Firebird combi 90, burner not firing for DHW when CH is on

[vhooda]

Hi, my 6 yr old firbird combi has had a new heat exchanger 6 months back and had to change the flow switch 6 weeks back when it burst and started leaking water. All was well untill I started putting CH on last week. Once the CH has come on the boiler firing sequence seems to get disturbed even if there is no CH but the switch is in on position (its got a manual 24hr timer and it only comes on in the night for couple of hrs.)

Normally, once I open a DHW tap the fan starts almost straight away follwed by burner within 30 secs. If the CH has been on in the night the firing sequence does not happen in morning, there is a audible click when I turn the tap on followed by fan after about 2 mins or so (later then usual) and that continues untill the hot water starts turning lukewarm. The burner does eventually come on after 5 mins or so but then it cannot make up for the lost time and I have to close the tap for about 5 mins before it builds up it store and turns hot again.

If however, I turn the central heating off and restart the boiler it returns to its normal firing sequence untill CH is turned on at some point.

Also I have noticed the CH can turn luke warm when the burner is not firing along with the DHW i.e if dhw and ch are on at the same time.

Only thing I can remember is that when the flow switch started leaking water it made all the wirings in the control box wet and I had to dry out everything before restarting 6 weeks back.

Need help pls as to wheres the poss problem.

 

[Agile]

At first sight its something interesting. In reality its probably something dull and boring.

If your boiler had a pump proving switch my first check is that its being fully operated AND the switch is low resistance when made. That requires both visual and electrical checks.

What I would do next would depend on the result of that test.

Why did the heat exchanger need changing?

Tony

 

[vhooda]

the heat exchanger just stopped heating water, it could only keep up if the tap was on a trickle any faster and water would turn cold, changeing it sorted the prob, so must have built up scale/gunk over the yrs

 

[vhooda]

whats a pump pproving switch?

 

[vhooda]

Hi sorry , I had a little look on google now, there is not mention of a pump proving switch on a firebird combi 90, I cant see it in any spares list as well. is this switch also known by some other name.
Look at the pressure switch on this link, is it what you mean and where should I look for it.http://www.firebird.ie/Portals/2/docs/Firebird%20UK%20spare%20parts%20pricing%20with%20pictures.pdf

 

[vhooda]

Hi, went to look for this pressure switch on the boiler, couldnt find it but noticed something else. There is a small drip of kerosene from somewhere around the burner, it seems to be dripping from where the kerosene inlet pipe is connected. Also I am not sure how much you should read into my correlation of CH switch in on position and change in firing sequence. I can see that sometimes just by switching the boliler off and restarting seems to correct the sequence. But it only gets screwed again only when the CH has come on.

I am no expert and obviously trying not to go down 'change everything and see if it works' trip with my boiler guy. I end up paying him for all the time he spent looking for the problem and all items he changes are not returnable.

 

[oilhead]

Firebird 90 wiring is not too difficult to understand. The DHW stat is fixed, and will take priority over CH until satisfied. If water in boiler is hot enough to satisfy DHW stat, when flow switch is activated, burner will not fire until the water is cool enough to switch stat again. Maybe your DHW stat has lost some calibration.
Also, when changing the flow switch, did you have to modify the wiring with a relay supplied with the new flow switch? If so, was the wiring done correctly?
You will not have a pressure switch or 'pump proving switch' on your boiler.

 

[vhooda]

Hi Oilhead thanks,

I am now convinced that it is nothing to do with the the CH being on, The buner fires up when DHW tap is opened on 60% of times other 20% nothing happens to burner and 20% the fan runs but no ignition.
If DHW thermostat is the problem how does it manage to get away 60% of times and also how does switching the boiler on and off correct the firing sequence.

The flow switch did not require any wiring change, it seems to be working fine, and if I take it off I dont get hot water.

Could it be the burner control box?

 

[Agile]

Any one of the several switch contacts in your boiler may be high resistance and only sometimes making adequately.

Every switch needs to have the resistance of its contacts measured.

Your problem is only on DHW so its not the burner control because thats common to CH and DHW.

Tony

 

[vhooda]

Hi Tony,

If the boiler is not firing it will not fire if demand for CH comes and the CH will remain luke warm as well like the DHW. You think that cd mean boiler control !

 

[oilhead]

If burner motor runs but with no ignition, on Riello burner, try a new photo cell. A common fault.

 

[oilhead]

Should have added burner will not go to lockout, but will sometimes run for hours