I just had a new Bosch highflow 21e natural gas continuous flow hot water system installed to replace an old 250 litre off peak system that was playing up.
Gas continuous flow hot water systems don't use a storage tank and instead just heat the water as it passes through the pipes using Natural Gas. I chose the system I installed because it has some significant advantages over the old off peak electric system.
Advantages
Price.
The first advantage is price. Not because natural gas is a cheaper form of heat than off peak electricity, because it isn't.
The price for my off peak electricity is 5.1c per KW Hour
The price for Natural Gas is 1.611c per Mega Joule and 1kW-Hour = 3.6MJ so a kilowatt of heat from Off peak electricity will cost 5.1c while the same amount of heat from gas is 5.8c
The real price benefit is that with a continuous flow system, you only heat the hot water you use, where a tank system reheats the water in the tank every night whether you use it or not. The whole time the tank is sitting there leaking heat, even if you go on holidays. About a third of the energy I was putting in to my electric hot water tank was getting lost this way. It's this loss that makes "on demand" gas hot water cheaper than off peak electric.
Another price advantage occurs, if you have a dual element electric system, when your system starts using peak power because you ran out of stored water. Peak electricity costs more than double the off peak rate where the gas system never costs any more per litre no mater how much you use.
Never Run Out Of Hot Water.
With my old off peak system, if you ran out of hot water you had to wait till the next day for more. The system was 250 litres, which ran out, on one occasion, after 4 adult showers and 1 bath. I did the evening dishes in kettle heated water that day. Too bad if one of the 3 kids that shared the bath grows up old enough to want to shower instead. The alternative is to have a dual element or peak only system that let you have nearly unlimited hot water but you pay extra for it. I say nearly unlimited because if you use all the hot water in the tank fairly quickly then the heater element in the tank is unable to re-heat water to a suitable temperature at the rate it would be used for a shower. It needs a rest for an hour or two in order to catch up. My continuous hot water system can heat 21 litres per minute (enough for 2 showers at a time) to a temperature suitable for showering. It can do this all day every day. And it never, ever, runs out.
Flushing the toilet will never elicit screams from the shower.
Normal tank hot water systems store the water at a temperature too high for straight hot water showering, you simply mix it with cold water at the shower to make it the temperature you want. The problem with this is that if someone else in the house uses either cold or hot water by flushing to toilet or using a tap, then the presure of the mix changes and you shower will suddenly become too cold, or worse, too hot, in an instant.
Continuous hot water systems have the option of a control pad that looks like this:
You set the temperature you want, turn on only the hot water, and it comes out exactly as you asked. No fiddling around wasting water while trying to get the right balance. No more worries that your toddler will play with the taps and accidentally scald themselves when they are having a bath. And no risk of the cold water going away leaving you in scalding hot because someone flushed the loo. The cold water cant be taken away because you aren't using any.
Some important things to note on the controller is are the priority button and the volume setting. When you go to have a shower, you press that button and control of the hot water temperature becomes isolated to that one controller. This means no one else can turn up or down the temperature while you are in the middle of a shower. It also means no-one else in the house can get water of any other temperature other than what you are showing with. If the hot water is already flowing somewhere in the house, you cannot get priority. This is so that the water temperature doesn't change unexpectedly for other people in the house. It is worth noting that if there are two bathrooms in the house and two people want to shower at the same time then whoever pressed the priority button first gets to decide the temp of the hot water. If it's too hot for the other person, then they can blend in cold water to make it the temperature they like. If it's too cold, then there isn't much they can do. You can release priority by pressing the priority button again when you have finished your shower. Another feature with the priority button is that when you press it the temperature changes to whatever it was when you last had priority. If, for example, you like to shower at 40 degrees but the when you wash up in the kitchen you like 55 degrees, The kitchen controller can be set on 55 and when you shower you press priority and it will jump to 40. After your shower you press priority again and it jumps back to 55. This saves you a lot of button pressing when you want a water temperature change.
The volume setting button lets you set an amount of hot water which when used an alarm sounds. This is handy for making sure you don't forget you were filling up the tub or perhaps to guilt your children into having shorter showers. Every controller in the house beeps about 5 times when the preset amount is used. You can set the amount in multiples of 10 litres.
