Hot water - constant or timed?

oldMcDonald

I have asked this on another thread, but after posting my question it dawned on me that this would probably be a better place to ask, so sorry for the duplicate post if anyone comes across it again on the boards:)

I'm really confused about this. We have a gas boiler with a programming box with one button for water, one for heating. We also have a water tank with a wall switch in a cupboard. I assume this is an electric heater - I have never used it.

I have the heating off for most of the year, but have the water on constantly, you can hear the boiler fire up when you turn the hot water tap on. Is this costing a lot to run it like this? Would it be cheaper to have a timer set for an hour or two a day, I assume the hot water is stored in the tank in the cupboard as this is always warm? I've never really given this much thought! This is a Housing Association house and so any leaflets to explain how to work the boiler / timer are long gone, this is the first time I have had a house with CH, so I just assumed the water should be left on.

I can't believe I have been so tight over the heating but never gave the water a second thought! I've always had an electric (immersion?) water heater until now - that was rarely turned on!

Sorry if this is a dumb question and thanks for any thoughts you share (so long as they are polite )

moonrakerz

As your C/H is off, read your gas meter, read it a week later and see how mush it is costing you.
My water heating is on 24hrs, it costs me about 50p a day.

jblack_2

It depends on how much hot water you are using and when.

It can be more economic to keep it on constant. It takes longer to heat a full tank of cold water than it does to heat the water as it's used. If it's on constant then it'll not fire up unless it needs to. 'Constant' is a bit of a misleading term.

HTH, J

culpepper

if the boiler feels warm it might be worth getting a lagging jacket for it.
We have our hot water come on twice a day. 1 hour early morning, 1 hour in the early evening and there are 4 people here to use it up. It shouldn't be on all day really , if you can hear it kick in every time you use some hot water it must be cooling down a lot between uses.
The immersion heater is the switch in the cupboard I expect but that is an expensive way to heat water as gas is cheaper .

TimBuckTeeth

jblack wrote: »

It depends on how much hot water you are using and when.

It can be more economic to keep it on constant. It takes longer to heat a full tank of cold water than it does to heat the water as it's used. If it's on constant then it'll not fire up unless it needs to. 'Constant' is a bit of a misleading term.

HTH, J

I can't think of any way that it would use less gas to keep it on constant. It is more convenient keeping it on constant and depending on the tank insulation, heating system and hot water usage there might not be much difference in cost but it can't be less than with timed.

jblack_2

culpepper wrote: »

if the boiler feels warm it might be worth getting a lagging jacket for it.

I think you mean the water cylinder.;).

culpepper wrote: »

It shouldn't be on all day really , if you can hear it kick in every time you use some hot water it must be cooling down a lot between uses.

There's no reason not to leave it on all day. It isn't burning gas all day. If it's an indirect hot water cylinder then as water is used it is automatically replaced by cold entering the bottom of the cylinder. The hot water you get from the tank never goes through the boiler. The water the boiler heats it on a circuit, much like a radiator circuit. I.E. if you use 10 litres from your hot water tap the boiler doesn't heat 10 litres. It heats the water in the cylinder 'element' until the cylinder thermostat is satisfied.

culpepper wrote: »

The immersion heater is the switch in the cupboard I expect but that is an expensive way to heat water as gas is cheaper .

At the moment. Up to £1000 per year in 2 years time for gas.:eek: :eek:

jblack_2

TimBuckTeeth wrote: »

I can't think of any way that it would use less gas to keep it on constant. It is more convenient keeping it on constant and depending on the tank insulation, heating system and hot water usage there might not be much difference in cost but it can't be less than with timed.

If you heat up a 120 litre cylinder once in the morning then later in the day it can use more gas than if you heat it once then allowing the stat to cut in and fire the boiler as and when it needs to just to 'top up' the temp. As you say it depends on the cylinder insulation etc and your water usage.

J

TimBuckTeeth

The heat loss is higher at higher temperatures, so keeping cooler water in the tank during periods when not in use (eg. over night) then less heat is wasted.

A water tank should not go cold during the day/night, if it does then better insulation is needed and the extra gas needed to keep it hot is even more.

If the temperature drops to 40C during the night then it does not use more gas to heat it from 40C to 60C in the morning than keeping it heated to 60C all night.
The amount is roughly the same, ignoring the extra heat loss from the 60C tank.

It is a similar argument to leaving the heating on all the time, it can't be cheaper than timed, although the heat lost from a water tank is a lot lower.

jblack_2

I agree with you Tim. But if hot water is on a timer to come on once a day at say 7am for 2 hours. Then it's used throughout the day and so is full of cold water by 7am the next day then the boiler has to heat a full tank from empty.

It is all dependant on the usage and set-up etc.

J

TimBuckTeeth

I have a combi boiler, so best of both worlds - hot water on demand without a timer and no heat loss from a tank.
It would be interesting for someone with a conventional boiler to do a test - timer vs. constant for a week and compare the gas use. It would have to be done scientifically though so the same amount of hot water was used, to be a valid comparison. Could get a bit tedious timing showers and counting sink fulls of water.

MarkyMarkD

Jblack

Your posts do not make sense.

It does not use more energy to heat up water from cold, than to heat up water constantly throughout a day.

Given that the water going into the cylinder is the same temperature under both scenarios, the actual energy used to heat up the water would be the same under both scenarios, if it wasn't for heat loss, and if boilers worked to the same efficiency irrespective of load.

But:

* heat loss is higher if the temperature of the water is higher, and a constantly heated tank of water will have a higher temperature - on average - than one which is heated on a timer.
* Boilers, particularly condensing boilers, work more efficiently under full load. Cycling on and off many times during the day, just to top up the temperature, is very inefficient. Heating the water a few times in the day, from a relatively cold temperature, is relatively efficient.


The amounts involved are relatively insignificant in either case, so it's something of an academic discussion.

The most significant savings are incurred by reducing the temperature to which the water is heated, to the minimum acceptable level.