I work as an energy auditor and have been recommending to people for a while now to change from gas storage to instant, particularly when the old storage needs changing anyway. I also of course recommend solar HWS and heat pumps where appropriate but try to be practical with clients where they don't have the capital or are low HW users. Anyway the discussion here is about the energy ratings on gas HWS and the relative merits of storage vs instant.
I was surprised to find when I looked in to it that the energy rating system for gas HWS is controlled by the industry. The energyrating.gov.au website has gas HWS on there but they hardly have any star ratings listed. They refer to the aust gas association for details but of course they don't say anything about what formula they use to relate star rating to efficiency or energy use. What they do say:
1. the energy use shown on the label refers to how much energy (in MJ) is used each year to provide 200L/day (or 37.7MJ of hot water).
2. This is equivalent to 13761MJ of heat needed as the absolute minimum to heat 200L/day. This agrees with the ballpark figure I use of 1 kWh per 20L of HW, which is 10 kWh or 36MJ per 200L.
3. So if a heater has an energy label showing 21,000MJ/year, as the 5* rated Dux prodigy does, then its has 7000MJ of losses per year. It only seems to be operating at 65% efficiency and its rated as 5* !
4. The AGA admits that some units are now calculating out as more than 6 stars but they can't give more than 6 stars with the current system.
5. What was surprising for me is that the instant systems don't seem to be much better rated. They publish a list of all approved appliances which is handy, http://www.aga.asn.au/uploads/148/06_2006.pdf, which shows the energy used and star rating. The best rinnai infinity HWS has a 5.9* rating but still uses 18969MJ and so is only 73% efficient. From these figures it would only save about 10% on gas costs compared to the 5* storage system
I've seen gas bills where the gas use has dropped by 50% when changing from storage to instant. Maybe this is because they are usually replacing an old, very inefficient, maybe even faulty storage system ? I tend to advise people that they will save about 30% on gas costs.
A poor quality, 2* storage system uses 26000MJ, so about 23% more than a 5* system.
I suppose we could work out the relation between energy used and star rating from the extensive data they publish.
I wonder if this lack of efficiency and small difference from instant to storage is related to the standard they use ? Supplying 200L/day is very high HW use! Thats about 10 4min showers with 9L/min shower heads. The efficiency of the storage system would be lower with lower water use as the losses would be about the same but energy for actual heating proportionally lower.
Would the efficiency of an instant system vary with amount of water used ? I can't think why it would.
If a 5* system uses an excess of 7000MJ/year over what is ideally needed to heat the water, this would be about 1944kWh per year and 5.3kWh per day - more than what I usually estimate as the standby losses from storage HWS. Then again at what storage temp do they doe the tests ? I heard somewhere it was 75C.
Worth an article in renew Lance ?
