Water too hot

22 May 2004
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United Kingdom

I wonder if anyone can shed some light on this? I have just moved into a house that is approx. 11 years old and uses an indirect unvented hot water cylinder (oso indirect 20 RI) to provide hot water to the taps. Unfortunately, I have come from a house that had a combination boiler and I haven't worked out how this new system works. The hot water is far too hot! The boiler is a Vaillant ECOmax 618 E and doesn't seem to have a water temperature control. I don't understand how the temperature of the water is controlled? Can anybody help - I'm worried that my children will burn themselves if I don't get this sorted.

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There will be a panel on the side of the cylinder and inside will be an adjustable thermostat. It can be adjusted but do not set the temp much below 60*C and be careful as it will be electrically live in there.(that is unless you isolate)
It's probably only due to the fact that combis very rarely deliver water at 60C, and you are not used to such hot water.
Make sure that the immersion heater(if fitted) is not switched on.
Hi Netview

You will also need to see if the expansion bubble is intact within the cylinder.
To do this, turn off the cold supply in the airing cupboard next to the cylinder, then go to teh nearest hot tap and open it. If the hot water runs out of the tap for less than 10 seconds and stops, you will know that you have to recharge the cylinder.. instructions are on the cylinder...or ask a registered unvented installer to do this.

It can be that if the bubble is not in situ that water is drawn off from mid way down the cylinder via a dip tube and water that is above the dip tube is being super heated.

The first issue is to ensure your cylinder is operating how it should in the first place and then problem solving can progress from there.
Thanks 45yearsagasman & osofarsogood. I will check both these.

Just to get my head around this system...does the tank continuously fill with cold water which is heated by some sort of heat exchange provided by the boiler...or...does the boiler heat water and deliver it to the tank on demand? If the later is the case, how does the tank control the temperature?
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As hot water leaves teh cylinder from the top outlet, cold water feeds in at the bottom.
You are correct that there is a heat exchanger or 26kw coil inside the cylinder which has hot water running through it supplied via the boiler. This coil transfers it's heat to the surrounding water in the cylinder.
There is a cylinder stat at the bottom of the cylinder as previously described by 45.
The tank stores the heated water until required.(by you turning on the taps)
The temp is controlled by the stat fitted to the tank.
If no one uses hot water the tank heats up and the stored water just sit there until required. It is possible to exhaust the supply of hot water if many long showers are taken one after the other and the stored water is all used before the boiler can recover it.
Thanks both. So from what you have said, I don't need to program the central heating thermostat for hot water on/off (even though this facility is available) as the tank will always store water at the temperature set on the tank thermostat.

Does that mean that if the water temperature falls below the set temperature it will call for heat from the boiler (even if it is in the middle of the night?).

Also, if we left the house for a 2 week holiday, how do I stop the water continuously heating unnecessarily?
you need to tell your programmer when you want hot water and how long you want it for.. i.e 90 minutes in the a.m. and 90 minutes in p.m... depending on your house requirements. If the cylinder needs replenishing it will ask the boiler and if this is during your DHW ON period then it will replenish.. if not then nothing will happen until the DHW ON period starts again.
some programmers have a holiday mode that will allow you to flick the switch to that mode and all previous timings will be ignored until you switch it off holiday mode.
you will also need to allow about 30 minutes for the cylinder to heat up for the hot water availability during that DHW ON period, as unlike a combi it isn't instaneous, although you may have hot water stored from the previous period.
these unvented cylinders are very well insulated.

If you use the HW timer on the boiler to come on (say) half an hour before you get up in the morning, and half an hour before you get home at night, and to go off after you usually have your baths and showers, it will save a little gas as the boiler will not be running for short periods to top-up (which is quite inefficient), but if you leave it on 24hours a day, and go on holiday, the heat loss from the cylinder is something like between half a kWh per day (about 2p worth of gas) and 2 kWh (about 8p worth) from memory, so I seldom bother turning mine off. You may have noticed that the airing cupboard is not very warm, due to the good insulation, what heat there is probably comes off the pipes as much as the cylinder. In winter, when the CH is on, heat losses from the cylinder and pipes just contribute to warming the house, so are not "wasted" heat.

Once heated, the cylinder will stay hot for a matter of days, and will easily hold enough water for washing up and so on. It will only get used up when you have baths and showers, depending on its size. Keeping it at 60C or thereabouts will prevent growth of microorganisms in warm water (you will have heard of Legionella, which is rare but severe. It cannot multiply in water above 50C and is killed at 60C or higher*.

*D Hailsham published the temperature table on //www.diynot.com/forums/viewtopic.php?p=1008964#1008964 which is quite informative
I'm worried that my children will burn themselves if I don't get this sorted.
The solution to that is to have blending valves fitted to each basin/bath etc., which will limit the outlet temperature to 40C or whatever temperature you want.

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