Can heating degree days be negative and how do you calculate cooling degree-days in Celsius and heating time and what is heating and cooling time constant?
Heating degree-days is the sum over the year of the positive differences between a conventional indoor temperature (often 18°C) and the daily average outdoor temperature expressed in Celsius. When the difference is negative, the day is not a heating day. With this definition, the degree-days cannot be negative.
Cooling degree-days have a similar definition.
Note that both concepts are used in calculation methods of heating or cooling energy needs that provide approximate results, valid only in climates where heating, resp. cooling is needed for a long period of time (e.g. 3 months or more). These concepts are no more used in ISO standards.
Sorry, I forgot the answer about the building or room time constant.
One definition is the ratio of its thermal inertia, in J/K (or Ws/K) to its heat transfer coefficient in W/K. The ratio is hence in seconds. This time constant expresses how fast is the reaction of the building or room to an outside temperature change.
The heat transfer coefficient is the sum of the transmission and ventilation heat losses.
Note that this concept does not take the internal and solar gains into account.
Another definition is the time needed to heat (or cool) a building or a room by 1 K when a given thermal power is given to (or taken from) it, either by the heating or cooling system, or by internal or solar gains. It is then the ratio of the thermal inertia to the thermal power.
See also the attached paper.
I agree with Claude Alain Roulet that if the temperature mean is above 65°F, we subtract 65 from the mean and the result is Cooling Degree Days. If the temperature mean is below 65°F, we subtract the mean from 65 and the result is Heating Degree Days. Because the result is below 65°F: 65°F - 29°F = 36 Heating Degree Days. HDD is set to zero if it has a negative value. HDD is used in calculations of weather futures contracts, which are used as a risk management tool by industries, such as construction and agriculture, whose operations are affected by weather conditions. Therefore, Heating Degree Days (HDD) can be calculated like this: 220 days times (65°F minus 27°F). And this is equal to 220 days times 38°F which is equal to 8,360°F days. We know the temperature difference, we know the HDD, and we can calculate the heat loss provided we have the R value. R-value is also given. It is calculated by taking a median temperature of 65°. This is the temperature that most houses do not need energy assistance to keep warm or cool. Next, the average temperature for that day is subtracted from the median temperature. This value is how many Degree Days represent the current day's weather. Base temperature has been determined as 18 °C for heating and cooling for India. For a hot summer month, if the average daily temperature is above 18°C for five days, you sum the total number of degrees above 18°C for those five days to get the number of degree-days of cooling in that month. Adding the values for each month gives you an annual total. Cooling degree days (CDD) is a measure that helps to simplify the cost of projected energy consumption. It is based on the number of days where the temperature is above 65 degrees Fahrenheit, and the number of degrees above 65. For India, annual cooling degree days are 3120 and annual heating degree days are 80. The optimal temperature setting for cooling in Celsius can vary depending on personal comfort and energy efficiency. However, a common recommendation is around 24-26 degrees Celsius. The time taken to change the temperature of a thermostat from an initial temperature T0 to the target temperature is called the heating time constant. The symbols representing the rate of change from T0 to the target temperature. The cooling time constant is defined as the time required to cool the machine down to 0.368 times the initial temperature rise above ambient temperature. If the motor is disconnected from the supply, there will be no losses taking place and so the final temperature reached will be ambient temperature.
Heating degree days (HDD) can indeed be negative. This happens when the average daily temperature is above the base temperature. The base temperature is a reference point that indicates when a building starts needing heating.
Here's a breakdown of your questions:
Heating Degree Days (HDD):
- Negative HDD: As mentioned earlier, HDD can be negative if the daily mean temperature is higher than the base temperature. In such cases, the heating system isn't required to maintain comfort levels.
Cooling Degree Days (CDD) in Celsius:
- Similar to HDD, Cooling Degree Days (CDD) are calculated using a base temperature, but for cooling needs. Here's the formula for CDD in Celsius:CDD = sum of (daily mean temperature - base temperature (cooling)) for all days where daily mean temperature > base temperature (cooling) Base temperature (cooling): This is the temperature above which a building requires cooling.
Heating and Cooling Time Constants:
- These constants are specific to each building and its cooling and heating systems. They indicate how long it takes for the building's temperature to react to changes in outside temperature or adjustments to the heating/cooling system.
- These time constants depend on factors like:Building size and insulation Type of heating and cooling system Ventilation rate
Unfortunately, there's no general formula to calculate them without considering the specific building details.