Baxi 105e - not modulating

[kktoday]

3 year old boiler. Customer reported poor hot water flow.
Checked burner pressure to find boiler is not modulating on CH or DHW. Burner pressure does not move from about 2 mbar.
21mbar available at inlet to GV. Suspected NTCs, Gas Valve, PCB
NTCs are fine: have run with spares outside of boiler and still getting minimum burner.
Checked voltage to modulating coil: Max is 7v DC. Min is 2v DC.
Thus assumed fault to be with gas valve so changed it.
Alas not the case. PCB seems to be the only other possible fault, although this appears to be giving the correct voltage.
I am suspicious. All help or suggestions appreciated.

 

[gas4you]

Have you checked the basic hydraulics, ie DHW diaphragm or heating diaphragm? Diaphragms can deteriorate after about 3 years. Is this in a hard water area? Is main heat exchanger scaled up reducing flow. Is pump running at full speed? Is modulating coil seated correctly and held in place with spring clip? Is 21mbar at gas valve standing pressure or working? If standing what does it drop to when running? What is working pressure at meter? Is burner pressure at 2 mbar at initial fire up or does it run at max then drop? Have you got approx 12 l/min flow through taps. Seems a strange one!

 

[ollski]

Is 7v all it requires to bring the gas rate up on the modureg?, I thought it should usually be about 16 - 18.

 

[kktoday]

It is hard water area but there doesn't appear to be any problem with flow through the primary circuit. If there was I would still expect some modulation. But I get none. Rads all heat nicely even on low flame, flow rate good at tap, diverter good, pump good. GV has been replaced. Burner pressure never rises above 2.5 mbar - slight spike on initial fire up. Not tested working inlet pressure. Can't imagine what else but pcb.

 

[ollski]

Burner pressure never rises above 2.5 mbar - slight spike on initial fire up. Not tested working inlet pressure. Can't imagine what else but pcb.

um, probably best to check you have more than 2.5 mb working pressure.

 

[kktoday]

ollski
manual says it runs between 1v and 9v. The valve itself says max 15v. Although Baxi said it needs 24v ? I'm going with what it says on the valve. At 7v I'd expect more than 2mabr.

 

[ChrisR]

Presumably you have the instructions - and are as mystified as everyone else. I have a feeling someone got an answer for what they meant to say, it might even have been me, but it's gone...

Voltage at modulating coil of gas valve is:
Max. burner press.approx 9V DC (13V DC LPG)
Min. burner press.approx 1V DC
Current at modulating coil of gas valve is:
(Use an instrument with average function for this
measure)
30 ÷ 230 mA DC (45 ÷ 310 mA DC LPG)

 

[Agile]

Your measurements seem to be misleading as you say volts on mod coil are 7 max and 2 min.

You also say burner pressure is never more than 2 mB.

Does the mod coil voltage vary from 2v to 7 v when you alter the temperature dial or something?

Have you really measured the inlet pressure at the same time as the boiler is operating at what should be the maximum power?

Please dont guess at the last question! It may indicate the problem!

Last, you dont just bung a gas valve in! It needs adjusting to the min and max burner pressures! That would have been the first thing I would have tested! Do you know how to and did you?

Tony

 

[namsag]

As Agile says check inlet pressure is still 21mb with boiler firing on hot water. It is not unusual to have a blockage in gas line, even main tap at meter not fully on. Have you tried adjusting gas valve pressure with little nut under cap

 

[Agile]

One guy spent THREE days and THREE hours a day changing gas valves because he had not bothered to check the gas pressure at the inlet when the boiler was firing.

Because three burners on cooker fired he thought gas supply was on! Well it was but restricted as a result of a faulty card gas meter letting by with no credit, great for cooking but not for a 28 kW boiler!

Tony

 

[baxpoti]

Thus assumed fault to be with gas valve so changed it.

As mentioned previously, this 'new' gas valve needs setting up,
It does not come factory set.

Do you know how to set this gas valve up?

 

[kktoday]

Thanks for replies.
Thinking about your comments and logic it does seem to be a supply problem
I will recheck working pressures next Monday. Have to admit I didn't check since cooker was working well and 21 mbar standing pressure, so assumed it was not gas supply problem.
Yes, I did try to set up the GV but even so the max pressure never rose above 2.5mbar so the max burner pressure nut will not be correctly set.
Will post back with more readings.

 

[bripl]

copper sulphide?

 

[gas4you]

Thanks for replies.
Thinking about your comments and logic it does seem to be a supply problem
I will recheck working pressures next Monday. Have to admit I didn't check since cooker was working well and 21 mbar standing pressure, so assumed it was not gas supply problem.
Yes, I did try to set up the GV but even so the max pressure never rose above 2.5mbar so the max burner pressure nut will not be correctly set.
Will post back with more readings.

What type of govenor have you on the meter? If its a J88, G910 or old donkyn then it needs changing by National grid anyway. I've had govenors that are jammed and full of rust. a silly thought, have your neighbours mentioned any gas problems lately?

I would be suprised if you really have a 21mbar working pressure on a combi unless it has been fitted by a 'perfect' installer who knows his pipe sizing inside out and your meter working pressure is 22mbar!

 

[Agile]

I would be suprised if you really have a 21mbar working pressure on a combi unless it has been fitted by a 'perfect' installer who knows his pipe sizing inside out and your meter working pressure is 22mbar!

I saw a nice new changed regulator giving 21 mB with a 28kW load !

Pity about the gas pipe though. The cowboy installer had left the 15mm under the floor and it was losing 2.5 mB from the meter. I had an interesting advice to give to the owner.

All installers have to be perfect because CORGI are very hot on pressure loss and if they find over 1 mB will serve a defects notice as well as turning the boiler off saying its At Risk. It does not make the installer look good to the owner.

A friend of mine was caught on that and had to pay £100 to appeal which was a waste of time and was made to retake ACS pipe sizing module. That could have been costly but the training centre he uses let him do that free and issued him a letter saying he had completed it satisfactorily.

Tony