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Proposed new regulations covering hot water systems have unleashed a war of words between the National Party and the Government.

National's Building and Construction spokesman Nick Smith said new regulations that would put tight constraints on water flows in household showers were unfair and unworkable.

"Labour's nanny state is out of control," said Dr Smith.

"They tell us how to live our lives by doing things like dictating what can and can't be sold in school tuckshop and what light bulbs we can use, and now how much water we can use when showering."

But Building and Construction Minister Shane Jones says Dr Smith was misrepresenting a proposed design standard for new homes.

"The plain and simple fact is that there are no restrictions on the amount of hot water anyone can use," Mr Jones said.

"There are no restrictions on the flow rate of your showerhead. There are no restrictions to the number of showers you can take or, the showerhead you use."

Dr Smith said the new showering regulations, which take effect on February 1 next year, were part of amendments to the Building Code Compliance Document NZBC H1 Energy Efficiency covering hot water systems.

To get any new building consent, the flow rate in showers would have to be approximately half that of existing household showers. It would also be illegal to change any shower head to increase flow rates after February 1, he said.

Mr Jones said Dr Smith's take on the issue was a complete fabrication.

"He is misrepresenting a design standard for new homes. That standard provides for hot water energy savings of 20 per cent over current averages using modern technology to save the home-owner money and reduce the possibility of blackouts and electricity shortages."

Mr Jones said the proposed amendments were only intended for new homes and additional water systems and would not apply to existing homes unless a whole new system was installed.

He said the Department of Building and Housing had consulted on the proposals. All submissions would be carefully evaluated before a final decision was made.

But Rheem New Zealand, the country's largest manufacturer of electric water heating cylinders, said the shower issue, which was aired on TV One's Close Up last night, was only one part of the story.

The company said the new regulations would significantly increase the cost of an electric water heater for a normal size house.

"The new regulations will require modifications to electric cylinders to make them heat pump or solar ready, which we estimate will result in a minimum $500 increase to the consumer," the company said.

"The tank modifications will require significant retooling and redesign work by manufacturers, and the associated costs will ultimately be borne by homeowners."