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Room Thermostat fo Central Heating

Control the Temperature in Your House with a Room Thermostat

Central Heating or Immersion Heater
It is usually cheaper to heat your hot water using your boiler if you have one. However if you are on an Economy 7 tariff it is cheaper to heat up the water overnight, during those economy hours. Using an immersion heater is the most expensive way of heating water and it’s especially expensive during the day if you are on an Economy 7 tariff, because while the electricity is cheaper during the night the daytime tariff is usually higher than normal.

Combi Boiler or Regular Boiler?
If you have a larger family then you will find that a regular boiler with a hot water storage tank is most cost effective, however if you have a smaller household, using less hot water, then a Combi Boiler is likely to be the most efficient (and cheapest) system for you. A combination boiler (Combi) heats water on demand rather than having a storage tank. If you are thinking of changing your boiler you may be eligible to take advantage of the Governments Green Deal Programme, read more here.

Water Heating with Economy 7
If you are on an economy 7 tariff your hot water system should be set to automatically turn on when the electricity is at its cheapest rate. You should make sure your tank is really well insulated so that the water stays as hot as possible for as long as possible.

Correct Use of Your Immersion Heater

If you run out of hot water during the day/evening you may need to put on the immersion heater, which is sometimes called a boost. To make this as economical as possible, you should run this for as little time as possible to get the hot water you need.

Immersion Heater – Mythbusters
You may have heard some people say that it is cheaper to leave the immersion on all the time rather than turn it on and off again. However, this is not really true. The immersion heater is usually positioned towards the top of the tank so that when it is turned on it heats the top layer of water in the tank. This means you quickly get a quantity of hot water that will be enough to wash up, bath the kids or have a shower. Once you have done that you can turn off the immersion heater and allow the normal water heating to commence on the next economy cycle.

If you leave the boost heater on then the convection current in the tank will start to allow the whole tank to be heated by the immersion, and because you will be using the higher rate electricity this will get expensive over time.

We did have an opportunity to try this out when the Economy 7 heater in our hot water tank broke a few months ago so, in the interests of DIY Doctor research, we did a controlled experiment for a couple of days and measured the amount of energy used and the relative price of the units used. Our findings agreed with the Energy Saving Trust’s recommendations.

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One Response to “Is Your Energy Bill Getting You Into Hot Water?”

  1. Padge says:


    Great entry! I’m just curious, how did you calculate the energy used?
    Did you use a smart meter with everything else turned off at the time of this experiment? Or, did you just time how long the immersion had been on and multiple and calculate the kw hours from that to get how much energy was used?
    The reason I ask is because, I have after reading this interesting entry, I noticed that you(maybe to keep it simple) didn’t make any reference to these two very important factors:

    A) Thermostat in the immersion heater.
    B) Insulated cylinders don’t just keep hot water hot, they also keep cold water cold.

    A. Immersion heaters don’t constantly use 3KW, they have a thermostat inside that cuts off the electricity when the water reaches the temperature it’s set to and doesn’t kick back in(turn on again) after the water has dropped 5-10 degrees below that.
    Therefore it uses a hell of a lot less energy than an appliance constantly using 3KW, especially if the cylinder is insulated, which leads me on to the other factor.

    B. Insulation works both ways, i.e. it keeps our houses warm in the winter(once we get the house warm inside) and it keeps them cool in the summer(it keeps the heat outside from coming in), despite what you may think, you’re better to have ac circulating cool air in your house than to open a window if it’s really hot outside.
    In hot countries near and south of the equator, they open their windows early in the morning before sun rise to let some cool air in, then they close the windows and pull their curtains/blinds during the day.

    -My point being here is that if you have let the water in your insulated cylinder drop completely, to say 15 degrees(or whatever your hotpress room temperature is), it will take a lot longer for your immersion heater to heat up that water than in a non-insulated cylinder, because on an insulated cylinder, the cold water will not have the benefit of dissipating it’s cold while being heated, like on a non insulated cylinder.

    I believe also, this ‘myth’ is to keep the sink/basin element of a dual element immersion on all the time. It’s shorter, uses only about 2KW of electricity and heats the water at the top of the cylinder mostly.

    I have not tried an expereiment to calculate it because me and the misses always just time it well and use just what we need but I think it’s really cool to see someone made the effort of doing this and recorded the results as well. Well done.

    If I was to try out a technique, I personally would, initially adjust the stat on the immersion down to just below 60 degrees, turn on the long bath element for about 2 hours to make sure the water on the bottom is good and hot too, stack as many towels, blankets on top of it to keep the heat in(as heat rises) and would have just the short sink element come one say 15 minutes every morning, maybe use the bath one for half an hour to 1 hour once a week top top up the cold water at the bottom.
    If we had kids to bath every other day, this would obviously increase a bit.

    I am open to getting thorn a new one so if anyone has their own 2 cents, I’ll hear them.

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