Cylinder touch test ?

[Ravel]

Could someone please tell me which part of the CH cylinder in a vented system should be the hotest.

I have poked my finger under the insulation jacket at the bottom, middle, and top, and found the top is definately in my case the hottest.

My reason for asking is having just bought a cylinder thermostat and followed the manufacturers instructions to the letter, by fitting it one third up from the bottom even though the tap water is too hot for a bath without adding cold, the thermostat does not switch over even when set at it's lowest 50 degs C.

I then removed the stat and felt the cylinder which was not as hot as halfway up and the top temp was too hot to touch, and I cant help feeling if the stat was mounted there it would do it's job.

I have tested the switching action using a digital continuity meter when applying an external heat source and it works fine, but my tank is obviously not reaching a high enough temp at the one third up point.

Any ideas please.

Regards Ravel

 

[gas4you]

The top of the cylinder will always be the hottest. Hot water rises.

Bottom is always cold as this is where the cold fill enters.

Cyl stats are usually set to 60C, as you say 1/3rd up, this would give around 65-70C at the top.

Is the cyl stat connected to a zone valve or what, if it isn't?

 

[Ravel]

Hello Dave, no it's not connected to anything atm, I just wanted to make sure it did the switching before spending any more money.

The result is it doesn't switch so my hot water heat input would be continous as the associated valve will be in a permanent flow condition.

Would it be ok to make a cut higher up the jacket and try it there, even though the instructions say otherwise.

Regards Ravel.

 

[ChrisR]

Opinions vary on where the stat should be fitted.
Remember two things happen;
one is stratification - this is a gradual temperature gradient down the cylinder, with as described, the top being 10 degrees or more hottter than the bottom
second is the effect of drawing off water. Cold comes in from the bottom, and you then get a step in the temperature rising up the cylinder. When the cold reaches your stat, it will certainly turn on. So if you have the stat high up, it won't know if all the water below it is much cooler, and you can have too little for a bath, say.

One issue is the thermal conductivity between the cylinder and the metal of the thermostat. Do get all the bits of polystyrene off the side, and make sure the stap is tight. I've used thermally conductive paste on mine, which helps. Something like grease or vaseline helps a lot. Without it there can be a big temp deifference between the cyl and the stat, and of course it would be very slow to notice a change.

 

[Ravel]

Opinions vary on where the stat should be fitted.
Remember two things happen;
one is stratification - this is a gradual temperature gradient down the cylinder, with as described, the top being 10 degrees or more hottter than the bottom
second is the effect of drawing off water. Cold comes in from the bottom, and you then get a step in the temperature rising up the cylinder. When the cold reaches your stat, it will certainly turn on. So if you have the stat high up, it won't know if all the water below it is much cooler, and you can have too little for a bath, say.

One issue is the thermal conductivity between the cylinder and the metal of the thermostat. Do get all the bits of polystyrene off the side, and make sure the stap is tight. I've used thermally conductive paste on mine, which helps. Something like grease or vaseline helps a lot. Without it there can be a big temp deifference between the cyl and the stat, and of course it would be very slow to notice a change.

Thanks for your excellent info, appreciated.

My main problem is to actually get the stat to turn off, or become open circuit

The stat is a Drayton HTS3 which like most other makes has a bi metal strip at it's rear but there is always at least 5 mm air space between tank and sensor which is provided by the plastic unit housing spacer, and i'm convinced that if the strip was in physical contact with the tank metal it would be much more responsive, but that's not how they tell you to do it.

Regards Ravel

 

[Agile]

You have already oveheated the water.

Connect the stat and let the boiler heat it and it should be fine when set to 55°C.

Tony

 

[ChrisR]

NB the hysteresis (on-off switching gap), is quite wide, perhaps 10 degrees.

 

[Ravel]

You have already oveheated the water.

Connect the stat and let the boiler heat it and it should be fine when set to 55°C.

Tony

I'm sorry Tony, and realise that's what should happen, but during the water heat up period the stat contacts are obviously closed, and eventually open when the desired temp is reached.

This is not occuring and no change of state takes place even with the stat set at it's lowest 50degs C because the cylinder temp at the choosen location never reaches that, but does higher up.

I think I'm going to move it to half way where my test thermometer shows around 63 degs, because where it is presently appears to be doing nothing.

Regards Ravel

 

[Ravel]

NB the hysteresis (on-off switching gap), is quite wide, perhaps 10 degrees.

Thanks ChrisR this condition is common with most thermostats, but I feel it has no bearing on this situation as it's not switching at all.

Regards Ravel.

 

[chrishutt]

I think I'm going to move it to half way where my test thermometer shows around 63 degs, because where it is presently appears to be doing nothing.

What temperature do you have at the present position of the cylinder stat? What type of thermometer are you using? What sort of heating coil does the cylinder have - where on the cylinder are the flow and return tappings to it?

If you've got 63°C half way up, you must surely have at least 55°C at the 1/3 up level, where a stat set at 50°c should switch OK. Millions of cylinder stats like yours are working fine at 1/3 up or less, so don't think in terms off changing the position, even less modifying the stat, just because yours doesn't seam to work.[/quote]

 

[Softus]

If the thermostat isn't switching then it's faulty. You can't rectify the situation by putting it in the wrong place.

And please don't say that it's new and therefore can't be faulty.

 

[bengasman]

Would this be a dual coil cylinder by any chance?

 

[ChrisR]

ChrisR wrote:
NB the hysteresis (on-off switching gap), is quite wide, perhaps 10 degrees.

Thanks ChrisR this condition is common with most thermostats, but I feel it has no bearing on this situation as it's not switching at all.

Sir if you knew as much about thermostats as I do you would have solved your problem by now.
I was simply filling in some of your lack of knowledge.

It sounds as though your thermostat needs testing but you have no means of measuring temperature.
However if you have a kettle, metal saucepan and mains supply I'm sure you can work out how to test it.

 

[Ravel]

I think I'm going to move it to half way where my test thermometer shows around 63 degs, because where it is presently appears to be doing nothing.

What temperature do you have at the present position of the cylinder stat? What type of thermometer are you using? What sort of heating coil does the cylinder have - where on the cylinder are the flow and return tappings to it?

If you've got 63°C half way up, you must surely have at least 55°C at the 1/3 up level, where a stat set at 50°c should switch OK. Millions of cylinder stats like yours are working fine at 1/3 up or less, so don't think in terms off changing the position, even less modifying the stat, just because yours doesn't seam to work.

No I'm afraid not it only reaches 45 degs C hence the no response, but this may be interesting just as a further test I switched on the immersion heater and after five minutes the stat changed over to off on my continuity test meter.

Regards Ravel.

 

[Ravel]

If the thermostat isn't switching then it's faulty. You can't rectify the situation by putting it in the wrong place.

And please don't say that it's new and therefore can't be faulty.

The new thermostat is not faulty because it switches perfectly when an external heat source is applied and at the correct temperature.

Regards Ravel.