Shower going cold

[Rick101]

Anybody have any advice for a shower plumbing issue?

Have an Aqualisa Colt shower. Unit is two years old and in good condition.
I have an issue where during a hot shower, the water will run cold for a maybe 10 seconds before returning to warm again.
I suspect it's something to do with flow, pressure or draw from the combo boiler (WB Greenstar 28i Junior).
All other taps in the house work fine with constant hot water.

The cold pressure is fairly low. The hot pressure is much stronger. On a grade of 1 to 10 I'd usually have the temp set around 3.
I have changed the cartridge and cleaned it up inside.
There is a hot pressure water dial and gauge which shows 2.5 bar
There is a cold pressure gauge in loft but not had chance to inspect that yet.

Any suggestions?

[GasEngineer]

Does it only run cold once and then works normally for the rest of the shower?

[Peanut Gallery]

I think I used to have that boiler - when I was washing dishes the water draw was too low for the boiler to modulate down to, so it would cycle off, let the water run cold, cycle on, burn my hands, cycle off, freeze my hands, cycle on, burn my hands.

To see if this is the case, either increase the flow of water through the shower or gently run a hot tap elsewhere in the house to keep the boiler working harder.

[Rick101]

GasEngineer said:

Does it only run cold once and then works normally for the rest of the shower?

I think so. runs hot for maybe 5 min, then a cold moment, then hot again.
Not usually in long enough to realise a 2nd time.

Will check.

Cold water pressure gauge shows 1 bar.

[OutInTheShed]

Peanut Gallery said:

I think I used to have that boiler - when I was washing dishes the water draw was too low for the boiler to modulate down to, so it would cycle off, let the water run cold, cycle on, burn my hands, cycle off, freeze my hands, cycle on, burn my hands.

To see if this is the case, either increase the flow of water through the shower or gently run a hot tap elsewhere in the house to keep the boiler working harder.

^This^

You may be able to increase the flow by checking there's no partially closed stopcocks (including the water co one at the meter!) etc, get a large bore shower hose and a shower head with more/bigger holes in it! There are special handsets and hoses for low water pressure.
At least, de-scale the shower head.
Maybe check if there's an inlet filter for the shower mixer, which could be a bit clogged.

Also some people install shower mixers with adaptors to the plumbing which are horribly restrictive.

Beyond that, it's possible for an aging boiler to have poor thermostatic control, causing an overshoot and shut off. Changing sensors, re-fitting sensors with better thermal contact (new grease etc) can help. Turning the target HW temperature down a bit might be enough.

You may want to consider checking the pressure and flow available to the house meets the water co's standard of service?
They may be able to sort it.

[Watcher of the skies]

Peanut Gallery said:

I think I used to have that boiler - when I was washing dishes the water draw was too low for the boiler to modulate down to, so it would cycle off, let the water run cold, cycle on, burn my hands, cycle off, freeze my hands, cycle on, burn my hands.

To see if this is the case, either increase the flow of water through the shower or gently run a hot tap elsewhere in the house to keep the boiler working harder.

When I hear about nonsense like this it always makes me wonder why people put up with combi boilers. They're only fit for small flats.

[GasEngineer]

Rick101 said:

GasEngineer said:

Does it only run cold once and then works normally for the rest of the shower?

I think so. runs hot for maybe 5 min, then a cold moment, then hot again.
Not usually in long enough to realise a 2nd time.

Will check.

Cold water pressure gauge shows 1 bar.

If its just happens once, it may be that your combi boiler has a pre heat function which gives a small amount of hot water immediately. If you are just washing your hands it's fine. If you run it for a while you can get a cold slug after the pre heat hot is used up and before the main hot is produced.

[OutInTheShed]

GasEngineer said:

If its just happens once, it may be that your combi boiler has a pre heat function which gives a small amount of hot water immediately. If you are just washing your hands it's fine. If you run it for a while you can get a cold slug after the pre heat hot is used up and before the main hot is produced.

Good point.

Also our shower can do similar if you're the second person to shower, so the plumbing is full of hot water, but the boiler is called to do central heating at a low flow temp between showers, which cools the heat exchanger.

I don't quite understand how the hot water has higher pressure than the cold, but I have known this cause issues on yacht plumbing systems, some thermostatic valves are UNSAFE if the cold pressure is low! The gauge on the boiler is the CH circuit pressure? Not the HW pressure??

The Aqualisa manual implies the HW temperature should be below 65degC.
https://www.plumbworld.co.uk/documents/aqualisa-co...

I think it would help to check what the sequence of events is. Does the boiler fire and then shut down and fire again? Does the boiler outlet go Hot-cold-hot?
I'd want to prove whether the problem was the boiler or the mixer.

