Economy 7 Trick

zeupater

st999 wrote: »

How does a freezer work? ....

..... The temperature is not kept at a constant temperature, it varies between 2 temperatures. It is called hysteresis.

That is why freezers work the way that they do, so you can sleep.

Hi

The discussion revolves around whether you can (safely) reduce the operating cost of the unit by utilising cheaper energy (E7) and cooling only at night, or possibly free energy in the form of solar pv and cooling only during the day.

I do agree with the point on automation being relevant .... Using solar pv, I believe that I could achieve a saving by switching the superfreeze option on the freezer on in the morning & off in the evening, thus decreasing the internal temperature to a level which will not likely rise above -18C before morning, but if it does, then the normal thermostatic setting will protect the food inside ....... but this energy saving measure depends entirely on my memory and not having a detrimental effect on the lifespan of the freezer ...

HTH
Z

noncom_2

I'm with Cardew on this one, and constantly have a similar debate with a colleague at work who believes it's more efficient to keep the underfloor heating on 24/7 on the grounds that "it takes more energy to heat it up again when it's cold than it does to keep it warm all the time" - I say it can't possibly.

However, the case of the car on the motorway cited as a parallel in the OP intrigues me..... clearly he is right that in that instance it is more efficient to keep the car running fast all the time than to let it slow down and speed up all the time. So what is the physical property that makes this case different from the "heating/cooling" case, when they appear to be superficially similar? Is it that there is no heat equivalent of inertia, or friction?

Just interested, and it's been too long since I studied physics to work it out.....

:-)

zeupater

noncom wrote: »

I'm with Cardew on this one, and constantly have a similar debate with a colleague at work who believes it's more efficient to keep the underfloor heating on 24/7 on the grounds that "it takes more energy to heat it up again when it's cold than it does to keep it warm all the time" - I say it can't possibly.

However, the case of the car on the motorway cited as a parallel in the OP intrigues me..... clearly he is right that in that instance it is more efficient to keep the car running fast all the time than to let it slow down and speed up all the time. So what is the physical property that makes this case different from the "heating/cooling" case, when they appear to be superficially similar? Is it that there is no heat equivalent of inertia, or friction?

Just interested, and it's been too long since I studied physics to work it out.....

:-)

Hi

There's a difference between the amount of energy consumed and the total cost of the energy consumed, even if more energy is required ... that's the basic premise of the OP, simply offsetting inefficiency against cost to make a financial saving.

Regarding the car analogy ... the main difference will be the effect of vertical acceleration caused by gravity on a mass to give it a weight, whether the road is flat, the drag coefficient of the vehicle and how heavy the driver's right foot is ..... all variables which don't really effect a stationary object in the same way.

HTH
Z

MFW_10YRS_4

noncom wrote: »

I'm with Cardew on this one, and constantly have a similar debate with a colleague at work who believes it's more efficient to keep the underfloor heating on 24/7 on the grounds that "it takes more energy to heat it up again when it's cold than it does to keep it warm all the time" - I say it can't possibly.

However, the case of the car on the motorway cited as a parallel in the OP intrigues me..... clearly he is right that in that instance it is more efficient to keep the car running fast all the time than to let it slow down and speed up all the time. So what is the physical property that makes this case different from the "heating/cooling" case, when they appear to be superficially similar? Is it that there is no heat equivalent of inertia, or friction?

Just interested, and it's been too long since I studied physics to work it out.....

:-)

I'm interested too. We have underfloor heating in part of our property and it is set up to keep the room at a constant 19C, which we were told is more energy efficient than turning it on and off and reheating the entire slab.

I could see the reasoning about re-heating the slab, especially in Winter where you don't want to sit around for hours waiting for the slab to warm up, which in turn then warms up the property. In summer time the property is above 19C from solar gain and so the heating stays off anyway.

grahamc2003

noncom wrote: »

I'm with Cardew on this one, and constantly have a similar debate with a colleague at work who believes it's more efficient to keep the underfloor heating on 24/7 on the grounds that "it takes more energy to heat it up again when it's cold than it does to keep it warm all the time" - I say it can't possibly.

However, the case of the car on the motorway cited as a parallel in the OP intrigues me..... clearly he is right that in that instance it is more efficient to keep the car running fast all the time than to let it slow down and speed up all the time. So what is the physical property that makes this case different from the "heating/cooling" case, when they appear to be superficially similar? Is it that there is no heat equivalent of inertia, or friction?

Just interested, and it's been too long since I studied physics to work it out.....

:-)

I haven't seen the comparison with a car, but there's no sensible comparison possible with a car on a motorway and heating/cooling a fridge.

The latter is concerned only with thermodynamics, the former with just about every engineering topic going - thermodynamics, aerodynamics, friction, accelerations in 3 dimensions, viscosity, dynamics of the Otto cycle and probably loads more.

Generally, the faster you go in a car above some minimum speed, the greater the cost per mile - and that's mainly due to aerodynamic drag and friction in various places. So if you vary your speed between 50 and 70 say, then you'll use less petrol than if you went 70 all the time.

