Educating the stove to the levels of the rest of the house

 by Steven Harris


I mentioned some time back that if the UK were to shift to low carbon energy we would have to start weaning ourselves off gas. For most people will mean starting to heat their homes and hot water with electricity.

Regular readers will remember that this was where we arrived at the £32 BILLION bill for increasing the capacity of the national grid, to carry all that additional energy.

I suggested that instead, some of that £32 billion (I’ll say it quietly this time) could be spent on a variable tariff, to try and shift that heating load away from the peak times – meaning the grid wouldn’t have to be enhanced as much. Today I would like to talk a bit more about how we could do this.

Let’s first look at the difference between heating with gas and electricity.

The trouble with electricity is that at the moment in the UK, it is mainly generated by burning gas and coal. By the time it gets to you as electricity, 70% of the original energy in that gas or coal has been lost – firstly (and mainly) as heat from the power station’s cooling towers, and then in wires as it zips around the country.

You could even argue that an electric heater is actually a 30-per-cent-efficient gas boiler, except that the gas is being burnt far away in a power station, not on your wall. (Modern gas boilers, on the other hand, have efficiencies of up to 85%.)

This will seem impossibly old-fashioned by the time we’re all in our clever homes. The general idea is that in the future we will generate less electricity by burning fossil fuels and more by renewable means.

Lots of people talk about biofuel and solar thermal. In fact, a log boiler stove is actually the main source of winter heat in my not-so-clever (yet) home. I supply some of the cleverness by switching to solar thermal in the spring, summer and autumn. I know I’m unusual, but more people are going this way. Green building magazine has a very good feature on this topic at the moment.

And now for my shocking confession. Although we try our best at home to make the log boiler stove our main winter heat source, we have been known to… arhum… switch on the electric immersion heater when we… you know… couldn’t be bothered to light the fire… Sorry!

But it’s really all my immersion heater’s fault.  The real problem is that it’s a bit stupid, and I want it to be clever! So what can we do about it?

As anyone who lives with solar thermal will know, the key is heat storage. As unfortunately you can’t guarantee that the sun will be shining when you want your bath, you have to collect and store heat when it does.  (Don’t let anyone convince you otherwise!)

So because your clever house has solar thermal (again, like PV, how could you afford not to?), you’ll already have thermal storage. From experience I would suggest the minimum size of store would be about 300 litres of water. A store of this size can just about fit in an airing cupboard, though admittedly there wouldn’t be very much room left for airing things.

To get really techie for a moment, for every degree of temperature you add to 300 litres of water, you can store 0.35 kWhrs of heat. (kWhrs are the units you will see on your electricity and gas bills).

From personal experience, I know that by the time you have exhausted the last trace of useful heat from the top of your store, it probably has an average temperature of around 30deg C. (40degC. at the top, 20degC. at the bottom) I have also had my whole store up to a slightly worrying (but still safe) 90deg.C. So we have around 60 degrees of temperature to play with.

So… 60 degrees X 0.35 kWhrs per degree = 21 kWhrs! With a kWhr of electricity costing 12.5p at today’s price, that’s worth £2.62.

What was that you said? ‘Big deal’?

Wait! Remember, in my clever house I have electricity at a variable tariff. This means that at times of day when generation is plentiful I may only be paying 2p per kWhr. Conversely, at peak tea time, the tariff may be 50p per kWhr. By being clever, my clever house would be able to buy 21kWhrs of heat for 42p rather than at £10.50! 

Now this is the extreme, and – before the engineers shout – ignores standing losses; but you can see the principle.

To put this into context, and using David Mackay’s rules of thumb, a typical bath will use 5 kWhrs and a shower 1.4 kWhrs.  Mackay puts the average national consumption of domestic heat (gas) per person (in 2000) for hot water, space heating (and gas cooking) at 21kWhrs. Very convenient for our purposes!

But please remember, my clever house is super-insulated and ultra-airtight, and has heat-exchange ventilation, so it will need much less space heat that your ‘average’ house. It also has underfloor heating pipes in its concrete floors, which can store even more heat away.

If the sun’s been out, my clever house may not even need to even spend 42p on cheap electric. Or it might decide that when prices are low it’s more worth its time to heat water with the spare electrical output of its microgen technologies, rather than exporting electricity to the grid. (There is already a clever box called Emma who can do this). Or the mood might have taken you to get all cosy and light the log boiler stove – so your store may already be hot.

Whatever your whims are and whatever the weather decides to throw at you, your clever house will listen and will find the cheapest and most sustainable way to make sure you never suffer a cold shower or have a cold house again.


Scene: a dark and stormy night

You: I think I will take a nice long bath.
House: well you didn’t put that in the diary did you! Have you any idea what that’s going to cost me now?
You: but we’ve got nothing on this evening so I have… erm… ‘plans’…
House: oh, I see… ;o) Maybe you should light the fire then?

Next time – how to talk to your clever house.