Energy savings by not heating the spare room

One of the recommendations for saving on heating at home is to turn down the radiators in rooms you do not use -  but how much does this save? The amount of energy you lose through the walls to outside depends on the difference in temperature between inside and outside. By turning off the heating in a room you reduce the inside temperature which reduces the difference to outside and hence the heat loss. So far so easy - but how low does the temperature get? There are a lot of factors involved and it is very hard to predict; much easier just to measure it. I have some thermochron ibuttons which can log the temperature at regular intervals over several days and I have used them to get some real data, in my house and in a friend's house. In my house, the (almost but not quite) unheated room was only 3.5 degrees cooler than the bedroom next to it, during a very cold period when the average outside temperature was -1C. In terms of temperature difference to outside, the unheated room was 20% cooler and so lost 20% less heat. When I measured my friend's house the weather was warmer, on average 6.5C but the unheated rooms were much cooler: up to 50% cooler than the other rooms and so losing 50% less heat. There are lots of reasons for this.


The ease with which heat escapes from the house to outside depends on the structure and insulation in the external walls, floor and and roof, the type of windows and how well they are fitted. The ease with which heat moves between rooms within the house depends on the structure of internal walls and floors/ceilings, and how well fitted the doors are (assuming they are closed).  The table below compares an unheated room in my house and two in my friend's house.

My house	Friend's rear bedroom	My friend's front bedroom
3.5C cooler on average	7C cooler on average	4C cooler on average
Two external walls, insulated with 100mm foam insulation.	Two external walls with no insulation	One external wall with no insulation
Ceiling adjacent to a heated attic.	Ceiling adjacent to an unheated loft  with 100mm insulation	Ceiling adjacent to an unheated loft  with 100mm insulation
North facing windows, modern double glazing, thick curtains left closed all day.	North facing windows, older double glazing, curtains open during the day	South facing windows, older double glazing, curtains open during the day
The door has a hole in it (originally for a lock).	A solid door	A solid door
Some heating from the ventilation system which cannot easily be turned off*.	No heating	Occasionally heated

* In our house we have MVHR installed which sucks air out of kitchens and bathrooms and uses it to warm up air coming into the house which is pumped into the other rooms. There are no controls on the vents - when they were installed they were balanced to give equal ventilation everywhere.

In my house the outside walls are well insulated and that makes it easier for heat to travel between rooms than to the outside. Hence our unheated room is only a little bit cooler than the rest of the house. In my friend's house the rear bedroom, which also has two external walls but without insulation and below an unheated loft is much cooler. Their front bedroom is warmer than the one at the rear, mainly because it has only one external wall but also because it faces south and perhaps because it is above the front office which is the warmest room in the house.
 

Chart showing temperatures over 3 days in different rooms in my friend's house. The external temperatures were recorded at a nearby weather station - the room temperatures were recorded using a thermochron ibutton at 15 minute intervals. In the house, the rear and front bedrooms are unheated. The front faces south and is considerably warmer The front bedroom is above the front office which is the warmest room. The rear lounge and front office are heated only when necessary - when my friend is at home and feels cold.

Looking at the chart of the recorded temperature you can see that the unheated rooms have an almost constant temperature, largely independent of the external temperature. I noticed this in my house too and it must be due to thermal inertia in the house itself - the brickwork in the walls stores a considerable amount of heat and only warms and cools slowly. Also brick walls conduct heat and are continuous from one room to another so the unheated room benefits from warmth conducted from below through internal and external masonry brick walls as well as through the ceiling/floor.

To calculate the approximate fuel savings, heat loss both by conduction and by ventilation is directly proportional to the temperature difference. If you halve the temperature difference between inside and outside you halve the heat loss and hence the fuel needed to replace it. In my friend's house the rear bedroom represents perhaps 10% of the external envelope so by halving the temperature difference they have saved 5% on their heating bill.