Less space used in your yard.
Even a modest 250 litre hot water system is physically huge. It has to hold the same volume of water as a large wheelie bin as well as surround it with a thick layer insulation. A Continuous flow system is about the size as the largest legal carry-on suitcase. It is easily fitted to an outside wall at any height. There are some limits to positioning restrictions, for example, it must be more than 30cm from the nearest window and there must be a meter of clear airspace in front of the exhaust vent but these aren't much of a challenge. This freedom of positioning allowed me to shorten my hot water pipes by 4 meters which reduces the time it takes for the water to come out hot from the tap Below is a photo of the side of my house where the system is installed. That's it attached to the wall about a meter from the ground and two thirds the way down:
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Environmental benefits
If you are switching from an off peak electric to a gas hot water system, the NSW government will give you a healthy cash subsidy. This is because there is a significant reduction in greenhouse gas emissions when using gas. The reason for this is that when you burn coal in a power station furnace and turn that heat into electricity, only 30% of the heat is turned into electricity and so only 30% of the heat form coal goes into your hot water. When you burn gas at your house to heat water in one of these hot water systems 93% of the heat from the gas goes into your hot water. This means there is a significant reduction CO2 if you use gas. It's as good as using electric boosted solar hot water but significantly cheaper to install. This chart from the rebate form shows the CO2 savings:
Disadvantages.
It takes longer from the time you turn on the tap to the time hot water flows out. Electric systems leak their heat right into the water in the pipe so the pipe is already warm. Continuous flow systems don't waste energy like this but it does give electric a head start in getting hot water to your tap. The other delay comes from the extra second or ''two it takes to light the gas and heat up the heat exchanger inside the system. The overall delay increase wasn't much for me, an extra 4 seconds. It used to take 7 seconds for warm water to come out of the shower head after I turned it on, now it takes 11. This is inconsequential to me.
The electronics of the system requires electricity. It doesn't use much electricity but if the power goes out then so does the hot water.
The preset temperature of the system means filling a bath to the right temperature is easy. If you are like me and occasionally have a long soak the bath will cool and you'll want to let some water out and fill it with more hot. To reduce the amount of water you have to let out, you'll want to have very hot water added in. My touch pad is not located within arms length of the bathtub, this is to prevent small children from playing with it, so I have to remember to turn up the temperature before I get in the tub to make reheating simpler. Otherwise reheating efficiently requires getting out of the tub to change the temp.
If you are turning the hot water tap on and off in short bursts, like when you rinse a few dishes, you wont get the consistently hot water that comes from a tank. this is because a small amount of cold water flows through the system in the time it takes to light the gas.
Things you might expect be a problem but aren't.
Its not silent but its very quiet. The unit has a fan that blows in air to be burned with the gas. Standing next to the system you can hear more noise from the shower on the other side of the wall than the fan itself.
The Insides
The proportions of the innards was a surprise to me. There is a large fan in the bottom to supply air to the burner. It is variable speed so that it can match the gas flow required at any given time. I presume the fan is required because the maximum burn rate of the heater is some 160MJ which is a huge amount of gas and would require a fair amount of air to burn efficiently otherwise gas may vent out the exhaust unburned. The size of the heat exchanger is remarkably small but given the heater is some 93% efficient, there isn't any need to make it larger to extract extra efficiency.
Power Usage
As I mentioned, the heater requires a power point to supply the fan and electronics inside the heater. Power usage requirements are very low. 8 watts on standby peaking to 50 watts when running, most of which powers the fan. This makes it very compatible with a cheap UPS. A relatively small and cheap 500VA UPS could power this on standby for about half a day and still give some 20 to 40 minutes of hot water time.
Cost
The quote from AGL to supply and install the system, including 2 controller pads was $1999. There were $730 worth of subsidies from AGL and the state government and that brought the cost back to $1,269. I have seen a similar Renia system advertised by Bing Lee for only $800. I'm fairly certain I could have had it installed privately for about $600. It was a very short run of copper for the gas and water pipes and the electrical setup was also straight forward. I don't know if you could still get the AGL subsidies if they didn't do the install but you'd still get the state government ones if you are converting from electric hot water.