Cylinder Very hot at top.

[Nestor_Kelebay]

I'm wondering if the problem isn't due to BOTH temperature stratification and the accuracy of the thermostat controlling the heating of the water.

Where I live, we don't have very many indirect water heaters. Most of the water heaters here are either gas fired or electrically heated tank style water heaters. But, we still get that temperature stratification you're talking about, but we call it "stacking" and it's often the result of using small quantities of hot water that are just large enough to fire up the water heater.

That's because every time you fire up a gas fired water heater, a lot of metal has to get real hot before the water gets hot, and that same metal keeps heating the water after the gas valve closes and the official water heating has stopped. What happens is that the hot steel at the bottom of the water heater continues to heat the water. Also, in a gas fired water heater, the flue gas goes up the inside diameter of the tank, and there'll be a spiral baffle inside there to assist in heat transfer from the gas to the ID of the tank, and that baffle must weigh a good 15 or 20 pounds (not to mention the steel of the tank it's in contact with). Also, if the water is partially insulated from the flame by an accumulation of scale in the bottom of the tank, that mass of scale will get very hot, too. And, every time the heater fires up, all this stuff will get very hot and will continue heating the water after the official heating has stopped.

And, of course, the result will be that the temperature at the top of the water heater can overshoot the temperature setting of the thermostat by quite a bit. And, using small quantities of hot water that keep the thermostat firing up the heater for short periods exacerbates the problem because of the increased amount of "unoffical heating" after the gas valve closes.

For example, this web site:

http://www.pmihome.org/tempcontrol.shtml

has this to say:

"Water heater thermostats were never intended to provide precise limits and controls on hot water temperatures. To illustrate this point, the thermostat dial calibration test of ANSI Z21.10.1-1998, which is the applicable standard for gas-fired water heaters, allows the temperature to vary 10 degrees above or below the thermostat setting. Additionally, the maximum temperature limit test of ANSI Z21.10.1 allows the outlet water temperature to rise 30 degrees F. above the thermostat setting. This provision accounts for the phenomenon known as “stacking” or “layering”. Stacking or layering occurs when hot water gathers at the top of the heater due to recurring short duration heating cycles caused by frequent number of small quantity hot water uses. Although the above example addresses gas water heaters, this phenomenon can also occur in other types of storage water heaters."

So, even a water heater that's within factory specifications can have an outlet water temperature as high as 30 degrees Fahrenheit above the thermostat setting. (!) That's a lot. It's certainly enough to make "hot" water in to "scalding hot" water.

Imagine how hot the water at the top of the heater could get if the thermostat was out to lunch too.

But, I don't know if indirect fired water heaters would have the same amount of "unofficial heating" from the steel tank and spiral baffle that our gas fired tank style heaters do.

 

[chrishutt]

I don't know if indirect fired water heaters would have the same amount of "unofficial heating" from the steel tank and spiral baffle that our gas fired tank style heaters do.

No, not at all. The heating element is just a coil of 22mm copper tube.

The cylinder does have a immersion heater but it doesn't work and is not plugged in.

The poster has already answered my early question about immersion heaters. He seems quite sure that it isn't working, so we can forget about that.

the water is more like 85....... 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.

The problem we have is the lack of objective observations from the poster. Why does he say 85C - just a wild guess it seems. You certainly couldn't hold onto that temp for any time at all. The temp may be around 60C, so perhaps there isn't really a problem at all.

 

[croydoncorgi]

The way you tell it, it sounds like there are separate pipes carrying hot water from the bottom of the cylinder to the top, and cooler water from top to bottom!

Convection in a cylinder doesn't work that way, any more than it does in a kettle. Hot water from the coil area will mix as it rises through the cooler water above, so the temperature of most of the water in the cylinder will tend to rise evenly AS LONG AS THE COIL IS HOT. As soon as the heat goes off, the convection process reverses BUT THE COLDEST AREA WILL BE NEAR THE SIDE OF THE CYLINDER, especially in this case which has poor insulation. So you WILL get cooler water sinking downwards near the sides and hotter water rising up the middle, without much mixing on the way. What we don't know is how long it takes to get to Ouch! at the top and 'cold' at the bottom.

The whole issue could be down to poor insulation.

 

[Softus]

As soon as the heat goes off, the convection process reverses...

Preposterous.

 

[croydoncorgi]

Preposterous.

OK - OK
You know what I MEANT: when the heat source turns off, the whole cylinder starts to cool by losing heat through the walls (which were stated to be badly insulated).

Of course the direction of convection doesn't 'reverse'. The 'reversal' is from heating to cooling.

Otherwise my statements stand (unless anyone knows different or has re-invented physics).

 

[Nige F]

2 essex flanges,bronze pump, bit o` pipe = destratification and no more arguments

 

[richardhill84]

Sorry chaps. Let me give you my apologies.

The HW was very hot due to a previous central heating problem i resolved in the day. Once i drained the cyl and let it refill, it is now the same temp all over.

The reason this was getting so hot is quiet interesting though.

