Combi diagnosis

[Martin Pentreath]

Hi to all,

My combi is on the blink. It's an Ariston Microgenus - the old model, it's = not a condensing boiler.

Symptoms: shower oscillates between hot and cold. In fact so do any of the = taps when opened. Aha! Clearly the secondary heat exchanger is clogged. I'v= e had enough experience of that one in the past, and that's not the problem= . I've cleaned out the secondary heat exchanger - no better. So I replaced = the secondary heat exchanger - no better!

So the question is, what else could cause this problem. It seems not to be = a fault with the (theormostatic) shower, because the same happens when any = tap(s) are opened, I've tried that one. Could it be the pump failing, and n= ot getting heat away quickly enough? Or I guess the sensor thinking the DHW= is hotter than it is, and shutting down the boiler when it doesn't need to= shut down? Or the PCB (which seems to be the standard diagnosis when all e= lse fails).

I could easily run up a bill of a couple of hundred in replacing parts that= don't need replacing, so any guidance would be welcome before I start lini= ng the pockets of the local plumbers' merchant. Good thing it's summer.

Cheers!

Martin

[Lieutenant Scott]

a condensing boiler.

taps when opened. Aha! Clearly the secondary heat exchanger is clogged. I've had enough experience of that one in the past, and that's not the problem. I've cleaned out the secondary heat exchanger - no better. So I replaced the secondary heat exchanger - no better!

fault with the (theormostatic) shower, because the same happens when any tap(s) are opened, I've tried that one. Could it be the pump failing, and not getting heat away quickly enough? Or I guess the sensor thinking the DHW is hotter than it is, and shutting down the boiler when it doesn't need to shut down? Or the PCB (which seems to be the standard diagnosis when all else fails).

don't need replacing, so any guidance would be welcome before I start lining the pockets of the local plumbers' merchant. Good thing it's summer.

On average your hot water is warm. Stop moaning.

[Martin Pentreath]

t's not a condensing boiler.

the taps when opened. Aha! Clearly the secondary heat exchanger is clogged.= I've had enough experience of that one in the past, and that's not the pro= blem. I've cleaned out the secondary heat exchanger - no better. So I repla= ced the secondary heat exchanger - no better!

be a fault with the (theormostatic) shower, because the same happens when = any tap(s) are opened, I've tried that one. Could it be the pump failing, a= nd not getting heat away quickly enough? Or I guess the sensor thinking the= DHW is hotter than it is, and shutting down the boiler when it doesn't nee= d to shut down? Or the PCB (which seems to be the standard diagnosis when a= ll else fails).

that don't need replacing, so any guidance would be welcome before I start = lining the pockets of the local plumbers' merchant. Good thing it's summer.

Haha! On average my palm is halfway between my shoulder and your face, stop= shouting.

[Lieutenant Scott]

not a condensing boiler.

taps when opened. Aha! Clearly the secondary heat exchanger is clogged. I've had enough experience of that one in the past, and that's not the problem. I've cleaned out the secondary heat exchanger - no better. So I replaced the secondary heat exchanger - no better!

a fault with the (theormostatic) shower, because the same happens when any tap(s) are opened, I've tried that one. Could it be the pump failing, and not getting heat away quickly enough? Or I guess the sensor thinking the DHW is hotter than it is, and shutting down the boiler when it doesn't need to shut down? Or the PCB (which seems to be the standard diagnosis when all else fails).

don't need replacing, so any guidance would be welcome before I start lining the pockets of the local plumbers' merchant. Good thing it's summer.

Insufficient information. What percentage is your arm extended? What is your reaction speed?

[Roger Mills]

a condensing boiler.

taps when opened. Aha! Clearly the secondary heat exchanger is clogged. I've had enough experience of that one in the past, and that's not the problem. I've cleaned out the secondary heat exchanger - no better. So I replaced the secondary heat exchanger - no better!

fault with the (theormostatic) shower, because the same happens when any tap(s) are opened, I've tried that one. Could it be the pump failing, and not getting heat away quickly enough? Or I guess the sensor thinking the DHW is hotter than it is, and shutting down the boiler when it doesn't need to shut down? Or the PCB (which seems to be the standard diagnosis when all else fails).

don't need replacing, so any guidance would be welcome before I start lining the pockets of the local plumbers' merchant. Good thing it's summer.

When you open a tap and get alternate hot and cold water, is the flow rate constant - or is it slow when hot and fast when cold? Is the boiler firing the whole time this is happening?

I don't claim to be an expert on combis - but answering these questions may help someone who is to home in on flow sensors, diverter valves, or whatever.

[A.Lee]

Blocked main heat exchanger, slow running pump, faulty temperature sensor on hot pipe/heat exchanger.

If the water from your radiators is pretty black, then the main heat exchanger is probably blocked with this gunk. Pump is a possibility, but for £80+, it is a bit expensive to change it to find it is not that. Temp. sensor - maybe not, if the water is set to a low temperature, at the boiler, does it still cut out after a few minutes whe4n running the hot water? If so, main heat exchanger is the most probable cause. Not worth changing it, it is new boiler time.

Alan.

[John Rumm]

While I appreciate its not the best time of the year for this, can it run the rads ok?

[Brian Gaff]

Hysteresis.

Obviously things are taking too long for the control loop and this manifests itself as oscillation. Where that item is is going to be hard to judge.

Brian

[newshound]

+1, with blocked HX top of my list too