Economy 7 Trick

MFW_10YRS_4

Cardew wrote: »

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!!

Ah, but if the kettle was really well insulated and cooled very slowly (and therefore warmed really slowly) then perhaps it would be cheaper to keep it topped up at an acceptable temperature?

That's the logic I'm thinking but I have no axe to grind either way. I'll research it and report my findings .

Just found this:

"Underfloor heating is best controlled with fully independent zone control. Basically, each room should have its own thermostat to control the air temperature. This allows you to balance the building to how you feel most comfortable living in it. Some rooms you may want warmer than others, i.e. lounges and bathrooms you may want to be much warmer than other rooms like kitchens or gyms."

"Underfloor heating should not be left running all the time, but likewise should never be allowed to cool down by more than a few degrees. This balance between on and off is created by adding a time clock, and an override thermostat to stop the system cooling down by no more than a few degrees. This small drop in temperature, whilst off is often referred to as a setback. The time control and setback can be done by a separate time clock and setback thermostat, or alternatively via programmable thermostats which have fully programmable time and temperature settings up on them."

Another Website:

"Should I leave UFH on all the time? Not at all – just like radiators you can switch the system off as needs be. However, the response time will differ from radiators although this does depend on the type of floor construction that is used. Within a screed floor for example, since there is a large thermal mass (due to the entire screed being heated) the system will need to be switched on an hour or so earlier but it will also take longer to cool hence the system can be switched off an hour or so earlier.
Some customers prefer to use Night Setback control (often abbreviated to NSB). It is designed to ensure that a system is never allowed to cool excessively and thus reduces the time taken to get back up to temperature. During the system ‘Off’ periods, the system is in effect in standby in that if the temperature in a given room drops 4°C lower than the thermostat set point, the system will run to maintain this reduced (or setback) temperature."

A couple more websites say a similar thing (re: Night Setback) so it looks as though it's more efficient to run UFH like normal radiators, but not to let the slab cool down too much.

Makes sense. I'll be sorting out our controls very shortly! Thanks guys!!

Ben84

I'm puzzled that people are suggesting this would work because the freezer won't increase much in temperature. If it's holding its temperature for a long time then the compressor won't switch on anyway because it doesn't need to.

You will only save energy if your freezer gets warmer - that is if your freezer needs to switch on and can't.

Ben84

Car analogies are not much use for things like heating. It's true that stop start driving uses more fuel, but a car isn't much like a boiler.

When you stop a car the motion of your car is converted in to heat by the friction of the brake pads rubbing against each other. You can observe the exact same effect from how power tools like sanders make wood very hot during use. The energy that was once petrol is now turned in to heat that is dissipated to the surrounding environment, so it is effectively lost and there is an energy penalty to stopping a moving car.

Your central heating however does not have brakes. When you turn it off the valve to the gas burner closes, the flame goes out and your gas meter stops turning. There's no penalty because no energy is converted in to another form that you can't use.

noncom_2

The website quoted above about "Night Setback" is typical of the confusion between "effective" and "efficient" (in energy consumption/cost terms). The only reason not to let the slab cool by more than a few degrees is that it will take less time to come back up to temperature - it will consume more energy to prevent it from cooling down.

Frankly it doesn't make sense to me: they suggest using independent thermostats for each zone (which is perfectly sensible) and timeclocks independent of any radiator-heated areas (again, a good idea), but then introduce this nonsense of Night Setback instead of just saying that you might need to switch the system on again in the morning a couple of hours earlier than you would otherwise.

The only rationale I can think of is that, say in Autumn, when the nighttime temperature is only a few degrees below the required daytime temperature, the slab/room will only take perhaps an hour to come up to the right level. So having the timeswitch kick in 4 hours early would seem to be a waste of energy. Whereas, in winter, when the slan will cool more rapidly overnight, it might take all that time to get up to a comfortable level.

This leads me to the thought that, as this winter approaches and the central heating system goes back on, I should gradually adjust the "pre-heating" time (ie the amount of time the floors switch on before we get up) to be longer as the average nighttime temperature drops: the (unattainable) ideal being that each day the system switches on at exactly the right time that the slab/room reach the required temperature at the same moment we get up. Equally, the "post-switch-off-cooling time" will get shorter as the nights get colder, and I can adjust the timers for this too.

Doesn't sound that hard to work out a rough guide of how long they should be for a given outside temperature, and adjust the clocks maybe once a month?

Although, surely there's a gap in the market for a "smart thermostat" which can learn over time how long it takes for a given heating system to warm up its environment to a given temperature, and how quickly it would then cool down again. All the user would need to do would be set the temperatures required in the zone at the given time, and it would intelligently work out what time to switch on or off the heating. Does such a thing exist?

podcake

Hello again all, I see I have created much of a stir lol.

Firstly whatever Cardew may have said I made it clear (certainly from my 2nd post) that I was referring to the total money saving, not the energy which we have gotten bogged down in.

Secondly I pointed out the car analogy to allow people to see how it works. That analogy roughly holds true for something that cools rapidly. Most freezers, however, will keep the temperature for much longer.

