Baxi 105e Combi-Boiler - DHW problem - No Gas Modulation

[Diomedes71]

Been without HW for over 6 months. Finally diverter valve started leaking so sprung my lazy ass into motion - changed 3 way divertor OK.

Central Heating 'appears' to be functioning except I notice that primary heating cct does not reach 80deg - only 70deg - ALL dials set at max (HW and CH)

Running HW with CH on with primary loop at 70deg results in surge of hot water at tap but LEDs slowly indicate falling primary temperature. Burner running all the while.

At sufficiently low rate (very low) I can run a bath to 40deg but if not careful HW flow will pull primary temp down to 30 deg resulting in very luke warm water.

Manual suggests checking mudulation. I observe non, but to be honest I can't say if it is stuck at max or min, no frame of reference !

I have tested terminal voltage at the Gas Valve modulating terminals to find 24V. THe manual suggest a maximum of 9v and a minimum of 1v. Thus I conclude both pcb AND gas valve are faulty???? As at 24V I should have high gas flow burning to feed heating cct BUT at the same time I must conclude that the pcb is faulty since manual says max 9v ???

I presume the HW sensor to be working since when water is running hot (55deg) on LOW flow and I turn the setting dial down - the burner cuts out.

Initially I had suspected a blocked heat exchanger as the system has never had any anti-corrosion fluid in it. I have been running some cleaner for 48 hrs in the system with little difference.

I really need to be able to force the modulation some how or get answers with regards to the anomollous voltages. In a forum post found by google it suggested that Baxi says the modulation voltage max is 24V.

 

[Andygasman2010]

What colour are the wires you're getting 24v across?

 

[Boilerman2]

Well it certainly 'aint the Modureg!! perhaps the OP should call a pro and NOT attemt DIY gas work!!

 

[Baxmax]

Deep....Very Deep.

 

[Diomedes71]

My uncle or cousin will do any gas work i need doing, but as an electrical service eng. Myself there is no harm me doing some diagnostics - especially since the blogs are replete with so called professionals struggling and changing parts willy nilly.

 

[Diomedes71]

Huh, can't see subject - WIRES ARE BLACK

 

[mfarrow]

From memory the boiler should be modulating on ignition - i.e. little flame followed by big flame when the thermocouple gets hot enough, usually after a second or two.

You could also calculate a rough flowrate/kW consumption by looking at the meter.

 

[Andygasman2010]

If you're confident you're getting 24vdc across the two black wires at the modureg then change the PCB.

Make sure you fit the heating and hot water temp control knobs in the correct orientation.

Tip: Run the hot water with the boiler off to make sure it's cold, turn the boiler on to be sure you get high gas initially, then pull off one black wire the burner should visibly drop to minimum. You could get an RGI to check the exact pressures.

 

[Agile]

It seems to me that he needs a reg gas engineer.

If he really does have a relative with the registration and insurance then he should be called!

 

[Diomedes71]

Gas regulation Test
1) sw. off boiler
2) run HW tap until very cold
3) sw. on boiler
- observed boiler fire - NO initial regulation (1-2 sec as suggested above)
- observed NO Regulation / gas flame change (2-30 seconds)
4) Disconnect / pull black control wire
- observed NO regulation / gas flame change

NOTE - absolutely definately getting 24V at regulation (2 bk wires)
CONCLUSION? - gas valve faulty ?


GAS FLOW RATES
Serv Manual Tech Data - (after 10 mins) max ft^3/hr = 128.12
Flow rate estimate Test - 6 min run (so mull 10 for hourly)
Meter rd start = 9912 01.5 ft^3
Meter rd start = 9912 05.5 ft^3 TOTAL = 4ft^3; per hr est. = 40ft^3

CONCLUSION = low water temp definately due to low burner output / regulation ! !! ? ? ?

QUESTIONS
Is the manual incorrect to suggest 1 to 9V control signal ? (did i say already, reading elsewhere about similar but not same flt, a thread suggested Baxi said 24V is the correct voltage for max demand)

If 24V is the correct MAX signal - but only getting low flow - My conclusion would be the valve is faulty - NOT the pcb - unless the valve is not fail safe - ie NOT fail safe = low voltage / op.cct = high flow

PS - promise to post final resolution. So many anoying threads were they never pay this most basic curtesy. Thanks for your help / good suggestions so far.

 

[Andygasman2010]

Gas pressure checks need to be carried out but we can't advise on those. You should get your cousin or uncle to do them, if they are qualified, or get an RGI in. I wouldn't spend any money on parts until this is done. Plus, you can't change the gas valve yourself.