Shower alternating hot & cold

[billm]

I've recently noticed the shower is alternating hot & cold about every 45 seconds or so. I've never noticed it before so think there may be a problem somewhere. Its fed via 2x 15mm pipes direct from the Combi boiler which was fitted about 8 years ago.

To test if it was the boiler causing this problem I ran the hot water tap in the kitchen sink which is next to the boiler. Its piped in 15mm from the boiler. I ran it full blast with hot water coming out then after about 45 seconds the boiler shut down for about 5-10 seconds and the water went luke warm then it fired up again and I got hot water again and the cycle repeats.

The boiler is an Ariston microGenus 27 MFFI and changing the water temperature setting on the boiler doesn't seem to have any effect.

Is this a problem with a boiler thermostat ?

 

[Fluffster]

It could be various issues. Most likely your heat exchanger has clogged up or your thermistors are defective.

Pete

 

[billm]

It hasn't had a service for years, could that clear it ?

According to the manual there is a Main Heat Exchanger and a Secondary Heat Exchanger.

And ideas which one is more likely to be clogged up ?

 

[Fluffster]

The secondary exchanger.

Pete

 

[billm]

thanks, will look to swap it out.

 

[billm]

So I removed the Secondary Heat Exchanger and it didn't look dirty at all, but I cleaned it out and put it back on and the problem is still the same.

I've been able to narrow the symptoms down a bit more:

The CH circuit is fine. But when drawing just DHW and running the hot tap full on in the bathroom even with the temperature knob on the boiler set to max (5) I can still hold my hand under the tap. The only way I can get the water really hot is to close the bathroom tap 1/2 way ie; reduce the flow.

I left the tap running full on and checked the boiler and it's only showing 60deg even after running hot water for 5 mins. I presume drawing DHW should make it go to 70 or even 80deg on the boiler if required ?

I'm now thinking it could be the DWH Temperature Sensor / Thermistor ? I could test the resistance of it in-situ when the boiler is running and drawing DHW. Anyone know what resistance it should read ? Or I could just swap it out and at less than £10 it's worth a go ?

If the sensor is OK then I guess it could be a faulty main PCB

The boiler is an Ariston microgenus 27MFFI

 

[onlyfitidealboilers]

Have you checked the shower head for scale?

 

[billm]

I might replace the shower unit just in case as it is quite old.

I think this is a generic DHW problem though now. My son spoke to a heating engineer about it and his reply was:

"It's probably just the water temp coming into his house has dropped so the boiler can't keep up. If it's a 27kw he would be looking at something like a 35deg rise at 10.5ltrs a min? "

Can someone translate what that means for me. I have just checked the hot water flow in the shower with the shower head removed, turned full on it's about 11 Ltrs/min and the bath is running about 12 Ltrs/min.

Does that mean the boiler is working fine but can't keep up with the flow so I have to slow the flow down to get hot water ? or is there a fault ?