Is it a good idea to use both weather compensation and setback at the same time?

Using both weather compensation and setback controls at the same time can be considered counterproductive. During a setback (a term that was previously referred to as ‘night setback’), a heat pump will be maintaining the lower temperatures set during the night or unoccupied times and when this desired lower temperature is met, the heat pump will switch itself off. There will be little heating demand until the ‘energised period’ commences which will either be triggered by a drop in temperature or when the system reverts to the normal heating demand.

Meanwhile, weather compensation modifies the flow water temperature in accordance with the outside temperature. If the outside temperature is cold, then the flow temperature increases and when the outside temperature warms up the flow temperature decreases.

To demonstrate how setback and weather compensation should be programmed, we’ve provided an example via the infographics below which are based on a typical school or working day during the winter months.

Night-time setup

7pm through 7am (evening until morning) with a traditional setback period between 10.30pm through to 6am

Generally, these are the hours during a 24 hour period that are the coldest part of the day. Therefore, if weather compensation is in place, the flow temperature in the system will be higher. If a traditional setback is applied during this period (between 10.30pm until 6am), this will mean that it is the most likely time to get the full design temperature into the heating system and, while the property will be allowed to cool at a faster rate due to the lower outside temperature, setback will switch the heat pump off for a large proportion of this period so the indoor temperature is less likely to be adequately achieved and maintained.

Daytime setup

7am through to 7pm (daytime) with a traditional setback period between 11am through to 3pm

Generally, these are the hours during a 24 hours period that are the warmest part of the day. If weather compensation is in place, the flow temperature will be lower than during the night, so the full flow design temperature is unlikely to be available during this period. In addition, if another setback period is applied (between 11am until 3pm), the heat pump will be switched off for a large period of time meaning that the indoor temperature is less likely to be adequately achieved and maintained.

Recommendations

A heat pump system should either be set to run the heat pump continuously with the weather compensation mode applied or, alternatively, if both weather compensation and setback facilities are to be applied, ensure all the following is programmed:

• make sure the weather compensation has been set to the correct heating design conditions
• reduce the setback temperature to a maximum of 2ºC including the hysteresis
• reduce the setback duration
• during prolonged cold spells, further reduce or remove the setback temperature and duration.