This article starts with a concise summary that is best-suited to individuals already familiar with TEDI. Readers can continue on past the summary for further details, or for more context if you are not already familiar.
Summary
TEDI is a measure of the heat required by a building. It is a metric that was introduced as a way to reduce the heat required by new buildings. This in turn is intended to reduce the greenhouse gas emissions associated with heating these buildings. TEDI is used in the City of Vancouver’s energy compliance limits and in the BC Energy Step Code.
Reducing the TEDI value of a building (reducing the heating requirement) can be achieved by making design changes.
Many of these improvements can lead to negative knock-on effects; these negatives always need to be considered.
Reducing the TEDI value without negatively impacting building occupants (people) should be the goal for every new building. Balancing of improvements and their negatives is required to achieve this goal. Understanding, and quantifying, these improvements and negatives is essential to achieving the optimum balance.
What does TEDI mean?
TEDI stands for Thermal Energy Demand Intensity.
It is a measure of the thermal energy (energy in the form of heat) required by a building for space conditioning and for conditioning of ventilation air. So, it is the heat load required to heat up spaces in a building as well as to heat up the outside air that is brought into the building. It is quantified on an annual basis, so when thinking about TEDI, one should imagine a building over the course of one full year.
TEDI is quantified on a floor area basis. It is the amount of heat load (in kWh) divided by the floor area of the building (in meters squared); the unit for TEDI is kWh/m2. As mentioned previously, it is on an annual basis. If a 20,000m2 building requires 600,000 kWh of heat and a 2,000m2 building consumes 60,000 kWh, both buildings have a TEDI value of 30 kWh/m2 per year.
TEDI was introduced to create performance limits for new buildings. The purpose of TEDI is to reduce the amount of heat load required by a building. Reducing the heat requirement reduces the energy consumption, which in turn reduces the greenhouse gas emissions during a building’s operation. So, TEDI is a way to reduce emissions in new buildings.
What influences TEDI?
The floor area of a building is constant. It is an important part of the TEDI equation, but it is always the same. Conversely, the amount of heating energy required by a building is constantly changing throughout the year. This is impacted by a wide range of factors. Some factors are relatively static while others are more dynamic.
A building’s envelope is a relatively constant factor that influences the amount of heat required on a continuous basis. At its simplest definition, the building envelope is the exterior shell of a building that repels the elements. The insulating properties of the walls in a building and the extent of window area are relatable examples of how a building’s envelope influences the heat required. The airtightness of the building envelope is another characteristic that has a significant impact on heating requirement; if more outside air enters a building through the envelope, more heating is required.
A useful guide that can aid in understanding a building’s envelope has been included in the further reading section at the end of this article.
The form and orientation of a building are constant. Once built, these do not change. However, they can have a large impact on the TEDI value for a building. These factors should be carefully considered during the early design stage. In relation to the form, or shape, of a building: a single storey, long, narrow building has a lot of envelope area relative to the amount of floor area. On the contrary, a multi-storey, deep, square building of the same floor area will have a lot less envelope area. In general, the higher the ratio of envelope area to floor area is, the more heat will be required, leading to a higher TEDI value. In the below image, all five buildings have the same floor area. However, the envelope to floor area ratio for the buildings varies from 1.5 to 2.9. This wide range highlights how the form of a building drastically influences the total envelope area; this has a large impact on the heat requirement. The options relating to the form of a building are often limited by the site. Sites in dense urban areas are often restrictive and can lead to small, narrow buildings; these will struggle to achieve a low TEDI value.
There is a link included in the further reading section at the end of this article if you are interested in how energy performance is impacted by this floor to envelope ratio.
Aside from the more static factors mentioned previously, dynamic factors cause the amount of heat required by a building to constantly vary. Throughout the year, it varies with the weather as the seasons change. It varies from nighttime to daytime each day. It varies throughout the day as people enter and leave the building; people generate body heat which warms the building. The equipment and lighting people use also varies depending on when they are in a building. The factors above are just a small sample of the many dynamic factors that impact the TEDI value for a building.
Additionally, there are factors associated with the mechanical systems within a building that must be considered. These are somewhat dynamic, but they are often predictable and can be designed for. The majority of new buildings require mechanical systems to deliver outside air into spaces occupied by people. This is ventilation air; it is required to ensure the indoor air quality is suitable for people to occupy the building. Often the outside air supplied to the building is fresh and cold, it needs to be heated up. The ventilation rates (the volume of outside air supplied into a building) are decided during design. The higher the ventilation rates, the more heat that is required to heat the outside air. The supply air temperature setpoint of ventilation air, which is the temperature the air will be heated to before being supplied into the building, is also important. Similarly, the temperature setpoint of each space in the building is important. Increasing the setpoint temperature for ventilation air and/or spaces will increase the heat required by the building.
The biggest standalone item that impacts TEDI is heat recovery for ventilation air. For all outside air supplied into the building, there is a similar volume of air extracted from the building. This extracted air is warm and stale. Heat recovery technology uses the heat in the outgoing stale air to warm up the fresh air. High-efficiency heat recovery units can recover over 80% of the heat in extract air; it avoids wasting all of that heat. Heat recovery technology is a guaranteed method of reducing a building’s TEDI value.
See the link in the further reading section at the end of this article explaining how heat recovery units work.
What does not influence TEDI?
The efficiency of heating equipment. It is important to understand that how the heat required by a building is generated does not have any influence on the TEDI value. The TEDI value only considers the heat load. It does not matter how this heat load is generated. The heat load is all that matters.
Take an office building as an example. It has a rooftop air handling unit that heats incoming outside air with a hot water coil prior to supplying air into the building. It does not matter how the water was heated. The TEDI value will be the same regardless. Whether that water was heated using a gas boiler with an efficiency of 80% or an air-source heat pump with a COP of 3 (300% efficient) doesn’t make a difference to the TEDI value for the building.
What is considered when calculating TEDI?
As previously outlined, TEDI is a measure of the annual heating required by a building for space conditioning and for conditioning of ventilation air. The previous section gave examples of what influences the TEDI value for a building. Hopefully it gave the reader an understanding of factors to be considered in relation to TEDI.
The following list, while not as easy to digest from the readers perspective, outlines what is considered in the calculation of a TEDI value:
- Thermal transmittance of above-ground walls and roof-ceiling assemblies
- Thermal transmittance of floors and walls in contact with the ground, or space that is not conditioned space
- Thermal transmittance and solar heat gain of windows, doors and skylights
- Air leakage through the air barrier system
- Internal heat gains from lighting, occupants, equipment etc.
- Heat recovered from exhaust ventilation.
The factors touched on in the previous section all fit into one or more of the items on the above list.
How can the TEDI value be improved?
Improving the TEDI value of a building means reducing that value. Reducing the value is achieved by reducing the heat load required by the building. Reducing the heat requirement of the building can be achieved by making improvements to the building. A sample of several potential improvements are shown in Figure 1.
Is reducing the TEDI value a positive thing?
Reducing the TEDI value reduces the heating energy consumed. This reduces the greenhouse gas emissions during a building’s operation; this is definitely positive. However, there are negatives that can be brought about by trying to reduce TEDI too far.
Many improvements aimed at reducing the TEDI value have negative knock-on impacts. Some examples are shown in Figure 2. Careful consideration of each improvement is required to balance against potential negative knock-on impacts.
By knowing the positive and negative impacts of each improvement under consideration, an optimized building design can be achieved. The optimum approach is one where the TEDI value is reduced as much as possible without negatively impacting the indoor environment of the building. After all, there is no point in constructing a building that does not require any heating if it is not a healthy building for people to use.