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Discuss Immersion heater only heats top of cylinder in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

I

immerse

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I have just got my system going with a home-built controller to use 'excess' PV generated electricity to heat my indirect hot water cylinder. This has a top entry 3kW Steeple 80027BT immersion heater (which I imagine is 27 inches long, I cannot find any technical data) and a Diamond WT18 thermostat, which I believe is 18 inches long and appropriate to the immersion heater. I use a step down transformer to deliver 1.5 kW into the immersion heater when there is sufficient 'spare' PV power available.

This all works as expected except that at the end of a long sunny day the water temperature is highly stratified, very hot at the top and only mildly warm half way down the 90 cm x 45 cm cylinder, where I would have expected the thermostat sensor to be positioned. There is plenty of energy available, about 12 kWh, enough to raise the whole cylinder from room temperature to boiling point. Conceivably this could arise because the immersion heater is not being driven to its design operating power - can anyone comment on this, please? (A number of other people, using either transformers or Triac load switching, comment that they get a full cylinder of hot water).

Alternatively, there could be some sort of fault with the heater or thermostat (although the max. temp. of the water is about the 70 deg C I set on the thermostat). Or maybe this arises because the heater is a long thin thing delivering heat over its whole length? If so, why do others not experience the same problem? Is it possible to purchase a 27 inch immersion heater wherein the whole power is delivered at or near the bottom end rather than along the whole length? This would lead no doubt to better convective stirring and less stratification.

Any advice would be gratefully received!
 
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Worcester

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Mentor
Arms
Best thing to do is to add a pump to circulate the water - a de-stratification pump - it's part of the stanadard installation proposals that we recommend, and using the Immersun controller has the facility to control that built in. The Immersun monitors the immersion heater, when it swithes off, starts up the de-strat pump, then a few minutes later switches the immersion back on. Voila - a ful tank of hot water :) All from "free" leccy!

pm me for more details.
 
I

immerse

  • Thread Starter Thread Starter
  • #3
Thanks for this advice. It seems to me that quite a feeble pump would suffice, since a cubic meter/hour should be plenty at what must be a very low pressure differential. What sort of pumps are used? I daresay canned-rotor glandless circulation pumps are a favourite, but there must be smaller/lighter/cheaper solutions?

Have had a look at Immersun. These PV immersion heater proportional controllers are almost at a reasonable price now!
 
Z

zeljuga

  • Thread Starter Thread Starter
  • #4
At 2.5 kw/24h Heat loss for top insulated cylinders(older ones up to 10kw/24h heat loss) all storey with dumping excess PV output in to it is highly questionable. As converting energy of photons thee time over accrues in conversion loss up to 95% roughly, not counting loss in cable resistance and hot water pipes/taps.
Storing Hot water at close to boiling point is another waste of energy as human can hardly recognise temperature difference between 45 C and 50 C. Where 45 C is ideal temp, for comfort of use and reduced heat loss in cylinder.On top of that you may incur scalding problem ( if storing at more then 75C) at point of hot water delivery which needs to be addressed separately.
Overall can of worms. But as good as it gets solution for now.
 

Worcester

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Mentor
Arms
S

sambotc

  • Thread Starter Thread Starter
  • #9
Yes you need a bronze pump, can't see a standard pump lasting long in this setting.

Never thought of the issues you've mentioned using this idea before now but it makes sense, top entry immersions are designed more as a back up. The thermostat being at the top of the cylinder is the issue (not just for this kind of set up)

These immersion switches could turn out to cost to much to install unless a simple product is found as a solution, bronze pumps are about £200
 
M

mountainman

  • Thread Starter Thread Starter
  • #11
Could it ne that someone has stuck an 11" immersion into the top of the cylinder?
 
S

sambotc

  • Thread Starter Thread Starter
  • #13
Out of interest what is the cylinders size/dimension? just in case you didn't know, you can remove the thermostat from the immersion element with out having to drain down.
 
I

immerse

  • Thread Starter Thread Starter
  • #14
To the responders of the last 3 posts: I give the tank size and immersion heater/thermostat specification in the first post of this thread. Although I can find no info about Steeple, the 27 in the model number suggests a 27 inch length, and the thermostat is definitely 18 inches long. If you think about the effect of convective heat transfer stratification is inevitable.
 
I

immerse

  • Thread Starter Thread Starter
  • #15
How about a plastic body to avoid the high cost of stainless or bronze? I see Grundfos make a model PU.120 for example which is plastic bodied for potable water applications. Unfortunately I cannot find it on the Grundfos website, the info is on the Euro Boiler Parts website. I'll bet someone will come back that this is only for cold water use.

Washing machine pumps are plastic bodied and obviously tolerate very high water temperatures, so plastic should be OK as long as the right material is chosen, with no leaching of plasticiser for instance. Does anyone know what washing machine pumps are made of? The material is white and looks like acetal or polyamide. Of course the building regs could outlaw such a solution on 'elf and safety grounds.
 

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