Shower alternates between hot and cold... Boiler is fine.

[ianmcd]

Everything was golden from October

so its Golden showers you are having problems with

 

[portuguesejohn]

so its Golden showers you are having problems with

I'll wait for things to get worse before going there...

 

[terryplumb]

Your bath tap/ mixer is duff. Fit a new one. Preferably thermostatic

 

[portuguesejohn]

Your bath tap/ mixer is duff. Fit a new one. Preferably thermostatic

I've just been on the phone with one of the installers (company called boxt) and that seems to be consensus as well, they say this is one of their most common queries.

Say I'd like to try changing the fittings myself, are there any considerations I need to be aware of besides the fact that it needs to be deck mounted (as opposed to wall mounted) and same width as the current one? I don't want to have this turn out into a bigger job than what it needs to be, would something like this type do the trick? (not necessarily this fitting, just found it online)

https://www.victorianplumbing.co.uk...n=googlebase&gclid=CJf70KbQx9QCFaW17QodoCkM0g

 

[bernardgreen]

Your bath tap/ mixer is duff. Fit a new one.

Duff only in that it does not take enough hot water from the boiler to allow the boiler to operate a continuous burn at minimum heat ( maximum modulation )

Preferably thermostatic

That could make the situation worse if the chosen temperature prevents enough hot water flow to keep the boiler "happy" cool.

 

[denso13]

Duff only in that it does not take enough hot water from the boiler to allow the boiler to operate a continuous burn at minimum heat ( maximum modulation )

What flow rate at what temperature rise is the OP actually getting?

 

[portuguesejohn]

Duff only in that it does not take enough hot water from the boiler to allow the boiler to operate a continuous burn at minimum heat ( maximum modulation )



That could make the situation worse if the chosen temperature prevents enough hot water flow to keep the boiler "happy" cool.

This is where I think something is a bit iffy with the shower - today, as the hot/cold cycle started, one of the things I tried was completely turning off the cold water tap, I then let max hot water run for a good 30 secs and didn't notice any more fluctuation. I then slowly started turning the cold water tap until it was a comfortable temperature and the temp remained perfect for the remainder of the shower.

 

[shambolic]

I have 3 en-suite And a downstairs WC/shower room and a wet room /boot room with a shower for dogs. my en suite has a20l minute rain shower head.
I have an Ideal Vogue 40kw and it’s perfect.
BUT, the hw side from the combi only feeds my en suite and the big shower. if the shower was off a cylinder (400litres) we’d get a combined time of 20 minutes in the shower before hot ran out.
with it being off the combi side it runs forever. the other bathrooms and kitchen are off an unvented cylinder fed by the combi through a zone valve, as is the underfloor heating and upstair radiators.
ps not read whole thing but I’d be changing the plate before anything else

 

[aptsys]

Your bath tap/ mixer is duff. Fit a new one. Preferably thermostatic

That just sounds like it'll probably mask the problem. If the shower is set to one setting, the temperature shouldn't vary if no other outlets are used unless there is a fundamental issue with the setup. The fact that slightly turning on a cold tap somewhere has a big effect on temperature suggests there is a restriction or the dynamic pressure is very poor.

 

[bernardgreen]

BACK GROUND CALCULATIONS INVOLVED IN THE HEATING OF WATER

Specific Heat of water 4200 J / kgram / °C ( J =Joule )
Weight of 1 ltr of water is 1 kgram
Specific Heat of water 4200 J / litre / °C
1 Joule per second = 1 Watt
To raise the temperate of 100 litres of water by 50 degrees C requires
4200 x 100 x 50 joules = 21,000,000 joules or 21 MJ ( Mega Joules )
How long for a 8 kW heater to lift 100 litres by 50 degrees C
8 kW - 8000 joules per second
Time = 21 MJ / 8000 = 2625 seconds = 43.75 minutes
These figures match experimental results for smaller volumes of static water being heated in a calorimeter by an immersed electrical heater.

Adapting to the calculations of energy and rate of energy when heating flowing water.

8 kW will take 43.75 minutes to heat 100 litres by 50 degrees C.
the flow rate is 100 divided by 43.75 = 2.2 litres per minute
48 kW ( 6 x 8 ) will take 7.3 minutes to heat 100 litres by 50 degrees C
the flow rate is 100 divided by 7.3 = 13.71 litres per second

Calculation of flow rate and temperature rise for various kW ratings and flow rates.

