Chlorine to blame for damaged hot water cylinders in Christchurch, new study finds

Wednesday, 3 October 2018

Christchurch plumbers and property managers are worried about the surge in failing hot water cylinders, and many blame the city's chlorination regime. (Video first published August 2018).

Chlorination is to blame for damaged hot water cylinders in Christchurch and could cause further problems to pipes and foundations, according to a University of Canterbury study.

More than 2000 cylinders have been replaced since June and a 45-page report by the department of mechanical engineering concluded chlorination was to blame.

'In the absence of a creditable alternative proposition, the chlorination of the Christchurch water supply by the Christchurch City Council has caused the failure of copper hot water cylinders.'

More than 2000 water cylinders in Christchurch have been replaced since June.

The investigation was commissioned by Woolston manufacturer Superheat and headed by Professor Milo Kral to establish the cause of a sudden spike in cylinder failures.

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Copper tanks are prone to pitting, a form of corrosion which causes tiny holes. If pitting occurs, cylinders usually cannot be fixed and have to be replaced at a cost of around $2000.

After analysing four defective cylinders, experts found multiple corrosion pits and concluded each unit failed due to leaks caused by pitting.

Evidence of chlorine was identified in the corrosion products of all pitting sites but was not detected on surfaces other than pits.

In his report, Kral said: 'Although a small population of exemplar cylinders were examined, the pitting observed was most likely due to the presence of chlorine and was influenced by the presence of iron in the sediments that settled to the bottom of the cylinders.

Due to changes in chlorination, corrosion of copper hot water cylinders was likely to become more common in Christchurch, with a reduction in the life of cylinders, Kral said.

Other factors may have also played a part, such as temperature, details of installation and other aspects of water chemistry, he said.

'Each cylinder installation is at least slightly different from every other, so patterns of failure may be difficult to determine.'

Superheat managing director Trevor Edwards supplied the University with four damaged hot water cylinders from three different manufacturers; Superheat, Peter Cocks and Rheem.

The cylinders were either 180 or 225 litre capacity and between three and 10 years old. They were removed from homes in Linwood Ave, Westmont St, Showgate Ave and Perth St.

They were from different manufacturers, different locations, and different ages and they all had the same problem, Edwards said.

'The combination of the chlorination and iron oxide has accelerated the corrosion on cylinders that would usually have a life of about 10 more years.'

Christchurch City Council head of three waters and waste John Mackie said council was working with cylinder manufacturers and researchers to find the cause of the problem.

'The issue of hot water cylinders pitting and leaking is not new and was an issue that was investigated in the early part of 2016, when there was no chlorine in the water supply,' he said.

They hoped to have a 'clearer picture' in a few weeks, he said.

'Given we have only seen the UC report today, we plan to have the research peer reviewed with the agreement of the researcher before making any further comment.'

Christchurch plumber John-Paul Bridger, of Aquafire Plumbing and Gas, first diagnosed the problem of chlorination and pitting in August after replacing two 180L low-pressure copper cylinders in the same week.

He said then the problem was just the 'tip of the iceberg'.

'When I examined one there were half a dozen holes.

'It couldn't be fixed, the whole unit had to be replaced. Unless the cause of this is diagnosed and a preventative measure is found, this could affect a lot more households.'

Prudence Morrall, director of The Good Girls Property Management, raised the issue after eight cylinders had to be replaced in homes the company managed.

She said on Wednesday she was looking forward to a response from Council.

'This is pretty conclusive stuff. I can't wait to hear what the council have to say about this because they have really dropped the ball on this issue.'

The University of Canterbury report warned more faults may occur and that pipe work was also at risk.

'The effects of pitting due to chlorination may be observed first in hot water cylinders because these are easily accessed and discovered once leaks occur.

'However, pitting in copper pipes will also become more frequent, and potentially even more damaging since leaks behind walls and within floors or foundations cause other damage and in locations that are more difficult to repair.'

Even if chlorination treatment stopped, the damage may already be done, the report said.

'Once pitting has initiated, failure may be inevitable.'