Why does your shower go cold mid shower

[bernardgreen]

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.

This was reportedly caused by their boiler shutting down while the shower was running.

Some people on this forum gave the advice to run another hot tap while having a shower

This advice did enable some people to have a shower without the shower water going cold.

Others found only that the flow of water from the shower was reduced and the water still went cold.

The reason for the boiler shutting down is that the temperature of the incoming water supply is too high and the maximum flow rate of water available to the boiler is too low for the boiler at that incoming water temperature.

The flow of water through the boiler has to be sufficient to remove the heat produced at minimum modulation without the exit water temperature exceeding a safe maximum

If the flow of water is unable to remove that amount of heat then the boiler will shut down on overheat.

The amount of heat that the flow of water can remove from the boilers depends not only on the flow rate but also on the temperature of the incoming water supply.


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

 

[ianmcd]

 

[Hot&Cold]

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

Bernard ,is it just your property

 

[bernardgreen]

Bernard ,is it just your property

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

As for Ian's comment,

Before being posted here the theory was put to a chap who for thirty years was designing boilers and heat exchangers, ( not domestic boilers ).

He agreed that it is a possible reason why boilers un-expectedly shut down in hot weather

He asked two questions which someone here ( Ian ? ) may be able to answer.

" Do manufacturers of modern domestic boilers specify limits for the temperature of the water to be heated ? "
" Do manufacturers of modern domestic boilers specify limits for the temperature of the air entering the combustion system ? "

As to ian's image

""trying to explain s**t"" could be expanded to ""trying to explain modern boilers and heating system""

 

[DP]

Bernard, can post figures to prove your post is without meat
In Glasgow we have high pressure, where I am, there is a water tower. Last week, outside the temperature was plus 30. Had a few calls where boiler worked ok for hw but there were non related issues. Last week and this week outside, now that temperature is moderate, I have attended three WB boilers with erratic hot water delivery, all due to fault well unrelated to what you write

 

[bernardgreen]

Bernard, can post figures to prove your post is without meat

It is a theory, not a result of investigation with recorded data.

For most boilers Δt ( uplift ) is rated ( claimed ) to be at least 35°C

Incoming at 20°C plus Δt of 35°C is 55°C, with adequate water available to the boiler this would be OK

But consider when the Δt is greater than 35°C at minimum gas burn. ( due to low water flow through the boiler )

If the incoming water temperature T(in) is such that T(in) + Δt is less than the setting of the over heat then the boiler will not (should not) cut out on over heat.

If the incoming water temperature T(in) is such that T(in) + Δt is greater than the setting of the over heat then the boiler should cut out on over heat.

The incoming water temperature here after the hot weather is about 20°C, previous it has been typically less than 10°C

 

[Razor900]

Now expand on your theory please Bernard. Not trying to catch you out I want you to work through it...

Apart from the newest most boilers will mod down to somewhere around 6kW at the bottom end hopefully we can agree on that as a base line?

How much heat input is that or more specifically how many litres of water will that input heat by one degree a second or minute?

If you say your water is stored at 20 degrees we can say it needs raising a further 30 degrees to be at the proper temp.

For arguments sake let’s imagine a low flow rate through the shower of 4LPM how do the maths stack up?

 

[bernardgreen]

Give me until Monday, more important things to do.

 

[Razor900]

Give me until Monday, more important things to do.

Clearly not

 

[Elkato531]

Using figures quoted for the Vaillant 831.

Max output of 31KW gives deltaT of 35 degrees with a flow rate of 12.7 litres per minute
Min rate is 8.7KW which is 29% of maximum. At same flow rate, this should produce 29% of temperature rise ie. 10 degrees.

Your incoming water would have to be up near 30 degrees for this to become a problem. Unless I've got the maths spectacularly wrong which is very possible. Other boilers will have a different turn down ratio of course but I don't think the Vaillant is out of the ordinary on that score.

 

[ianmcd]

Bernard read Alpha solar smart and how the solar valve works, the pre heated HW from the solar Cylinder goes straight to the taps until the temp drops to 50 C then it is diverted via the boiler to be topped up, so hardly think that water at 20C would be a problem for the boiler

 

[JP_]

I assume this is a combi boiler problem?

I like my set up - standard boiler with tank in loft, and cold water tank in loft. The cold water tank only feeds the shower, nothing else. So people turning taps on or flushing loos doesn't steal the cold water.

 

[ianmcd]

I assume this is a combi boiler problem?

I like my set up - standard boiler with tank in loft, and cold water tank in loft. The cold water tank only feeds the shower, nothing else. So people turning taps on or flushing loos does steal the cold water.

Think you might want to edit that

 

[JP_]

ah, added n't

Well spotted!

 

[Hot&Cold]

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