Combi Boiler bother

[mikep]

Can anyone help to solve this problem.

I have a Vokera Maxin 28e combi boiler. This serves a 5 bedroom house in the following way (as I understand it). The combi boiler serves an upstairs en-suite (shower, basin) and a downstairs utility room (washing machine, sink) directly. We are able to store domestic hot water via an indirect hot water cylinder and this is heated by the combi. This serves a main bathroom, (shower, bath and basin) and a downstairs cloakroom.

The central heating is again via the combi and serves 14 radiators of varying sizes. The problem is that the combi continues to trip out interemittently and in a pattern that is very difficult to fathom.

Having looked at these forums, I noticed that the safety valve overflow was leaking and have replaced that. Additionally, the flow switch has been replaced. I have checked the expansion vessel as told by these forums, this reads OK (PSI = 13 when boiler is empty) and have descaled the heat exchangers. I have had a look at the circuit board and can't see any obvious cracks, though this is not to say that there isn't any.

There does not appear to be any leaks and the system now does not lose pressure (as it did before replacing the safety valve). What I do notice is that the pressure can rise to just below 3 bar, then the boiler ceases to fire and cools. It may refire once or twice on cooling to about 2.3 bar, but then gives up the ghost and trips out.

Is the noiler big enough for the house. Does this system seem feasible. Is this a simple problem or is it something more serious

 

[oilman]

If your pressure goes from 1 bar to 3 bar, your expansion vessel is too small.

Unless you have a 3 storey house with a radiator in the loft, 1 bar air pressure in the expansion vessel is unecessarily high. It only has to be 0.5 bar for a 2 storey house and 0.7 bar for 3 storeys. The cold water pressure is then set to 0.2 bar above the air pressure, and should not rise by more than 1 bar (if that) when hot.

 

[mikep]

Does this mean set the air pressure to 0.5 bar, then fill the boiler until the pressure gauge reads 0.7 bar? Have tried with similar pressures and the boiler tripped out, hence increased the pressure to 1.2 bar.

Would this mean replacing the expansion vessel or simply attaching a bigger one?

 

[oilman]

If the pressure change is that big, you can just attach an additional expansion vessel to the system. You cannot have to much expansion space.

 

[DP]

Pressure vessel is correctly sized. Let the no water pressure in the vessel be 0.7 bar and then top up the boiler to 1.25 to 1.5 bar pressure.

When you say the boiler trips out, do you mean the red light comes on?

 

[mikep]

Yes, the red light comes on. This will cause the boiler to fire usually without problems but can come straight back on and I have to let the boiler cool for a while (about 15 mins)