PERFORMANCE TANKLESS WATER HEATERS
suffer. To capture these impacts, short- term tests were grouped into three
categories: ● Defining modelling parameters – One goal was to provide better parameters for the development of TWH simulation tools. Representative heat transfer parameters of the TWH were defined to go beyond a simplified ‘on/off’ model, capturing transient effects. For example, focused testing defined the ability of the heat exhanger to retain heat between firing (thermal capacitance, kJ/K). For the low and high-efficiency TWH, this capacitance was as much as 24.5 and 11.2 kJ/K (12.9 and 5.9 Btu/°F) respectively. Also, when defining the combined heat transfer coefficient, UA, it was found that the maximum quantity of heat that
Table 1: Characteristics of TWHs Tested Description
Non-condensing Condensing 1 Condensing 2
Condensing with small Buffer Tank (BT) Min Baxi_WaterHeaters_CIBSE_HalfDPS_April2013 14/3/13 15:29 Page 1
3.224 2.784 5.832 4.982
58.584 58.584 51.287 58.584
Firing Rate (kW) Max
Certified Performance EF
0.82 0.93 0.92 0.95
Max Litres per second 0.271 0.277 0.265 0.322
at ∆T (K) 43 43 43 43
The widest range of condensing
FASTflo PLUS 54kW
ECOflo 36 – 118.7kW
could be stored by the low-efficiency heat exchanger was 264 kJ (250 Btus). Depending on frequency of hot water draws, this heat may be recoverable.
● Short term on/off tests – With the exception of bathing (including showers), most hot water draws are intermittent, causing TWHs to come on and off in a rapid fashion. A hot water draw pattern was designed to capture this effect, with six one-minute long draws spaced apart
by progressively longer durations from a 30 to a 360 second delay. By varying the draw rate and temperature rise (Tout
– Tin )
and using a mass flow meter to observe changes in firing rate, the TWH response was monitored over all tests. In general, TWHs will monitor a draw for three seconds before opening the gas valve,
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Established values. 0845 070 1055
Baxi Commercial Division 42 CIBSE Journal April 2013
www.cibsejournal.com
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