Top Mounted Vertical Immersion

[John Carroll]

 

Does anyone have any views /experience on/of  the above re unstable HW outlet temperatures even with a perfectly operating thermostat which is the same length as the heating element.

My views.

With this top mounted immersion you will only heat the full immersion length of the cylinder from a fully cold cylinder.
20/25% of the HW must then be drawn off before the stat will cut back in, for example, with a stat setpoint of 60C and assuming 50C cut in, (hysteresis of 10C), and a cold mains of 15C then ~ 22% of the HW must be drawn off to enable the stat to cut in.                                                                                                                                     

If 22% of the HW is drawn off, just enough to get the the stat to start calling at 50C  then the top 78% will/can still be at 60C and the remaining 22% at 15C then the actual final HW temp will be far? higher than 60C because the vertical element will heat the cylinder without any destratification and you might end up with 78% at 70C and the remaining 22% at 25C on stat cut out. If the above cycle is then repeated can it lead to still higher temperatures even though the stat is still only sensing the average of 50C/60C cutin/cutout temperatures?, if not, then easy enough to take a few tests over a few days and set the stat appropriately to give 60C.

 

I have a 53 year old dual element top mounted immersion but only use the shorter element which heats 30L, it doesn't have the rod type stat but has a surface mounted one on top of the immersion so I can't carry out any tests, almost the full 30L has to be used before the stat cuts back in but it does give a very stable HW temperature of 65C.

 

 

[SteamyTea]

Draw a few pictures.

I think I know what you are saying, but not sure.

 

If your cylinder is 53 years old, that betters my old one by 2 decades.

[John Carroll]

Photo of typical top mounted single element immersion below.

My Cylinder is only 14 years old, the original, 39 years, which I removed and installed a twin coil (Solar+oil fired) in 2011. I just removed the original immersion but the element had no limescale whatsoever so I desided to re install it in the present cylinder where it sits today.

 

 

[marshian]

Similar arrangement for me but only time I use the immersion is when the HW/CH system is drained down for changes in the summer.

 

My thermostat is set to 55 Deg C it's only a fairly short element maybe 40 L but that's enough HW at 55 deg for two showers or one shallow bath (I have a tank temp display and from memory the thermostat was quite accurate v the temp sensor)

 

I can't help with re-heat set points as we heat only when (or just before) we need it so I've never noticed what hysteresis is applied

[John Carroll]

That's interesting, sometime maybe when using the immersion only, you might switch it on until it cuts out, note the displayed temperature, run off ~ 15L of HW to get it to cut in again, then again note the displayed temperature when it cuts out again.

[marshian]

6 hours ago, John Carroll said:

That's interesting, sometime maybe when using the immersion only, you might switch it on until it cuts out, note the displayed temperature, run off ~ 15L of HW to get it to cut in again, then again note the displayed temperature when it cuts out again.

 

I don't think that would tell you anything meaningful unfortunately

 

This is what I have on the tank (Library picture from a legionaires cycle to 61 deg C - followed by pouring a bath) the top one is looking at the top of the tank - the bottom one where the boiler tank thermostat is

 

 

I'd need to put another temp probe in or move the bottom one up to roughly where the immersion element ends to get a better idea of hysterisis

[SteamyTea]

This is what happens with my DHW cylinder.  It is on E7 and only uses the lower element, but you can see that there is quite a pronounced temperature gradient after switch off (5AM).

 

With a bit of extrapolation/guesswork, you could estimate that there is a 15°C DT below the element.

 

@John Carroll are you hoping to heat less water and once you have drawn of a bit, the element kicks in and extends the life of usable DHW enough for a shower?

If so, you are only putting in 2.8 kW which will only give you a small lift in temperature (does depend on shower flow rate).

 

[John Carroll]

All I am interested in is the HW cylinder outlet temperature while using a top mounted vertical electric immersio

2 hours ago, marshian said:

 

I don't think that would tell you anything meaningful unfortunately

 

This is what I have on the tank (Library picture from a legionaires cycle to 61 deg C - followed by pouring a bath) the top one is looking at the top of the tank - the bottom one where the boiler tank thermostat is

 

 

I'd need to put another temp probe in or move the bottom one up to roughly where the immersion element ends to get a better idea of hysterisis

I'm only interested in the HW (top) displayed temperature when using the (IF) you have a top mounted vertical immersion, this also incorporates its own thermostat,

"the bottom one where the boiler tank thermostat is", this is only used when you use the heating coil to heat the cylinder.

Edited Wednesday at 19:22 by John Carroll

[John Carroll]

44 minutes ago, SteamyTea said:

This is what happens with my DHW cylinder.  It is on E7 and only uses the lower element, but you can see that there is quite a pronounced temperature gradient after switch off (5AM).

 

With a bit of extrapolation/guesswork, you could estimate that there is a 15°C DT below the element.

 

@John Carroll are you hoping to heat less water and once you have drawn of a bit, the element kicks in and extends the life of usable DHW enough for a shower?

If so, you are only putting in 2.8 kW which will only give you a small lift in temperature (does depend on shower flow rate).

 

Again, thanks, but a horizontally mounted heating element heats the cylinder in a completely different fashion to a top mounted vertical one.

[SteamyTea]

3 minutes ago, John Carroll said:

horizontally mounted heating element

Forgot that.

But my lower element is horizontal.

 

I am not sure how much difference it will make in practice, when heating, the water becomes quite turbulent. A combination of density and inertia does that.

[marshian]

33 minutes ago, John Carroll said:

All I am interested in is the HW cylinder outlet temperature while using a top mounted vertical electric immersio

I'm only interested in the HW (top) displayed temperature when using the (IF) you have a top mounted vertical immersion, this also incorporates its own thermostat,

"the bottom one where the boiler tank thermostat is", this is only used when you use the heating coil to heat the cylinder.

 

Oh I get that and 'it's why I might (in the interests of "science" ) move the bottom sensor up to a position on the outside of the tank level with the end of the immersion

 

It's a top mounted vertical immersion in a 115L tank and it has it's own internal thermostat just as yours does

 

Question will be at what point the internal thermostat signals re-heat and after how much water has been drawn off?

 

gut feel from how we've used it before is you can pretty much use 70% of the water before it will kick in again and try and reheat so maybe 40 litres

[marshian]

28 minutes ago, SteamyTea said:

Forgot that.

But my lower element is horizontal.

 

I am not sure how much difference it will make in practice, when heating, the water becomes quite turbulent. A combination of density and inertia does that.

 

I don't get a lot of turbulence with a vertical top mounted element and you certainly get a very defined stratification with minimal heat transfer to water - down side to my element is it's quite short, maybe less than 1/3 of total tank height so it really only heats about 50 litres max - just enough for two decent showers or a shallow bath

[John Carroll]

16 minutes ago, marshian said:

 

Oh I get that and 'it's why I might (in the interests of "science" ) move the bottom sensor up to a position on the outside of the tank level with the end of the immersion

 

It's a top mounted vertical immersion in a 115L tank and it has it's own internal thermostat just as yours does

 

Question will be at what point the internal thermostat signals re-heat and after how much water has been drawn off?

 

gut feel from how we've used it before is you can pretty much use 70% of the water before it will kick in again and try and reheat so maybe 40 litres

 

If you can move that as far up to the cylinder top as possible and run on the electric immersion and drain down as you say maybe 20/30L to get the stat to cut in, then wait until the stat cuts out, and repeat this test say one or ideally two more times  then that will provide the info I'm after hopefully.

Edited Wednesday at 20:19 by John Carroll