DIY Doctor UK DIY Forums

Hi,
I have a biasi 24s combi boiler. The central heating works fine, but at present the DHW temp is too low even though DHW stat is turned to full !, i have confirmed the DHW flow switch is fine as the boiler continually runs once a tap has flow. The boiler when supplying DHW never goes off on the stat as it never reaches this point. I can only think it could be the stat or the hot water probe, most probably the probe. I will be doing some resistance checks across it tomo. Can anyone shed some more light, i understand could be probe, stat, Control board PCB or maybe even the DHW heat exchanger reducing the efficiency due to limescale etc , do these heat exchangers significantly reduce efficiency, the boiler is only 5 years old ?

Thanks in advance for any advice.

Leo

Check CH flow, with the system cold, run the hot tap.

If there's heat going down the CH pipes then it's the diverter at fault.

Could also be gas pressures/modulation


htg

ok, will check diverter v/v tomo. Thank you. Gas modulation seems fine on c/h. This diverter v/v if it is passing into c/h flow will i notice the temp rise quite fast or will i need to leave a hot tap flowing for a good ten mins. Once again thanks in advance.

[quote="htg engineer"]Check CH flow, with the system cold, run the hot tap.

If there's heat going down the CH pipes then it's the diverter at fault.

Could also be gas pressures/modulation


htg[/quote]

ok thanks, will check diverter v/v. If this v/v is passing will i need to run a hot tap for a good ten mins before i notice heat going into the c/h flow. The modulation of gas seems fine on the c/h side of things. Thanks in advance again.

Leave it for 10 mins or so, even 30 mins the CH pipes should not get hot. They will once you close the tap but this is just the pump getting rid of excess heat.

If the burner pressures are ok and the modulation is ok, then it has to be the diverter - where else can the heat be going ????

You need a manometer to check gas pressures - not just guess work. It could even be the governor on the gas meter at fault.


Htg

hi, checked the diverter v/v and it is fine, checked the temp probes and the resistance readings are fine as per biasi temp/resistance graphs, cleaned dhw filter all ok. The stat potentiometers appear fine when tested too ??? The DHW gets upto nearly temperature and then the burner ramps down, if i increase the tap flow the boiler ramps up to try and raise the temp, so everything appears normal apart from still not getting to set point temp. Is all i can come up with now is the PCB. Its like eveything works as should but the stat isnt setting as hi as it should/used to.

Cheers.