Newbie Question - Water Heater on or off?
Comments
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Sorry about the late relply Mike. It's as Jill says, 30 minutes would be ample time
Kerry
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Can you explain please?
Me too, I'm very interested
OK here goes.
Anything that is hotter than its surroundings will dissipate heat into the atmosphere. You can lessen this with insulation but there is no such thing as 100% insulation.
The greater the temperature difference between the object and its surroundings the greater the heat dissipation will be.
The temperature of your hot water container will go down when it is switched off so will dissipate less heat at such times and therefore use less energy than if on all the time.
Basic rule of Physics is: Energy put in equals energy got out.
Having said that I never turn mine off when on site as the electricity price is fixed and I would probably forget to switch it back on in time.
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I read Astro76's post on page 3 with a certain amount of disbelief! But no one else has remarked on it! Astro did you really mean to say that you are using grey water to top up your toilet flush????
Fine in theory but how long do you expect to be able to use the flush before something jams in the pipes and causes a blockage?
The fact that no one else has commented on it suggests that Ive got hold of the wrong end of the flush!
TF
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I saw that TF and wondered if he only showers into the aquaroll - my kitchen waste would not be good! BUT seeing so many onsite who use watering cans to fill the loo flush - and LEAVe them under the caravan - I have seen the number of insects that collect
in watering cans and they are BAD news!
I use an old 5 litre water bottle and keep the stopper firmly ON! 0 -
I saw that TF and wondered if he only showers into the aquaroll - my kitchen waste would not be good! BUT seeing so many onsite who use watering cans to fill the loo flush - and LEAVe them under the caravan - I have seen the number of insects that collect
in watering cans and they are BAD news! I use an old 5 litre water bottle and keep the stopper firmly ON! Pippah, I'm one of the watering can under the van brigade. I'm pleased to read your comment and now certainly see your point. I will put it away from now on.
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Can you explain please?
Me too, I'm very interested
OK here goes.
Anything that is hotter than its surroundings will dissipate heat into the atmosphere. You can lessen this with insulation but there is no such thing as 100% insulation.
The greater the temperature difference between the object and its surroundings the greater the heat dissipation will be.
The temperature of your hot water container will go down when it is switched off so will dissipate less heat at such times and therefore use less energy than if on all the time.
Basic rule of Physics is: Energy put in equals energy got out.
Having said that I never turn mine off when on site as the electricity price is fixed and I would probably forget to switch it back on in time.
We are discussing which costs the most, leaving it on relying on the insulation to keep it warm until the thermostat kicks in turning on the power for a short time verses turning it off and reheating from cold.
The laws of conservation of energy equally to both senarios.(We would be in trouble if it didn't) But which uses the least power over a 24 hour period. Trials on domestic systems suggest leaving it on and the numbers get more convincing the better the insulation
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Can you explain please?
Me too, I'm very interested
OK here goes.
Anything that is hotter than its surroundings will dissipate heat into the atmosphere. You can lessen this with insulation but there is no such thing as 100% insulation.
The greater the temperature difference between the object and its surroundings the greater the heat dissipation will be.
The temperature of your hot water container will go down when it is switched off so will dissipate less heat at such times and therefore use less energy than if on all the time.
Basic rule of Physics is: Energy put in equals energy got out.
Having said that I never turn mine off when on site as the electricity price is fixed and I would probably forget to switch it back on in time.
We are discussing which costs the most, leaving it on relying on the insulation to keep it warm until the thermostat kicks in turning on the power for a short time verses turning it off and reheating from cold.
The laws of conservation of energy equally to both senarios.(We would be in trouble if it didn't) But which uses the least power over a 24 hour period.
Trials on domestic systems suggest leaving it on and the numbers get more convincing the better the insulationThat depends totally on how often it runs. This time of year our room heating comes on for half an hour twice a day. However the two rooms we use normally have the doors close dwhereas often back door is left open occasionally,
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The greater the temperature difference between the object and its surroundings the greater the heat dissipation will be.
