abstract
Converting from central storage water heaters to central instantaneous water heaters is a sensible initial measure to improve energy efficiency and potable hot water hygiene in existing large systems (e.g. hotels, apartment blocks). There are performance limits (minimum and maximum output), electric energy consumption, and control quality issues that require an objective testing and assessment procedure. Up to now, there is no standardized test procedure and no transparency in the market, except for capacity. In a research project, the test procedure previously elaborated for small instantaneous water heaters was further developed and applied to 9 large products with a hot water output of up to 100 l/min including circulation pumps. The products are assessed according to key performance indicators. The control quality, also known as comfort, evaluates the control behavior of temperature fluctuations caused by load changes. Most products show a high control quality for heat pump applications with low storage temperatures, but only few products achieve it for solar thermal applications with high storage temperatures. The performance factor indicating ratio of transferred heat with electric energy demand is higher than 500 for all products, ranging up to 1200. The heat transfer capability allows temperature reduction in the storage and increases its stratification, which allows a higher share of renewable heat generators. Here, the products range from A to G.