Hot water cylinder temperature difference

mp3guy

I have a fairly large hot water cylinder (about 160cm tall). It has a fairly standard setup, indirectly heated by my oil boiler with a cylinder thermostat and a dual element immersion on top.

I'm having problems with stratification since installing a hot water diverter for my solar PV.

The boiler cylinder thermostat is mounted about 1/4 of the way up from the bottom of the cylinder. When I divert PV energy into the immersion, eventually the internal thermostat on that opens (at 63c, measured at the tap) and all is good. But the cylinder thermostat down towards the lower end still only reads 40c (it's a new thermostat, double checked this measurement on the tank side also).

Is my only solution installing a destratification pump? Are these easy to retrofit?

I checked the in flow and out flow temperatures of the coil and it's ~65c in and 61c out, is that expected?

John.G

Assuming that the PV divert feeds the "bath" element then if the end of this element (depending on its length) is higher than cylinder stat then the water below that will be cold/cooler, also the immersion stat rod length will influence the immersion cut off.

The coil 65/61C will give a very rapid heat up time.

Lenar3556

Would fitting an second (initial) pre-heat cylinder allow for more efficient/complete utilisation of this excess PV?

Is it the case that that you are not able to utilise much of this excess PV because your cylinder is effectively already at temperature? And necessarily so, as you need a constant supply of hot water.

A destrat pump is going to have an energy consumption in itself, and may not unleash that much greater thermal capacity within the cylinder.

You could also check the element, and see could you replace it with a longer one to reach closer to the bottom of the tank.

John.G

Top mounted immersions will really only heat ~ 70% of the cylinder volume in fairly standard 1050 mm high cylinders, the immersion lengths are 24",27",36" & 42", if the installed one is 27" then increasing it to 36" will give 33% increase in the hot water volume and a 42", a 55% increase but very unlikely that a 42" can be installed without fouling on the heating coil but a 36" should possibly fit, if not already installed.

mp3guy

Lenar3556 wrote: »

Would fitting an second (initial) pre-heat cylinder allow for more efficient/complete utilisation of this excess PV?

Is it the case that that you are not able to utilise much of this excess PV because your cylinder is effectively already at temperature? And necessarily so, as you need a constant supply of hot water.

A destrat pump is going to have an energy consumption in itself, and may not unleash that much greater thermal capacity within the cylinder.

You could also check the element, and see could you replace it with a longer one to reach closer to the bottom of the tank.

Well I can dump about 2.5kWh of heat into the immersion element before it reaches temperature while the water down near the boiler stat still reads 40c. So if I get a pump I can mix that water and put more energy into the tank.

Not too concerned about pump power usage as it will only be intermittent and probably around 100W.

If I'm looking at getting a longer heating element, I don't think a pump is that much more difficult.

John.G

The pump will definitely realize the greatest benefit as by calculation 2.5kwh will only heat ~ 50 litres from 20C to 63C. Even if the HW cylinder is only 200 litres, by using the destrat pump you can potentially soak up ~ 10 kwh from the divert.

Tree

So did you get a destratification pump in the end OP?

We have a 167 L slim tank, and the diverter is saying max temp reached after ~2.5 kWh which definitely isn't a full or half full.

mp3guy

I didn't bother!

John.G

(Reply to Tree's post)

IMO, to realize the full diverter benefit means either having two separate heating elements, one mounted high up in the cylinder, the other mounted low down, the upper one would be fed from the normal mains supply to heat say 30 litres which will consume 1.6 kwh, the other, lower element will be supplied by the diverter to heat, in your case, a minimum of 167-30, 137 litres (if top 30 litres is already at 60C) and require 7.2 kwh from cold, or 8.7 kwh if the whole cylinder requires heating, or you maybe utilizing the "bath" element of a top mounted dual immersion.

What is you're set up and where is the the thermostat mounted that tells the diverter that the water is up to temperature?, should be able to figure it out then.

If the thermostat is part of the dual immersion and if the bath element is utilized then then a large volume of the water will get to say 60C depending on element length and stat setting. If the element is in the lower part of the cylinder then the stat must be located slightly above this to give a true reading or it can be integral to the element.

Tree

The new tank (apparently the pump wouldn't fit in beside the old one) came with its own single element immersion/integrated thermostat as far as I can tell. The side of it says max immersion length is 700mm, which seems small to me, but I am not a plumber. Gas central heating is going to put some watts into it, but I don't tend to run that from April to October as the amazing zoning on our 20yo house means it's all or nothing, no seperate loop for the water.

John.G

The height (dimensions) of the cylinder are probably stamped on it, I'll assume 1060 mm so the 700mm immersion will heat roughly 100 litres, allowing for the conical top. 100 litres from say 25C to 60C requires 4.1kwh, or working backwards 2.5kwh will heat 100 litres to 60C from 38.5C, so the normal best performance is around 4.1kw but if only using the immersion in the summer then possible to absorb 5.2kwh if whole cylinder fell to 15C. You can fit a longer element, (be careful not to foul (hit) the coil) and realize further savings but your present coil is ~ 27 ins and I doubt if anything longer than 30 ins will fit, not worth the expense IMO.

Of course, you may only have a 11ins element, in which case you will be only heating ~ 60 litres, so absorbing 3.1kwh from 15C to 60C, well worth in this case to install that 27 ins one.

Tree

It's the SEIE48X18IND so 1200 tall . The pump for the shower coming out lower also reduces the amount of accessible hot water by about 15cm. The immersion that came fitted is a TIH 650, so in theory it's already max length :-/

I think "you'll get two good showers out of that easily" was an inaccurate statement by the plumber :-/