Tuesday 27th October 2009
More efficient?
I’ve heard this a few times over the years, and it always bamboozles me. Yesterday it cropped up again, when someone was referring to an immersion heater, and said:
It’s more efficient if you leave it on all day – that way it’s just topping it up. It uses more electricity if you let it cool down and then have to heat it all the way back up again.
I’ve heard similar stuff about leaving the central heating running all day too. My questions are – why do people believe such drivel? Why is it so widely perpetuated? And actually how would you explain this to someone in a way they’d understand? If you just though about mentioning heat flux, then that’s not going to get you anywhere, but does anyone have a good analogy or explanation I can use in future?
Large immersion heaters do have an energy storage time-delay effect which could be put to good use: http://www.guardian.co.uk/technology/2009/jun/17/immersion-heater-renewable-energy …but that only works if we have a “smart grid”.
Nope. I’ve heard that one quite a few times times. It’s clearly incorrect, but not that easy to explain why.
I also used to have a housemate who would insist that leaving the iron on for 20 minutes without using it was absolutely fine because it only really loses heat while you’re ironing. gah!
Comment by Harry Wood — 30/10/2009 @ 2:36 am
So at a basic level its not true because heat transfer rate increases as the temperature difference driving it increases i.e. you loose more heat when your house is warmer than when its cold – so not only do you lose less heat on the period while the house is allowed to be cold but you lose less heat during the cooling down/heating back up periods than if it was at its optimum temperature the whole time and all heat energy lost is energy the central heating will replace.
The only thing I can think of that could feasibly throw a spanner into this theory is the response of the control mechanisms. There will be some delay between the sensor detecting that its at the correct temperature and the heating system responding (electric sensors are quick to respond but pipes of hot water tend to be pretty slow). In this delay the system will overshoot on the temperature (then undershoot, and oscillate until it settles down). So if you get a much bigger overshoot when heating your house up from cold than what you get when its in a more settled state then it could be better to leave your heating on for short periods. I feel I’ve just reached the “need some experimental data” threshold.
Comment by Nia — 30/10/2009 @ 10:22 pm