Degree Days

Degree Days

Weather Data for Energy Professionals

Degree-Day Base Temperature - Choosing the Best Values for Heating and Cooling

The choice of base temperature is a fundamental consideration for degree-day-based analysis. Heating degree days will only accumulate (have non-zero values that increase over time) when the outside air temperature drops below the heating base temperature. Similarly, cooling degree days will only accumulate when the outside air temperature rises above the cooling base temperature. (If this is confusing, please see our introduction to degree days.) The base temperature(s) you choose will affect your figures greatly, so it's important to choose them in a way that is appropriate for the analysis you are doing.

Different countries have different base-temperature conventions, like 65°F in the US and 15.5°C in the UK, but these are really just traditional values that have lingered on from the days when degree days were disseminated in print and it wasn't practical to provide them in a wide range of base temperatures. Now you can get degree days in any base temperature you want (certainly you can through Degree Days.net), and it's usually best to choose the most appropriate base temperature(s) for each building you are analyzing. These are sometimes called the balance point(s) of the building.

You can use regression analysis to find the base temperature(s) that give the best statistical fit with a building's energy-consumption data (our regression tool does this automatically), but you should usually favour base temperature(s) that make logical sense as well. This article will help you estimate base temperature(s) that make logical sense for a building.

One base temperature for heating, another base temperature for cooling

We say "base temperature(s)" because, for buildings with both heating and cooling, the heating base temperature and the cooling base temperature are usually different. Typically the cooling base temperature is higher than the heating base temperature. The reasons for this will become clearer as you continue through this article.

For a building with only heating, you'll only need to work with heating degree days (HDD), so you'll only need to estimate the heating base temperature. Similarly, for a building with only cooling, you'll only need to work with cooling degree days (CDD), so you'll only need to estimate the cooling base temperature. For a building with both heating and cooling, you'll need to estimate the heating base temperature and the cooling base temperature as you'll be working with both HDD and CDD.

Simple definitions of the heating and cooling base temperatures of a building

These are simple definitions, but estimating the base temperature(s) of a building is more complicated. A common error is to assume that the base temperature is simply the thermostat set point (i.e. erroneously thinking that the heating base temperature is the inside temperature below which the building needs heating, which is not the case). But the need for heating and cooling is driven by changes in the outside temperature, and the relationship between the temperatures outside and inside a building is complicated by various factors that should be considered when estimating the degree-day base temperature(s) of the building:

Key factors that influence the base temperature(s) of a building

Thermostat set point(s)

The thermostat set point is the inside temperature that the building is heated or cooled to.

Buildings with both heating and cooling often have 2 thermostat set points:

It is typically a good idea for an HVAC system to have a "deadband" like this, as it saves energy and reduces the chances of the heating and cooling systems fighting each other. Naturally you should consider the heating set point when estimating the heating base temperature and the cooling set point when estimating the cooling base temperature.

Many buildings also have different thermostat set points for unoccupied hours. This is discussed further below when we consider intermittent heating and cooling.

The thermal properties of the building

The main factors are generally as follows:

The thermal properties of the building determine the extent to which the factors described below affect the base temperature(s):

Internal heat gains

Most buildings contain people and equipment that generate heat. For example, lights and office equipment like computers, monitors, projectors, and photocopiers all generate heat. Manufacturing processes can generate a lot of heat, and residential equipment like televisions, dishwashers, and washing machines generate heat too.

Internal heat gains lower the heating base temperature of the building, as the "free heat" means the heating system doesn't have to work as hard. (Remember that a lower heating base temperature means less heating degree days and less heating is required to keep the building at its heating thermostat set point.)

Internal heat gains lower the cooling base temperature too. But, whilst internal heat gains make things easier for the heating system, they make the cooling system work harder as it has to cool the building more to counteract the extra heat generated by people and equipment. (Remember that a lower cooling base temperature means more cooling degree days and more cooling is required to keep the building at its cooling thermostat set point.)

Internal heat gains naturally vary over time, as people come and go and equipment is switched on and off. So for degree-day purposes we think of them in terms of an average.

Also, the average heat from people and equipment would logically be thought of in units of power (e.g. kW), but, for degree-day purposes we think of them in terms of the average temperature increase they cause within the building (assuming the cooling system isn't countering it!).

Thus we have the concept of average internal heat gain - the average number of degrees of free heat provided by people and equipment within the building. To estimate an actual figure, we also need to consider the thermal properties of the building:

A well-insulated building with a low ventilation rate does not easily let the heat energy from people and equipment escape. This makes for a bigger average internal heat gain (measured in degrees). So a well-insulated minimally-ventilated building will typically have lower base temperatures than a poorly-insulated or highly-ventilated building. Though bear in mind that many HVAC systems provide cooling by increasing the ventilation rate, meaning that the internal heat gains do not reduce the cooling base temperature as much as they reduce the heating base temperature.

