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Where did the Heat go?

We often hear tenants say that when they turn the heater off the heat just disappears.  This is a fair point as heat will always disappear from warm air (even in a brand new house), but what is a reasonable time for it to disappear?  This depends on various factors which are discussed below.

The first question is how long was the heater on for?  A heater (eg bedroom electric heater or living room heat pump) heats the air.  The air then heats everything in the room.  This includes you as well as all the furniture (beds, dressers, couches, benches, wardrobes, desks etc) plus the wall linings, ceiling linings, doors, curtains, windows etc.  So if you only have the heating source on for a short time to heat the air then the heat from the air is quickly absorbed by everything in the room.


This photo has a thermometer showing a heated air temperature of 22.9 degrees.  It also has a laser thermometer showing that the couch is only 16.4 degrees.  If the heat pump was turned off at this point, then the air temperature would drop as heat is absorbed by the couch (and everything else in the room) until the temperature of air and room contents equalises.

The next question is how big is the space you are trying to heat?  If you have a very large bedroom with high ceilings then it will take a long time to heat that volume of air (and most of the heat will rise to the ceiling level).  So if this is the case then it will take some time and cost some power to heat the whole room (and everything in the room).  If you have a large bedroom and you are studying at your desk for long periods then perhaps localised heating with a small heater just beside you is a way of keeping yourself warm economically.

Shouldn’t the insulation keep the heat in?  Insulation will definitely help to keep the heat in if it actually gets the chance to keep the heat in.  Remember that the insulation is on the other side of the ceiling lining, wall lining or flooring.  So for the insulation to do its job of trapping heat in you first have to heat the wall lining, ceiling lining or flooring to a temperature where heat is actually passing through the linings.  You have to heat a room for some time to heat plaster board or MDF wall and ceiling linings.  The same goes for the floor where you have to heat through the carpet and the underlay and then 20mm thick flooring before the insulation will do its job.


We often hear from tenants that they are only using heat pumps and heaters during their “Free hour of power”.  One hour of heating is not going to significantly raise the temperature of the wall, ceiling and floor linings to allow the insulation to do its job and help retain heat.  Especially if you start with the inside room temperature the same as the outside air temperature.

The insulation will be working though when the outside temperature drops as it will slow the cooling process of the cooler outside air (meaning whatever inside temperature you have inside it will at least help keep that heat in overnight).

We have often suggested to tenants to open the doors and let some heat into the house on sunny winter’s days.  These non-heated houses are still cold from the night before and the closed doors and windows along with the insulation is actually keeping the houses colder inside than the warm day temperature outside.

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