When i changed my pump and Motorised valve, i had to close four stop cocks. 1 from the pomp to MV, 1 on the HW side of the MV and 2 on the CH side of the MV.

After fitting the new pump and MV, i simply opened all the stop cocks again. What i didn't realise (and for the life of me i don't understand why the system has been plumbed in this way), on the CH side of the MV there are 2 stop cocks that branch from a tee piece. One feeds to the Hot/TRV side of the rads and the other (very bizarrely) feeds the return side of the rads. Therefore when both cocks are open, the CH system is effectively short circuit. I think this created pressure back through the MV to the HW side. (just a guess).

I have now closed the stop cock that drives the return side of the rads and the central heating and HW have never been so good. I guess this stop cock was always closed but why would you have a CH HOT feed to the return side of the rads i don't know. Any ideas????????

 

[chrishutt]

It's a by-pass valve. Some boilers require a by-pass to ensure that there is adequate flow through the boiler heat exchanger even if all the TRVs on the radiators are shut. If you have TRVs on your rads, you may find the boiler overheating occasionally if the by-pass is not adjusted correctly.

 

[richardhill84]

arrr that makes a bit more sense.
I always thought it was odd that all the rads had TRV's.

Do you think it is a good idea to change one rad to a manual valve?(assuming i keep the stop cock closed)

 

[chrishutt]

Yes, especially if you have, or intend to have, a room stat, in which case the rad heating the space with the room stat should not have a TRV. Otherwise choose a rad that doesn't need to have it's heat output reduced in mild weather - e.g. a bathroom rad that dries/warms towels.

 

[richardhill84]

Thanks for that.

The system does not have a seperate room thermostat but i was thinking off installing one now everything seems to be working.
I'll put the stat in the front room and remove the TRV from the front room rad.

Thanks again.

 

[Softus]

The way you tell it...

Hm, let's have a look at the way I told it:

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.

No mention of pipes up and down the tank there, and yet from the above you've managed to infer the following:

The way you tell it, it sounds like there are separate pipes carrying hot water from the bottom of the cylinder to the top, and cooler water from top to bottom!

You are nuts. Absolutely barkingly so.

Here's another little conundrum:

Of course the direction of convection doesn't 'reverse'. The 'reversal' is from heating to cooling.

According to what I was taught about fluid thermodynamics, the cooling doesn't wait for the period of heating to end, it happens all the time - in fact, the rate of cooling is at a peak at the instant that heating stops, and then decreases over time. So, there is no "reversal", as you define it. I'm increasingly curious about the physics that you've been studying.

Moving on to the simple task of framing coherent sentences:

Preposterous.

OK - OK
You know what I MEANT:

So, if I've got this straight, it's OK for you to make a ludicrous interpretation of something unambiguous that I wrote (see above), but you expect me not to take your words at face value. IMHO that approach has very little chance of yielding a successful outcome.

Here's something we can agree on:

The whole issue could be down to poor insulation.

Yes, but there might be absolutely no problem at all, bearing in mind the following is the closest thing we have to a description of any symptom:

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.

To strike at the heart of this topic, I believe that chrishutt is on the right track, viz:

the water is more like 85....... 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.

The problem we have is the lack of objective observations from the poster. Why does he say 85C - just a wild guess it seems. You certainly couldn't hold onto that temp for any time at all. The temp may be around 60C, so perhaps there isn't really a problem at all.

This is the all-important question, and it looks like we're still waiting for it to be answered...

 

[bab]

if u only remove the head & fit the decorators cap in the open position the valve wont need changing.But also if u hav a fire in the lounge that you use the room stat will turn the heating off when that rooms hot, Best in a room without other heatsources.

 

[Softus]

Sorry chaps. Let me give you my apologies.

I stand corrected - we now have the answer...

The HW was very hot due to a previous central heating problem i resolved in the day. Once i drained the cyl and let it refill, it is now the same temp all over.

I very much doubt that it is.

The reason this was getting so hot is quiet interesting though.

When i changed my pump and Motorised valve, i had to close four stop cocks. 1 from the pomp to MV, 1 on the HW side of the MV and 2 on the CH side of the MV.

After fitting the new pump and MV, i simply opened all the stop cocks again. What i didn't realise (and for the life of me i don't understand why the system has been plumbed in this way), on the CH side of the MV there are 2 stop cocks that branch from a tee piece. One feeds to the Hot/TRV side of the rads and the other (very bizarrely) feeds the return side of the rads. Therefore when both cocks are open, the CH system is effectively short circuit.

I have a hunch that this is a bypass.

I think this created pressure back through the MV to the HW side. (just a guess).

I don't agree with your conclusion.

I have now closed the stop cock that drives the return side of the rads and the central heating and HW have never been so good. I guess this stop cock was always closed but why would you have a CH HOT feed to the return side of the rads i don't know. Any ideas????????

I have a hunch that this is a bypass. If so, it should not be fully closed.

 

[chrishutt]

Softus, you're repeating what's been dealt with. You're so absorbed in your private debate with croydoncorgi that you've overlooked developments in the original thread!