Generating hot water often leads to massive energy loss and even overheating; find out how to design your system to make it as efficient as possible.
In this article we cover:
- The problem with hot water
- How to design your system to reduce heat loss
- How to design your system to avoid overheating
- Top tips for appliances and overheating
- Details on system sizing
The average domestic hot water (DHW) system contributes to both overheating and inefficiencies. This is particularly true in multi-residential buildings where there can be long lengths of hot water pipes.
To reduce heat loss from your hot water system, the following strategies should be employed:
Flow temperatures should be kept at the minimum you require, i.e. do not distribute at high temperatures and mix down at point of use. For example, oil or gas boilers will heat water to a high temperature which will then need to be cooled down if your heat emitter is underfloor heating. Heating water to cool it down leads to inefficiencies. Take advice from your plumber.
Minimise pipe runs. The length and complexity of pipe runs should be minimised by coordinated and intelligent room layouts. This is best achieved by reviewing the design at concept stage to ensure the most effective layout and distribution pathways.
This is often achieved with vertical distribution (but is dependent on building geometry) with outlets close to risers. Where hot water or communal heating is circulating year-round, reducing flow and return temperatures is very beneficial.
In particular, the distance between the primary heat source and the hot water storage vessel (if present) should be minimised as these pipes will need to be large.
In non-circulating systems, pipe diameters can be reduced to small bores (outside diameter of approx. 10mm) by use of a radial distribution system whilst still maintaining satisfactory levels of flow and pressure. It is important that these systems are designed appropriately to address pressure drops.
The volume of stored hot water should be optimal (not oversized) and the storage temperature should be as low as is practicable whilst ensuring there is an appropriate periodic (weekly, or even monthly) sanitisation cycle to deal with legionella. A storage temperature of around 50degC is sufficient to provide for hot showers.
Match the heat source to the hot water storage volume, and the associated hysteresis setting should be optimised to avoid rapid cycling.
Secondary circulation systems should be avoided (again achieved by efficient layouts). Where they are required, the return pipework should be small bore, and the circulation times minimised in line with the building’s usage patterns.
Overheating from appliances
Many appliances and devices (dryers, refrigerators, freezers, dehumidifiers, ovens, computers, etc.) within a building generate heat, which can significantly impact the risk of overheating. These devices are often added after construction and may not be part of the building’s fixed services, making them difficult to model. When in doubt, assume the worst-case scenario in the PHPP (the PassivHaus methodology used to design buildings to comply with the passive house standard, see passiv.de) model. Appliances that are always on, even at a low load, can have a disproportionate impact on heat gain.
