alpha 240p primary flow valve microswitch

[jellybelly]

Hi
I have a hot water problem with my alpha 240p.
(in hot water mode only)
When hot water tap is turned on ,the DHW micro switch turns on but the primary flow microswitch does not turn on,this is due to the pin from the primary flow valve not rising to push the switch.(DHW & PRIMARY FLOW VALVE diaphrams have been replaced)

(In central heating mode)
The primary flow pin does rise & the switch gets activated.(without the hot water tap being opened)CH all works ok.
When i turn the hot water tap on the primary flow pin then goes down (the primary flow microswitch goes off) & i get no hot water.
DHW microswitch remains on.

My question is,why would the primary flow pin be up in CH mode but go down when i turn the hot water tap on & stop the hot water working.
Any suggestions would be gratefully accepted.

Thanks
Jellybelly

 

[Rob]

Probably a blockage in the HW circuit. Most likely in the plate HE!

 

[jellybelly]

Thanks
can you give me some more info about how the water gets to the DHW HE.
Below is something i read about the diverter valve assembly.

When a domestic hot water tap is turned on, the flow of water is sensed and causes the diaphragm, via a plastic plate, to act on two stainless steel shafts, one causing the 3 Way valve section to move to the hot water position, the second shaft projecting from the centre of the diaphragm chamber operating on the micro-switch.

Could it be that the 3 way valve is not diverting CH water to the DHW HE.?Would a stuck 3 way valve stop the CH water getting to the DHW HE.?
If the 3 way valve was stuck would this stop the shaft from operating the primary flow valve microswitch.?
Sorry to ask so many questions but id like to understand all possible reasons why the primary flow shaft isnt operating the primary flow microswitch & i have understood that a blocked HE could be the reason.
Thanks again
Neil

 

[Terrywookfit]

If the pin is not operating the micro switches it is probably the diaphragm that has failed!

 

[Agile]

Sorry to ask so many questions but id like to understand all possible reasons why the primary flow shaft isnt operating the primary flow microswitch & i have understood that a blocked HE could be the reason.
Thanks again
Neil

Like many DIYers you should not be trying to understand how it works as much as whats necessary to fix it.

Suggesting that the plate HE is blocked is just possibility. We helped a lady recently with exactly the same problem.

First check the pump inpeller to see if there is dirt in the eye.

If that does not effect a total cure then do a finger test on the pump torque and if thats ok then its going to be a blockage in the DHW path and the plate HE is the most likely.

Tony

 

[bengasman]

Don’t know this model by heart, but primary flow switch should work regardless of ch or dwh mode.
If you recently had diaphragms changed, I suspect it was not done properly.

 

[Agile]

No Ben, its operation is dependent on the flow rate and will differ according to the resistance of the CH or DHW circuits.

Tony

 

[bengasman]

No Ben, its operation is dependent on the flow rate and will differ according to the resistance of the CH or DHW circuits.

Tony

I see. Seems odd. Any idea why it was done that way?
Maybe my view is a bit simplistic, but as far as I can see primary flow switch ensures flow through prim hex, doesn’t matter if it is for ch or dwh.
Glad I mentioned not knowing this model, now I only seem a little thick

 

[Agile]

The flow switch has to ensure there is adequate flow regardless of which path its taking.

Thats the only way it can do that effectively on any boiler.

Tony

 

[jellybelly]

Thanks for all the info,but i have a final question before taking bits of the boiler apart.
You mention checking pump impeller/torque.(see below comment.)

''First check the pump inpeller to see if there is dirt in the eye.

If that does not effect a total cure then do a finger test on the pump torque and if thats ok then its going to be a blockage in the DHW path and the plate HE is the most likely. ''

If the pump had an impeller/torque problem would this not affect the CH system,as well as the hot water system.(the CH is all working ok)
Can a pump problem affect one & not the other?

Thanks again

 

[Gasguru]

The demands on the pump can be more when the boiler is operating in DHW mode. In DHW mode the pump is circulating water at a rate to dissipate 30KW of heat energy. In heating perhaps only 10KW so considerably less. A plate heat exchanger requires a high head pump even when new let alone when partially blocked with corrosion debris.

Does the pump run quietly in DHW mode or does it sound as though there's air trapped in the boiler? The differential diaphragm sits between the negative side of the pump and the flow side of the primary heat exchanger. Depending on the spring rate under the diaphragm cover, trapped air can have a detrimental effect (since it's compressible) as the hydraulic effect on the diaphragm is reduced.

Remove the pump head and feel the torque of the impeller. Briefly run the pump so as not to cause damage.

Check the plate is clean on the primary side.

I've had this problem on other boilers with similar connections of the differential sensor and it has sometimes been a partial blockage on the primary heat exchanger (the washers were slightly smaller bore than the pipe connections and crud had built up on the edge).

 

[bengasman]

The flow switch has to ensure there is adequate flow regardless of which path its taking.

Thats the only way it can do that effectively on any boiler.

Tony

Indeed. I am wondering now if I have stumbled over a fundamental flaw in my understanding.

If a flow sensor of whatever kind detects (sufficient) flow through the primary hex, adequate dissipation of the produced heat is ensured, presuming that the boiler is working under the conditions that it is designed for, is how I always understood it.
Most boilers have the maximum ch at about the same level of the max dhw production with only a few exceptions like the response.

It is beyond me how a different flow limit would be needed or desired unless somehow coupled with the throughput of the gasvalve thus allowing the boiler to partially keep working when something starts to give up.

Am I that far off??

 

[Agile]

The flow switch has only one operating point.

Its purpose is solely to ensure that there is enough water flow to enable the temperature sensing elements to control the boiler safely.

Tony

 

[jellybelly]

In reply to comment below.
''Does the pump run quietly in DHW mode or does it sound as though there's air trapped in the boiler?''

The pump runs quietly in DHW mode.

Q:If the pump operates the primary flow shaft which activates the microswitch in CH mode(& working) why would it not be able to operate the same microswitch in DHW mode.Does the primary flow shaft/microswitch need a higher pressure/pump throughput in DHW mode than it does in CH to operate the same switch.(if so then i understand why my logic was incorrect)
If this is the case then i will check the pump before checking for a Heat exchanger blockage.
Thanks

 

[jellybelly]

Hi
Ive changed the DHW heat exchanger & it has made no difference.The primary flow shaft still dosnt move & does not activate the primary flow microswitch.
What do i change next.?
Thanks