Why does your shower go cold mid shower

[ericmark]

In recent weeks there have been several pleas for help from people who could not have a satisfactory shower because the water would go cold while they were showering.

Bernard has not claimed it was his shower having a problem.

remember typing this
I have a shower with full mains water pressure.

The green / orange "box" in the top of the cylinder is a "coil" with a large surface area.



Read more: https://www.diynot.com/diy/threads/replacing-my-thermal-store-and-boiler…help-in-specifying.455158/page-4#ixzz5vNThOXfA

bernard,at certain times your bespoke sidewinder could be allowing warmed water back into the cold water main. seen it a few times

How does this have any bearing on the problem when Bernard has not claimed the problem is his.

I had a Main 7 gas water heated fitted in last house, and in the main it worked OK, however some shower heads did cause a problem in the summer due to not enough water going through the head. I drilled out some of the holes in the head, cured.

My mother had a Worcester Bosch Greenstar 28i this also had a problem, however it was not the same, it was caused because it heated up a small store inside the boiler to reduce the time between turning on tap and getting hot water, selecting eco mode switched this off, so shower worked better, but it also resulted in if you turned hot tap on just a small amount to for example rinse dishes, the boiler would not fire up. So answer was Eco off but run shower longer, it would go hot, cold, and hot again and stay hot.

Even the washing machine seems to have a problem tumble drying cloths when in coming water too hot, it uses cold water to condense water.

So from my very limited experience yes hot water feed can be a problem.

Legionella bacteria grow best between temperatures of 20°C – 45°C with optimum growth temperature being 35°C – 40°C. High temperatures (minimum 60°C) kill the bacteria. Legionella bacteria are found in low numbers in natural aquatic environments, for instance, lakes, rivers and ground water.

This to my mind is a far bigger problem than if the shower works, my cold water is never anywhere near 20°C and even the water in the water tower after being in the sun all day is not that hot, while painting it, I was unfortunate enough to get wet with the water while filling a real boiler water tank, one that actually boils water. OK not a big water tower, but seems odd water allowed to get that warm?

The shower is the big problem with legionella as the water is atomised so breathed in, the one case I know about at work, was the guy washing down trucks using the hot water designed to make concrete where water temperature is critical for correct curing time. It was because water was atomised so he was breathing in the fine spray.

 

[nwgs]

This morning the temperature of the incoming water here is 20 deg C

20 plus let’s say 36, can’t see any combi overheating at 56.

I’ve got a 40kw Duotec. All hot water outlets are working fine ?

In theory Bernie you’re saying the boiler can’t move the heat fast enough? And the addition of incoming cold water is the issue.

 

[Hugh Jaleak]

No not just mine, the supply to this area is from a large water tower which warms up in hot weather.

Not 100% sure where the supply would come from for that area, but assuming it is a Water Tower, given the volume of water contained, and the throughput of water in hot weather, I'd be very surprised if there was any noticeable rise in temperature of the stored water, then after it's been mixed with water in the supply and finally arrived at the tap, it'll be pretty cold again! Does seem strange logic...

My supply comes from a concrete tower at the top end of the town, running the tap for a few seconds usually brings colder water from the pipework under the floor to the tap for that refreshing drink on a hot day!

 

[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 lites 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 sesond
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 sesond
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 sesond
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 sesond
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 sesond
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 sesond
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 sesond
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 sesond
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 diffence 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 condtion (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 condistions when the incoming water temperature is beyond the ability of the boiler to modulate down sufficiently to prevent the safety cutout from operating.

 

[bernardgreen]

Not 100% sure where the supply would come from for that area,

Milton Earnest Water tower. Temperature of incoming water this morning is 69.5°F ( mercury thermometer )

Bernard has not claimed it was his shower having a problem.

No problems at all with my shower, but then there isn't a combi involved.

 

[bernardgreen]

Bernard ,is it just your property

I have not asked my neighbours. But unless there is a geothermal event in the neighbourhood I can see no reason why my supply would be warmer than the supply to other properites,

 

[ericmark]

My Main 7 gas water heater I think 7 to 23 kW output, and I can confirm if the shower head is too small, in the summer the shower will go cold, cure is a larger shower head, also it depends on how hot you have set the water to come out.

So 4 to 13 litres/minute at 25°C temperature rise to 2.5 to 6 litres/minute at 55°C temperature rise is what the installation instructions say. So by those figures your 4 litres a minute should not cause a problem. So the shower head I had used was designed for an electric heater not gas, just not enough flow.

 

[bernardgreen]

bernard,at certain times your bespoke sidewinder could be allowing warmed water back into the cold water main.

Very unlikely, volume of expansion of water in coil is far less than volume of water in pipe between coil and kitchen where supply enters the building.[/QUOTE]

 

[ericmark]

Oh and yes my boiler is not a combi boiler, two independent boilers, one for domestic hot water, the other for central heating. Both getting on a bit, would guess Main 7 fitted around 1981. The central heating boiler older around 1978.

And I would personally be worried at water arriving that hot, legionnaires could develop at that temperature.

 

[bernardgreen]

Clearly not

Oh yes, still feel exhausted.

 

[Hugh Jaleak]

69'5F = circa 21°C! Good grief, does seem rather warm for mains water. Perhaps it was affected when Kier moved the Tower in the 50's....

 

[bernardgreen]

Perhaps it was affected when Kier moved the Tower in the 50's....

Just spoke to a lady at Anglian Water and while she was surprised it was that warm there is nothing they can do about the temperature of the water going to the treatment works from the reservoirs

 

[Hugh Jaleak]

A possible programme of underground refrigeration is being looked at for the next budget forecast though.