What are flow temperature and weather compensation?

Q: What are flow temperature and weather compensation?

Flow temperature is how hot the water going to your radiators is. The lower it can be while the rooms stay warm, the better the COP. Weather compensation lets the heat pump raise it on cold days and lower it on mild ones.

Updated 29 September 2026

Short answer: flow temperature is how hot the water going to your radiators is. The lower it can be while the rooms stay warm, the better the COP. Weather compensation lets the heat pump raise it on cold days and lower it on mild ones.

Your Cosy heating runs in one of two ways. On a fixed flow temperature, the water goes out at the same temperature whatever the weather. On weather compensation, there are two settings: a minimum flow temperature used in mild weather and a maximum used in the cold. The heat pump picks a point between them from the outdoor temperature.

If the rooms are comfortably warm, lowering the flow temperature a little often improves COP. If they start to feel cool, it's gone too far. Change one setting at a time and give it a couple of weeks.