Bivalent Heating: Heat Pump and Gas Boiler by Outdoor Temperature

A heat pump runs most of the year and the gas boiler stays blocked. Only when it has been freezing for a longer time does the system switch to the gas boiler and block the heat pump, and it switches back only after a longer mild period. One Equation smart rule and two statistics of the outdoor temperature do this without any script.

The example uses −5 °C for 3 hours to switch to the boiler and 0 °C for 12 hours to switch back. Thresholds and durations are yours to choose.

You need:

• an outdoor temperature sensor,
• an output that turns the gas boiler on,
• an output that blocks the heat pump.

How it works#

• The Maximum of the outdoor temperature over the last 3 hours is −5 °C or lower only if the temperature stayed at −5 °C or lower for the whole 3 hours. One warmer reading inside the window raises the maximum and the countdown starts again.
• The Minimum over the last 12 hours is 0 °C or higher only if the temperature stayed at 0 °C or higher for the whole 12 hours.
• Between the two thresholds the rule has to remember what it did last. That is the LastState variable: the previous result of this Equation. Without it, the boiler would switch off as soon as the temperature rose from −6 °C to −2 °C, although 12 hours above zero have not passed yet.

Both statistics use Period type Period to now: a rolling window that ends at this moment and moves on with time. Current period is a fixed calendar period (the current hour from the full hour, the current day from midnight) that starts again at its beginning, so it cannot tell how long a condition has lasted.

Create the statistics#

Open the detail of the outdoor temperature sensor, then Statistics, and add two statistics:

Create the statistics before the smart rule. The variable picker of the Equation lists only statistics that already exist on the device. A variable whose statistic does not exist stays empty and the rule never reacts, without showing an error.

Create the Equation smart rule#

Variables#

Add the outdoor temperature sensor twice and pick a statistic from the Statistics part of the list each time:

• Maximum, Period to now, 3 hours — rename the variable to TMAX3
• Minimum, Period to now, 12 hours — rename the variable to TMIN12

Equation#

TMAX3 <= -5 OR (LastState AND !(TMIN12 >= 0))

The operators are AND, OR and ! — && and || do not exist in the TapHome script. !(TMIN12 >= 0) keeps the boiler running while the 12-hour minimum has no value yet.

If result is TRUE, perform following actions:#

• Gas boiler — turn on
• Heat pump block — turn on

If result is FALSE, perform following actions:#

• Gas boiler — turn off
• Heat pump block — turn off

Behaviour over time#

The grey lanes are what the rule watches: the outdoor temperature (shape only, without a scale) and the two statistics. A striped bar is a window counting down, a dashed outline is a countdown cancelled by a warmer reading. The blue lanes are what the rule switches. The time axis is in hours and to scale.

1. The temperature drops below −5 °C and the 3-hour countdown starts.
2. A warmer hour (−3 °C) cancels the countdown. The 3 hours start again when it is below −5 °C once more.
3. After 3 hours below −5 °C the result is TRUE: the boiler turns on and the heat pump is blocked.
4. The temperature rises to −2 °C. The maximum is above −5 °C now, but LastState keeps the boiler running.
5. The temperature reaches 0 °C and the 12-hour countdown starts.
6. After 12 hours at 0 °C or above the result is FALSE: the boiler turns off and the heat pump runs again.

Alternative with two smart rules#

The same works without LastState, with two Equation smart rules: one only switches to the boiler (TMAX3 <= -5, actions only for TRUE), the other only switches back (TMIN12 >= 0, actions only for TRUE). Both act on the same outputs, so check their order on the outputs. The pattern is described in Hysteresis equation.

Things to keep in mind#

• Accuracy. The Core recalculates the statistics about every minute, also when no new reading comes. The 3-hour window follows the readings to about a minute. The 12-hour window is built from hourly blocks, so the switch back can come about an hour earlier or later.
• Partial window after creating the statistics. A new statistic has no history yet. If you create the rule during frost below −5 °C, the boiler turns on immediately; during the first 12 hours the minimum covers only the time since the statistic was created.
• Saving resets LastState. Every saved change of the rule starts it again with LastState FALSE. If you save it while the boiler runs and the temperature is between −5 °C and 0 °C, the system switches back to the heat pump. A restart of the Core does not reset it.
• Manual changes stay. A manual change of the boiler or the heat pump block is not overridden while the result stays the same; the rule takes the outputs back when its result changes.

See also — Statistic values in smart rules, Hysteresis equation, Equithermic regulation, Boiler cascade.