New air emission regulations from September 1 threaten the use of full wetbacks because they are unable to meet efficiency standards.

From that time, woodburners must convert 65 per cent of wood burned into room heat.

Householders using wetback fires as their sole hot water source may soon face rising power bills and cold showers, wetback manufacturers say.

Air emission regulations coming into effect on September 1 may spell the end for full wetbacks because they are unable to meet thermal efficiency standards.

Wetback retailers warn that power supplies in remote provincial areas are so unreliable that many people will go home to cold water once wetbacks go.

Under the new national standard, woodburners must convert 65 per cent of wood burned into room heat.

Particle emissions must be less than 1.5 grams per kilogram of wood burned.

Manufacturers told NZPA that while it would be possible to produce a wetback that met the 1.5 emission rate, it could not achieve 65 per cent heat efficiency unless energy used to heat water was also counted.

Although some households do not use their wetbacks to full effect, others produce so much hot water that it is channelled into under-floor heating and radiators.

Without a wetback, some householders would pay an extra $1400 a year on power, manufacturer Barry Kernot told NZPA.

Open fires, which are four times less efficient than woodburners, and coal fires are exempt from the regulations.

Fourteen water-heating models offered by the Environment Ministry as alternatives to wetbacks are a "joke", wetback manufacturers and retailers say.

But Government officials say cleaner air should take priority over the survival of what they deem an inefficient and outdated water-heating system.

Inglewood woodburner manufacturer Peter Butler told NZPA that householders in provincial areas such as Masterton, Central Otago and Northland often relied on wetbacks for their hot water.

"There are lots of areas in the Far North where the maintenance is not done - a tree comes down, the power lines come down, and it takes two days to put it up again," he said.

Water-heater models posted as alternatives on the Environment Ministry website did not solve the problem, as they were "booster wetbacks", able only to supplement an electricity-based hot-water system.

"They're an absolute joke," Mr Butler said.

A full wetback could save a household's hot water bill for six months of the year and see a return in two years, he said.

Householders could spend the same amount installing a booster without seeing a "noticeable" difference in power savings, he said.

The new environmental standard had "huge holes" in it, as theoretically a coal-fuelled wetback would still be allowed, he said.

"It's absolutely ridiculous. As a fuel, coal is the worst fuel you can get."

Householders were being forced to rely on electricity, which was often less efficient than wetbacks.

Although householders preferring non-electric water heating could still buy European "cooker" models, they cost up to $14,000. New Zealand-built wetbacks cost about $1000 installed.

NZPA was told that at least 30 per cent of woodburner sales were for full wetbacks.

The availability of cheap wood made wood fires a primary energy source in provincial areas.

Kaitaia manufacturer Mr Kernot said 4000 full wetbacks were sold in New Zealand annually.

He said the air emissions move would eventually contradict the Government's environmental aims as power suppliers would have to look to coal or gas to meet increasing power shortfalls.

"At the end of the day the most environmentally friendly way of heating your home is with a woodburner, and the most environmentally friendly way of heating water is with a wetback," he said.

Home Heating Association national secretary Ed Hawkes said it was working with the ministry and the Energy Efficiency and Conservation Authority (EECA) to get full wetbacks approved.

"The EECA are plugging electricity, but we're plugging wetbacks," Mr Hawkes said.

Ministry spokeswoman Sue Powell was unable to explain why water-heating was not included in efficiency calculations for wetbacks.

She said people wanting to rely on woodburners to heat all their hot water would find it difficult under the regulations, as low emissions requirements meant new models could not be "damped down" over night.

It was significant that manufacturers are the only ones to complain, she said. "We did wide consultation - including with industry - for over two years."

The difference between boilers and wetbacks was minimal, as most households using woodburners to heat water also used electricity, she said.