electrical: immersion heater with two heads

Just trying to set the temp. on an immersion heater - which has two thermostats/elements (?) http://tinyurl.com/l9b9772 There are cables to both that could carry 13A each no problem. I don't have any info on this settup and just want to reduce the temperature a few degrees as the water is too hot for the user (not me).
A slight change to the top one set-point seems to result in quite a large change +/- 5 deg or so, so don't want to fiddle too much. Is there a "proper way" to set these things? Thanks
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Is one above the other? If they're both at the top, one may be a long reach one to the bottom, and the other a short reach one just heating the top.
If so, the top one is to heat up enough for wash basins or a shower, and the bottom one for a bath. Alternatively, the bottom one is used to heat the whole tank on cheap rate electricity, and the top one for topping up during the day on expensive electricity if the hot water runs out. With some setups, both are used when heating the whole tank on cheap rate.
They should probably be set to same temperature.
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Andrew Gabriel
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You have two separate immersion heaters there, each with own thermostat.
It is probably so that small or large amount of water can be heated to meet requirements.
Or there might be alternative prices/sources of electricity, (ie some energy/moneysaving setup.) Normally it's best to set the thermosts just high enough to meet the needs. (To save electricity)
Typically the lower one would be set so that there was enough hot water for a bath. And the upper one set so that water is hot enough for hand/dish washing. A lot depends on whether only one heater canbe turned on at a time or both.
You have to mess around with the settings so that there is enough water without wasting electricity. There is no one-setup-suits-everybody.
You're probably getting large temperature swings because the two heaters are adversely interacting.
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Just discovered a scrap of paper showing what these are (which was basically answered by the responders) - but just to be complete.
There are two elements- one above the other. Only one can be on at once. One is fed from the wall-mounted timer, the other comes on when the Boost button is pressed. However Boost is only available when the wall-mounter timer is off. ie they cannot both come on at the same time. I think the worlkd "boost" is confusing. To me it sounds like both elements can come on at once to heat the water in less time. In fact it just heats the water with the other element. Mention is made of econ7 - this place has *no* E7 - just a single tariff.
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