energies Article Heat Loss Due to Domestic Hot Water Pipes Anti Hamburg 1, *, Alo Mikola 1 , Tuule-Mall Parts 1 and Targo Kalamees 1,2   Citation: Hamburg, A.; Mikola, A.; Parts, T.-M.; Kalamees, T. Heat Loss Due to Domestic Hot Water Pipes. Energies 2021, 14, 6446. https:// doi.org/10.3390/en14206446 Academic Editor: Gerardo Maria Mauro Received: 3 September 2021 Accepted: 30 September 2021 Published: 9 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 nZEB Research Group, Tallinn University of Technology, 19086 Tallinn, Estonia; alo.mikola@taltech.ee (A.M.); tuule.parts@taltech.ee (T.-M.P.); targo.kalamees@taltech.ee (T.K.) 2 Smart City Centre of Excellence (Finest Twins), Tallinn University of Technology, 19086 Tallinn, Estonia * Correspondence: anti.hamburg@taltech.ee; Tel.: +372-53-419-274 Abstract: Domestic hot water (DHW) system energy losses are an important part of energy consump- tion in newly built or in reconstructed apartment buildings. To reach nZEB or low energy building targets (renovation cases) we should take these losses into account during the design phase. These losses depend on room and water temperature, insulation and length of pipes and water circulation strategy. The target of our study is to develop a method which can be used in the early stages of design in primary energy calculations. We are also interested in how much of these losses cannot be utilised as internal heat gain and how much heat loss depends on the level of energy performance of the building. We used detailed DHW system heat loss measurements and simulations from an nZEB apartment building and annual heat loss data from a total of 22 apartment buildings. Our study showed that EN 15316-3 standard equations for pipe length give more than a twice the pipe length in basements. We recommend that for pipe length calculation in basements, a calculation based on the building’s gross area should be used and for pipe length in vertical shafts, a building’s heating area-based calculation should be used. Our study also showed that up to 33% of pipe heat losses can be utilised as internal heat gain in energy renovated apartment buildings but in unheated basements this figure drops to 30% and in shafts rises to 40% for an average loss (thermal pipe insulation thickness 40 mm) of 10.8 W/m and 5.1 W/m. Unutilised delivered energy loss from DHW systems in smaller apartment buildings can be up to 12.1 kWh/(m 2 ·a) and in bigger apartment buildings not less than 5.5 kWh/(m 2 ·a) (40 mm thermal pipe insulation). Keywords: DHW heat loss; DHW circulation; energy performance 1. Introduction Nearly zero energy (nZEB) apartment buildings have a relatively higher share of energy use for domestic hot water (DHW) because of reduced heat loss from the well- insulated building envelope, the use of ventilation heat recovery and LED lighting systems. DHW energy consumption can be divided between energy used to heat the water and en- ergy consumed by system losses. Bøhm and Danig showed [1] that in apartment buildings the heat losses from the hot water system correspond to approximately 65% of the energy consumption for domestic hot water and the cause of these heat losses should be further investigated. Later, Bøhm specified [2] that most of the energy demand for DHW is lost in the circulation system. As the system’s apartment building’s DHW heat loss was 23–70%, its efficiency was 0.30–0.77. Gassel [3] showed that if the DHW circulation is constantly in operation, this equates to 15 kWh/m 2 ·a energy consumption, the circulation share being 19% of total DHW heating demand. Horvath et.al [4] showed that when the specific DHW annual heat demand is between 23.2 and 32.2 kWh/(m 2 ·a), the distribution and circulation losses are between 5.7 and 9.9 kWh/(m 2 ·a). Zhang et al. [5] indicated that recirculation loop pipes heat loss represented about one third of a system’s fuel energy consumption and the average overall system efficiency was only about 34%. Similar results were found in the study by Marszal-Pomianowska et al. [6], where DHW accounted for 16% to 50% of total DHW heating consumption. Huhn and Davids [7] showed that the energy losses from Energies 2021, 14, 6446. https://doi.org/10.3390/en14206446 https://www.mdpi.com/journal/energies