A lot of these debates result from people's differences of what 'efficiency' means. Engineers generally have a pretty strict view of what it means, but others may think things are more efficient because they are more comfortable! (ref to the underfloor heating debate).

noncom_2

MFW_10YRS wrote: »

I'm interested too. We have underfloor heating in part of our property and it is set up to keep the room at a constant 19C, which we were told is more energy efficient than turning it on and off and reheating the entire slab.

I could see the reasoning about re-heating the slab, especially in Winter where you don't want to sit around for hours waiting for the slab to warm up, which in turn then warms up the property. In summer time the property is above 19C from solar gain and so the heating stays off anyway.

This seems to be a very common problem.... that installers will tell customers it is more efficient to run the system constantly than to let it switch on and off. I suspect what they mean is "more effective", and they do it in order to avoid a flood of complaints from people who don't appreciate the fact that it will take several hours for the underfloor heating to reach the correct ambient temperature, in contrast to the few minutes from radiators (in a well insulated house).

However, the argument that you are wasting energy by heating up the slab seems entirely spurious to me. The energy/heat put into an underfloor system gets to the desired room via the tiles/wood floor it is installed in. Some of the energy put into the pipes or electric elements will be lost into the ground, but hopefully not too much if the system is well designed. The rest heats up the actual floor. The floor then heats the air in the room above until the thermostat switches it off.

The amount of energy required to keep the slab at a constant temperature (given that it loses heat to its environment) is, as Cardew says, necessarily greater than the energy required to let it cool down and then reheat, so it cannot be more energy efficient to keep it switched on all the time.

The problem is that, from cold, it may take the slab 3 or 4 hours to get hot enough to heat the room around it significantly, and if you set your heating to come on at 7am and the room is still cold at 10am you're not going to be happy. Therefore you need to set the timers differently from any "radiator heated" rooms, and have the underfloor heating come on much earlier, say 4am so it's quite warm by 7am when you get up. Conversely, however, you can switch off the underfloor heating at 8pm and the slab will start to cool down, but it will do so by losing heat mainly to the room above it, effectively continuing to heat it for several hours after it's been switched off. Overall, the system doesn't need to be on for longer than a radiator driven one, you just need to take account of the long time lag when heating AND cooling down.

Cardew

noncom wrote: »

I'm with Cardew on this one, and constantly have a similar debate with a colleague at work who believes it's more efficient to keep the underfloor heating on 24/7 on the grounds that "it takes more energy to heat it up again when it's cold than it does to keep it warm all the time" - I say it can't possibly.




:-)

To use the analogy I posted above, ask your colleague if he went away and the house was empty for 2 years would it be more efficient to keep the underfloor heating on 24/7 for those two years or switch it on when he returned.

If he says keep the heating on for the two years - just hit him!!

So how about 1 year? 1 month? 1 week? 1 day? 1 hour? at which point would the law of thermodynamics not apply?

If his theory was correct, we would all keep kettles and saucepans of water simmering because it was cheaper to keep them on constantly than let them go cold and have to heat them up.

noncom_2

Ah yes, but then he's got an Aga too which is pretty much the same thing.......

:-)

MFW_10YRS_4

noncom wrote: »

This seems to be a very common problem.... that installers will tell customers it is more efficient to run the system constantly than to let it switch on and off. I suspect what they mean is "more effective", and they do it in order to avoid a flood of complaints from people who don't appreciate the fact that it will take several hours for the underfloor heating to reach the correct ambient temperature, in contrast to the few minutes from radiators (in a well insulated house).

However, the argument that you are wasting energy by heating up the slab seems entirely spurious to me. The energy/heat put into an underfloor system gets to the desired room via the tiles/wood floor it is installed in. Some of the energy put into the pipes or electric elements will be lost into the ground, but hopefully not too much if the system is well designed. The rest heats up the actual floor. The floor then heats the air in the room above until the thermostat switches it off.

The amount of energy required to keep the slab at a constant temperature (given that it loses heat to its environment) is, as Cardew says, necessarily greater than the energy required to let it cool down and then reheat, so it cannot be more energy efficient to keep it switched on all the time.

The problem is that, from cold, it may take the slab 3 or 4 hours to get hot enough to heat the room around it significantly, and if you set your heating to come on at 7am and the room is still cold at 10am you're not going to be happy. Therefore you need to set the timers differently from any "radiator heated" rooms, and have the underfloor heating come on much earlier, say 4am so it's quite warm by 7am when you get up. Conversely, however, you can switch off the underfloor heating at 8pm and the slab will start to cool down, but it will do so by losing heat mainly to the room above it, effectively continuing to heat it for several hours after it's been switched off. Overall, the system doesn't need to be on for longer than a radiator driven one, you just need to take account of the long time lag when heating AND cooling down.

Thanks for your post, it has given me something to think about. I'll research this as I really can see the logic on both sides.

Cardew

MFW_10YRS wrote: »

Thanks for your post, it has given me something to think about. I'll research this as I really can see the logic on both sides.

I suggest you start with the laws of thermodynamics;)

Seriously there surely can be no question that keeping it on 24/7 is more expensive.

Why do you not keep a kettle boiling 24/7 rather than boiling it when required - it is exactly the same principle!!