If my memory serves me correct ( and it is a long time since my school days
) this is "Newtons Law of Cooling"0 -
Newton's cooling law in convection is a restatement of the differential equation given by
Fourier's law:- d Q d t = h · A · ( T ( t ) - T env ) = h · A ? T ( t ) {\displaystyle {{\frac {dQ}{dt}}=h\cdot A\cdot (T(t)-T_{\text{env}})=h\cdot A\Delta T(t)\quad }}
where
- Q {\displaystyle Q}
is the thermal energy in
joules - h {\displaystyle h}
is the
heat transfer coefficient (assumed independent of T here) (W/(m2 K)) - A {\displaystyle A}
is the heat transfer surface area (m2) - T {\displaystyle T}
is the temperature of the object's surface and interior (since these are the same in
this approximation) - T env {\displaystyle T_{\text{env}}}
is the temperature of the environment; i.e. the temperature suitably
far from the surface - ? T ( t ) = T ( t ) - T env {\displaystyle \Delta T(t)=T(t)-T_{\text{env}}}
is the time-dependent thermal gradient between environment
and object - I remeber it well !!
0 - d Q d t = h · A · ( T ( t ) - T env ) = h · A ? T ( t ) {\displaystyle {{\frac {dQ}{dt}}=h\cdot A\cdot (T(t)-T_{\text{env}})=h\cdot A\Delta T(t)\quad }}
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Just to put another perspective on this - we tend to turn our water heater on and off as required - not for reasons of economy or efficiency - but to ensure we don't exceed the current draw from the bollard. We often stay on CLs and CS's and some of them
only provide a supply rated at 10 amps. It's quite easy to inadvertently trip the circuit breaker if the water heater / kettle / battery charger and perhaps the microwave are all switched on at the same time. If this isn't a consideration, then I agree it
makes sense to let the thermostat do its job.0 -
Just do whatever takes your fancy, to many variables to pin it down to a definitive answer. Whether you are in or out most of the day, whether you shower in or use facilities, whether you have loads of dishes and wash them in or use facilities, whether you
mind waiting 30 minutes for it to warm up or you want instantaneous, whether there's a z in the month and finally whether you should give two hoots anyway.Myself I'm not interested in efficiency, if on a CL and no facilities then its on permanently, but if on a CC site then its as and when.
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Newton's cooling law in convection is a restatement of the differential equation given by
Fourier's law:- d Q d t = h · A · ( T ( t ) - T env ) = h · A ? T ( t ) {\displaystyle {{\frac {dQ}{dt}}=h\cdot A\cdot (T(t)-T_{\text{env}})=h\cdot A\Delta T(t)\quad }}
where
- Q {\displaystyle Q}
is the thermal energy in
joules - h {\displaystyle h}
is the
heat transfer coefficient (assumed independent of T here) (W/(m2 K)) - A {\displaystyle A}
is the heat transfer surface area (m2) - T {\displaystyle T}
is the temperature of the object's surface and interior (since these are the same in this approximation) - T env {\displaystyle T_{\text{env}}}
is the temperature of the environment; i.e. the temperature suitably far from the surface - ? T ( t ) = T ( t ) - T env {\displaystyle \Delta T(t)=T(t)-T_{\text{env}}}
is the time-dependent thermal gradient between environment and object - I remeber it well !!
If only RRM had put it in these simplified terms it would have made soooooo much sense soooooo much sooner. Thanks for clearing it up HD, I've always had a soft spot for Fourier
0 - d Q d t = h · A · ( T ( t ) - T env ) = h · A ? T ( t ) {\displaystyle {{\frac {dQ}{dt}}=h\cdot A\cdot (T(t)-T_{\text{env}})=h\cdot A\Delta T(t)\quad }}
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I hate to cool off the apparent fantastic increased interest in physics on CT but there is a huge error in the first line of that equation.
it should be dQ/dt not dQdt (ie written as a fraction - even though of course differentials are not fractions but can be treated as such in their solving)
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Well spotted, Corners.
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thank you both - it's a service I provide
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Bummer, I never did like that Fourier
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The answer is 42, it always is!
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We switch our water heater on and it says on until we leave. We have an Eco setting, but like most less power options, it's a bit useless especially for washing up after dinner.
Good point earlier about the watering can for the toilet flush. I had been leaving ours under the van but will now store inside to stop bugs entering the flush system.
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Even more 'bugs and slime ' in the flush tank!
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I you are on EHU bear in mind richardandros's comment -
'It's quite easy to inadvertently trip the circuit breaker if the water heater / kettle / battery charger and perhaps the microwave are all switched on at the same time.'...yes, really easy
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