Very rough estimates: a sparsely-populated building with limited equipment and poor insulation might have an average internal heat gain of just 1-2°C (about 2-4°F). A well-insulated building with more people and a fair amount of home or office equipment might have an average internal heat gain of 5°C or so (9°F or so). A server room packed with racks of hot computers might have an average internal heat gain of 15°C or more (27°F or more).

Solar gains

Sunlight on a building causes it to heat up - we call this heat "solar gains". Solar gains can be good and bad: the extra heat can reduce the heating energy consumption, but it can also increase the cooling energy consumption.

Pretty much all buildings experience solar gains to some extent, but some more-modern buildings are designed to really make the most of them, with large windows to let the solar gains in, and clever heavyweight construction to allow the thermal mass of the building to warm up slowly with solar gains in the day (taking heat from the inside space to prevent overheating when the outside temperature is peaking) and then release the heat slowly into the inside space later when it is more useful.

For degree-day-based analysis we tend to think of solar gains a bit like internal heat gains: an average number of degrees of extra heat provided to the building. Of course in most climates the solar gains will vary day to day and season to season, but, unless they are particularly large, it's not an unreasonable approximation to think of them as an average.

So, like for internal heat gains, solar gains will typically lower the heating base temperature (good because it means less heating) and lower the cooling base temperature (bad because it means more cooling), and the extent of the effect will depend on the thermal properties of the building.

A heavyweight building may introduce sufficient time delays to capture the good (reduced heating) but little of the bad (increased cooling). In fact, a heavyweight building may even reduce the cooling requirement (increase the cooling base temperature) if its construction means that the building fabric cools down at night and remains cool for long enough to cool the interior space during the day. Heavyweight buildings are complicated and their base temperatures deserve more careful consideration!

Very rough estimates: a typical building with a small to moderate amount of sun-facing glazing is unlikely to have average solar gains of more than 1°C or so (about 2°F), but a heavily-glazed building that has been designed to make the most of solar gains could expect a considerably greater number. A heavyweight building may be designed such that the heating base temperature is affected a lot more than the cooling base temperature.

Intermittent heating/cooling

Some buildings are heated/cooled to the same temperature(s) 24/7, but many have intermittent heating/cooling to match occupancy hours. Typically during unoccupied hours the HVAC system is left on, but with a lower heating thermostat set point and a higher cooling thermostat set point than during occupied hours. (These are often called "setback temperatures".)

An intermittently heated/cooled building will generally use less heating/cooling energy than one which is heated/cooled to the same temperature(s) 24/7. The reduction is not as great as many people instinctively assume, because, when a building cools down (or warms up) during unoccupied hours, it usually requires extra heating (or cooling) to bring it back to the desired temperature for occupied hours. But overall:

The extent to which intermittent heating/cooling lowers the heating base temperature and raises the cooling base temperature depends mainly on the following factors:

Very rough estimates: for a well-insulated day-occupied building, intermittent heating might reduce the heating base temperature by 2-4°C or so (about 4-7°F), and for a similar night-occupied building, intermittent cooling might raise the cooling base temperature by a similar amount. The effect will typically be smaller if the insulation is worse or the hours of occupancy are reversed, or longer.

Refrigeration

Some commercial buildings have refrigeration/freezer areas that are powered by a central cooling system. For analysis of the energy consumption of such cooling systems, you'd typically use cooling degree days with a low base temperature, taking the average internal temperature of the refrigerated spaces as the set point.

If the energy consumption of the refrigeration cooling system is metered together with the energy consumption of the space cooling system (which cools the normal occupied spaces of the building), you can expect the refrigeration to lower the overall cooling base temperature, more if there is more refrigeration, less if there is less. This is the main circumstance in which you will often get a building with an overall cooling base temperature that is lower than the heating base temperature - it's being dragged down by refrigeration cooling on the same meter as space cooling.

Estimating the numbers

Base-temperature estimation is a skill that develops with experience of analyzing data from lots of different buildings. This article primarily aims to highlight the main factors that should be considered; the rough °C/°F estimates we have included are just approximate generalizations - they can't be expected to apply consistently as buildings vary considerably and there are a lot of factors involved.

It is possible to estimate base temperature(s) more rigorously by modelling the sources of heat and the thermal properties of the building. This is, however, a step beyond what many people are willing to invest in simple degree-day-based analysis, and it needs to be done well to get good results. Also, it inevitably still requires approximations because many of the factors involved vary day-to-day or with the seasons. The whole idea of a building having a constant base temperature (or one for heating and another for cooling) is an approximation - a useful approximation, but we should remember that real-world buildings are a lot more complicated than any simple model implies.

We often prefer to start with the energy-consumption data, using our regression tool to automatically test regressions with lots of heating/cooling base-temperature combinations to find the ones that give the best statistical fit. The general guidance on this page is then useful to help choose the best base temperature(s) from the shortlist generated, or to determine that it is necessary to investigate the building and its metered data further before a good model can be developed.

Generate Degree Days or Run Regressions Now

© 2019 BizEE Software Limited - About | Contact | Privacy | FAQ | Desktop App | API