Considering Thermal Mass
This is somewhat of a whistle stop tour through thermal mass, what it is, what it does, control strategies, and design considerations. My hope is that this raises awareness to concepts, leading to further investigation and understanding.
This blog post is by no means a comprehensive explanation of thermal mass as it pertains to heating system design, and the built environment.
If you have anything to add, or questions to ask, don’t hesitate to email at: hello@kirtonrenewables.co.uk
Thermal mass
The thermal mass of a material is the capacity it has to absorb, store, and release heat. Important factors are the specific heat capacity of the material, its size, and density.
Thermal mass can be used to store and release heat, smoothing out thermal variations in a building caused by environmental circumstances, or occupancy (when used correctly). Think solar gain (another blog post will address this), the use of appliances, or simply the amount of people in a building, and how active they are inside it.
Shock absorber
The additional capacity for heat energy storage in a high thermal mass building, corresponds with elongated heat-up and cool-down times. Absorbing excess heat and making up the deficit when extrinsic factors are no longer present, or at night. This is a result of a buildings thermal inertia when the heating and insulation strategies align with the thermal mass of the property.
Inertia
The speed at which the temperature of a mass equalises with its surroundings, is its thermal inertia. This is dependent upon the insulation values (U-values) of outer surfaces, available surface area, and thermal mass (Y-value), to give a less than exhaustive list of factors.
Controls
High thermal mass buildings are well suited to weather compensated control, due to the gentle warm up and cool down with minimised fluctuation in temperature during heating periods. This avoids waste and maintains the efficiency of the heat source (heat pump or boiler, for example) as overly high temperatures are not used to achieve comfort in the property.
Balancing act
There is a balance to be struck between thermal mass and the usage patterns of a property. A highly insulated – low thermal mass property may benefit from a more reactive approach where system design and control is concerned, such as the addition of internal reference to accommodate for temperature fluctuations caused by body heat or the operation of appliances. However, internal reference applied alone and/or inappropriately in a high thermal mass building, may bring about temperature swings and inefficient operation of the heat generator.
Times of variable occupancy or irregular use may necessitate a more responsive system with internal reference, but it should be noted that this is inherently a less efficient approach, and in most cases, efforts should be made to maintain steady state heating.
In short, we must design and install to the demands of the building and its thermal characteristics, avoiding wild fluctuations in emitter and air temperature.
Emitters
Emitter selection is a key design decision in any property, and - put simply - we can place ourselves between two main approaches, responsive heating, and steady state heating.
Underfloor heating is highly efficient, slow responding, and makes use of radiant heat (for a greater portion) and significant thermal mass. In contrast, radiators typically have lower thermal mass, can be used at higher temperatures, and make use of convection to essentially mix the air in the room, to deliver heat.
Both UFH and radiators sit within a spectrum, and it should be noted that radiators can be designed into a system that makes use of steady state heating, lower temperatures, and a to a greater proportion, radiant heating in place of convection (yet another blog to be written here!).
Using what you have
For the most part, we inherit the thermal mass of a property, but we can move the needle a little with finishes such as stone, or hard wood flooring, and our furniture choices.
When selecting a control strategy, it helps to be aware that the first 50mm of a wall is the most effective, and beyond 100mm there are diminishing returns. The thermal mass should be in contact with the internal space, not covered by insulation. This may not be possible in every situation, but if you have a choice and the myriad of influencing factors balance to allow it, keeping your thermal mass exposed to the heated space, can be beneficial. Now thit does not go to say that insulation should not be used, only that where it is used is important!
Warmer places and times
To effectively use thermal mass in warmer climates, it is common practice to use tiled floors and external shades over windows. With low winter sun the floor is heated in the day and emits heat through the evening. When the sun is high the floor is shaded, allowing buffer storage from other heat sources.
Weather compensated underfloor heating can make great use of this, as the circulating water can distribute collected heat, around the house.
Measure and design
The inescapable truth is that we must carefully consider the characteristics of each building and its inhabitants, measure to establish our requirements, and design our systems to achieve comfort, efficiency, and longevity!