What is Thermal Mass?
Thermal mass is the capacity a material has to absorb, store and release heat, it is directly linked to the specific heat capacity of the material and its size and density. This is not to be confused with insulation values, for example a slab of concrete can be of high thermal mass but not provide a very average to low insulation value, whereas gram for gram mineral fibre insulation the opposite would be true. So, How can thermal mass be utilised when heating your home?
Thermal mass can be used to
effectively to store heat, and excess heat in times of peak demand from sources of heat such as solar gain, machinery or even body heat to smooth the temperature within a property over periods of
time, and absorb peaks and troughs throughout the day, but is only efficient and effective in the correct circumstances.
Because of the additional energy stored within high thermal mass buildings this will result in both a longer heat up time, and in turn, a longer cool down due to the heat being released back in to the cooling property, this is because when used effectively its slows the properties ‘thermal inertia’.
Thermal inertia is the speed at which a body of mass equalises temperature with its surroundings and is dependent on insulation (u value) of the outer surfaces, surface area, and thermal mass (y value) among other factors. This ‘low thermal inertia’ gives us the benefit of keeping cooler in summer whilst effectively storing heat to release at night time.
Because of the way high
thermal mass properties work they are much more suited to a weather compensated control with a gentle warm up and cool down and little fluctuation in temperature during the day. This ensures minimal
waste and no overly high boiler temperatures which are inefficient. However a balance has to be drawn between the thermal mass of the building and its usage pattern, if the if the occupancy is low
(either not used often or not used by many people) or the pattern of use is irregular a method using internal reference may be used as a for a faster response and more instantaneous heat up. This is
an inefficient approach, however the intermittent use and erratic temperature swings of a building are inefficient in their own right, especially in buildings of high thermal mass.
On the other hand if you have a property that has very good insulation, yet low thermal mass a control utilising internal reference would be more suited, to counteract internal influences such as body heat or appliances, and a weather compensated control less desirable, however usage and occupancy should again be considered when selecting control.
Underfloor heating is a
type of heating that although is extremely efficient is also very slow reacting, as the floor its self is used as a thrermal store. This makes it a perfect accompaniment for high thermal mass
buildings. If the building is going to be used intermittently a different emmiter type should most definitely be used, preferably ones that warms air directly and doesn't use as much radiant
In buildings of lower thermal mass underfloor heating is less desirable but again usage should be taken in to account, these are simply ideals and no hard and fast rule, but again, the lower the system volume the faster the heat up and in turn responsiveness.
Usefull thermal mass.
The thermal mass of a
property is usually something which is inherited, however can be slightly added to by the likes of hard wood furnishings and floor coverings. When selecting a control strategy and accounting for
thermal mass you should be aware that the first 50mm of the walls is the most effective and after 100mm the effects go beyond diminishing returns, it should be in direct contact with the internal
space and not seperated by insulation.
In order to effectively use thermal mass, it is common practice in warmer climates to use tiled floor and shades over windows externally. This has the effect of warming the floor during winter months when the sun sits lower in the sky line, for later release at night, yet shades the floor from radiation during the summer, allowing a buffer storage for heat from other scources.
This can be utilised very effectively with a weather compensated underfloor heating as the circulating water can spread collected heat around the house.
Please contact your local ecotechnician for more information.
was written by ecotechnician Adam Chapman from Chapman Plumbing and Heating.