Cycling/oscillating hot water temperature with constant flow from tap

[d000hg]

My wife mentioned this a while back just after the BG person did a service, but I couldn't reproduce it myself until the other day.

She said that the shower temperature was repeatedly going from hot to cold - so cold she has to jump out. I ran the hot tap yesterday and it did the same thing, it would run at full temperature (so hot you can barely stand to keep your hand in) for 20-30s then cool down to just lukewarm over about 10s, stay there for ~20s then get back to hot. It did this several times with the whole cycle being perhaps a minute.

What might be the cause of this, I didn't want to call out a plumber in case they cannot repeat it and charge me a call-out fee as it doesn't seem to always happen? Could it be some setting the engineer changed or is that just a coincidence it happened right after they were here?

The boiler is a Baxi combi, about 2-3 years old and it passed the service without any reported issues (and the one the year before).

Any help appreciated, obviously it's not a great time of year for cold showers

 

[ianmcd]

so you have a Baxi Combi,unfortunately Baxi make lots of combis which one do you have might help

 

[d000hg]

Baxi Duo-tec combi 40 HE A

 

[Gasguru]

Can you replicate the problem at a hot water tap?

 

[d000hg]

I confirmed it in the bath, my wife did at the kitchen tap too.

 

[GasElectricalRepairs]

Blockage probably in the plate heat exchanger or the pump not running fast enough. Unlikely to be the pump , with it only being 2 to 3 years old.
Sounds like you need someone to look at it.
Check the pump speed if it’s adjsutable. If so set it higher.

 

[d000hg]

Blockage probably in the plate heat exchanger or the pump not running fast enough. Unlikely to be the pump , with it only being 2 to 3 years old.
Sounds like you need someone to look at it.
Check the pump speed if it’s adjsutable. If so set it higher.

It's actually installed 2014 when I checked but still kind of new. Is there any chance the BG person could have adjusted something or would you chalk the timing to coincidence?

Thanks for your help

 

[GasElectricalRepairs]

If there is a speed control on the pump , he may have turned it down. If not you need the pump checking first , blocked filters , blocked plate .

 

[d000hg]

The plumber got us to watch the display as the hot tap was running over the phone, you could see it get to 90 then cut out until it cooled then turn back on, repeatedly.

His diagnosis is that the heat-exchanger is blocked... we have a magna-clean thing but I'm not sure this was ever serviced and we have quite old heating pipes which have been worked on a bit in the last few months, repeated system re-filling.

Sounds quite reasonable to me, just thought someone might have been interested Will report back after he visits.

 

[Thomato]

The plumber got us to watch the display as the hot tap was running over the phone, you could see it get to 90 then cut out until it cooled then turn back on, repeatedly.

His diagnosis is that the heat-exchanger is blocked... we have a magna-clean thing but I'm not sure this was ever serviced and we have quite old heating pipes which have been worked on a bit in the last few months, repeated system re-filling.

Sounds quite reasonable to me, just thought someone might have been interested Will report back after he visits.

What was the result of the visit, please? We have been having exactly the same from a Vokera Unica HE.
We are in a hard water area, so after some tricky searching I suspect the heat exchanger...

 

[ianmcd]

What was the result of the visit, please? We have been having exactly the same from a Vokera Unica HE.
We are in a hard water area, so after some tricky searching I suspect the heat exchanger...

you will have to start your own post, it is against site rules to piggy back existing posts and for very good reason, the Unica and the baxi are two completely different beasts , when you start your own post Put Vokera Unica in the title and that will draw it to the attention of the Vokera lads

 

[d000hg]

I don't mind answering in case anyone finds this in the future. The main heat exchanger was indeed blocked.

 

[Thomato]

you will have to start your own post, it is against site rules to piggy back existing posts and for very good reason, the Unica and the baxi are two completely different beasts , when you start your own post Put Vokera Unica in the title and that will draw it to the attention of the Vokera lads

Thank you, Ian! I've been suspecting heat exchanger on the Vokera and this seems like a good site, so I'll pop it in a new thread. Thanks for the tip.