Cylinder Very hot at top.

[richardhill84]

Hi,

My hot water is getting very (scalding) hot even though the hot water cylinder thermostat is set at 55-60 degrees.

If i feel the cylinder where the thermstat is placed (about a third of the way up) the cylinder is warm/hot (I guess about 55-60 degrees).

However, if i feel the cylinder at the top, it is red hot. (This is the temperature it is coming out of the taps at).

Can Hot Water cylinders break down over time or is there another explanation for this?

Thanks for your help.

 

[chrishutt]

it is red hot.

You are exaggerating a bit I think. So how hot is the water? 60C feels very hot, some might say scalding, or even red hot if they were prone to exaggeration. Have you tried turning the stat down a little? Does it make a difference?

 

[richardhill84]

It's that hot that i can't put my finger under it for more than a second or two. It also seems to make opening and closing the taps harder.

The stat is currently set to 55.

I have just felt the cylinder again now and it feels almost tepid where the stat is but still very hot at the top. Why would this be? Should't the cylinder be roughly the same temp throughout?

 

[chrishutt]

Should't the cylinder be roughly the same temp throughout?

Not to start with. I assume the water is heated by a coil fed from the boiler. The boiler water in the coil will be at around 70C, depending on the boiler thermostat setting. The coil will heat the cylinder water around the coil. This water will then rise to the top of the cylinder.

So to start with the top will be hot and the lower part still cool. Gradually the proportion of hot water will increase until it reaches down to the level of the cylinder stat. When the temperature of the water at this level matches the setting on the stat the stat switches and shuts off the boiler (or motorised valve).

I wonder if your cylinder is actually being heated by an immersion heater which does not reach down very far? This might explain the apparent discrepancy between water temp and stat setting. The external stat does not control the immersion heater.

 

[richardhill84]

Thanks.

The cylinder does have a immersion heater but it doesn't work and is not plugged in. It is pretty old and still has a red jacket on it!!!!!

Like you say, initially the top of the cylinder would be the hottest and then the rest of the cylinder when heat up when the water in the coil reaches it.

However, i turned my Hot water off at 4pm today (5 hours ago) so i would expect the cylinder to be evenly warm all over.

Note that no water is getting past the 3-way motorised valve as the pipes leading top the cylinder are cold.

 

[chrishutt]

i turned my Hot water off at 4pm today (5 hours ago) so i would expect the cylinder to be evenly warm all over.

Only if no hot water had been used since well before 4 pm! You really need to observe what happens during the hot water heating cycle. Does the cylinder stat switch as expected and does the motorised valve move as expected?

 

[brumylad]

maybe be coil has corroded and the cental heating water is mixing with the water in the cylinder?

Are any overflow pipes overflowing ?

Is the hot water form the tap getting dirty

Are you needing to bleed your radiators more than usual?

 

[Softus]

maybe be coil has corroded and the cental heating water is mixing with the water in the cylinder?

This is a good thought, but it would be more common for water to pass into the coil, not out of it.

Are any overflow pipes overflowing ?

Is the hot water form the tap getting dirty

Two class-topping questions!

Are you needing to bleed your radiators more than usual?

Hm. A split coil would not be the sole cause of air accumulating in the radiators, although if the situation persisted, and all chemical inhibitor was displaced, then eventually gas would result from corrosion.

richardhill84 - if your water is too hot, why not turn down the thermostat? If you can't hear the thermostat click on and off as you turn the dial, then it's very likely to be faulty.

Aso, FYI, the water in the cylinder will never all be at the same temperature (as long as it's liquid), because the warmest water will always rise to the top, by virtue of convection.

 

[richardhill84]

No water running through the overflows.
Hot water is clean
Rads are NOW fine. (see below)

As a bit of background, i had a problem for the last two weeks where my downstairs rads would not work if the upstairs were on. I fixed this by changing the pump, DIY powerflush, balancing the rads, generally tinkering until it worked etc......

I also had a problem where my hot water was getting red hot even if the Hot water was switched off on the programmer. I tracked this down to the 3-way motorised valve as the pipe on the Cylinder side of the valve was hot even when it was off on the programmer or closed manually. I have change the MV and it seems fine now.

 

[richardhill84]

richardhill84 - if your water is too hot, why not turn down the thermostat? If you can't hear the thermostat click on and off as you turn the dial, then it's very likely to be faulty.

The thermostat is down to 55 but the water is more like 85.

