Worcester 28i RSF Hot water temperature fluctuation problem

[Tbone]

Hi all,

Hope you can help before I start ordering expensive parts.

Just recently the hot water temperature fluctuates every 30-60s or so.
This happens at every tap/shower.
I checked the boiler and as hot water demand comes, the boiler is firing up., Then eventually the boiler switches off then re-starts etc in this cycle.
I checked the flow turbine and it seems ok.
I checked the domestic hot water sensor. Before water flow, this sits around 4.0vDC. As the hot water is supplied it drops steadily to 3.2vDC, and this is when the boiler switches off (ie flame goes out).then as the temp cools, the voltage rises to 3.8vDC, then the flame re-ignites. It then continues on this cycle.
Does anyone know if these voltages are normal for this sensor ?

I replaced the control PCB about 2 years ago, so this is newer than the rest of the boiler.

Thanks for your help.

Tbone.

 

[Homesafe]

There is scale or debris in you dh exchanger

 

[Tbone]

There is scale or debris in you dh exchanger

What can i do about that ?
Clean or replace ?
Also, I dont live in an area known for limescale.

 

[keego]

Hi all,
I checked the domestic hot water sensor. Before water flow, this sits around 4.0vDC. As the hot water is supplied it drops steadily to 3.2vDC, and this is when the boiler switches off (ie flame goes out).then as the temp cools, the voltage rises to 3.8vDC, then the flame re-ignites. It then continues on this cycle.
Does anyone know if these voltages are normal for this sensor ? Tbone.

To check the NTC properly you will need to record what voltages you get over a range of temperatures, Worcester tech support will give you the information tomorrow.
The other way is to spend £10 on a new NTC & see if it does the trick.

 

[Tbone]

OK, part ordered.

 

[Tbone]

Sensor fitted and seems to have made very little difference. Boiler switches on when water is flowing then off at 3.4volts then on at 4volts etc etc.

Can anyone help me ?

 

[Agile]

Thats because the DHW side of the HE is blocked!

Measuring the voltage is useful and easy but the resistance is what they are specified as.

The voltage depends on the PCB design.

Tony

 

[Tbone]

Cheers Tony,

So what can i do about it ?

 

[Tbone]

Surely if it was blocked this would mean restricted flow ?
Flow is good as far as I can tell.

Also, if it was blocked, wouldnt this just be the same condition as lower flow demand of hot water ?

 

[Agile]

I expect you are over thinking!

Try thinking of heat transfer!

Tony

 

[Tbone]

Maybe you reckon I am overthinking, I am actually just trying to understand.

One day, DHW is fine, absolutely no problems.
Next day, while having a shower, the water temp starts fluctuating every 30 secs, found out to be coinciding with the boiler firing/extinguishing.
We live in an area with 0% limescale. I checked the gauze filter on the incoming water pipe, it was as clean as the day it was new.
There is no restricted flow. I increased the flow from the isolation valve and this made no difference.
I also replaced the temp thermistor and checked out the flow switch.

Surely if it was a biuld up of particulate in the heat exchanger then this would manifest itself over time ?

Also, what is the mechanism for the boiler to switch off if the heat exchanger did indeed have a build up - DHW thermistor ? With the increased flow, this is not getting up to a high temperature.
There is alos a sensor on the heat exchanger but this works perfectly when the CH is on.

Please dont feel that I am dismissing suggestions, I just want to try to understand.
I realise that people have experience of this problem in a limescale area, and I dont want to just blindly go down that road without undertsnding why and analysing all reasons.
I am tempted to buy a new control PCB.


Thanks again for all your help, it is really appreciated.

 

[Tbone]

Right. Update with this.

I decided to check the over temp sensor. which would be the thing tripping if the Heat exchanger was going over temp.
I removed it and put it on a hot plate and started to increase the temp. Eventually it went open circuit and the re-set knob jumped out. All ok.

I then fitted this again but still the same problem. So I then wanted to check while it was running. So I pulled it out and closed the cover over the wires, only using 2 screws, so cover wasnt 100% closed.
Boiler worked perfectly, no over temp.
SO I re-fitted the sensor properly and closed the cover properly. Tested - Problem returned.

Now I started to think - whats causing this now. I then backed off 1 screw on the cover slightly, so that the seal was not 100%. Now the boiler worked perfectly, if I tightened the cover screw, the problem returned.

So do I have a faulty air pressure sensor ?
Any way to check ?

 

[Agile]

We dont give advice on gas/combustion issues.

Nor do we recommend that as DIYer opens the combustion chamber!

The only advice is to get a gas reg professional.

Tony

 

[Tbone]

OK, Understood.

Actually, the air pressure sensor is not inside the chamber.

Thanks for your help any.

 

[jgphGAS]

you will prob find you have water in your pressure switch happens a lot on these boilers , its the auto air vent that leaks and runs into the switch.