works fine, DHW worked only if CH was on first. If tap opened without CH being on i get no hot water.

Whats been investigated/replaced so far:

On investigation the DHW differential pressure diaphragm/pin/switch were operating correctly when a tap was turned on which initiates the pump however the flow pressure diaphragm pin to prove pump flow and start burner only partially comes out and stops about 1mm short of activating the switch. Strangely though when the CH is activated on its own the flow pin comes out no problem and activates burner switch, if the DHW flow is turned on at this point the pin stays out and hot water is evident.

At this point while i was getting hot water i noticed that the temperature on the feed out to the radiators was rising quite rapidly and an audible noise could be heard of restricted water flow from the diverter valve area.

CH pressure was as normal at 1 Bar and expansion vessel was tested ok at 0.5 bar with circuit drained. There had also never been any overheating or high pressure problems.

These finding are what led me to suspect a faulty diverter valve with possible diaphragm issues so I purchases an overhaul kit for the diverter assembly then stripped and reconditioned all service parts within. The old valve parts were certainly gummed up and not moving freely however both diaphragms appeared to be serviceable. I still replaced these for new.

I also replaced the auto air bleed as it was leaking.

After rebuild and start up the issue appeared to get a little worse with the DHW not working at all, even if CH was turned on first.

CH works fine and activates flow switch promptly

If DHW turned on with CH on, flow pin reduces slightly deactivating flow switch.

If DHW turned on without CH,flow pin comes out but still stops 1mm short.

If i by-pass the flow switch with hot tap running the burner fires up and i get hot water. I also no longer get the radiator feed pipe heating up or the audible flow restriction noises so the valve overhaul certainly fixed those issues.

Looking for some thoughts on the possible causes?

Could i possibly have a week pump where the back pressure in the radiator circuit is generating a slightly better reaction at the flow diaphragm where as once the diverter valve is activated the smaller boiler circuit has less back pressure therefore dropping the circuit pressure slightly and in turn the flow diaphragm pin?

The pump certainty sounds OK but i have not inspected the impeller blades which i think i will do tomorrow.

Is there anyway to test pump head pressure accurately? Or is it possible to diagnose from the system gauge? I have a manometer but would be grateful if any one has some guidance on where to attach into the system and what fitting required.

I am ruling restricted flow through the primary exchanger as CH operates correctly and i am also ruling out restricted flow in the secondary exchanger as to my mind this would actually create a better pressure at the diaphragm rather than worse and create over temp issues. (I may be wrong here)

I may also be miles out with everything here :dunno: so would be very grateful for any help you can offer folks.

I have a video of the action at the pins, will upload ASAP.


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PostPosted: Wed Jan 30, 2019 9:55 pm 
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For anyone interested this problem is now fixed.

I removed the motor/pump assembly but visually there were no obvious signs of deterioration on the pump impellers. I also ran the motor/pump with it dismounted and it seemed to run fairly well.

Despite this all the symptoms (in my head anyway) still led me to believe i had weak motor/pump so i decided to order an new assembly.

Once fitted and running the diaphragm pin was shooting out no problem and the systems runs better than it has in years!


Hope this helps someone else who may be struggling out there. ::b


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