Biasi Garda M90F.32S - Cycling Burner Operation during DHW

[GoodDIYjob]

Boiler was serviced about 1 year ago, before we purchased property by BG.

Yes I know, I don't think their service was very thorough either based on the inspection I have done and the things I have rectified since.

I suspect it hasn't been shown a huge amount of love and attention over the years.

Currently, when using the DHW in conjunction with a thermostatic shower valve in the shower, the boiler cycles the burner with the flame extinguishing and subsequently re-lighting.

The boiler doesn't seem to do this when drawing hot water only from a tap.

The water supply flow and pressure in the house is very ordinary. It's the supply pipe from road meter to internal stop cock that's at fault. Original and steel or iron and must be very constricted.

Water supply at kitchen tap cold is about 7.5lpm cold only. When drawing hot only, about 6lpm. Forget trying to do simultaneous demand measures because any concurrent demand will cause a dribble at the secondary demand point.

I have read a few postings that I can find on the subject but circumstances are not the exactly the same as this.

Given this only happens with the thermostatic shower valve and not hot water only draw off, is this likely to be:
*mains water flow issue related?;
*modulating gas valve minimum pressure setting;
* gunked up DHW heat ex?

Anything, I can do to eliminate any of the above?

Edit. Ps also measured resistance of thermistors. At cold temperature, their resistance is in line with the graph in the manuals.

 

[Agile]

I don't greatly like thermo shows with combi boilers as they can work against each other an cause the temp to cycle.

It might JUST be this. If so try turning the boiler temp knob down. They should be used at about 2/3 of the max but many people turn everything up to max.

It can also be a blocked plate HE and that will be evidenced by a high reading on the temp gauge. Normally it should be 55-65 C on DHW. If higher then a blocked plate. At 75 c or higher then its intervention time!

Tony Glazier

 

[GoodDIYjob]

OK, thanks for the reply. I'll look at the temperatures with DHW demand.

The DHW setting is within the "safe zone" recommended in the manual not up at max. Think about 4. It is producing water about 44-45C at the taps.

I did try to turn back slightly before posting but it didn't behave as expected, initially as I turned it back it went up to high 40s. Turning a bit further back has ended up being exactly the same as before.

I can turn right down and see what it does.

 

[Agile]

Sometimes the knob shafts are missing/snapped/180 degrees out of position!

Tony

 

[GoodDIYjob]

OK, results in.

Drawing hot water only from a tap - boiler temp at constant 60C.

Using the thermostatic shower, it slowly creeps up, until the flame extinguishes about 70C.

The DHW temp control on boiler barely appears to be functioning. It's definitely seated on the shaft and moved from aligned min to max settings.

However, turned to min water was still 42-43C. Turning back up the 4 or 5 the water was at 44C. Almost no control.

Could this point back to the min flow rate of water - i.e. boiler not able to control temperature at that flow rate or min gass pressure set too high or just a fault on the temp control circuit?

 

[Agile]

There are many possible causes and we cannot advise on the gas ones in detail.

But the first problem is that you need to inspect the inside of the pcb and that the pot is really being turned by the knob!

Your mains flow rate is not helping you. Are you sure its not restricted by the stopcock?

If you tell me something about running the hot tap then I would also like to know at what flow rate!

If you reduce the flow rate then does the flame get smaller?

By the way, BG don't normally service a boiler. Its usually just a "safety check" using a flue gas analyser.

Tony

 

[GoodDIYjob]

OK, I understand that.

Yes, shaft turning inside plastic pot mounted on PCB. Just doesn't seem to do anything. No visible sign of anything fried nearby diodes and a capacitor look ok physically.

Hot water max flow about 6lpm. Temp at 42-43 at tap.

Well that would make sense then. That's what BG would of done, which is why so many other obvious things were left.

It's hard to tell with gas flame size. Honestly looking at it come up from cold I really don't see much flame size difference. Would probably be easier to check with two people.

Should a boiler service test for (and adjust) min / max and ignition gas pressures? As you know it's #hit# and miss finding a good heating engineer.

post scrpit edit: replaced mains stop cock about 9 months ago, helped very marginally. Had Thames Water test at meter and pressure and flow were good. from Memory, flow rate about 30lpm. I have a test ticket somewhere. The supply pipe that comes up into the kitchen looks small and very old.

 

[Agile]

The knob shaft could always be 180 degrees out but lets ignore that for the moment. If it was then you could feel a discontinuity as the wiper runs across the end stop.

You could measure the mod coil voltage in situ. It should vary between about 6v to 15v

Any GOOD engineer would always check min/max/ign gas pressures. Most would measure the max but very few would measure the min/ign!

You could read the FAQ about measuring gas rate and then do that on DHW as normal and then with one wire pulled off the mod coil.

BUT there are two different makes of ignition PCB and one of them turns off the gas if the mod coil is disconnected.

If yours disconnected the gas then you could simulate a mod coil with a resistor of 270 ohms!

Tony Glazier

 

[GoodDIYjob]

The temp adjustment for DHW is a 180 degree movement sweep, so yes could be 180 out. But, you can feel the stops at each end of movement, so assume this must be right as is.

Further tests.

Gas mod coil reistance 80.2ohms. Service manuals says 80 ohms.

Operating voltage, using DHW. A bit variable between runs but seems to start off about 5.5V to 6V and track down quickly to 3V and ends up at 2.2V.

On one run it was as low as 1.9v ending voltage after running for a few minutes.

Gas usage. Did tests with the mod coil connected and without. Gas still flows with a connection removed.

They were identical. 028 thou in 60 seconds. So 1.68m3 per hour. Looking at the service manual it says gas rate 1,64 (min)- 3,73 (max) so seems like the boiler is modulating to minimum gas rate.

Any suggestions for a good boiler engineer to service this as a start, in Guildford?

 

[GoodDIYjob]

On the DHW temp control - Is there any way I can further test this? Is the only fix a replacement board or can the pot be de-soldered and replaced as I think replacement of the whole board may be not the best use of limited funds.

The temp control really doesn't seem to work as you'd expect (it seems to make practically no difference at all even at min and max) but has settled back to 42C or thereabouts so is usable.

Thanks

 

[Agile]

It sounds as e gas valve adjustments are wrong.

But so is the voltage to the mod coil!

It could be that someone has fiddled the gas vale to do a bodge repair to cover a PCB fault.

But we are not allowed to encourage DIY repairs on gas/combustion aspects.

But it really needs a good boiler engineer. Unfortunately diagnosing these kind of faults are only within the remit of very few engineers!

Tony