Hot water – more than meets the eye

by Richard Keech
orginally published December 2016, re-blogged 2019-07-20
A couple of times recently on this list, the question of the relative merits of solar hot water vs heat pump hot water has arisen. There’s an oft-overlooked factor to consider when comparing these and that is the temperature set point, and more broadly how the temperature is regulated. Temperature regulation of heated water systems is a matter which has serious impact on energy consumption, as well as on biological safety (due to legionella) and on burn safety (due to the risk of scalding).

How hot? – competing requirements. The biological-safety imperative motivates relatively high storage temperatures, whereas the burn safety imperative motivates lower delivery temperatures. In Australia we’ve settled on regulations which achieve these seemingly contrary goals by generally a) storing water at greater than 60C, and b) ensuring water delivery for personal hygiene situations (mostly bathrooms) at either no more than either 50C or 45C depending on the situation (AS3500.4 Section 1.9.2). The temperature gets dropped back down by mixing in some cold water using so-called ‘tempering’ valves. As someone who was badly scalded as a child, I know first-hand how important this. It’s still OK under the regulations to have kitchen and laundry at much higher temperatures, and some people prefer that.
Killing Legionella. It’s common knowledge that we regulate water at >60C to kill Legionella bacteria. But is that really necessary? In the USA they get by with 50C as their regulated minimum storage set-point temperature with no apparent problem with Legionella in hot water storage. Legionella bacteria can, according to the WHO, survive and multiply in the range 25C – 45C. Higher temperatures will kill Legionella, but the actual required temperature depends on exposure time. This is embodied in Australian Standard 3498 (Table 1) which outlines another temperature control regimen to reliably kill Legionella, as follows:
Minimum exposure period as a function of temperature
Temperature: Minimum exposure period
>70C: 1s
66C: 2min
60C: 32min
55C: 6h
Weekly sterilizing pulse. This alternative temperature control regimen is now (since 2010) a legal alternative to simple thermostatic storage at >60C (AS3500.4 section 1.9.1). What it means in practice is that all the stored water needs to be subject to a sterilizing pulse of heat based on this table at least once every seven days. The rest of the time it’s OK to allow the water to be as low as 45C (regulatory testing will fail if modeled delivery temperature ever falls below 45C).
Solar and biosafety. It’s common to hear proponents of solar hot water claim that their systems are incredibly efficient and they hardly ever need to boost in winter. However when you dig deeper you often find that it’s a dirty secret that this efficiency is achieved by manually boosting on the basis of subjective need. In other words, it’s very likely that someone who chooses to do their hot water boosting by hand is not achieving the minimum legal temperature control according to the standards. Clearly this imposes a risk of heated water that is sometimes not biologically safe during cooler months. How big that risk is, is debatable. But it’s not zero risk.
Biosafe controllers. The old-school way of hot water systems staying safe is simple thermostatic regulation above 60C. In practice this usually means a 65C set point, allowing for a conservative margin of error. However very few manufacturers seem to take into account that it’s possible to save energy by controlling according to the table above. I’ve got a solar controller (from Senztek) that has an optional biosafety mode that allows for this style of smarter temperature control based on a periodic sterilizing pulse. This isn’t hard to achieve and can potentially save a lot of energy and avoid many hours of unnecessary hot water boosting, while remaining within legal biosafety parameters.
Cold water biosafety. Just to show that things are always more complicated than they seem, it’s important to know that Legionella isn’t only a problem in hot water. Low levels of Legionella are normal and acceptable in water supplies according to the WHO. However Legionella can grow above 25C, and in summer it’s not uncommon for some cold-water pipes to spend a lot of time above 25C. There are documented cases of fatal Legionelosis arising from exposure to Legionella in cold water supplies.
Learnings. Hopefully the learnings from this are:
  • Be very careful manually boosting your hot water – you may be putting your family at risk;
  • Comparing the energy performance of different hot water systems is really only valid if the temperature control regimen is accounted for, otherwise it’s apples and oranges;
  • There’s un-met potential for manufacturers of all types of hot-water systems to be more efficient by adopting smart controllers which keep systems at lower temperature much of the time.
Perhaps I’ll leave consideration of quantifying the potential for energy savings for another post.