Worcester 280 - Intermittent hot water

[Knightrider]

Please can anyone help? The boiler has recently had the heat exchanger changed due to scale. Pretty much since then if the boiler has been running (ie Heat) & HW is demanded only warm water comes from tap. The burners appear to be fully on. Also had burners switch off when HW demand if Heat is on & hence no HW.

Any suggestions gratefully received.

 

[COG]

who fitted the heat exchanger and why cant he fix it ?. when you make a hot water demand when the boiler is fired up in heating turn off the tap and the flames should go out for at least 12 seconds till the diverter valve moves back over,lift the cover off and look in the spy hole.

 

[Knightrider]

who fitted the heat exchanger and why cant he fix it ?.

Bl**dy good question, but if he wont meet appointments!? anyway, yes the boiler does 'turn off' for 12 secs or so, but when it relights the DHW (the majority of the time) only comes through warm. If the tap is shut down to just over a trickle the water heats up. Where do i find the divertor valve? and how can i tell if the valve moves fully to DHW? The only point of reference i have is the DHW Out from the H/Ex.

Thanks for your help in anticipation.

 

[DP]

Locate two 22mm pipes leading to boiler. Feel these pipes for heat. When these pipes are cold, run a hot water tap. Do any of these pipes get hot?

Why was the heat exchanger changed? What were the symptoms of failure?

 

[COG]

your diverter valve is located behind the control panel the right,it has an indicator on the side marked "a" and"b" the small lever should move fully between the two.Did you check the flow rate at your taps it should be 10m lt per min full on,your fitter could have left the restrictor out.

 

[Knightrider]

Locate two 22mm pipes leading to boiler. Feel these pipes for heat. When these pipes are cold, run a hot water tap. Do any of these pipes get hot?

Why was the heat exchanger changed? What were the symptoms of failure?

Thanks DP, when the heating is off the 2 pipes you mention (C/Heating) do not get hot. I have since found that when the heating is off the DHW heats fine, it seems to be when the DHW is called for whilst the heating is running - then the water only heats to warm.

The H/Ex was changed due to severe scaling (on primary & secondary sides (the boilers normal running temp was 98 deg!)) & needless to say, the DHW output was next to nothing.

 

[Knightrider]

COG & DP - i may have misled you abit - it's a 350..........sorry

 

[COG]

ARRGGH. got to think again now and i've just had another stella . the 350 does not have a diverter valve it uses two pumps ,so scrap all mentions of div valves passing ,check your flow rate it should be 12 lt per min at max,again if the resticter has been left out the flow rate will be too high and temp too low

 

[Knightrider]

sorry to drive you to drink........i have roughly measured the water output as 8 litres/min.
cheers!!!

 

[COG]

Hic, at 8 lt it should be around 54 deg C at the taps? if not check you have full gas pressure (tricky if your not corgi etc ) make sure both yellow wires to the modureg are conected and the gas iso valve to the lower left is fully on,apart from that i cant think why you would have poor hot water without knowing the gas pressures, if the pump runs even a little slow the boiler will shut down and you say you have a new heat exchanger. Are you sure it is new , it should be the plate type not the old white tube type

 

[Knightrider]

thanks for the reply COG

the 2 yellow wires are connected & the gas valve is fully on, but alas no hot water.

what do the two sensors on the DHW outlet from the H/Ex do? if one of them senses water temp & has been 'scalded' in the past due to the old scaled H/Ex - could they be telling the boiler to run 'warm'.

 

[COG]

The sensor for the hot water could be out of call as you say ,its a simple job to replace it just shut off the mains water and isolate the boiler power turn on a hot tap to let any water out then undo the sensor with a 11mm spanner ,you will find it on the hot eater outlet pipe from the heat exchanger it shold have a green head with a green plug atached.