Turning down the radiators in unused rooms

To reduce our heating bills we are often advised to turn down radiators in rooms that we are not using. However, this can be a bad idea if you have a heat pump. The adjacent rooms leak heat into the colder room which means the working radiators have to work harder. This is generally OK with a boiler but not with a heat pump which gives better efficiency at low temperatures. With the radiators working harder they need to run hotter which often means you end up using more energy rather than less [1]. I have done some modelling to see what this effect looks like. The savings on the overall heating demand is probably smaller than you might think – typically 3.5-5.5%. On the other hand the impact on the radiator heat demand surprisingly high – 20% or more.

House model: 2 storey semi/detached with one room unheated.

For this investigation I used a fairly simple model of a house with two floors, each with four rooms and a hall. The unheated room is the back left (blue in the diagram). There are two versions: detached and semi-detached. In the detached model, the unheated room has two external walls (both with windows). In the semi-detached model the left hand wall is a party wall so the unheated room has just one external wall (see diagram). I have assumed that there is some air mixing between most rooms but not at all with the unheated room because the door is kept shut. This is a best case as in practice there will be some leakage, which means more heat loss from the adjacent rooms. However, unless the unheated room is very draughty this should not make too much difference.

Plan of the first floor in the model showing the unheated room.

Adjacent rooms cool a little more quickly

The temperature in the unheated room depends on the balance between the rate that heat leaks out through walls and roof and the rate that heat leaks in from the adjacent rooms. Also when the heating goes off the heated rooms cool more quickly but this effect is small. The chart below shows one case for a few days in cold weather: the unheated room (blue) averages about 15C. The adjacent room (green) cools slightly more quickly than in the normal case (red - all rooms heated).

 

Three days in a model run from January, showing temperatures in the unheated room and an adjacent room.

Annual heat savings mostly 3.3% to 5.4%, depending on type of walls. 

A solid wall without insulation leaks more than a cavity wall and so the temperature in the unheated room will be lower and overall heat savings greater. Windows lose heat faster than walls so more window area also loses more heat. Roof and floor are usually less significant, unless you have poor roof insulation. This table shows the actual results from my model in six cases: detached and semis, with filled cavity walls, uninsulated solid walls, or solid walls with external insulation (EWI). In all cases the internal walls are solid with plaster on both sides. (I also modelled it with stud walls and they made little difference because the heat leakage is similar unless you insulate the air gap, which is not standard practice.) 

Case	Jan front left (heated) (°C)	Jan back left (unheated) (°C)	Annual heat savings (%)	Increased radiator demand (%)
semi-Cavity	19.2	16.0	3.3	26
semi-EWI	19.4	16.5	3.3	26
semi-Solid	19.0	14.8	3.8	24
det-EWI	19.1	15.8	4.3	24
det-Cavity	18.9	15.0	4.5	23
det-Solid	18.4	13.4	5.4	19

Overall the annual heat savings vary between 3.3% and 5.4%. The savings on the gas bill will be less because hot water heating comes on top of this. Savings are highest in the case where the room has most heat loss (detached with solid walls), and the lowest temperature. This case also has the most heat transfer from the adjacent rooms. However since the radiators are already working harder the relative increase in demand from radiators is actually less. 

(For geeks) Crude estimate of savings without a model, based on mean room temperatures.

You can make a crude estimate as to how much heat is saved over the year by considering how much cooler the room is compared to the other rooms. Heat loss from each room is proportional to the temperature difference to outside. In January it is typically 5°C outside and the heated rooms average 19°C while the unheated room averages 16°C. That implies that the heat loss from the unheated room is only 79% ((16-5)/(19-5)) of the heat loss from the other rooms. We have saved 21% for that room. In the detached model, there are eight main rooms all with two walls so we can crudely estimate that the unheated room counts for an eighth of the outside surfaces: an eighth of the walls and windows, a quarter of the roof but none of the floor. So we estimate our heat savings as 21% * 1/8 = 2.7%. However this figure refers to all heat sources including electrical appliances and solar gains. When you take account of these the radiator heat savings go up a bit.

With a heat pump, efficiency loss of 6-8% wipes out the reduced heat loss.

The model I have been running uses gas heating with a combi-boiler so the radiators have no trouble maintaining the temperatures in the adjacent rooms. However, the radiators are working harder to do so and with a heat pump this difference is critical. With a gas boiler you are probably feeding the radiators at around 65°C or hotter in cold weather but a standard heat pump will supply at most 55°C and even then it will not give you very good efficiency. You need to go lower. That generally means larger radiators - either wider or thicker, double instead of single panel. Your installer will ensure your radiators are large enough assuming that all the rooms are heated normally. In the example runs above, turning off the heat in one room will increase the demand in adjacent rooms by 20-25%. That could mean you need to increase the radiator flow temperature by 4-5°C, with a 6-8% loss in efficiency. It more than wipes out the 3-5% savings you thought you would get. This chart shows the effect of flow temperature on efficiency, for a typical heat pump.

Variation in efficiency with external temperature for a typical heat pump.

Tips for energy saving with boilers do not always work with heat pumps.

What this means in practice is that, if you are in the habit of turning down the radiator in the spare room, you should turn it on again after having a heat pump installed. Either that or insulate the walls and floor/ceiling to minimise heat leakage from the rest of the house. This is an interesting example where what we learned about energy saving with gas boilers has to be modified for heat pumps. They are a different game entirely.

[1] Why NOT to ZONE Heat Pumps! (SURPRISING RESULTS!!) (Heat Geek)