Assume the safety cutout operates at 60 degrees C,
(1)
Flow rate 4 litres per minute
Incoming water temperature is 10 degrees C
4 litres per minute = 0.0667 litres per second
Raising the water temperature by 50 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.0667 x 50 = 14,000 joules per second = 14 kW

(2)
Flow rate 4 litres per minute
Incoming water temperature is 20 degrees C
4 litres per minute = 0.0667 litres per second
Raising the water temperature by 40 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.0667 x 40 = 14,000 joules per second = 11.2 kW

Assume the safety cutout operates at 50 degrees C,
(3)
Flow rate 4 litres per minute
Incoming water temperature is 10 degrees C
4 litres per minute = 0.0667 litres per second
Raising the water temperature by 40 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.0667 x 40 = 14,000 joules per second = 11.2 kW

(4)
Flow rate 4 litres per minute
Incoming water temperature is 20 degrees C and flow rate is 4 litres per minute
4 litres per minute = 0.0667 litres per second
Raising the water temperature by 30 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.0667 x 30 = 14,000 joules per second = 8.4 kW

Assume the safety cutout operates at 60 degrees C,
(5)
Flow rate is 8 litres per minute
Incoming water temperature is 10 degrees C
8 litres per minute = 0.133 litres per second
Raising the water temperature by 50 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.133 x 50 = 14,000 joules per second = 27.93 kW

(6)
Flow rate is 8 litres per minute
Incoming water temperature is 20 degrees C
8 litres per minute = 0.133 litres per second
Raising the water temperature by 40 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.133 x 40 = 14,000 joules per second = 22.34 kW

Assume the safety cutout operates at 50 degrees C,
(7)
Flow rate is 8 litres per minute
Incoming water temperature is 10 degrees C
8 litres per minute = 0.133 litres per second
Raising the water temperature by 40 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.133 x 40 = 14,000 joules per second = 22.34 kW

(8)
Flow rate is 8 litres per minute
Incoming water temperature is 20 degrees C
8 litres per minute = 0.133 litres per second
Raising the water temperature by 30 degrees C will cause safety cutout to operate
Energy per second 4200 x 0.133 x 30 = 14,000 joules per second = 16.75 kW


The difference is power rating between condition (3) and condition (4) is 2.8 kW and is due to the difference in the temperature of the incoming cold water.
If the boiler concerned cannot modulate down below 8.4 kW then the safety cutout will operate when the incoming water temperature is 20 degrees C as in condition (4) . It will work normally when the incoming water temperature is below 20 degrees C as in condition (3)
Other boilers will have different sets of values for the conditions when the incoming water temperature is beyond the ability of the boiler to modulate down sufficiently to prevent the safety cutout from operating.

Read more: https://www.diynot.com/diy/threads/...o-cold-mid-shower.526681/page-2#ixzz6V5SUt9oo

 

[denso13]

BACK GROUND CALCULATIONS INVOLVED IN THE HEATING OF WATER

Yea, I know. What is the OP actually getting though?

 

[bernardgreen]

What is the OP actually getting though?

Looks like he is getting too much heat from the burning gas which means the water passing through the boiler is too hot and the boiler is shutting down.

Do the maths.

 

[denso13]

Do the maths.

Ok, tell me the flow rate, temperature rise and any boiler output settings and I'll try.

 

[bernardgreen]

Ok, tell me the flow rate, temperature rise and any boiler output settings and I'll try.

The parameters specific to the OP can only be provided by the OP.

Perhaps my comment "Do the maths" was not the best way to make the point.

Worcester 30i combi

From that the minimum possible heat output from the burner can be found ( at maximum modulation ).

If the input water temperature is known then the minimum flow rate necessary to keep the output water temperature below maximum can be calculated. If for any reason that minimum flow rate cannot be achieved and maintained then the boiler is going to cycle ON and OFF

 

[denso13]

The parameters specific to the OP can only be provided by the OP.

Quite, you probably knew that's where I was going with this.

If the input water temperature is known then the minimum flow rate necessary to keep the output water temperature below maximum can be calculated. If for any reason that minimum flow rate cannot be achieved and maintained then the boiler is going to cycle ON and OFF

You have been pretty adamant that this is what is going on, no ifs, no buts. You also said the "proper" solution is to fit a hot water cylinder.

You cannot possibly know those things for sure without further investigation.