Help with understanding my hot water

nes04

Hi,

I have moved into my first flat in London with my partner. It is in an apartment block that is around 5 years old and it does not have a gas supply. For hot water, it has an unvented water cylinder installed. I have two switches next to it - one is for the 'heat exchange' and one for the 'cylinder'.

At my parents house and at university I have been used to having a combi boiler with electric showers so I am very confused about the water cylinder that is installed and how it works. I see I have a Danflo thermostat which can control the central heating and the hot water on and off.

1) When the water is heated and some of it is used, does cold water enter it straight away and therefore cool down the existing water? Or does the cylinder not refill until it is empty?
2) Question 1 leads me into this... how often do I need to turn the hot water on for each day? Should it be on for most of the day or just the morning or all night? This must be an expensive way of heating water?
3) Will the central heating not use some of this water?

Sorry for the stupid questions - I am just very very confused

AndyPK

1) Heat rises, so the hot water is at the top of the tank, and is drawn off from the top. So, no, cold water entering the tank (at the bottom) doesn't really cool it down.

2) Say an hour in the morning, and 2/3 hours in the evening. Depending on yours and your partners routine in the morning. (It will shut off when upto temp)
(If you have economy 7 electric supply, the other way round)


3) No

Cardew

The advantage of an unvented water cylinder is that you will get Hot water at mains water pressure.

As indicated above, you can experiment with the length of time you need to have the immersion heater on each day.

However a modern system like yours will mean that(by law) the HW tank will be very well insulated and thus the heat loss will be small.

That heat loss will probably be marked on the tank. I have just had a large tank of a similar type to yours fitted. With the tank on permanently and the temperature kept at 65C(higher than you would require) the heat loss is 54 watts. Thus over 24 hours the heat loss would be 1.3kWh.

Now you are going to have some heat loss however careful you are with having it switched on for the optimum time, and you will not be having the water at 65C. So in the real world having it switched on 24/7 or for 2 or 3 hours a day, means a loss of around 0.5kWh a day(say 5p).

The other factor is that heat loss warms the fabric of the property so isn't really 'lost' for much of the year.

The overall point is that the length of time a modern HW tank is heated isn't critical. Many people waste more electricity in a year by boiling a full kettle every time they make a cup of tea!!

footyguy

AndyPK wrote: »

...
3) No

:huh:

Assuming the CH the OP refers to is via water fed radiators, then how is the water heated?

I would say if Cardew has it right, especially if the design of the system is to allow for mains pressure hot water, then the OP should leave the power on for the hot water all the time. That will be expensive as it is powered by electricity, but if there is no draw (either hot water or space heating) then the thermal losses should be low, and so whilst the power is on permanently, the electrical heater will not be (as it is thermostatically controlled and so only heats when needed)

If the CH is a water fed design, then yes, when the central heating is operated, it will draw (recirculate) hot water directly from the system. (This is probably the same hot water as that used to heat the mains pressure hot water via a heat exchanger)

Cardew

nes04 wrote: »

1) When the water is heated and some of it is used, does cold water enter it straight away and therefore cool down the existing water? Or does the cylinder not refill until it is empty?
2) Question 1 leads me into this... how often do I need to turn the hot water on for each day? Should it be on for most of the day or just the morning or all night? This must be an expensive way of heating water?
3) Will the central heating not use some of this water?

Sorry for the stupid questions - I am just very very confused

Lots of people seem to misunderstand about how Hot water Cylinders work.

If you have 'wet' Central Heating to radiators(i.e. hot water fed from a gas/oil/electric/lpg/solid fuel boiler), the hot water in the cylinder you get fed to your taps is not mixed with the hot water to your radiators.

The hot water from the boiler is on a separate pipe which enters the tank. This pipe inside the cylinder is in the form of a coil which transfers the heat from the hot water in this pipe to the water in the cylinder - i.e. indirect heating.

http://www.gasapplianceguide.co.uk/images/Gledhill/InDirect%20Cylinder%201cX.jpg

That same hot water from the boiler can be diverted to the radiators.

Owain_Moneysaver

nes04 wrote: »

For hot water, it has an unvented water cylinder installed. I have two switches next to it - one is for the 'heat exchange' and one for the 'cylinder'.

Unusual terminology rings warning bells.

Does this flat have a centralised boiler? The 'heat exchange' switch might control a valve for bringing central heat into your cylinder, and the 'cylinder' switch a local immersion heater as a backup?

Ask your neighbours / porter / managing agent or factor. Do you have a heat meter or receive a bill for heating?

Using the 'cylinder' immersion might be a lot more expensive than heat from the heat exchanger (which might actually be a central gas boiler).