Ravenheat RSF820/20 Combi. DHW not hot enough.

[garfield133]

I hope someone can help me diagnose this problem.

Symptoms
The DHW temperature never gets hot enough for the DHW stat to switch the burner from high to low. Even at low flow rates it never gets above 55C at the tap.
The main burner cuts out (leaving just the pilot) until temperature gets down to about 30C at the tap and it then fires up again until approx 55C.

Occasionally after drawing hot water the pilot light goes out.

Central heating works fine.

What I have tried so far.
1 Cleaned pilot burner jet and slightly increased gas to pilot. - Pilot still cuts out occasionally.
2 Replaced thermocouple and overheat stat (single replacement part). Pilot now cuts out only very occasionally.
3 Temporarily bypassed High Limit stat - DHW now gets hotter but overheat stat trips regularly as expected. Removed bypass.
4 Replaced High Limit stat - No improvement
5 Checked Automatic Air Valve causing it to leak water. Would not re-seal. Replaced with new valve.
6 Checked diverter valve. Operates to spec at between 2.5 and 3 litres per minute. Suspected may be partially blocked. Dismantled valve-end, removed a bit of debris and cleaned but no significant blockage. Reassembled. - No improvement.
7 Checked pump is running and in correct direction.
8 Fan runs at Hi and Lo speed as expected.

Conclusion
Either two faulty Hi Limit stats - What temperature should this open at? - I guess 85-90C. Can possibly test this with a clamp on pipe thermometer. (unfortunately the temperature gauge on the boiler has not worked for years)
OR Something is preventing adequate heat transfer from the primary CH water to the DHW circuit in the heat exchanger. We are in a soft water area so scaling is unlikely. Is it worth removing the heat exchanger and flushing through using a hose pipe?

I would be grateful if anyone could answer my couple of questions or suggest any other ideas.

Other forum users appear to have had similar problems, on other boiler types as well, but I cannot find any conclusive answers to say what solved the problem.

Many thanks in anticipation.

 

[Agile]

Can you explain your involvement in trying to repair this boiler please?

The hot water should not exceed 55°C.

You say the burner never reduces to low rate, even with the stat at minimum? Dont you hear it click?

Do you mean it clicks the second stage to burner off WITHOUT switching to the low setting first? Have you been fiddling with the gas valve adjustments?

I will need you the do a temperature survey of primary and secondary flow and return temperatures if I am going to be able to help you further!

Tony

 

[Slugbabydotcom]

There should be at least 3 l/m coming through the DV. Did you check the filter on the inlet? On the outlet there is a flow rate adjuster.
Eliminate the primary side of the DV. In the summer position [heating system cold] turn a hot tap on. Does the flow pipe get hot ?
The DHW stat wont knock the flame down to low until it reaches 60ºC but how accurate are your measurements 55 at the tap could be equivalent to 60 at the DHW stat.

Theres plenty of stuff to go through and you will need the right tools to go much further as Tony says but my strongest suspicion here is a faulty modulator coil.

 

[Agile]

The DHW stat actually measures the temperature on the primary flow pipe and assumes a small temperature loss in the secondary heat exchanger which is usually about 5-15°C

The modulating coils fail less often than people fiddle with the settings!

Tony