Combi shower going Hot-Cold-Hot problem

[buildersteve]

Hi to all.
I have a Baxi 105e instant combi boiler that has served us well since 2004. In that time it has of course had interval services and has been pretty much bullet proof. There is also a proper water softener in the house that feeds everything, except of course, the cold feed over the kitchen sink. The plate heat exchanger was replaced about three years ago.
Then a couple of weeks ago the shower was running cold all the time. The gauge on the front was in its usual place at 1.5 and the hot water was running fine from the taps.
I called Brit Gas for a fix and the guy changed the DHW diaphragm and the temp sensor and said the boiler was in good working order.
The following morning the shower ran hot at first but then ran cold for about ten seconds, then went back to hot. So, I had a recall from Brit Gas who again said the boiler was fine and that I should look at buying a new bar mixer as he thought the problem was with that.

I've now changed the bar mixer for one of these https://www.amazon.co.uk/gp/product/B07XNWDFV7/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
and I thought that there were too many 90 degree bends in the feed to it so I've re routed the pipework taking out most of the 90s. This has made a good improvement to the problem. The shower does go cold but only for a couple of seconds.

I've run the shower whilst looking at what the boiler lights display and the shower goes cold more or less when the boiler gets to its top temp and the burner cuts. Then, it re lights and the hot water returns. The other hot taps in the house run fine and don't go cold at all.
Is this a common problem? Why would it run well for all those years, then start playing up? The engineers both said the boiler was ok, But is it?
Sorry for a long post but I'm at a loss now on exactly what to do next.
Thanks. Steve

 

[Theweeknd]

Hi to all.
I have a Baxi 105e instant combi boiler that has served us well since 2004. In that time it has of course had interval services and has been pretty much bullet proof. There is also a proper water softener in the house that feeds everything, except of course, the cold feed over the kitchen sink. The plate heat exchanger was replaced about three years ago.
Then a couple of weeks ago the shower was running cold all the time. The gauge on the front was in its usual place at 1.5 and the hot water was running fine from the taps.
I called Brit Gas for a fix and the guy changed the DHW diaphragm and the temp sensor and said the boiler was in good working order.
The following morning the shower ran hot at first but then ran cold for about ten seconds, then went back to hot. So, I had a recall from Brit Gas who again said the boiler was fine and that I should look at buying a new bar mixer as he thought the problem was with that.

I've now changed the bar mixer for one of these https://www.amazon.co.uk/gp/product/B07XNWDFV7/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
and I thought that there were too many 90 degree bends in the feed to it so I've re routed the pipework taking out most of the 90s. This has made a good improvement to the problem. The shower does go cold but only for a couple of seconds.

I've run the shower whilst looking at what the boiler lights display and the shower goes cold more or less when the boiler gets to its top temp and the burner cuts. Then, it re lights and the hot water returns. The other hot taps in the house run fine and don't go cold at all.
Is this a common problem? Why would it run well for all those years, then start playing up? The engineers both said the boiler was ok, But is it?
Sorry for a long post but I'm at a loss now on exactly what to do next.
Thanks. Steve

It sounds like you have a block plate heat exchanger.

Why was it replaced 3 years ago?

 

[buildersteve]

Hi. Thanks for the reply.
The water is hard here in Kent and it was replaced once before but the guy who replaced it used an exchanger that had less plates than the boiler was originally fitted. I got so fed up with all the problems with the heating and the washing machine that I had the softener and a new exchanger fitted about three years ago. Its worth noting that in all this time the shower has performed well and has never cut to cold during a shower.
With what you are saying, it may be that something may have got into the exchanger causing a slight drop in water flow entering the heat exchanger. I bet there's no way to back-flush it while its in place? When I re routed the pipework to the shower I noticed that there was no buildup inside the pipes, so the softener is working.

 

[Theweeknd]

Hi. Thanks for the reply.
The water is hard here in Kent and it was replaced once before but the guy who replaced it used an exchanger that had less plates than the boiler was originally fitted. I got so fed up with all the problems with the heating and the washing machine that I had the softener and a new exchanger fitted about three years ago. Its worth noting that in all this time the shower has performed well and has never cut to cold during a shower.
With what you are saying, it may be that something may have got into the exchanger causing a slight drop in water flow entering the heat exchanger. I bet there's no way to back-flush it while its in place? When I re routed the pipework to the shower I noticed that there was no buildup inside the pipes, so the softener is working.

the water ways are so small on a plate to plate heat exchanger that the water transfer when blocked isn't fast enough, this then causes the hot water temperature to rise quite drastically causing the burner to go off, then once it cools back down the burner lights up again.

