PERFORMANCE TANKLESS WATER HEATERS
Tankless water heating is certainly not a new technology, with European and Japanese manufacturers introducing products to the US marketplace in the 1970s
● The successful marketing of non-effi cient TWH benefi ts, specifi cally promising ‘endless’ hot water3
Tankless water heater performance testing This surge in TWH popularity has brought a better understanding of the technology’s performance limitations, with discrepancies in ‘delivered’ (actual use by end users) versus ‘rated’ (expected) effi ciency. This has been characterised by several recent studies of TWHs, conducted because of the minimum hot water ‘draw rate’ requirements and the start-up sequence delays found in the technology. These defi ciencies prompted some energy effi ciency codes to be changed.
The State of California had downgraded the effi ciency of tankless water heaters in its residential effi ciency codes
One prominent case for non-prescriptive compliance saw the State of California’s Residential Building Energy Effi ciency Standards reduce its TWHs effi ciency requirement by a 0.92 ‘de-rating’ factor4
– a
reduction to 92% of the manufacturer’s stated effi ciency. By design, TWHs are ideally best suited
for – and most effi cient during – ‘steady state’ operation, that is when there is a continuous steady draw off of hot water. This means that the effi ciency of the system is reduced when hot water is drawn from the system in small amounts, for short periods of time or intermittently. Unfortunately, recent fi eld studies have shown that, in comparison with the hot water ‘draw’ pattern* US products are rated with5
, actual hot water use consists largely
of this type of use. A recent survey and analysis of residential hot water use in North America6 – representing 159 homes across the US and Canada – found that: 90% of hot water draws are less than 10.4 litres (2.75 US gallons); 50% of hot water draws last one minute or less; 90% of water withdrawals from a system are taken at less than 11.4 litres (three US gallons) per minute;
and 75% of all draws occur within less than 15 minutes of one another. These studies have, however, primarily focused on daily energy effi ciency. This led the Gas Technology Institute (GTI) in Chicago, Illinois, to evaluate the performance of several products in greater detail under controlled conditions in its Residential Appliance Laboratory, focusing on these short-term usage effects. A battery of tests was devised to investigate short-term and daily simulated use which, in addition to energy effi ciency, focused on: water-side pressure drop; delays to fi re and deliver hot water**; and outlet water temperature stability during variable loads. Four TWHs were evaluated: one minimum effi ciency TWH (non- condensing) and three high effi ciency TWHs (condensing), one of which had a small buffer tank onboard. Their specifi cations are shown in the table (see page 44). Key highlights of the research are presented in this article.7
The results TWHs perform best (most effi ciently and stably) during ‘steady state’ operation; however, as real hot water use is highly transient, installed performance – compared to ‘rated’ performance – may
40 CIBSE Journal April 2013
www.cibsejournal.com
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