What is TPI ?

What is TPI ?

TPI – (Time Proportional & Integral)

Modern controls are designed to ensure that you get the best energy efficiency possible from your heating system. TPI is an algorithm function in your room thermostat that ensures your boiler fires for the shortest possible time in order to reach and maintain your room at the comfort level selected:

How it works :

Firstly, there are a few things you need to understand about a TPI room thermostat: It has a simple memory included and based on historical temperature data the thermostat can ‘learn’ how long it takes to heat up a room to a desired setpoint temperature and then maintain it. It also has a basic calculation capability so that it can work with the learned data and current temperatures to predict how long heat must be supplied ie. how long the boiler needs to be fired for.

Similar to PWM (Pulse Width Modulation) it works according to cycles. Cycle length is measured in amounts per hour so 3CPH = 3 Cycles per Hour.

TPI in action :

A TPI room thermostat works by continuously measuring the room temperature and calculating a difference between the measured and target setpoint temperatures. When the difference is large (known as outside proportional band) it fires the boiler continuously. When the temperature difference begins to decrease, the TPI software in the thermostat comes into action. It performs a series of calculations where it calculates how long the boiler needs to be fired to reach and maintain the setpoint temperature. The calculation is based on the previously learnt characteristics of the room in which the controller is placed and the current measured temperature. This ‘intelligent’ control will mean that the heating is supplied for just long enough to achieve the set temperature and no longer. This increases energy efficiency.


What you can notice when using TPI :

In normal operations you will not notice any difference in how your heating system functions. You may, if you listen carefully, hear the boiler firing for shorter and shorter periods as it approaches the correct temperature. When the room setpoint is reached, you will notice that the boiler is fired for short periods in order to maintain the temperature. The thermostat is calculating the rate at which the room is losing heat and topping up with the precise amount required to counteract it and stop the room from cooling down.

The Thermostat can overheat a room when the setpoint temperature is reached for the first time (this is because the thermostat tries to reach its setpoint as quickly as possible). This is known as the ‘overshoot’. The Thermostat stores room temperature historical data to calculate the rooms heat loss characteristics. By reading those parameters, the thermostat "learns" a room. You will find that the initial overshoot is higher during that learning period, but that it will quickly learn the characteristics and become more accurate.

How does this differ from Non-TPI Thermostats ?

In non TPI thermostats (commonly older devices), the boiler will continually fire until it gets past the set point and then when the temperature drops to a specified amount below the set point the boiler is then fired again. This creates greater temperature fluctuations and less control of your comfort levels.



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