@ Cardew specifically:

I can absolutely and categorically say that this statement is not true.

Quote: it is generally the case that it is better to keep the difference in temperature 'topped up' because you are just using a small extra bit of energy every so often.

you cannot, because your 'definition' of 'better' is quite clearly not the one I made:

to clarify:

you thought i meant better was better in terms of energy usage, what I mean is in terms of food hygiene and wear and tear on the freezer

So, in actual fact it IS true, not categorically NOT true.

and also, you cannot say this as true:

I can absolutely and categorically say that this statement is true.

Quote: So if your desired set temperature is -20C and you allow it to rise to, say, -15C it will take less energy to bring it back to -20C than it would have taken to have maintained it at a constant -20C

because you do NOT know for every specific freezer. If the freezer never warmed up it would not take more energy to maintain-20, in fact, depending when the motor/compressor comes on it might not even START to work until -15 is hit.. in which case you have just stated:

' it takes less energy to cool a freezer from -15 to -20 than it does to keep a freezer cold at -20 (when the pump only comes on at -15)'

and one final point:

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

because a kettle uses (approx) 3KW continually through its heating cycle where a freezer has a little device called a thermostat which is designed to keep the food at a safe temperature...


The OP was asking about the money saving of the scheme, and suggesting it as a method to save money, not specifically save energy.. The whole idea round this system is to use the energy when it is cheapest to the final costs cheaper. Not to save energy.. My feeling was that :
1) you will wear the freezer out faster
2) the food will be wasted sooner

hence its a false economy.. They FEEL like they are using less energy because the freezer is off for a while, but it has to make that 'cool' that is lost back up.

at the end of the day letting the freezer warm up and then refreezing is using less energy, okay, i'll accept that it takes less energy to leave the freezer till it is room temperature and then let it cool off again. But that defeats the whole point of a freezer. it is there to keep food FROZEN at a SAFE temperature..
as far as the SHC of air changing, i'm not sure about how much it fluctuates (I'm aware that temperature and moisture content will change it, but having looked at some data it changes by as little as 10% in the higher scales, if the chap who mentioned that can inform me i'd be most appreciative)


Lets leave this amicably and just agree to disagree.. My thought is that a freezer is there to freeze food, and you could safe lots of money by not having one. Working it hard will wear it out faster and warming up the food will make it unsafe.

Now I'm sure we both have better things to argue than the specifics of the efficiency of thermostats/insulation in freezers. (i'm certainly sure I should be packing for my holiday as i'm off later today).

Chris

grahamc2003

podcake wrote: »

Hello again all, I see I have created much of a stir lol.

You haven't created a stir, you've taken part in a discussion.

However, in several respects, your contribution has been technically incorrect, and continues to be in your latest post. If you were just passing comments, then probably no one would have picked you up on them, but you are supposedly explaining/teaching the physics of what's going on, and, while doing that, it's best to be correct in what you say.

There are numerous examples, and here's one which may surprise you. Heat loss (or gain) isn't proportional to the temperature difference (please no links to websites which say it is!)- it is a function of the temperature difference (and quite a complex function at that).

podcake

I was referencing the generally used form of Newton's law of cooling ( to quote)

Sir_Isac_Newton wrote:

"The rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings."

I accept it is not entirely true in all cases, ( some might say most) but I did say in my first post that I had simplified things, and it does provide a relatively workable solution..

anyhow I'm sure I'll see you all in 10 days and we can continue the discussion then

also, if people reference the points where I'm wrong I'll happily concede the point to them, I'm not an immovable object

Cardew

grahamc2003 wrote: »

However, in several respects, your contribution has been technically incorrect, and continues to be in your latest post. If you were just passing comments, then probably no one would have picked you up on them, but you are supposedly explaining/teaching the physics of what's going on, and, while doing that, it's best to be correct in what you say.

Amen!

Someone else take over please!

MFW_10YRS_4

To be fair, people have convinced me that it's not cost effective to keep an UFH system on all the time, constantly keeping an insulated concrete (or in our case, Limecrete) slab between two fixed temperatures. Why then is it suddenly cost effective to keep an insulated fridge on all the time between two fixed temperatures.

Just to play devil's advocate and keep the discussion going.

zeupater

MFW_10YRS wrote: »

To be fair, people have convinced me that it's not cost effective to keep an UFH system on all the time, constantly keeping an insulated concrete (or in our case, Limecrete) slab between two fixed temperatures. Why then is it suddenly cost effective to keep an insulated fridge on all the time between two fixed temperatures.

Just to play devil's advocate and keep the discussion going.

Hi

If you're talking about a refrigerator, you have to keep the contents between two fixed temperatures within a narrow banding. A temperature much above 6C and you're into temperatures where the preservative benefits of refrigeration start to reduce and at 0C your liquids begin to freeze ... considering thermostats could have an operating range of +/- 1C to 2C around the mid set point, there is little margin for operation other than a regular 'top-up' and this is why the set point is usually around 3C or 4C. Freezers have a much wider range in which they can operate, the temperature being a compromise between the operating cost, the contents storage period/quality/safety and the failure rate of the components.

HTH
Z