Hello
We had our 3 heaters on last night to remain active up to 21 degrees but the temperature recorded last night was 7 degrees.
Has anyone been able to keep their S16 mini greenhouse at a constant 20 degrees in order to support the sensitive seeds like aubergines and tomatoes?
Thank you

I think additional insulation will be your only option. Remember that the temperature is air temp rather than soil temp.

For an outside greenhouse, with minimal insulation, and less than 200W of energy going in, it's unrealistic to expect a "warm" environment on cold nights. In winter, it's reasonable to expect the S8 and S16 to prevent frost, and in most situations a few degrees warmer than ambient. As Moe mentions, the soil temperature ( just above the heaters ) will be warmer than the air temperature recorded by the sensor.