Balance point temperature

The building balance point temperature is the outdoor air temperature required for the indoor temperature to be comfortable without the use of any mechanical heating or cooling . Internal heat sources due to electric lighting, mechanical equipment, body heat, and solar radiation along with building envelope heat loss characteristics may offset the need for additional heating although the outdoor temperature may be below the comfort zone. When a building has significant solar or internal heat gains, the balance point temperature may be lowered below the thermostat temperature, and the building is considered internal load dominated. When a building has a high rate of heat transfer through the building envelope, the balance point temperature is usually near the thermostat temperature, and the building is considered skin (or envelope) load dominated. The balance point temperature is a consequence of building design and function, not climate .

Balance point temperature in reference to ideal indoor temperature and building heat gains and losses

Balance point temperature in reference to ideal indoor temperature and building heat gains and losses.

Mathematical Definition

The balance point temperature is given mathematically as:
Equation 1: t_balance = t_thermostat - ⁠Q_IHG + Q_SOL/U_bldg⁠
Where:

• t_balance is the ambient outdoor temperature which causes the heat transfer across the building envelope to balance the building heat gains at the desired thermostat setting.
• t_Thermostat is the building thermostat setting.
• Q_IHG is the internal heat generation rate per unit floor area due to occupancy, electric lighting and mechanical equipment. This internal heat generation is not constant due to variability in occupancy, lighting, and equipment operation schedule but is largely considered constant to a first order approximation.
• Q_SOL is the internal heat generation rate per unit floor area due to solar radiation. This internal heat generation is not constant due to solar variability with time of day and year but is largely considered constant to a first order approximation.
• U_bldg is the rate of heat transfer across the building envelope per degree temperature difference between outdoor and indoor temperature per unit floor area. This heat transfer can vary due to variations of fresh air ventilation rate but is largely considered constant to a first order approximation.

Building Characteristics

Internal load dominated buildings are usual compact with a low surface-area-to-volume ratio. These buildings usually have many rooms with only zero to one exterior walls in each room. Due to usually higher internal heat gains, internal load dominated buildings do not require passive solar heating (except in cold climates) but rather require passive or mechanical cooling. For large buildings, an internal load may not be able to sufficiently buffer envelope heat exchange throughout the building especially in perimeter regions. Large office spaces, schools and auditoriums are typical examples of internal load dominated buildings where the balance point temperature is set around 50°F (10°C) . Skin (or envelope) load dominated buildings usually have a spread out building form with a high surface-area-to-volume ratio. These buildings usually have few rooms with two to three exterior walls in each room. Due to typically lower internal heat gains, skin load dominated buildings can require both passive or mechanical heating and cooling depending on the climate. Residences, small office buildings and schools are typical examples of skin load dominated buildings where the balance point temperature is set around 60°F (15°C) .

Solar gains can hamper internal load dominated buildings, contributing to overheating, while helping skin dominated buildings that lose heat due to poor envelope design. Therefore, architects and building designers must strategically control solar gains based on the building characteristics .

Case Study

Architects and building designers should holistically consider building function and envelope design before construction; however, balance point testing of a building post construction is important to ensure thermal design integrity. Properly understanding and characterizing skin or internal load dominated buildings can help facilitate overall energy consumption savings by properly setting thermostat temperature to avoid excessive heating or cooling. A balance point test study was performed on Kroch Library at Cornell University in Ithaca, New York to better understand how its unique, underground location attributed to the building performance and whether Kroch Library was a skin or internal load dominated building. Hypothesis testing was on the basis that Kroch Library was an internal load dominated building with a low balance point.

The study investigated three primary modes of energy exchange within the building:

1. Heat transfer through the building envelope was developed by determining glazing surface area through a relation to the total building surface area and using architectural drawings to extract glazing material and U values.
2. Heat transfer through the building envelope was further delineated through three building envelope components: ground heat transfer rate, wall heat transfer rate, and roof heat transfer rate. In each component, architectural drawings provided surface area or perimeter dimensions and material compositions to which U or R values could be inferred.
3. Heat gains through internal loads were compiled through several components:

• • Lighting Heat Gains: Counted total number of light fixtures and determining wattage for each fixture
  • Occupancy Heat Gains: Assumed 40% of the occupancy allowance set by the initial architectural plans and selected predetermined activity levels from ASHRAE standards
  • Equipment Heat Gains: Counted total number of computers and other office equipment with idling heat released values inferred.

The result of the cumulative energy flows determined that Kroch Library is an internal load dominated building

Degree Days

Use of heating degree days (HDD) and cooling degree days (CDD) is the practice of counting the number of days each year for which it is necessary to use energy to heat a building or cool a building. Although degree days are calculated based on recorded energy use in the building, the balance point temperature of the building determines whether a building will annually have more HDD or CDD. A low balance point temperature (relative to the local climate) indicates that the building will be more likely to need additional cooling, while a high balance point temperature indicates that it is more likely to need heating. Ideally, a building should be designed such that the balance point temperature is as near as possible to the average outdoor temperature of the local climate, which will minimize both the CDD and HDD

Modeling

In order to determine the balance point temperature, either before a building is made or when trying to optimize the building, it is often necessary to create a mathematical model of the situation. The most important part of this for balance point temperature is often internal loads, which do not have a linear relationship to balance point temperature, making modeling a challenge. In the past, semi-parametric systems have been put in place to solve this problem . An example of this is the work carried out Jeffrey Dubin in 2008. His resulting model equations are shown below. In these, assuming that Equation 2 may be used to approximate the amount of heating energy needed to be put into a space in order for it to remain within the comfort band, then Equation 3 may be used to model the balance point temperature, given varying internal thermal characteristics (insulation levels, thermal loads, etc).
Equation 2: Q = w₀ + w₁ * (t_i - t_o) + w₂ * (t_i - t_o)²
Equation 3: t_b = t_i + ⁠w₁/2w₂⁠(1 –(1 * 4w₂w₁²w₀)^1/2)
Where:

• Q is heat loss per hour due to indoor-outdoor temperature differential
• w₀, w₁, and w₂ are functions of the dwelling's thermal characteristics (due to size, insulation, and interior heat load, etc)
• t_i is the interior temperature
• t_o is the outside temperature
• t_b is the balance temperature

A more common use of balance point in modeling is to use the balance point as a base by which to calculate another factor, such as the energy demand of buildings due to various stressors , or natural ventilation’s effect on indoor particle concentrations .