E9 overheat error on zone valve switch

[Joe50]

Hi All
I have a Worcester Bosch FS 30CDI with a traditional header tank and hot water cylinder. The heating circuit is divided into 3 zones: downstairs, upstairs and the hot waterr cylinder coil all controlled by Honeywell two port valves.

When the boiler is set to Max about 86deg and is heating the downstairs zone switching in a cold upstairs circuit drops the boiler temperature to 74deg and this is followed by an E9 error. This same error does not occur when the hot water cylinder coil is switched in. The fault also occurs if the upstairs is switched in when the system is heating the downstairs zone plus hot water.

Reducing the boiler temperature down to 75deg and the fault does not reoccur.

Any ideas appreciated?

 

[Chuck]

When the boiler is set to Max about 86deg and is heating the downstairs zone switching in a cold upstairs circuit drops the boiler temperature to 74deg and this is followed by an E9 error.

I suspect that the circulation pump is at too high a setting. Not uncommon as these boilers are delivered with it set to maximum but the correct setting is as low as possible as determined during commissioning. Will need to be reset by a good heating engineer, possibly with advice/guidance from Worcester Bosch Tech Support.

 

[Joe50]

Thanks, Chuck your suggestion seems to a done the trick no more E9 errors on zone valve switch. I reduced the pump speed from 3 to 1 and balanced all the radiators on the system, all seems fine now. I am curious, why does too high a pump speed cause E9 errors on these boilers?

 

[Chuck]

I am curious, why does too high a pump speed cause E9 errors on these boilers?

A detailed answer is quite complicated but a simplified version is:

In steady-state, the boiler heats the water to the set-point temperature by supplying energy at a rate that exactly balances the power being emitted by the radiators. If the power emitted by the radiators increases, the returning water drops in temperature, this is detected by the boiler, which increases the heating rating to return the water to the set-point temperature.

This mechanism, known as 'negative feedback' is straightforward as long as changes happen slowly compared with the time the boiler needs to respond to the change. In some circumstances when the changes happen too rapidly the system the system will over-react and the water temperature will overshoot the set-point before recovering and, in extreme cases, may even oscillate around the set point and never stabilise.

Your problem had two possible solutions. Either (a) modify how the boiler responds to changes, or (b) slow down the rate at which changes propagate in the external (to the boiler) system.

Solution (a) is best avoided in domestic contexts due to the complexity of getting it right. Solution (b) is preferable where it is feasible because, as a side-effect of the lower circulation rate, your heating will be quieter and less prone to corrosion.

 

[Joe50]

Thanks, Chuck I appreciate your explanation, I'm an old retired automation controls engineer and appreciate the physics. I used to use a lot of process control PID controllers in PLC and SCADA systems in the past so I should have twigged.

 

[Knappers]

Why do you have your boiler set to max, unless your rads are undersized around 65 is what I'd aim for?

 

[Chuck]

Thanks, Chuck I appreciate your explanation, I'm an old retired automation controls engineer and appreciate the physics. I used to use a lot of process control PID controllers in PLC and SCADA systems in the past so I should have twigged.

You'd probably think of the boiler as a PI controller and the process would be an integrator (the water heat capacity) and a cyclic time-delay (the time for a dose of hot water to travel around the radiator circuit). Easy enough to control with respect to setpoint changes with a process sensor at the boiler so the time-delay is kept outside the loop. But such a system is not (typically) stable with respect to rapid load disturbances like your zone valve opening, which is why you were seeing overshoot.

Anyway, I hope you are enjoying a warm and temperature-stable retirement.

 

[Knappers]

Not sure what your after there so I've just given you a special badge for trying hard

 

[Joe50]

Why do you have your boiler set to max, unless your rads are undersized around 65 is what I'd aim for?

Well, I would aim for it too, but I deliberately undersized the boiler in relation to the house size thinking it would have lower overall running costs with more condensing running before modulation taking place during the heating cycles. I also factored in that the house has benefited since it was built in 1972 with extra roof and cavity wall insulation plus the addition of modern double glazing all of which must have reduced the heat load. However, when my wife says she is cold on the occasional very cold winter day I please her and turn the boiler temp up form 70deg to Max.