A family of five is in a stable condition in a Sydney hospital after being treated for carbon monoxide poisoning from an open coal fire lit to keep warm at home.

It is believed the family, sleeping in one room of the unit in Liverpool in south-western Sydney, lit the fire on the floor and went to bed leaving the doors and windows closed.

Paramedics treated two children aged six and 11 while their father rushed a third, aged one, to hospital.

Two of the attending police officers suffered breathing difficulties, and one was taken to Liverpool hospital.

The authorities are reminding people to make sure that there is sufficient ventilation in rooms where an open fire is burning.