A "ghoulish" proposal to use heat from cremations at a Cardiff crematorium to help warm its chapels is backed by many bereaved families, says the council.
Cardiff council is looking into using emissions produced during cremations at Thornhill Crematorium to heat the building.
The idea has been criticised as "disrespectful" to families who have lost loved ones.
A similar proposal is being considered in Worcestershire.
Cardiff council said a large majority of families who responded to questionnaires sent to them following funerals supported the idea.
However, a 62-year-old woman from Rhiwbina in Cardiff, who did not want to be named, said she went "utterly ballistic" when she received a questionnaire following the death of a cousin.
"I thought it was inappropriate to be asked this in the bereavement period. The actual concept is quite despicable. I'm really against this," she said.
"You go into the church and the thought that somebody's loved one was being burned and [generating] the heating you're experiencing is really, really ghoulish."
She said she was not against cremation itself but was strongly opposed to "using the body for reheating a building".
Councillor Nigel Howells, executive member for sport, leisure and culture, said there had been a positive response to the proposal.
"Each year, questionnaires are sent to around 3,500 applicants following a funeral service asking if they would find the proposal to recover heat from the cremation process to provide heating for chapels objectionable," he said.
"The return rate for the last two years has been around 13%, and an average of 85% of respondents were in favour of the proposal."
He said new filtration equipment at Thornhill Crematorium reduced air pollution while capturing waste energy.
"As part of the process, heat from emissions produced during cremation pass through pipes contained within a cold water boiler, and heat from this water could be passed through the heating system within the crematorium building," he said.
"The harnessed heat is not generated directly from the cremation of the remains of the deceased but from the filtration process.
"It would be possible for the harnessed heat to be used to heat the existing crematorium building and chapel. The use of a heat exchanger would reduce our carbon emissions and heating costs."
He added that the council's bereavement services would "further investigate" the proposal this year, but the local authority was not "actively considering it at the moment".
Independent Cardiff councillor Jayne Cowan said she believed it was a "ludicrous" idea.
"If they speak to the wider community they will realise that people think it's just unacceptable," she said.
"It's just disrespectful for people who have recently lost their loved ones to use the heat generated from the crematorium to heat buildings.
Unison union regional manager for Wales, Dave Galligan, said there were "sensitive issues" to be considered.
"The ecological point of view is we can't afford to waste energy but the humanitarian part says it's slightly distasteful.
"It's probably a debate that needs to be heard."
A similar scheme in Worcestershire which could see heat from cremations used to warm a swimming pool has also caused controversy.
Local authorities are looking to reduce emissions of mercury produced from cremations.