28 Aug 2014

Six Questions on Heat You Never Thought To Ask

Q1 What is heat?

Heat is a by-product of ‘work’ going on or, if you like, energy being spent. Heat is most commonly found where one substance is in the process of breaking down into its constituent parts. Our bodies (like our houses) leak heat and this leaked heat must be replaced, which we do by eating. Calories are just another measurement of energy. The colder it is outside our bodies and our houses, the more heat we leak and the more energy we have to take on board.

Q2 What is ‘Feeling warm?’

The rate at which we lose heat determines how hot or cold we feel. ‘Feeling cold’ is a signal that we are losing high and potentially dangerous amounts of heat; ‘feeling warm’ signals that all is OK.

Q3 What determines how warm we feel?

The insulating capability (U value) of our clothes (or duvets, or houses)
The temperature of the surrounding air
Wind speed (wind chill factor)
 Level of water vapour around
Whether our skin is wet or dry
How much heat is being ‘given off’ (radiated) by surrounding objects (including the sun).

When assessing heating systems, we use air temperature as the man indicator of background comfort but it is important to be aware that air temperature is just one of several factors at play. Anyone who has ever had a thermostatic control dial in their home will be well aware that what’s warm on a dry day can be 2° or 3°C too cold on a wet or a windy day.

Q4 How does Heat Move?
Heat transfers via three different methods: conduction, convection and radiation. Conduction is the passage of heat through a solid – the classic example is the poker placed in the open fire that soon gets too hot to hold. Convection is what happens to heat when it transfers into a gas (typically air) – it rises. Radiant heat is the glow you feel on your face when you are standing near a bonfire; the air temperature may be minus 10°C but you feel as warm as toast. We don’t often feel conducted heat but most heating systems deliver a mixture of the other two, convection and radiation.

Convected heat (or warm air) is characterised by being very responsive – i.e. you feel warm very quickly – but it can also be rather unpleasant, drying the throat and watering the eyes – think of the fan heaters in cars. In contrast, radiant heat you hardly notice. We experience it from things like underfloor heating systems, night storage radiators and Agas. Despite their name, radiators deliver a mix of all three forms of heat. The air convects through them, they are hot to touch (conduction) and you are aware of their warmth if you sit nearby (radiation). All heating systems deliver heat by all three methods but the mix varies according to the delivery system.

Q5 So what’s the perfect heating system?
I haven’t really been much help here, have I? You just need to understand that you must make a series of compromises and your aim is to make the least bad compromise.

Q6 Watts it all about?
Finally a word about how we measure power output, because I know people find it confusing, not least because there are different systems of measurement in operation. Here I try to plump for one, the watt (W), and its big brother the kilowatt (kW) which is 1,000 watts. These are measurements of power, rather than energy used. If you want to a measurement of energy used, you need to express it as so much power per hour, which we routinely call kilowatt hour or kWh.

Why the capital W in the middle of kWh? It's a strange convention to do with the watt being a unit attributed to James Watt, inventor of the steam engine. We seem to routinely refer to watts or kilowatts with a lower case w, but when it's written shorthand it becomes kWh.

To make it even more complex, there are other units used for energy measurements and one you frequently come across is the British Thermal Unit or BTU which, as you might guess is an imperial unit. The Americans still use it. What many people (myself included until recently) don't realise is that the BTU is a measurement of energy rather than power so its the equivalent of a kWh. If you want to know the power output of a boiler, you need to divide by hours. Thus:

1W = 3.41 BTU/h            1kW = 3,410 BTU/h

Let's finish with a table

1 litre
1 litre
1 litre
1 litre
1 kg

1 comment:

  1. If 1 litre of road fuel contains round about 10kWh, that's equivalent to a carthorse working for 12.8 hours (1 horsepower = 750W), or an Olympic cyclist maintaining his max sprint power for 80 hours (1 cyclistpower = 120W), or 500 hours of maintained hard work by a slave, say 50 days at 10 hours a day. All that in 1 litre. What happens when the pumps dry up? Are we organising for orderly Energy Descent? Definitely not.