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Outdoor Weather Compensation vs Load Compensation vs TPI

April 24, 2021

The proper control of heating systems ensures an optimal indoor climate, long life of the system, fewer maintenance activities, higher efficiency, lower operating cost, and improved equipment safety. These central heating systems can be boilers powered by gas, oil, or LPG. It can also be heat pumps. Several control methods are popular in the heating industry as they provide the best compromise of functions. The choice of which control method is used often depends on the building usage characteristics and the owners' preference.

Like most engineering systems, there is a trade-off of the desired parameters within the control method selected. The compromise is often between energy efficiency and comfort levels. For example, the control method that focuses on an optimal indoor temperature may not be the most energy-efficient or cost-effective solution. This article will discuss in detail the pros and cons of each control method.

The control method for centralised heating systems comprises standard temperature control and advanced temperature control. The standard temperature control is the basic ON/OFF control. The heating system is switched OFF when the upper limit setpoint is reached. It is switched ON when the temperature falls below the lower limit. The advanced temperature control can provide a more precise temperature regulation while being energy efficient. There are three main types:

  1. Outdoor weather compensation control

    The outdoor weather compensation control uses an external temperature sensor to estimate the heat demand in a building and control the heat from the heating system (boilers, heat pumps). A relationship curve is defined between the boiler's outlet temperature and the outdoor temperature of the building. The curve ensures that the lower the outside temperature, the higher the outlet temperature of the boiler.

    The outdoor weather compensation control has no feedback connection with the indoor temperature. The curve connects only the outdoor sensor with the boiler. The curve is usually fixed but can be adjusted or shifted to achieve a desired indoor temperature. This tuning of the curve is done during the commissioning stage of the heating system installation.

    The outdoor weather compensation control adjusts the boiler outlet temperature before the indoor temperature starts to change. This is because of the building's thermal mass, where the effect of a change in the outdoor temperature sensor is delayed before it results in a change in the indoor temperature. The outcome is a proactive energy-efficient system that minimizes boiler heat delivery when it is not needed while ensuring indoor temperature requirement is met.

  2. Load compensation control

    The load compensation control replaces the external outdoor temperature sensor in weather compensation control with an indoor temperature sensor. This method allows the heating system to adapt to the indoor temperature as it changes. It is called a reactive system as it requires changes in the comfort levels before the heating system can compensate. The quick response minimizes energy consumption with minimal effect on comfort levels.

    The load compensation control is a more straightforward system to operate. It does not require tuning of a curve in the commissioning stage. The temperature setpoint can be set by the user as desired. The load compensation control adjusts accordingly to building insulation, solar radiation, and other heat sources in the building.

  3. Time proportional and integral control

    The standard temperature control can be improved using a time proportional and integral control (TPI). Remember that the temperature swings between constant upper and lower limits define the OFF and ON positions in the standard temperature control. In the TPI control, the temperature swing is minimized by continuously reducing the swing to a desired internal temperature setpoint. An algorithm is used to calculate the amount of heat needed based on the previous measurement and the time taken to reach the setpoint. The algorithm continuously corrects the deviation until the desired temperature is reached.

    The pros and cons of the three control methods

    Condition Outdoor weather compensation control Load compensation control TPI control
    Automatic regulation with changes in building fabric, oven, insulation etc. No. Regulates with outdoor temperature. Yes. Indoor temperature compensated. No. The setpoint must be adjusted.
    Automatic regulation with changes in weather. Yes, and proactive. Corrects indoor temperature before it is experienced. Partly, but reactive. Slow response to weather changes. No. The setpoint must be adjusted.
    Changes in indoor heating demand due to periodic building use. No. Regulates with outdoor temperature. Yes. Quick response to intermittent use. Yes. Quick response without temperature swing
    Manage undesired changes in temperature, e.g., window opened. Yes. Independent of indoor activities No. Attempts to increase/reduce heating No. Attempts to increase/reduce heating
    Ease of installation Hard. Expert required for tuning and commissioning Easy. Easy to install Medium. Coefficients need to be set for integrals. 
    Speed of response to different comfort levels Slow. The compensation curve needs to be adjusted Fast. Setpoint needs to be changed Fast. Setpoint needs to be changed
    Rate of the cyclic firing of heating system (This affects boiler life expectancy) Fewest of the three control methods. Changes in outdoor temperature are slow, and the compensation curve accounts for rapid changes A bit more than Outdoor weather compensation. The indoor temperature sensor is more likely to change frequently due to several events, e.g., cooking heat, open window etc. More than the other two control methods but significantly less than standard temperature control with high swings.
    Energy efficiency Highest efficiency, but compromise on comfort level. Lower energy efficiency when compared with weather compensation control. But a better comfort level. Lowest energy efficiency but precise temperature control. Hence highest comfort levels.

In addition to the advanced controls for the heating system, there are other ways to improve the heating system's energy-efficiency while ensuring high comfort levels in the building. For example, the comfort level for outdoor weather compensation control can be improved by installing an indoor temperature sensor to raise the heating system's temperature despite the compensation curve.

The load compensation control can be improved by combining it with a communication protocol called OpenTherm. OpenTherm is a language that the heating system uses to communicate with thermostats. This language allows the thermostatic controller to control space heating systems and domestic hot water systems. It also allows the control of heating equipment and controllers across different manufacturers as the protocol is manufacturer independent.

Conclusion

The choice of what type of temperature control method a building owner should choose can depend on several factors like building function (commercial, housing, office etc.), comfort requirements, desired energy-efficiency, energy cost and investment capital. As with most engineering systems, the heating control methods compromise comfort levels or energy efficiency. Regardless of the choice of control method, communication protocols such as OpenTherm offer the possibility to improve the heating system's performance while maintaining high comfort levels.

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