If you heat your house with LPG in the South Island, you may have noticed the bottles seem to be running out sooner.

Paul Pollard of Invercargill did: he told us he was getting a lot less gas from his cylinders this year.

In the previous two winters his family would get 25 to 28 days of gas per 45kg cylinder from their supplier Yunca. Nova now supplies the Pollards’ gas as it has bought Yunca’s LPG business. The first two bottles this winter from Nova have lasted 14 and 20 days respectively. The Pollards aren’t using the LPG differently, so Paul wanted to know if Nova’s LPG was different from Yunca’s.

Yep, Nova’s LPG is different. The Yunca mixture was around 90 percent propane and 10 percent butane; Nova’s is about 55 percent propane and 45 percent butane. This change in the mixture is why Paul seems to run out sooner. The boiling point of butane is much higher than propane. So in the cold winter temperatures of Invercargill, the butane doesn’t boil readily – and doesn’t form butane gas. It’s staying in the cylinder as a liquid.

Peter Gilbert of the LPG Association explained that for the last five years LPG suppliers have been importing enough propane from Australia to supply an “almost exclusively propane-rich product to the South Island during winter. This has now changed with 90,000 tonnes of LPG coming from the Kupe field, which is an approximate 55/45 mix of propane and butane”. This is what the South Island has been receiving this winter.

Possible solutions

Nova and the LPG Association told us they were working on a solution to increase the propane content for LPG supplies to the coldest parts of New Zealand, but they couldn’t give us a date when this will happen. We suspect this won’t be until next winter.

Peter Gilbert has also suggested two possible interim solutions. We don’t think either is good enough.

Suggestion 1: Customers who use a lot of LPG could change to a 4-bottle supply system.

Even if a customer has enough space to do this, it’s likely to mean extra bottle-rental charges. As well, any LPG storage over 100kg requires an HSNO location certificate: this can cost $200 to $300 and needs renewing annually. (The LPG Association has been trying to get this requirement removed so that consumers are more willing to take up the 4-bottle option.)

Suggestion 2: When customers notice the red “empty” indicator they could gently rock the cylinder to assess how much liquid LPG is left. If they do hear liquid LPG in the cylinder, they should wait: some of the remaining liquid will evaporate, the regulator will flip back and the cylinder will supply more gas.

Now there’s a comforting thought for a cold winter night … just wait and the heater should work again! We think “gently rocking the cylinder” is impractical. A part-full 45kg capacity LPG cylinder could weigh over 50kg – and there’s also the risk of hoses breaking or bottles tipping over. However, waiting should work if you’re not using a lot of gas at one time.

We say

• LPG customers can’t switch to another supplier with a better mix – so the LPG industry should immediately change back to the 90 percent propane mix during the coldest parts of winter and early spring.
• Petrol and diesel have a “winter specification”. We think LPG should also have a winter specification limiting the amount of butane in supplies delivered to cold areas. Currently the amount of butane in any LPG mixture must not exceed 50 percent.
• If your LPG bottles seem to be running out sooner, insist that your supplier weighs your “empty” cylinders when it exchanges them. Ask for a credit for the unused LPG – otherwise, you could be sending back $30 to $50 worth of unburned gas. Nova has now credited Paul for some of his unused LPG.

Boiling points of LPG

With the arrival of supplies from the Kupe field, local LPG now contains more butane: the current mix is around 55 percent propane and 45 percent butane. Of the approximately 45 percent butane in the LPG mixture, 15 to 20 percent is likely to be n-butane and 20 to 25 percent is isobutane. It's the n-butane that's the main cause of any low temperature problems. Propane boils at -26°C and isobutane boils at -12°C, so neither are likely to cause too many problems even in the depth of winter. It’s the n-butane that is likely to cause trouble as it boils at 0°C.

But that’s not taking into account the latent heat of evaporation. When any liquid boils or evaporates it takes in nearby heat. That's why when you rub alcohol-based sanitiser on your hands it feels cold – the alcohol sucks heat out of your skin as it evaporates.

When LPG gas is drawn out of a cylinder to fuel instant hot water or gas-fired central heating, it is replenished by evaporation from the liquid LPG in the cylinder. The evaporation takes heat from the LPG, and cools the remaining liquid – quite rapidly if the demand for fuel is high.

If the outside air temperature is already low, the LPG temperature can drop below zero, especially during periods of high fuel demand – for example, if the supply is to an instant hot water system, central heating or barbecue.

Below zero, the n-butane won’t boil, and stays in the cylinder after the propane and iso-butane have been used. Peter Gilbert says “the regulator on a twin 45kg bottle system should then swap to the second cylinder to give the original cylinder some time to recover. When gas is again available from the original cylinder, the regulator should automatically swap back. Long periods of below freezing temperatures combined with high LPG use can result in some of the n-butane being left in the cylinder”.

If you barbecue in colder weather get your bottle filled where it's weighed first and you're charged for what's put in. If you “Swappabottle” for a flat fee you won’t get any credit for butane you send back.

More from consumer.org.nz

• LPG prices - our 2009 report looking at uncompetitive pricing

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