[Crumpet]

Have you seen whether the flame is cutting out on the boiler when it goes cold?

We’re having the same issues with the same shower and pretty sure it’s a blocked heat exchanger in the boiler that’s causing it to overheat and cut out.

[OutInTheShed]

How old is the boiler?

There are a lot of things that can go wrong with an older boiler causing various flavours of instability.

My first house may years ago had a combi boiler and a simple 'diverter on the bath taps' shower. That worked great but only if you adjusted the two bath taps exactly right in the correct sequence!

Current house has a combi and a generic thermostatic shower which I really can't fault. It just works.

[Aluminati]

Surely the hot pressure is down to the cold supply ?

[Rick101]

Crumpet said:

Have you seen whether the flame is cutting out on the boiler when it goes cold?

We’re having the same issues with the same shower and pretty sure it’s a blocked heat exchanger in the boiler that’s causing it to overheat and cut out.

Just an update and had chance to get up in loft whilst Mrs was in shower.

The flame indicator was off when she was in shower. It came on for a time but then went off again so thinking a draw issue.
We tested the sink tap, just next to shower and taking tap water gave a fairly quick flame on and off as required.
I increased the hot pressure slightly. On the last test this didn't make any difference and shower still ran cold momentarily after a few minutes.

I have now increased the pressure using the PRV of hot to 3.0 bar and cold to around 2.8 bar so so how that does over the next few days. I think 3.0 is the max recommended.

Edited by Rick101 on Thursday 21st August 07:25

[megaphone]

The pressure gauge on the boiler is to indicate the pressure in the central heating system, has nothing to do with the hot water. Should be around 1 bar when the heating is off/cold.

Try turning down the flow temperature of the HW, should be able to do this on the boiler controls. I have mine set to 60c and lower it in the summer to 55c. This will stop the boiler over heating and shutting off, may also save you some gas.

Edited by megaphone on Thursday 21st August 09:06

Edited by megaphone on Thursday 21st August 09:29

[Sanderling]

It sounds like you have increased the central heating pressure? When cold it should be at the lower end of the green section (1/1.2 bar) so that as the system heats up there is room for the water expansion and increased pressure.

It is common for combi boilers to cycle on and off when the secondary heat exchanger is blocked. Very hard water areas or crud in the system can quite easily block it or reduce flow enough to cause the boiler to run over target temperature and then shut down. It wouldn't shock me if that's the case after only 2 years.

[Glosphil]

We live in a hard water area & have a WB 55CDi combo boiler what was installed in 2013. We bought the house in 2018. The boiler has just had its 4th heat exchanger fitted. Each time the improvement to flow through hot taps & the 2 showers is massive.

We are planning to have a water softener installed but the position of the mains water into the house & the layout of the water pipes makes installation difficult.

[Sheepshanks]

megaphone said:

The pressure gauge on the boiler is to indicate the pressure in the central heating system, has nothing to do with the hot water. Should be around 1 bar when the heating is off/cold.

Try turning down the flow temperature of the HW, should be able to do this on the boiler controls. I have mine set to 60c and lower it in the summer to 55c. This will stop the boiler over heating and shutting off, may also save you some gas.

On a combi, wouldn't that just make it shut down more? Unless the burner cn modulate low enough, I suppose.

I guess it depends on incoming cold water temp and hot water flow rate - could be at this time of year a shwer doesn't need that much hot water adding, unless you have the shower blasting out.

We stil have an electric shower in one bathroom but it's fed off the balanced cold supply and in the recent hot weather it's really been a little too hot even on max flow. Our other shower is off pressurised stored hot water, so no issue with that.

[Rick101]

To be clear, I haven't adjusted the boiler or allowed more water into the boiler to change pressure. It remains at 1.5 bar.

The valves I adjusted, I think, manage the flow to the shower only.

[C-J]

Following may be irrelevant, especially as my boiler is a conventional one (not combi) - but sharing just in case.

My Aqualisa something or other was often having temperature drops.

I was close to giving up - and it was only by chance that I realised after months and months that the cause was a sticking valve in the loft cold water tank - causing it not to fill much or fast, meaning that the cold water pressure was low and dropping during showering.

I didn't see any issues in the taps - hence didn't realise for ages.

£10 for a new CW float arm thingy and all sorted.

So worth just checking that CW tank etc fine - ignore if you pressure checks etc cover this.

[Rick101]

Lat few have all been good so increasing the pressure from the PRV's seem to have done the trick.
Issue must have been draw wasn't strong enough from boiler and it was cutting out.

Hopefully that's it's sorted. Cheers all.