28Cdi diverter valve

[Agile]

I've had the boards fail on the cdis like this before, turn the hw pot down below half and the boiler will work perfectly but over about half way up the boiler fails to modulate and drops out. Won't explain hot water going round rads though.

That can happen when someone has changed a PCB and managed to fit the knob shaft 180 degrees out of position!

 

[garyo]

Thanks for all the ideas guys. Last night I fitted my replacement heat exchanger that I'd descaled and flushed in all directions. I think this has made the hot water marginally warmer, but not by much.

I left the boiler switched off yesterday so that everything was stone cold when I came back from work. When I switch a hot tap on full, there is no hot water circulating around the CH system. As before, the burner seems to fire up to what looks like full strength flame, then quickly (maybe 2 seconds later) drops back to a low flame height. With the tap on full, the water temperature is more or less cold. With the tap set to a low flow, I can just about get a reasonable temperature.

When the CH demand is on, the boiler is capable of running at a very high flame height - way higher than I see with hot water demand.

I've already replaced the DHW temperature sensor. Tonight I'll take some temperature readings from the plate exchanger and do the resistance checks against the gas valve that are suggested.

Thanks!

Gary

 

[ollski]

I've had the boards fail on the cdis like this before, turn the hw pot down below half and the boiler will work perfectly but over about half way up the boiler fails to modulate and drops out. Won't explain hot water going round rads though.

That can happen when someone has changed a PCB and managed to fit the knob shaft 180 degrees out of position!

Maybe Tony although I'm talking about a fault developing over time on an exsisting board. Only had 3 but one was last week so it's fresh in my mind.

 

[garyo]

My thermocouple readings are as follows. I assume this means the heat exchanger is in pretty good condition?

With hot water tap on low flow:

HW Out: 41 degrees Loop out: 43 degrees
Loop in : 47 degrees HW In: 19 degrees

With tap turned on almost full (no noticeable difference in flame height):

HW Out: 33 degrees Loop out: 38 degrees
Loop in : 44 degrees HW In: 17 degrees

I've just done the resistance checks for the gas valve and DHW sensor, and they're all in spec.

I think I'll buy a PCB unless anyone has any other suggestions?

Thanks as always!

 

[Maxsys]

One other symptom i had was if the tap was fully on the boiler would cycle switching on and off, it was witching off because the hot water could not get through the exchanger, then when it cooled a bit the sensor would turn the boiler back on, do you have this as well?

I cannot comment on the temps, hopefully Tony will be along shortly. They do however look a bit cold to me, did you measure the temps on the outlet pipes from the boiler, or on the actual exchanger itself?

 

[garyo]

Hi Max,

Nope - I don't see behaviour like that - I rarely put the taps on full flow nowadays though so I'll try it and watch out for it.

Cheers

Gary

 

[mysteryman]

If there was debris in the primary side of the plate heat exchanger, there may be debris throughout the primary side of the boiler. If so, you need to powerflush the boiler and heating circuit.

 

[garyo]

Someone nearby was selling an old 28CDi for £50 so I thought it was worth buying to swap the PCB over. Apparently it was fully working and replaced with a newer model.

I've swapped the PCB over this made no difference. I also thought I'd swap over both CH sensors and the DHW thermister too just in case.

The behaviour is identical with the new PCB, but something I noticed is that sometimes - maybe once every 30 minutes, if I wiggle the hot water temperature knob while the tap is running, then the flame height really ramps up and at one point I got it to stay in this steady 'fixed' state for around five mintes (until I switched the tap off). At this point the plate exchanger temps were up at 72 degrees.

So I'm not sure if the other PCB is also duff, or does this point to the gas valve being sticky and wiggling the temp knob sometimes frees it up?

Gary

 

[garyo]

I think I've convinced myself that my new PCB must be faulty too, given that the boiler seems capable of modulating the gas supply perfectly when in CH mode....