Glowworm 30SXI burner modulating before reaching temp

[philce]

We have a 30SXi that a local engineer cant seem to get to the bottom of a strange issue we have, its not a problem as such, but is not working as it used to!

When the boiler fires up from cold (on CH or HW) the burner goes to full power and the temp on the panel starts to rise as expected, however within a couple of minutes the temp reaches approx 50c and then the burner and fan start slowing and speeding up again approx every 30-60 secs until the water temp reaches the setpoint (70c) and then behaves as expected by modulating down to maintain. As the burner fan is running at full speed its quite noticeable that its dong this for about 15-20 mins while the water gets to temp.

The flow/return pipes are hot/cold as expected, there is no flow issue, mag filter checked and cleaned.

The PCB was swapped by Glowworm a few years ago due to a fault that blew the board. The engineer tried a different PCB but that did exactly the same thing, the flow/return thermisitors have also been swapped. Its just as baffling for the engineer who also wants to solve it!

Now I don't want to spend loads of money on a 15 year old boiler, but this is a weird problem that is annoying and I'm worried it will develop into something more serious?

Any Ideas/pointers?

Cheers

 

[ScottishGasMan]

How many radiators is it heating?

 

[polesapart]

Has your engineer checked that the return filter (internal to this boiler, not the external magnetic filter) is clear?

 

[philce]

The small black filter under the pump has been checked (was clear due to magnetic filter, that also was cleaned).

We have 10 rads connected, the system has evolved due to extensions being built, so not optimum design, but all rads get hot at roughly the same time so I dont think its a flow issue.

As I said before, the flow pipe from the boiler isn't getting rapidly hot, and the temp display on the panel is showing sensible numbers (as in i mean the flow pipe is at the temp its showing).

A powerflush was mentioned, but wouldn't the flow pipe get rapidly hot if the flow was restricted?

 

[ScottishGasMan]

Yes the flow pipe would get very hot very quick if circulation was poor.

Next time your gas fitter is round tell him to range rate the boiler. 30kw is probably too much for what your system is able to flow, setting it at 15 to 20 roughly will probably make it a more steady heat up

 

[philce]

Thanks, I'll get a message to him, hopefully he can do that to see if it sorts it.

Cheers

Yes the flow pipe would get very hot very quick if circulation was poor.

Next time your gas fitter is round tell him to range rate the boiler. 30kw is probably too much for what your system is able to flow, setting it at 15 to 20 roughly will probably make it a more steady heat up

 

[ianmcd]

Thanks, I'll get a message to him, hopefully he can do that to see if it sorts it.

Cheers

When he comes in tonight from the pub