Help! No DHW on Alpha 240P boiler

[tcm]

I’m a beginner on boilers. Before I drained down the boiler, I had DHW. I had the problem of over-pressure on the central heating reaching 2.5 bar after about 8 mins of switching on, which caused water to flow out of the external flow pipe.

Therefore, I drained down the boiler and recharged the expansion vessel (EV) to 0.8 bar. As there was no water coming out of the EV valve and the 0.8 bar held constant using a foot pump, I assume that the EV was OK. I re-pressurised the boiler to 1.0 bar but noticed that the boiler pressure was dropping. Immediately, went out of the house and saw water flowing out of the external flow pipe. The boiler pressure is now at 0 bar. I think the PRV is damaged which needs replacing. I already know that.

Now I’m not getting any DHW because the burner is not coming on when I turned the hot tap on. The green indicator light for hot water with a “tap” symbol came on on the boiler when the hot tap was on. The pilot gas light is on. I heard clicking in the boiler as I turned the hot tap on.

1. Can someone point out why no DHW?

2. Will I be able to replace the PRV without removing the pump first?

 

[Paul Barker]

8.35 PRESSURE RELIEF VALVE OR DHW INLET MANIFOLD - Fig. 35
1. Remove the DHW heat exchanger as described in section 8.34.
2. Disconnect the pump unions and withdraw the pump.
3. Disconnect the pressure relief valve outlet fitting, central heating return and mains water inlet valves.
4. Disconnect the two small flow sensing pipes from the manifold, and the small by-pass pipe from the rear of the diverter
valve.
5. Remove the clips (accessible from under the boiler) retaining the by-pass pipe to the manifold and diverter valve and
withdraw the pipe.
6. Remove the screw securing the manifold to the chassis (accessible from under the boiler) and remove the manifold.
7. Unscrew the pressure relief valve from the manifold. Fit the replacement valve using a small amount of jointing compound.
If replacing the manifold, transfer the drain point fitting to the new manifold.
8. Re-assemble in reverse order. Ensure that all seals are in good condition and in position and that the by-pass retaining
clips are correctly fitted and locate in the slots.
9. Refill and pressurise the system. (Refer to Commissioning, section 5.1).

then refill system and see if you have hot water.

 

[Rob]

all that to change a PRV

 

[corgiman]

all that to change a PRV

that and you MUST make sure that you do it whilst in GREEN trousers

 

[Agile]

We used to supply and fit a PRV for only £13 in addition to our diagnostic fee.

If its really that complicated then perhaps we need to review our charges ( which are used to compile the RPI incidentally ).

I thought we were meant to wear dark BLUE trowsers?

Dont forget that Paul used to do nursing and repair washing machines with cow gum!

Tony

 

[corgiman]

We used to supply and fit a PRV for only £13 in addition to our diagnostic fee.

If its really that complicated then perhaps we need to review our charges ( which are used to compile the RPI incidentally ).

I thought we were meant to wear dark BLUE trowsers?

Tony

Well paint me pink and call me wrong

THATS why its so complicated I have the WRONG trousers gromit, I am off to munch some cheese

 

[tcm]

Thanks to the quick reply from Paul. Ok I'll follow section 8.35 of the manual when I receive the PRV next week.

Before I drained down the boiler, the PRV allowed water to go out at 2.5 bar. Am I correct to say that after I drained down the boiler, the PRV got damaged, because it is now allowing water to go out at any any pressure?

I have actually re-pressurised the boiler to 1 bar (pressure slowly dropping to 0 bar within about 15s) and quickly turned the hot tap on but the burner did not come on. Therefore, if I follow section 8.35, will I really get DHW?

I forgot to mention that I disconnected the EV pipe from the diverter valve to allow water to come out of the EV. There were some gurgling noises and water came out of the opened connection (either through the EV pipe or from the diverter valve). After draining, I re-connected the pipe. Could this action affect the DHW?

Thanks in advance.

 

[ChrisR]

The system pressure isn't connected to the DHW directly. Nothing to do with tap water pressure either..

The 3 bar the prv's are supposed to open at isn't very accurate, and neither ar the gauges.
Once they have let water by though, they are inclined to leak.

