Is it more cost effective to leave the immersion on all the time?

macman

gordon2 wrote: »

Oh I agree, switch it off or have it timed. I just think the comparison that Cardew trots out is a bit missleading.

I have an immersion but it has not been switched on for years, last time was when I wanted to see if it would be cheaper to heat my hot water by gas or electricity, as I have a hot water system that has a pilot light on all the time which uses 4 kWh of gas a day and there is no timer to switch the gas on and off so my hot water is on all the time.

My conclusion was to leave my pilot light on and the hot water on all the time as it was still cheaper than using an immersion.

In the summer with no heating I use about 20 kWh of gas a day for hot water and cooking.

Cardew's comparison is perfectly valid, he was just taking the OP's proposition to it's logical conclusion in order to demonstrate it's absurdity.
Your own comparison is not relevant in this case, as the OP did not mention burning a pilot light as their alternative to running an immersion 24/7. The alternative was presumably to use the immersion on a timer or on-demand-which is the economic approach.

gordon2

Are you sure it's 20kWh.

Yes I am.

So 2 showers and a bath a day and cooking for 2 would cost you less than 80p a day?

Actually I agree with you it is a bit much, but the rubbish hot water system I have uses that.

Gareth56

I suppose you could work out from physical data how much energy (in Joules) is used to raise the temperature of whatever volume your tank is of water from say 20 degC to say 70 deg C.

For example:-

To heat 1kg of water from 20degC to 70degC takes

4.19kJ/kg.degC x 1kg x (70 - 20) = 210kJ


Then I suppose to see how much [approximately] it costs to keep the volume of water in your tank at the set temperature of the immersion heater, take an electricity meter reading on one day then try to limit any high using electrical appliances keep the immersion heater on for 24 hours then take another reading the next day. Then just work out the kWh used. Admittedly it's not gong to be very accurate if you use TVs, electric kettles etc but it'll give you a good approximation if you keep those to a minimum.

Then just compare the two figures.

Physically it does take less energy to heat water that is already hot'ish to a slightly higher temperature rather than heat cold water up to the same higher temperature. It becomes a moot point deciding whether it's cost effective leaving the immersion on continually just to maintain the already hot water at a set temperature rather than reheating a tank full of cold water each time.

Cardew

Gareth56 wrote: »

It becomes a moot point deciding whether it's cost effective leaving the immersion on continually just to maintain the already hot water at a set temperature rather than reheating a tank full of cold water each time.

It doesn't!

It really really doesn't!

Gareth56

Cardew wrote: »

It doesn't!

It really really doesn't!

QED:D


However I suppose the only definitive way to prove the hypothesis is to carry out the experiment over a normal usage time interval. Also I'm not too sure which Law of Thermodynamics is supposed to apply here.

HappyMJ

Gareth56 wrote: »

QED:D


However I suppose the only definitive way to prove the hypothesis is to carry out the experiment over a normal usage time interval. Also I'm not too sure which Law of Thermodynamics is supposed to apply here.

The first law. Experiments at home never work properly. You need the exact same conditions. The exact same ambient temperature. The exact same amount of water usage and so on.

grahamc2003

Gareth56 wrote: »

QED:D


However I suppose the only definitive way to prove the hypothesis is to carry out the experiment over a normal usage time interval. Also I'm not too sure which Law of Thermodynamics is supposed to apply here.

No, you don't have to experiment. It's a direct application of a thermodynamic Law (not a hypthesis).

Heat loss is a function of temperature. The higher the temperature, the greater the heat loss. That's all you need to know. It follows from the second law from which you can derive that net flows of heat energy are from hotter to cooler bodies only.

The higher the temp, the more the heat loss and the more electricity you'll use to maintain the temperature.

Gareth56

HappyMJ wrote: »

The first law. Experiments at home never work properly. You need the exact same conditions. The exact same ambient temperature. The exact same amount of water usage and so on.

Given certain data you can calculate the amount of energy required to raise a known mass of water from T1 to T2 thereby avoiding the need for any experiments. Where it gets difficult but not impossible is determining the amount of energy over a certain time period to maintain that temperature.

grahamc2003 wrote: »

No, you don't have to experiment. It's a direct application of a thermodynamic Law (not a hypthesis).

Heat loss is a function of temperature. The higher the temperature, the greater the heat loss. That's all you need to know. It follows from the second law from which you can derive that net flows of heat energy are from hotter to cooler bodies only.

The higher the temp, the more the heat loss and the more electricity you'll use to maintain the temperature.

I would have thought the degree of insulation would also have to be known in that the amount of heat loss from an uninsulated hot water tank is higher than that from an insulated one.

Cardew

Gareth56 wrote: »

I would have thought the degree of insulation would also have to be known in that the amount of heat loss from an uninsulated hot water tank is higher than that from an insulated one.

There will always be heat loss! All better insulation does is slow that heat loss.

This is why the 2 years - 1 year - 1 month - 1week - 1 day - 1hour analogy is relevant.

If you concede that it will use more energy to maintain the hot water temperature for two years, you have agreed that the laws of thermodynamics are correct.

It then follows that for any other period of time those laws still apply!

macman

If all the hot air that has been expended on this topic could be saved, we could probably decommission all those older generating stations rights now...
I've decided to leave my central heating on all summer-this will surely save on my gas bills by avoiding warming the house up from cold come the colder weather...