Flued gas heaters

We assessed the Energysavers in an expanded version of the thermal comfort laboratory we use to test electric heaters — 5m wide by 6m deep and featuring a 2.4m-high insulated ceiling. Both heaters are floor-mounted units.

Like heat pumps, these units are costly to shift once installed.

We fitted them halfway along a wall. After stabilising the lab’s temperature at 8°C, we cranked the heater’s thermostat to maximum and started timing.

Heat-up time: In just over 5 minutes, the 559FT raised the temperature by 5°C, and within half an hour had our lab at a balmy 23°C. This excellent result wasn’t surprising as the 5.3kW unit is designed to heat rooms as large as 77m², and its performance is comparable to a similar-sized heat pump. The 309FT, which has a lower heat output (3.1kW), took almost twice as long to achieve a 5°C temperature rise and only increased the temperature to 16°C after half an hour.

Evenness: both gas heaters warmed the room as evenly as a heat pump would, with just over 1°C variation between the warmest and coldest parts of the room.

The experience: Heat pumps and gas heaters both warm through convection, using a fan to circulate warm air, rather than by radiating heat directly at you, like a woodburner or bar heater. However, a heat pump can take a while to get going, especially as the outdoor temperature falls (in the same way your car’s heater blows chilly air for the first few minutes on cold mornings).

In contrast, the Energysavers instantly produce a jet of hot air. Consumers who are frustrated their heat pumps emit a type of heat that’s too “soft” are likely to prefer the instant, powerful heat of a gas convector.

Noise: We measured the sound levels of the Rinnai heaters and found them slightly quieter than heat pumps, though there’s still a perceptible fan noise throughout the room.

Rinnai general manager sales and marketing Mark McCutcheon says the majority of customers for its Energysaver heaters “already have piped natural gas and want to maximise the value of their connection”. Note that reticulated (piped) gas is only available in the North Island.

The idea is if you’re already paying fixed daily gas charges for powering your water heater and cooktop, you should get your money’s worth by using gas to keep warm as well.

A heat pump with the same capacity as the Rinnais costs about the same to buy and install. And while electricity costs almost 3 times as much as gas, an average of 26¢/kWh vs 8.8¢/kWh, heat pumps convert every unit of power into 3 or more units of warmth.

The larger Energysaver 559FT, with an efficiency of 83%, costs 60¢/hour to run from natural gas (based on the North Island average). A heat pump with the same output costs less than 50¢/hour in electricity. So, in economic terms, you’re better off going with the heat pump, even if you’re already paying fixed gas charges.

However, in some parts of the North Island, gas is much cheaper than average. In Taranaki, close to our natural gas reserves, prices are the lowest in the country, generally less than 7¢/kWh. This brings the running cost of the Rinnai Energysavers to about the same as a heat pump.

Complicating matters, Genesis Energy offers flat rate gas plans where you pay a fixed monthly price for a capped amount of gas per year. If you’re using significantly less than your quota, then it might be worthwhile opting for a gas heater.

The Energysaver heaters can also be powered by 45kg LPG cylinders, available throughout the country. Mr McCutcheon says the breakdown of Rinnai gas heater customers is 4 to 1 between natural gas and LPG. Based on the average price of $100 per replacement bottle, the Energysaver heaters cost twice as much to run off LPG as they do from natural gas. Unless you’ve somehow wrangled a very cheap cylinder hook-up, the economics of powering them with LPG are a bit grim.

How easy heat pumps are to use is a major drawcard. Push a button on a remote and within minutes your home is cosy. Bells and whistles such as timers and WiFi control further automate the heating process. How do gas heaters compare? (Note: the Rinnai Energysaver 559FT and 309FT have the same control panel).

Starting: both gas heaters have an instant electronic start we found reliable.

Controls: the Energysavers have a push-button digital control panel, allowing you to set the thermostat (16-26°C), select an “economy” mode, activate a child lock or operate the timer. There’s also an indicator light showing when you need to remove and clean the filter. We found the controls easy to understand and use. There’s no remote, so you have to wander over to the heater every time you need to make an adjustment. The electronic controls and starting mechanism mean the heater needs a mains connection, and won’t work in a power cut.

Changing the dust filter: The filter is readily accessible from the top of the heater and easy to remove, clean with a vacuum and replace.

Timer: The Energysavers feature dual timers. You can set each timer to provide 2 periods of heating each day. Setting the timers is simple.

Overall, they’re as easy to use as heat pumps, but the lack of a remote is frustrating. Switched-on users may also want a WiFi control option.

Since last year, we’ve observed a 3.8% rise in piped natural gas prices nationwide. Over the same period electricity prices remained steady. Rinnai’s Energysaver heaters should last for upwards of 15 years all going well, but are you betting on the wrong horse if you invest in a gas appliance?

New Zealand is self-sufficient for natural gas, with 100% of our reserves produced from onshore or offshore wells in the Taranaki. But a report by the International Energy Agency (IEA) says only about 10 years of domestic consumption “can be covered with current reserves” including the declining Maui gas field.

Meanwhile, a study by the Ministry of Business, Innovation and Employment on the future of electricity demand and supply predicts slow growth in demand for electricity. Technological advances like the increasing availability of energy efficient appliances and falling price of solar PV are likely to ease pressure on the grid.

New Zealand enjoys a large base of cheap electricity generation sources like hydro and wind power, coupled with what the report describes as an ‘excess supply of committed new generation’. In English, this means some powercos hold resource consents for new geothermal and wind power plants, which they’ll build when they become economically viable.

In our view, the only thing you can be sure of is natural gas is more of a gamble than electricity. That said, large increases in natural gas prices in the short term are unlikely.

Both Rinnai Energysavers are good heaters: they’re cheap to run, heat up quickly and evenly and are easy to use. As the 559FT has a significantly higher heat output than the 309FT, at a similar upfront cost, we think it’s the better option for homes needing to heat a decent-sized living area.

But the low running cost of heat pumps, combined with their comparable purchase price and the range of models (which means you can find one for nearly every size living area), means they are the better option for most of us. That said, the Rinnai Energysaver 559FT can be a good bet if you tick the following 3 boxes:

• You get cheap per-unit rates for natural gas — about 7¢/kWh before prompt payment, or you’re on a fixed rate plan and aren’t hitting your yearly quota.

• You have a decent-sized living area to heat (but not larger than 77m²).

• You want guaranteed instant heat even on the coldest mornings.