Pressure Jumping

[Big Lad]

I have a Bosch Worcestor 28si Combi. I have recently replaced the heat exchanger and everything works fine except when I run the hot tap, after a minute or so the pressure gauge on the boiler starts jumping up and down approx .1 bar and the is a slight surging sound from the boiler, Is this alright or is something else wrong?

 

[doitall]

To change the primary heat exchanger you need to remove the combustion chamber.

Not an enthusiasts job.

Have you refitted the sensors.

 

[kevplumb]

is that the bit you burn the wood in

 

[Big Lad]

Yes I have replaced all the sensors, I also removed the cover off the combustion chamber to gain access to the heat exchanger, replaced heat exchanger then re-fitted combustion chamber cover. the boiler is operating correctly i.e hot water ok, central heating fine, no noises etc it is just that the gauge keeps pulsing/jumping.

 

[doitall]

is that the bit you burn the wood in

Just you wait till april fools day.

This will be an everyday occurance

 

[Big Lad]

Yes

 

[doitall]

Yes I have replaced all the sensors, I also removed the cover off the combustion chamber to gain access to the heat exchanger, replaced heat exchanger then re-fitted combustion chamber cover. the boiler is operating correctly i.e hot water ok, central heating fine, no noises etc it is just that the gauge keeps pulsing/jumping.

Maybe so but have you tested the integrity of the combustion chamber

assuming it only plays up in hot water mode you need to check the mains supply filters etc.

 

[kevplumb]

smells of ev

 

[Big Lad]

The mains filter has been checked recently during the service.
Also I dont understand how the mains filter would affect the pressure gauge as this is detecting the pressure of the sealed side of the system. It only happens when the hot water is running, its as if the burner that is heating the hot water flowing through the heat exchanger is also heating the central heating side of the exchanger (which I imagine it must do) this is somehow causing the gauge to surge slightly as if the central heating water is boiling/ expanding or whatever, or have I got this totally wrong?

 

[doitall]

The only link between the domestic hot and the heating is the domestic hot water flow switch and the pcb.

Mains water goes into boiler through the flow switch which operates a micro switch that controls the pump etc

Therefore the only way the gauge can surge is via the pump shunting on/off

 

[kevplumb]

Therefore the only way the gauge can surge is via the pump shunting on/off

you dont have a shunt pump on a combi

 

[doitall]

Therefore the only way the gauge can surge is via the pump shunting on/off

you dont have a shunt pump on a combi

Why not

flow switch controls the pump

 

[Big Lad]

I have just checked the pump and it is not shunting on and off, I have checked for voltage at the pump when the surgeing occurs and have also inspected the shaft of the pump and there is no movement.
I can hear a slight surging noise from inside the boiler which ties up with the gauge. Is this anything I should be worried about? or shall I just live with it as it doesnt appear to affect the operation of the boiler

 

[kevplumb]

you are having a laff john

 

[doitall]

I have just checked the pump and it is not shunting on and off, I have checked for voltage at the pump when the surgeing occurs and have also inspected the shaft of the pump and there is no movement.
I can hear a slight surging noise from inside the boiler which ties up with the gauge. Is this anything I should be worried about? or shall I just live with it as it doesnt appear to affect the operation of the boiler

whats the flow switch doing when it surges.

Could be worth a phone call to the makers