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The Tankless Water Heater
Tankless hot water heaters are also referred to as on demand hot water heaters because they don't store hot water, but rather they heat it on demand and this makes them far more energy efficient than the traditional storage cylinder.
When the hot tap is turned on cold water enters the unit, which activates a flow sensor. The flow sensor activates the heating element, called a heat exchanger, which heats the water to the required temperature before it leaves the hot outlet. Large electrically powered units require a large amount of energy when operating, which imposes a limitation on the maximum size that can be installed within a home. In contrast, gas units just burn more gas to heat large volumes of water. Thus they are generally used for larger whole-house systems and commercial applications.
System Sizing
There are three variables that must be considered when sizing the unit or units needed for any particular site.
- The volume of water the unit is required to heat, measured as flow rate in Litres per minute (LPM)
- The temperature of the cold water entering the unit
- The desired temperature of the hot water exiting the unit
Which Type of Tankless Water Heater to Use
There are two basic types of tankless water heaters, Point of Use and Whole House. The type you select is based upon your intended use, maximum water flow rate, location and cost.
- Point of Use Tankless Water Heater
A point of use system will take a feed from the cold supply line almost directly below the cold tap, possibly in the cabinet, and deliver hot water to the corresponding hot tap immediately above.
In a system like this, the cost of plumbing from a storage cylinder to the hot tap is eliminated. This includes the cost of the pipe, fittings needed, lagging and labour. It also avoids the ongoing cost of hot water runoff while waiting for the temperature to stabilise, which is particularly bad in cold areas and where long pipe runs are involved. These savings are offset by the cost of providing power or gas to the point of use system.
- Whole House Tankless Water Heater
The term “Whole house” simply means that these units have higher flow rate capacity and can handle more than one fixture at a time. It does not necessarily mean that one whole house unit can supply a whole home.
The size and number of whole house tankless water heaters needed for a whole house is largely driven by flow rate and that is determined by the number and types of fixtures you may have running at any one time.
Maximum Water Flow Rate
The maximum water flow rate imposes a limit on the total number of units that can be installed because the water supply must be capable of delivering at least the minimum flow required by each tankless system when all faucets connected to the supply are open. This is to avoid an extreme but not impossible situation.
An example might be:
- Dad is washing the car with the hose out front and Joe is washing his footy socks with hot water in the laundry
- The swimming pool is being topped up and the sprinkler system out the back is watering the garden.
- Mum is in one shower and Mary is in the other
- The dishwasher and the washing machine are both going
- Meanwhile while little Jim decides to play with the water in the kitchen just because he can…
If you were that dad, what do you think might happen to your day when the flow rate falls below the minimum and the shower unit switches its element off causing the girls to get a sudden, icy cold, shower?
Groundwater Temperature Impact on Size
Groundwater temperature varies roughly according to the latitude of the site. The further south the locality, the colder the water supply will be. As the groundwater gets colder any water heater will produce less hot water per unit of energy because it must warm the water more to reach the desired temperature.
For example, for a traditional storage heater, if the groundwater temperature in the Invercargill is 8.3°C and in Auckland it is 13.5°C, the usage is 60 Litres per day and the required temperature is 60°C – the temperature rise required in
- Auckland (60 - 13.5) = 46.5°C resulting in an energy demand of 3.60 GJ per year
- Invercargill (60 - 8.20) = 51.8°C resulting in an energy demand of 4.03 GJ per year
Which is a percentage difference in energy required of about 12.7% between the two localities.
This means a tankless (or any) water heater in Invercargill must be rated about 12 - 13% larger than an equivalent system in Auckland that has been designed to serve the same number and types of fixtures at the same temperature with the same occupancy and water consumption rate.
Using the data above, if the energy savings produced by a tankless system were 50% then in
- Auckland the energy demand would reduce to 1.80 GJ per year
- Invercargill the energy demand would reduce to 2.03 GJ per year
Thus the savings in energy required is greater in the south than in the north by about 12.7%.
Note: This example does not take into account the effects of:
- The difference in power prices or
- The greater standing losses due to the colder average temperatures experienced in the south.
Determining The Temperature Rise Required
- A tankless water heater is sized by rating its temperature rise in degrees C at a given flow rate expressed in LPM.
- The temperature rise is the difference between the hot water exiting the heater and the incoming cold water.
Importantly, the slower water flows through the unit, the hotter the water can be.
If you want your shower to be 40.5°C and you live in Auckland where the minimum groundwater temperature is 13.5°C, then you need a unit capable of 40.5-13.5=27°C temperature rise
Based on the manufacturers data for the EEMAX EX190TC the unit above will supply up to about 10 Litres of hot water per minute in winter in Auckland
The same unit in Invercargill will deliver about 8 Litres per minute because the temperature rise required is 40.5-8.3=32.7°C.
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