Well this is what I assume is causing it as it goes hot and cold and that you have mentioned the burner does go off.

You can flush them ( mostly by taking them out) but then its better of replacing it while you're at it.

 

[buildersteve]

Thanks. I agree. If its to come out, then a new one would probably be a better decision as they aren't exactly dear to buy.
Looking at the installation manual its probably possible to back flush it but a bit of a job in itself to drain, disconnect the piping and arrange a drain away.
I see on ebay that the exchangers are about £35 for a 22 plate. Are these ok or do I need a genuine Baxi part? Thanks.

 

[ianmcd]

with the heating off turn on a HW tap and hold the heating flow and return pipes below the boiler and see if either is getting hot

 

[buildersteve]

Thanks Ian. Yes, I had the conversation with the Brit Gas engineer when they came the second time. He looked at that and said all was well so he didn't change any parts.
I've run the other hot taps and none of them go cold. The boiler is in a cupboard in the bathroom so I can see what's going on. When the hot tap on the bath is turned on the boiler gets to its top temp and then the burner cuts, but the cold break doesn't happen.

 

[ianmcd]

Thanks Ian. Yes, I had the conversation with the Brit Gas engineer when they came the second time. He looked at that and said all was well so he didn't change any parts.
I've run the other hot taps and none of them go cold. The boiler is in a cupboard in the bathroom so I can see what's going on. When the hot tap on the bath is turned on the boiler gets to its top temp and then the burner cuts, but the cold break doesn't happen.

OK next thing to do is run the problem tap and another Hot tap at the same time and see if the boiler stays lit constantly

 

[buildersteve]

Just done that. Water pressure on an outside tap is just over 4 bar so opening a second hot tap is not good for the shower.
With the shower running and a basin hot tap running the burner takes a lot longer to cut but when it does, the water remains hot, even on the cut duration.
Another thing that neither engineer checked was the filter on the cold feed tap on the boiler. I didn't know it had one until I read it in the manual but I might have a look at that tomorrow.

 

[ianmcd]

Does sound like you have some sort of circulation, problem, try turning the DHW temp selector down to about 3/4 and see if the boiler modulates rather than cycles , when the shower is running

 

[buildersteve]

Turned the DHW pre heat down to 3/4. It took a bit longer to for the burner to cut and the shower went cold for about four seconds, so when the temp went down to 50 degrees (three lights) without turning it off, I lowered the temp to 1/2 the scale and increased the temp a bit on the bar. The temp went up to the 70 degrees (5 lights) and just stayed there constant. Didn't go up or down. I kept it running for at least two shower durations and it didn't move. I think the temp of the shower would have been ok for a shower.

 

[ianmcd]

you are half way there , just find a balance between the boiler potentiometer and the shower bar, they are conflicting with each other and the boiler cant get the eat out quick enough , so is switching off rather than modulating down

 

[buildersteve]

Many thanks for that Ian. If what you say about the boiler is true then perhaps a plate heat exchanger swap will probably be the next big thing. I suppose the problem will only get worse with time but I'll look into what the softener is doing in case its loosing its usefulness.

 

[ianmcd]

Many thanks for that Ian. If what you say about the boiler is true then perhaps a plate heat exchanger swap will probably be the next big thing. I suppose the problem will only get worse with time but I'll look into what the softener is doing in case its loosing its usefulness.

I wouldnt swap it , buy some good cleaning fluid and remove and clean the existing one, there are a few good cleaning agents available , but be carefull using them, they work very well but can be very aggresive

 

[buildersteve]

Yes a good clean and flush through would probably do it but I'm not a Gas Safe person so it wouldn't be me who does it. The boiler has to be removed from the mounting to do it I think, and its about 50kg. As mentioned in post 4, if its got to come off the wall I may as well change exchanger for a new one.
Looking about the web, Many people have this problem when running a shower from a combi boiler. It could be that the boiler has to be in better than "pretty good" condition for the shower to work properly. I also have an electric shower in the downstairs toilet/shower room and that doesn't hold a candle to that of a combi boiler run. shower.