Shower hot then cold then hot again

Anne8584

Recently after turning on the shower it gets to the set temperature fairly quickly then suddenly runs cold then hot again. It will only do that once then its fine. Is this an issue with my combi boiler? This was fitted 5 years ago. Might a power flush fix it?  

Jeepers_Creepers

A combi boiler's hot supply cycling from hot to cold to hot to cold repeatedly is a classic symptom of a partially blocked plate-to-plate exchanger. I wonder if this cycling would continue if you stayed in the shower for longer rather than appearing to only do it once - it usually would.

If it is this, then a power-flush won't fix it, but it probably will remove the root of the cause - which is sludge in your pipes and rads which is slowly building up inside your P2P and blocking it off. Once blocked, you either need to clean it out (I've done this seemingly successfully using hydrochloric acid (used for cleaning up concrete!) but that's a 'going there' type fix!). Usually you'd almost certainly also need your P2P exchanger replaced.

That's IF this is the cause.

Does this also happen with running the hot tap for filling your sink?

When it runs cold, does any error code appear on the boiler's display?

coffeehound

Wonder if it might be that the "ECO" setting has switched off following the boiler being switched off?  Does this coincide with a power cut or the boiler being switched off, Anne8584?

Anne8584

I don't notice the issue with hot water at the sink.
Eco setting is on, boiler hasn't been switched off.
Its a Worcester Greenstar Si Compact, installed Feb 2017, so 4 years ago not 5 as I said above. It should therefore be under warrantee, but when I checked the website the certificate says my warrantee expired Feb 2021, surely that isn't right? as this came with 5 years cover and the installer registered it. 

Jeepers_Creepers

Anne8584 said:

I don't notice the issue with hot water at the sink.
Eco setting is on, boiler hasn't been switched off.
Its a Worcester Greenstar Si Compact, installed Feb 2017, so 4 years ago not 5 as I said above. It should therefore be under warrantee, but when I checked the website the certificate says my warrantee expired Feb 2021, surely that isn't right? as this came with 5 years cover and the installer registered it. 

IF it's due to 'sludge', then the warranty is moot anyway as this is not a boiler fault, but one of the system not being prepared properly. As part of enabling that warranty, the installer should have carried out some form of system clean - a 'powerflush' or similar - and then installed a magnetic filter to catch any subsequent nasties. I think you'll find most warranties don't cover any 'sludge'-derived issues...

Something to check - when someone else is having a shower (or just run the shower with no-one in it) keep an eye on what the boiler is doing. If the flame light goes out shortly before the water runs cold, and then comes back on afterwards a few seconds before the hot is restored, then chances are it's a P2P issue. You will most likely find that - if you leave the tap running for long enough - it'll happen again. And again.

Anne8584

The installer did a power flush, they used Suregraft as a cleaner and inhibitor. I saw the power flush being done. The boiler was last serviced a year ago, no problems detected then, a service is now due again. 

Jeepers_Creepers

I can't really say any more from this side of the screen.

Chances are it's a partially-blocked P2P exchanger, but it could be other things too (like a partially stuck divertor valve for example). I guess it could even be as simple (and cheap) as a faulty temp sensor. I guess it could even be a faulty thermostat in your shower, but it doesn't follow that pattern.

(Haven't heard of Suregraft in this context).

I guess a good next move would be to contact WB themselves and ask their opinion. This will probably lead to taking out a fixed-price repair from them - around £300.

If it turns out to be an inherent boiler fault - a failed part -  then you should have good grounds on which to challenge them on the cost under the CRA 2015. It would clearly not be of 'satisfactory quality' for a 4-5 year old boiler.

neilmcl

Sounds to me it could simply be the case of there being a bit of hot water in the system initially, possibly because the CH is/was on, then when that's used up it'll go colder until the boiler itself brings the hot water back up to the require temperature. I've always found the best thing to do when running a mixer shower in a combi boiler household is to first run the hot tap in the sink first until it's gets up to max temperature then turn on the shower.

Anne8584

I ran the shower for a while, part filled the bath, then ran the hot tap for a bit, while observing boiler readings. The light was on throughout. When the shower went on the temp rose to just above 60 degrees then after a minute or so dropped quickly to 54.5 and stayed there. I then turned of the shower and turned the tap on. When the hot tap was on the temp went up to 65 degrees then down to 62.5 up to 63 then down to 62.5 and stayed there. 

neilmcl

Jeepers_Creepers said:

A combi boiler's hot supply cycling from hot to cold to hot to cold repeatedly is a classic symptom of a partially blocked plate-to-plate exchanger. I wonder if this cycling would continue if you stayed in the shower for longer rather than appearing to only do it once - it usually would.

If it is this, then a power-flush won't fix it, but it probably will remove the root of the cause - which is sludge in your pipes and rads which is slowly building up inside your P2P and blocking it off. Once blocked, you either need to clean it out (I've done this seemingly successfully using hydrochloric acid (used for cleaning up concrete!) but that's a 'going there' type fix!). Usually you'd almost certainly also need your P2P exchanger replaced.

That's IF this is the cause.

Does this also happen with running the hot tap for filling your sink?

When it runs cold, does any error code appear on the boiler's display?

It's not doing it repeatedly though, according to the OP it only does it the once then heats up normally. As I said, it's probably nothing more than there being some residual hot water in the pipes initially.

Anne8584

The CH is still turned on, although controlled a thermostat so not always on now. It isn't on today.