PERFORMANCE TANKLESS WATER HEATERS
and it takes five seconds for the unit to begin heating water. This may not seem long, but for 30-second draws the end user may not receive hot water at the desired temperature. To limit this impact, one product bypassed certain control sequences for draws within five minutes of one another to reduce these delays by half.
●Temperature stability during variable loads – Using a hot water draw pattern with step changes in flow rate from steady state, low flow to high flow – for example, 5.7-11.4 litres per minute (1.5 to 3 gpm), and vice versa – the outlet temperature stability was observed. As the draw rate abruptly decreases, a small outlet temperature overshoot may be observed (< 2.8K) as the TWH adjusts and residual heat from the heat exchanger is withdrawn. As the draw rate increases abruptly, significant temperature undershoot can be observed, up to 6K, as some TWH products will interrupt firing to adjust controls and ultimately avoid scalding outlet temperatures.
Daily simulated-use testing Using the standardised daily hot water draw pattern of 242 litres per day (64 gallons/day) and two ‘realistic’ patterns generated from field data, a mid pattern of 246 litres per day (65 gallons/day) and a low pattern of 114 litres per day (30 gallons/day), the delivered efficiency, gas and electricity consumption, and operating economics of the TWHs were estimated. Using the Energy Factor (EF) as a metric of delivered efficiency, with the ratio of energy output and energy input accounting for transient effects8
, the results are summarised in figure 2, with some
observations: ●Water heater efficiency is a strong function of daily hot water consumption; however, draw intermittency and number play a role. For the ‘standard’ and ‘mid’ draw patterns with similar daily volumes drawn, a loss of efficiency and average delivered temperature are seen with the more intermittent ‘mid’ pattern.
● Onboard buffer storage tanks are a double-edged sword. The TWH with a buffer tank has an option to schedule its tank thermostat. GTI performed the
water heaters from Andrews
NEOflo 25kW
MAXXflo 30 – 120kWkW
SUPAflo 145 – 550.7kW
The delays to delivering hot water are as much a function of the water heater itself as the piping layout
Leading edge technology.
www.andrewswaterheaters.co.uk
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April 2013 CIBSE Journal 43
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