Immersion or Gas central heating for my shower?

Dara Robinson

I live in shared rented accommodation and a discussion in the house broke out recently about bills and we got talking about the water for everyone's showers. Normally we put on the heating for an hour when someone wants to have a shower but the discussion is because when there is a few people wanted showers at different times then it gets out of hand and that some are saying that putting the immersion on for 30 mins or so would be a much cheaper alternative as our place does not separate hot water heating and the central heating.

So.... which is best? Central heating for an hour or immersion for an hour??

Any thoughts?

eth0

if you don't actually need the heat from the central heating i'd say immersion

dardhal

Dara Robinson wrote: »

I live in shared rented accommodation and a discussion in the house broke out recently about bills and we got talking about the water for everyone's showers. Normally we put on the heating for an hour when someone wants to have a shower but the discussion is because when there is a few people wanted showers at different times then it gets out of hand and that some are saying that putting the immersion on for 30 mins or so would be a much cheaper alternative as our place does not separate hot water heating and the central heating.

So.... which is best? Central heating for an hour or immersion for an hour??

Any thoughts?

You can roughly calculate the energy cost for both options, which will you a more precise idea of what is the answer to the original question.

The immersion tank I used in my former apartment had a 3.75 KW power rating, but the tank was very large (maybe 150 litres or so), and there is no point in heating the tank for only one hour, as the energy released is not enough to heat the tank water but for some degrees (and you'll end up having a rather cold shower). A completely different story is the "power showers", which only heat the water as it comes out of the tap, and for that reason, they haver higher power ratings, but at the same time they don't waste energy heating water to be stored for hours and then slowly loose temperature to the environment.

Take a KWh (unit) average price of 18¢ to calculate the hot water heated by electricity cost, except if using some kind of night saver plan, when units (KWh) are around the 9¢ each mark.

For gas, it's more difficult to theoretically calculate cost. You can take readings both before and after the test (measured in cubic meters), multiply by 11.1616 (conversion factor between gas cubic meters and KWh), and then by the KWh cost for gas, which is around 5.5 ¢. Theoretically you could calculate the consumption of the boiler by looking at the rated power (23 KW is a typical value) and assuming it doesn't modulate power and works at the rated power continuously, which is usually NOT the case.

By the way, I don't see the point in putting the heating on to have a shower. Boilers start automatically whenever they are needed, and all boilers in the market (that I know of) have different circuits for domestic hot water and heating, so as soon as you open you hot water tap, the boilers starts itself. Some boilers can work at the same time in hot water and heating mode, and other prioritize hot water against heating (that is, is heating is on, and you need hot water, the heating function just stops until hot water is no longer being used).

Dara Robinson

dardhal, while I appreciate the information its not practical in my situation. We live in rented accommodation and there is almost always someone else there and I would have no way of excluding their electricity use from any tests I ran. Its just not practical
Also, we do not have a boiler or an electric shower. As mentioned, we have central heating connected to the tank and electric immersion

Bruthal

dardhal wrote: »

By the way, I don't see the point in putting the heating on to have a shower. Boilers start automatically whenever they are needed, and all boilers in the market (that I know of) have different circuits for domestic hot water and heating, so as soon as you open you hot water tap, the boilers starts itself. Some boilers can work at the same time in hot water and heating mode, and other prioritize hot water against heating (that is, is heating is on, and you need hot water, the heating function just stops until hot water is no longer being used).

The different circuits that boilers have, that you speak of, depend on how the system was installed, rather than the boiler itself. Different circuits, or zones as they are called, simply have motorised valves controlled by a thermostat in the area that is being heated by the circuit controlled by that valve, so the usual would be upstairs rads, downstairs rads, and the hot water cylinder, each of which has a stat, and a seperate channel in a 3 channel time clock (for a 3 zone installation).

So if it is just set for heating the water, it will maintain the water temperature by cycling on and off, but does not actually just come on when a tap is turned on. So without using any water at all, and with the radiator circuits off, the boiler would still cycle on and off, although a lot less than if hot water is being drawn off, unless its a tankless, on demand water heater.


For the OPs question, it is usually cheaper to heat the hot water cylinder with the gas than with the electric immersion. A 3kw immersion costs about 55 cent to run for an hour, assuming the stat does not switch off, which for the first hour of heating the cylinder from cold, it probably wont.

From doing the very test that dardhal mentioned, taking before and after meter readings, I found heating the cylinder for an hour with the gas cost about 25 cent when heating from cold.

Bruthal

Dara Robinson wrote: »

dardhal, while I appreciate the information its not practical in my situation. We live in rented accommodation and there is almost always someone else there and I would have no way of excluding their electricity use from any tests I ran. Its just not practical
Also, we do not have a boiler or an electric shower. As mentioned, we have central heating connected to the tank and electric immersion

I did a few tests anyway about a year and a half ago, during that big snow. First was a single radiator and hot water cylinder from cold for an hour, and next was all radiators in the house, and hot water cylinder for an hour from cold.

The single rad and cylinder worked out at about 25 cent for an hour from cold as I mentioned in the last post, but the cylinder and all 9 radiators in the house worked out at about 50 to 60 cent for the hour from cold.

So without being able to keep all the radiators off, it would be about the same using either the gas, or the 3kw immersion on the bath setting, to heat the water for an hour.

But you could manually turn off all the radiator valves at this time of year and so only heat the cylinder with the gas.

Dara Robinson

Thanks man, thats very useful. Issue here is that while we can have everyone turn off their radiators we cant do the electricity tests as like I said too many people in the house. No chance of getting them to not use electricity at all for a few hours while a test is ran

I am still keen to hear what others say also.

Bruthal

Dara Robinson wrote: »

Thanks man, thats very useful. Issue here is that while we can have everyone turn off their radiators we cant do the electricity tests as like I said too many people in the house. No chance of getting them to not use electricity at all for a few hours while a test is ran

No need to do a test on the electrical immersion. A standard immersion has a 2kw element for the sink, and a 3kw element for the bath. A unit of electricity costs 18.3 cent incl vat.

So the immersion on bath for 1 hour costs around 55 cent (18.3 x 3) when heated from cold.

And the reason you want everyone to turn off radiators is not just to do this test with the gas, its simply so you are only heating the hot water cylinder, as you seem to have said the system does not have separate circuits for the cylinder and radiators. No point in having the radiators on in the summer if you want to heat the cylinder with the gas boiler.

To do the gas test yourself, read the gas meter before and after you run the boiler for an hour. Take the before reading away from the after one, taking account that the digit in the red window is 1/10th of a unit.

Multiply the answer x 11.267, and multiply this by 0.04786, and the answer you get will be the cents it cost to run the gas boiler for the duration according to the meter readings.

This all assumes the electric immersion for an hour heats the boiler to the same degree as the gas boiler for an hour will. But overall, using the gas boiler should be a fair bit cheaper than using the immersion, once the gas boiler is not heating all the radiators as well.

recedite

You'll find gas is a lot cheaper than electricity for heating a cylinder of water, but not if you're also heating the radiators in summer with the windows open.
Turn off each radiator individually at its valve, and away you go.