What thermostat temperature settings and heating degree days can teach you about your heating bills

Heating Degree Days

This is the fifth post in our Beginner’s Guide to Heating Bills, it follows average heating billsheating fuel useheating cost comparisons and home heat loss.

Have you ever heard that ‘reducing your thermostat setting by 1°C (1.8°F) can cut your heating bill by 10%’?

Do you know why?  Because the heat loss in a body is proportional to the  difference in temperatures . . 

I’ll try it in English.

A home loses heat in two ways.

Conduction: through the fabric of its walls, floor, roof, windows . .

Ventilation: hot air squeezing through gaps under doors, out windows . .

When you dig into calculating heat losses you’ll find that both conduction and ventilation losses are a function of the difference between internal and external temperatures.

Understanding Heating Degree Days

We can measure the difference between internal and external temperature over a period of time with something called heating degree days (HDD).

For example if the outside temperature was 10°C all week and you kept your home at a constant 20°C inside, that would be 7 days x 10 degrees difference adding up to 70 heating degree days (HDD).

The image at the top of this post shows the heating degree days over a year in five of the world’s major cities.  It also shows how these figures change between 5 different base temperatures, from 20°C down to 16°C.

The first thing to notice is that Rio has virtually no heating degree days at any base temperature, because it has a warm climate.  LA is still pretty good, Madrid is getting chilly in winter and New York and London are pretty cold.

No mystery there, that is just the difference in climate.

The Thermostat Temperature Setting Connection

But let’s instead take a look at London by itself.

At a base of  20°C London has 3,634 heating degree days (HDD).  If you drop the base to 19°C London this falls 9.4% to 3,293.  From 19°C  to 18°C it drops by 10.1% more to 2,962 HDDs. From 18 to 17 it’s a further 10.8%. And from 17 to 16 it’s 11.6% again.

So for every 1°C (1.8°F) you reduce the internal temperature of a building the number of heating degree days falls by between 9.4% and 11.6%.  Still with me?

This is where the idea that ‘reducing your thermostat setting by 1°C (1.8°F) can cut your heating bill by 10%’ comes from.  Because the heat loss of a building is proportional to difference between internal and external temperatures.  So in a cold climate the amount of heat loss falls by about 10% for each degree C you reduce the temperature of a building.

As we noted in the last post buildings also gain some heat from the sun, people and appliances, so the reduction in heating needs is actually a little higher than this figure.  In the calculations at least.

In the real world

In real life we don’t keep constant temperatures in buildings throughout the day, or throughout the year.  But this doesn’t change the principal that:

‘heat loss is proportional to the difference between internal and external temperatures’.

The colder your house is in winter, the lower your heating needs will be.   If you are smart you will apply this principal to times of day and zones within your home, rather than just being cold all the time.

But if you want more comfort for your dollar you’ll need to look at the other parts of the heat loss equation.  That means improving your home’s insulation and making it more airtight.

Or better yet . . .  the World Cup, the Olympics, the beaches, no heating bills . . .

Move to Rio!!!!!!!!

The last post in the series is about house insulation.

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  • TedKidd

    Have you ever wondered why you can’t milk a spherical cow?


    Because it in NO WAY represents a real cow… But Data folks with no connection to how homes TRULY perform will promise the milk from spherical cows to folks who foolishly follow their advice.

    • Lindsay Wilson

      I like data, that is why I use it instead of ‘metaphors’ http://yorkland.net/downloads/setback.pdf

      • TedKidd

        You make your whole argument based upon one study? Not much depth of knowledge or understanding in that.

        What happens if your basis is an absurd study making absurd conclusions?

        Sorry Lindsay – let me go deeper. How leaky were those houses? Hmmm.
        How oversized was the equipment? Hmmm.
        What about cycling losses? Hmmm.
        How much duct leakage to outdoors? Hmmm.
        Think those issues might have impact on savings?
        What really WAS the cause of those savings? You conclude a thermostat?

        How about this study which suggests thermostats save so little as to be unseeable on your monthly bill: $25 year savings, p3 http://bit.ly/bigthermostatsavings

        I think you like to look at bad data and jump to simplistic conclusions. Looking deeper takes years of understanding. Wearing a green ball cap is much easier…

        • Lindsay Wilson

          Each of these is a valid point, and anyone with a bit of sense that digs in to the topic will get there. But they don’t dislodge the central idea that heat loss is proportionate to the heat differential, with a few caveats on moisture, ducting . . . I didn’t make any grand claims in my piece, and you sent me off to a snarky spherical cow link? I think my audience is sensible enough to understand that heating bills are not so simplistic. I’ve simply tried to give people an accessible piece that explains heating degree days and losses.

          • metawatch

            I agree and appreciated your article! I have developed an extensive furnace monitoring device, and have direct evidence of real savings linked to furnace temperature settings,as a key variable in cost savings. Of course other,more costly home improvements reducing leakage will also aide in reducing costs (tedkidd)