250l UV Tank

[Madrab]

To give you an idea of the internal setup of that cylinder -



As you can see ( 1st pic isn't really to scale as the immersion and coil would usually be smaller, relative to a 250L cylinder) their design isn't as efficient as a normal vertical cylinder will be at heating the water through convection and the immersion would take an awful long time to heat that volume of water up. That and the immersion's stat will usually shut it off a lot sooner and more than probably temp at the stat pocket (stat for the coil/CH system) reaches set point. Also, can I recommend you increase the cylinder stat to at least 55deg

 

[Johntheo5]

Where are those two immersion stats located?????,
The immersion heating element will only heat directly as far down as the bottom of the element, eventually in will slowly heat up the remaining layers of water below it due mainly to conduction or whatever so the cylinder stat will rarely if ever be satisfied if using the immersion(s), also even though Madrab's schematic may not be to scale, it does look as if that cylinder pocket is actually below the coil, the stat (pocket) is normally located at least a little above the bottom of the coil otherwise it will give very inaccurate temperature control.

 

[johnwa]

Hi John the first of Madrabs sketches is the right layout and the second immersion is in the same location at the other end.
I appreciate you guys helping and I am trying to grasp the way the tanks water circulation works! still trying
The tank stat was closed this morning after 7 hours of the thermostat being on, so I did a boost on the Hive and it took less than half an hour for the tank stat to open so I guess there is some activity from the 240v side of things. I will fit the new Intelligent Thermowatt immersion which they should have fitted at the factory! they sent me one after the tank was installed so I will have to drain the tank top a bit and fit it.
Do you think the second Immersion would be worth wiring in? my 78-year-old head says it will give a more uniform heat across the top of the tank? will it help somehow with the bottom?

 

[Mister Banks]

You will need a qualified engineer to work on an unvented cylinder. This is not a diy job.

 

[johnwa]

Thank You,
The tank was installed by a qualified engineer with a certificate, the electrical side is not down to a plumber, I have had 55+ years in all forms of the electrical/electronics trades so I am happy to do my bit and get it checked. I built my house it took me 3 years and "all the trade jobs" were done by myself in conjunction with Building Regs.

 

[Madrab]

grasp the way the tanks water circulation works

Do you mean how the water is heated within the cylinder?

That works simply by convection, the coil heats the water it's in contact with by conduction, it rises and displaces the colder water and that circulates down - that water movement then creates a convection current and the water circulates like that, absorbing heat until it heats up to the stats set point.

Given the volume of water that sits below the coil, which is significantly more in a horizontal cylinder than a vertical one, the % of hot water they actually store is usually significantly less.

 

[johnwa]

Hi Rob thanks for the info so in effect, if the boiler is not switched on the bottom of the tank will never get hot using the immersions? warm maybe?

 

[Madrab]

Hi Rob thanks for the info so in effect, if the boiler is not switched on the bottom of the tank will never get hot using the immersions? warm maybe?

Yes ... that's why the immersion element(s) in an indirect cylinder are only intended as a stop gap when it's configured as an indirect (heated by CH).
When it comes to directly heated cylinders - where the primary heat source is the immersion heaters, then usually the 2 immersion heaters would work together to heat a larger portion of the cylinder though again, it can only heat down as far as the actual length of the element.

In a vertical direct cylinder - that has 2 immersion elements as well - there is a main element in the bottom of the cylinder that heats the whole cylinder and one in the top 1/3rd that provides a quick boost say for sinks or a shower. Usually with these cylinders the only energy source is electric so the bottom element is run overnight on cheap rate to keep the costs down.

Your cylinder is the Tempest model that can be both - That's by design from Telford

 

[Johntheo5]

Hi John the first of Madrabs sketches is the right layout and the second immersion is in the same location at the other end.
I appreciate you guys helping and I am trying to grasp the way the tanks water circulation works! still trying
The tank stat was closed this morning after 7 hours of the thermostat being on, so I did a boost on the Hive and it took less than half an hour for the tank stat to open so I guess there is some activity from the 240v side of things. I will fit the new Intelligent Thermowatt immersion which they should have fitted at the factory! they sent me one after the tank was installed so I will have to drain the tank top a bit and fit it.
Do you think the second Immersion would be worth wiring in? my 78-year-old head says it will give a more uniform heat across the top of the tank? will it help somehow with the bottom?

Yes, I think using both immersions should give more uniform heating but can't see it helping much with heating the water below the bottom of the elements, the problem with top mounted immersions on any type of cylinder is that the heat emitted per unit length of the element is practically the same throughout so if say the upper 1/2 of the element is immersed in hot water at say 50C and the bottom 1/2 in water at 35C, the element will start heating (assuming set to 50C) but since the rod stat is measuring a average temperature then a good portion of the cylinder top might end up at say 60C or higher with a gradual reduction in temperature maybe down to 44C/45C which will still open the stat at 50C. A immersin installed horizontally at the bottom of the cylinder will give a almost even temperature rise throughout the cylinder. Some hold that the hot water will rise to the top of the cylinder but thats certainly not my experience with 3 separate heating sources in my vertical vented cylinder. I have a solar coil right in the bottom of the cylinder, a (boiler) coil probe further up and a top mounted immersion (with another probe) that only heats 30L, I have often seen the top 30L at say 50C, the next probe further down at say 40C and the solar probe at maybe only 25C in the morning, when the solar coil starts heating the cylinder the probe at 40C will remain exactly at 40C until the solar probe is at 40C, they will both then rise at exactly the same rate (within 0.1/0.2C), until both reach exactly 50C, the whole cylinder will then start rising in temperature completely uniformly, I allow my cylinder to heat to 80C but I have a anti scald thermostatic mixing valve on the HW outlet.

 

[Mister Banks]

Isn’t heating the cylinder to 80c just like heating the house and then opening all the doors and windows. That’s a lot of wasted energy.

 

[Johntheo5]

Heated to 80C only with the solar (themal) panels, it increases the cylinder HW capacity "thermally".

 

[Madrab]

Do you think the second Immersion would be worth wiring in? my 78-year-old head says it will give a more uniform heat across the top of the tank? will it help somehow with the bottom?

If your primary heat source is the CH and the coil then no - As suggested - you shouldn't ever need to use the immersion if the CH is working. With an indirect system, which yours is, immersion is a backup only and shouldn't every be used unless the CH system fails, it's a huge waste of money.

 

[Madrab]

Heated to 80C only with the solar (themal) panels, it increases the cylinder HW capacity "thermally".

If there is access to available 'free' energy (ETST system) then would be silly not to use it - increasing the 'heat capacity' of the cylinder by using it as a thermal store then in turn reduces the need to use energy that costs directly at point of use. In effect it's increasing the amount of 'usable' temp HW in the store for the given volume, using the TMV.

 

[johnwa]

If this is a lame question sorry! On dipping the tank when I fitted the Thermowatt learning immersion/stat there was another 100mm of depth, why don't they fit a longer immersion?

 

[Johntheo5]

Possibly because its one of the standard length elements, did you measure the actual length of this one and the other one?.