A mother-of-two from Cornwall died after scalding water poured out of a water tank through a ceiling and on to her bed as she slept, an inquest has heard.
The couple had lived together in the house for three years
Sharon Minister, 30, was admitted to hospital with 45% burns after the accident at her housing-authority owned home in Penmere Road, Penzance in May 2002.
She died from her injuries nine days later.
Tests carried out by the Health and Safety Executive (HSE) later showed that a fault in the property's immersion heater had allowed the cold water tank to fill with scalding water.
Ms Minister's partner, Mark Nicholas, who was sleeping next to her at the time, survived the incident but suffered severe burns to his face and body and needed a skin graft to his foot.
I realised that Sharon was experiencing the same pain as me
Mr Nicholas, 27, told the inquest in Helston he was unemployed and had lived with Ms Minister, a cleaner, in the Penwith Housing Association-owned property for three years.
Ms Minister's children Jessica, nine, and Victoria, six, were in the house at the time but were unharmed in the accident.
Mr Nicholas said the house was gas centrally heated, but the couple used the electric immersion heater for their water.
On the night of 29 May, 2002, Ms Minister said she had switched off the immersion heater before the couple went to bed. Mr Nicholas said he was later woken up by a "massive shock".
He said: "I was screaming at the top of my voice. I realised that Sharon was experiencing the same pain as me."
The injured couple were both taken to Bristol's Frenchay Hospital, where Ms Minister died of septicemia caused by scalding on 8 June.
A police inspection of the property showed the immersion heater in a cupboard in the main bedroom was switched on.
David Pritchard of the HSE told the inquest he had carried out tests on the cold water tank from the loft and the immersion heater's hot water tank from the couple's bedroom.
He said a faulty thermostat meant the water was able to reach boiling point instead of stabilising at 60C.
The hot water then flowed into the cold water tank in the loft until eventually that too was filled with boiling water.
Mr Pritchard said: "I understand that the tank wasn't fully supported. In my view this would explain why this tank slumped and released its water."
Mr Pritchard said it was not clear why the thermostat had failed.
Mr Nicholas told the inquest he heard the hot water tank in his room "bubbling" on many occasions in the months leading up to the accident but had not reported it to the housing association.
David Cory, HSE inspector, told the inquest that Ms Minister's roof had been inspected in 2001 but was not repaired.
Since the accident 703 properties owned by Penwith Housing Association have
been investigated for faults.
Of those 17 had problems with the water heating system, one of which had a
similar defect to the cold water tank in Ms Minister's home.
Mr Cory said new thermostats had been fitted in the association's properties to replace the model which went wrong in Ms Minister's home.
The faulty model did not bear the British Safety kitemark.
The inquest was adjourned until Wednesday.