Aso, FYI, the water in the cylinder will never all be at the same temperature (as long as it's liquid), because the warmest water will always rise to the top, by virtue of convection.[/quote]

Very true but the cyl is cold where the stat is and hot at the top. (i.e. I can touch and hold the cylinder for 5-6 seconds at the top before before i have to let go because it is that hot.)

 

[croydoncorgi]

Once the boiler starts to heat the hot water by pumping hot water through the coil in the cylinder, ALL the water in the cylinder will quickly get to an AVERAGE temperature due to convection! You're putting heat in at the bottom of the tank which heats the cold water there, which rises, with a stirring effect.

If the water at the top of the cylinder is VERY hot and the bottom is cool or cold, my best guess is that the boiler is not even trying to heat the hot water and all the work is being done by the immersion heater fitted at the top of the cylinder. Due to its position, there are no convection currents to stir the water, so the bottom stays cold.

There's no other valid explanation of your described symptoms.

 

[Softus]

The thermostat is down to 55 but the water is more like 85.

Let's get specific then - could you actually measure the temperature and post the result here?

...Very true but the cyl is cold where the stat is and hot at the top. (i.e. I can touch and hold the cylinder for 5-6 seconds at the top before before i have to let go because it is that hot.)

I doubt that you could hold it for 5-6 seconds if it were at 85 degC.

Just to recap - has something changed recently, or has it always been this hot (however hot "this hot" might be)?

 

[Softus]

Once the boiler starts to heat the hot water by pumping hot water through the coil in the cylinder, ALL the water in the cylinder will quickly get to an AVERAGE temperature due to convection!

There is, of course, an average temperature, but this isn't the same thing as all of the water being at the same temperature. If the water in the cylinder has been heated to be above room temperature, then the water at the very top will be hotter than any of the other water in the tank.

If the water at the top of the cylinder is VERY hot and the bottom is cool or cold, my best guess is that the boiler is not even trying to heat the hot water and all the work is being done by the immersion heater fitted at the top of the cylinder. Due to its position, there are no convection currents to stir the water, so the bottom stays cold.

I disagree. Whilst the bottom of the immersion element might well be above the bottom of the coil, it isn't true that convection doesn't occur with the immersion heater on - it very much does occur.

 

[croydoncorgi]

Sounds like you haven't looked too closely at what I said - or at too many cylinders lately!
In 90% of cases of top-mounted cylinders, the element only reaches part-way down, so the water at the bottom stays cold.

I stand by my other statement - if the water in a cylinder is heated by a coil at the bottom (where most are), ALL the water in the tank will be at roughly the SAME temperature from a few minutes after it starts to heat until it 'stratifies' due to hot water flowing out of the top and cold in at the bottom. If no water is used, unless a cylinder is very badly insulated, it will NOT stratify significantly until quite a long time after heating stops. Any other result would defy laws of physics. Before you contradict this again, please recheck your facts.

 

[Softus]

Sounds like you haven't looked too closely at what I said - or at too many cylinders lately!

If we have our wires crossed then I'm sure we can put it right, but on both of the above points you're quite wrong. However, I'm not sure how just looking at a cylinder can tell either of us anything about the temperature gradient inside it.

In 90% of cases of top-mounted cylinders, the element only reaches part-way down, so the water at the bottom stays cold.

I completely agree, and I never intended to imply otherwise. I'd even go so far as to say that I actually didn't imply otherwise.

I stand by my other statement - if the water in a cylinder is heated by a coil at the bottom (where most are), ALL the water in the tank will be at roughly the SAME temperature from a few minutes after it starts to heat until it 'stratifies' due to hot water flowing out of the top and cold in at the bottom.

I fundamentally disagree. For a side-mounted element at the bottom of the tank, the heated water above it is NOT all at the same temperature. The water behaves just as it does in a kettle, where the heated water, that is less dense than the cooler water above it, rises to the top and displaces that cooler water. This process of convection continues while the element is energised, and when the element is switched off it continues until a state of equilibrium is reached, which happens when there's an even temperature gradient from the top to the bottom of the cylinder.

If no water is used, unless a cylinder is very badly insulated, it will NOT stratify significantly until quite a long time after heating stops.

I disagree, for the reason I outlined above.

Any other result would defy laws of physics. Before you contradict this again, please recheck your facts.

We are clearly working to different sets of the laws of physics. I'm happy to agree to disagree, if you like, but until you explain why you believe you're right, I'm going to stick with my set.