For DHW to work, the pin must come out of the red bit and operate the microswitch:

 

[Paul Barker]

You can sometimes reseal the prv by opening it under system pressure conditions, better still when water is hot. Sometimes this dislodges a little crud. However I always change a suspect one as I don't want a call back.

You could just change the plunger/seal spring red knob half. Need some small thin jawed grips and it tends to waste the old one removing it.

I suspect many a gas fitter has gone away with a nice clean brass manifold section and a grip jaw chewed insert.

You can't start to analyse further faukts until you get it holding sensible pressure, but yes you are likely looking at dhw side of diverter valve work.

 

[tcm]

Thanks to replies from ChrisR and Paul.

When I turn the hot tap on, pin from diverter valve comes out and pushes against the microswitch. I have checked the pump and it runs. However, when I checked PCB M2 terminals 5 and 6, there was no 240V supply to the gas valve! According to the manual, it is related to the DHW temp sensor.

I'm going to collect a CH temp sensor later, which is the same one as the DHW temp sensor. I believe the current CH temp sensor is faulty before I drained down the boiler. I'll try the new temp sensor later and report today. It is so strange, because the DHW system worked before I drained the boiler, even though the system pressure was well under 1 bar.

 

[Rob]

Can some one clear something up for me please. Whats the best procedure when you find one of these boilers letting by to CH when HW is on.

Am i right in saying when flow switch spindle comes out the internal spindle should also? And that removing the flow swicth head would mean the internal spndle should spring forward?

I phoned potterton (same DV as perf.) when i had this problem and he told me it had nothing to do with spindles, if the flow switch makes then the DV is working!

It could only be a blockage in the water ways preventing poor/no HW. Also had already checked HE for blockages.

However removing the flow swicth and giving the internal spindle a bit of prodding, got it all working again.

Also whats included in the DV service kit, was told it was just the diaphragms??

thanks

 

[tcm]

I have tried using a new DHW temp sensor but still burner would not turn on.

- Checked the modulating coil at about 6.5V when demanded and 1.4V when not demanded which I think is OK.
- Checked pcb M3 connector, terminals 17 and 19 for fan at 240V at slow speed but will not go to 440V when demanded. Not OK.
- Checked pcb M2 connector, terminals 5 and 6 for gas valve, 240V NOT available. Not OK.

I have now replaced the faulty CH temp sensor with the new one. The DHW temp sensor is the same one.

With these new clues, can anyone make any suggestions as to why the burner will not come on for DHW?

 

[tcm]

At last, I found the problem. It was due to the primary flow valve not being able to maintain constant pressure on the pin and as a result the primary flow microswitch was not activated constantly. To be precise, initially the pin pushes against the microswitch for about a second and the pin retracts into the valve! Hence, the fan and gas valve were always disabled.

When I turned on a hot tap and used a screwdriver to enable the microswitch, it fired up the burner. WONDERFUL FEELING! I'm a happy man.

I guess now, I have to open the primary valve in the diverter valve assembly to check the diaphragm. I guess when I was draining down the boiler last night, dirt must have clogged up the valve. I'll open it and check tomorrow. I wasn't paying too much attention on the primary switch because I thought it was for the CH side. I was wrong. It really pays to follow the troubleshooting flowcharts exactly in the manual. Also, the boiler pressure is at zero but you can still obtain DHW!

I'll now ask a Corgi engineer to replace the PRV for me early next week as it is quite an arduous task (well certainly for me)!

Thanks again to everyone who contributed

 

[tcm]

BTW, you need both DHW and primary flow microswitches to be enabled in order to enable other functions, so that the burner can be fired up for DHW. I hope this helps other users in the future.

 

[Agile]

Thats obvious to anyone who understands boilers!

Dont bother touching the system flow diaphragm! If it works for Ch then it would work on DHW.

If its only failing on DHW then that indicates a blocked plate HE.

Regarding an earlier question about rads getting warm on DHW, that happens because the unseen back manifold part of the diverter is not following the pin in the front. Its pushed forward by a spring at the back and if this part is dirty then it may not move fully or at all.

I clean them with chemicals but you can take it all out and disassemble it and put silicone grease on the seals to make it move freely.

This often sticks if the DHW diaphragm has been left failed for many weeks/months.

Tony Glazier