The numbers your water system has to hit do not change when the clocks do. Cold stays cold, hot stays hot, all year round. What changes is how hard each one is to achieve, and which one is quietly slipping while you are watching the other.
In summer the pressure lands on the cold side. Warmer mains, hot loft spaces and sun on exposed risers nudge cold storage and outlets up toward the band where Legionella starts to multiply. In winter it flips. Hot water sheds heat through long runs in cold plant rooms and unheated voids, so the furthest outlet that sailed through in August can fall short in January. Then come the quiet weeks, summer holidays and the Christmas shutdown, layering stagnation on top of both.
So the seasonal task is not to relax the rules or tighten them. It is to know which threat the calendar is loading onto your system, and to read your monitoring with that in mind.
The targets stay fixed; the threats rotate
HSE’s hot and cold water guidance sets the benchmarks most schemes are built around: keep cold water below 20°C where that can be achieved, store hot water at 60°C or above, and distribute it so it reaches roughly 50°C at the outlet within a minute, or 55°C in healthcare premises [1]. Those figures read the same in July and December. Your risk assessment decides how they are monitored and what action a reading outside them triggers [2].
What the season changes is the direction of drift. Recognise that, and you stop chasing one odd thermometer reading and start asking the better question: can this system hold control through the conditions it is actually facing now?
What summer and winter do to your system
Laid side by side, the two seasons pull on almost opposite parts of the system.
| Summer | Winter | |
|---|---|---|
| Main pressure point | Cold water creeping up | Hot water falling short |
| Why | Warmer incoming mains, hot loft and roof spaces, solar gain on tanks and risers | Heat loss through long runs in cold voids and plant rooms; calorifiers working harder to recover |
| Outlet most likely to fail | Cold sentinel furthest from the tank, or any cold tap in a warm space | Hot sentinel at the end of a long, poorly lagged run |
| Stagnation trigger | Summer holiday voids, seasonal closures | Christmas and New Year shutdowns, areas frozen off |
| The easy own goal | A cold tank sitting in an unventilated, unshaded loft | Dropping storage or flow setpoints to save energy |
Pin it above the bench: it tells your team where to look first by time of year, rather than working through every outlet with the same flat expectation.
Summer: keep the cold side honest
Cold water is the half that gets taken for granted, and summer is when that bites. The cold sentinel furthest from storage, and any tap routed through a warm ceiling void or a plant room, are where you will see the creep first. Check the cold tank itself too: is it genuinely cold, or baking in an uninsulated loft with the sun on the roof all afternoon? Lagging, ventilation and a bit of shade do more for a summer cold tank than any amount of extra sampling.
A cold outlet reading 21 or 22°C in a heatwave is not a paperwork nuisance to initial and move past. As general HSE guidance, cold water is expected below 20°C where achievable [1], so a reading above that is the season telling you the storage or routing needs attention. Note what you found, note why, and decide what closes it out.
Winter: do not let the hot side coast
Hot water is the half everyone trusts, which is exactly why winter catches people out. A far outlet that delivered comfortably warm water in summer can drift below target once the pipework around it is running through a cold building. The usual culprits are unlagged or badly lagged runs, a circulating loop with a weak return temperature, and a calorifier that cannot keep up when demand and heat loss both spike on a cold morning.
The mistake is reading 48°C at the far tap, calling it near enough, and moving on. Pull the trend across the autumn and you may well see it stepping down week on week. That is the signal to fix the lagging or the circulation before the reading becomes a clear failure in January.
The quiet weeks are the real trap
Reduced occupancy is its own hazard, and it arrives twice a year. A building that empties over the summer break or the festive shutdown leaves water sitting still in pipes and outlets, and stagnation in little-used outlets is a recognised driver of Legionella growth [4]. The instinct to pause the flushing rota while everyone is away is precisely backwards.
When the building empties, flushing should ramp up, not stop. Tie flush tasks to the closure calendar so unused outlets still get run off through the void, and make sure the work is recorded rather than assumed. If an area is isolated for frost protection, log that decision and how it is brought back into service. Keeping that evidence in order is its own job, covered in Keeping records of flushing and sampling activities.
Trade-offs worth naming out loud
Seasonal pressure tempts people into fixes that solve one problem and open another.
Energy-saving setpoint cuts are the big one. Lowering storage or flow temperatures to trim a winter fuel bill can pull hot water below the level your scheme relies on. If a change like that is on the table, it belongs in a risk-assessment review first, not a quiet adjustment at the plant [2][3].
TMVs deserve a second thought in summer. They protect against scalding, but the blended, low-use sections downstream of them sit in tepid territory, which is the worst place to be when ambient temperatures are already high. Over-correct for one season and you can manufacture a new risk for the next.
Read the trend, not the snapshot
A single reading tells you about one outlet at one moment. A season of readings tells you whether the system is holding or sliding. That is where logged data earns its place: a continuous record across the warm and cold months surfaces drift a monthly clipboard check will miss, as Using data loggers for water temperature monitoring sets out. Whichever way you capture it, write down the decision and not just the task: “cold sentinel ran warm in the August heatwave; tank re-lagged and re-checked; back within range” is worth far more at review than a tick in a box.
Common questions about seasonal temperature control
Does a cold tap above 20°C in a heatwave count as a failed control?
A reading above the cold-water benchmark is an exception your scheme should act on, not something the weather excuses [1]. Heat does not change the target; it changes how often you will brush against it. Record the result, find why that outlet or tank ran warm, and let your risk assessment decide what action and what frequency follow.
Why does my hot water pass in summer but fall short at the far tap in winter?
Almost always heat loss. The same pipe run loses more heat when it passes through a cold building, so a far outlet that just made target in summer can drop below it in winter. Look at lagging on the run, the return temperature on any circulating loop, and whether the calorifier is recovering fast enough under cold-morning demand.
Should flushing carry on during a seasonal shutdown when no one is in the building?
Usually yes. An empty building means still water, and stagnation in little-used outlets is exactly what flushing exists to prevent [4]. Your risk assessment sets the frequency, but a quiet period is a reason to keep the rota running, not to suspend it.
Before you act on any of this
The benchmark figures above are general HSE guidance, not settings you can copy from a web page into your plant room. What counts as a pass, how often you measure, and what a failed reading sets in motion all come from your own risk assessment and written scheme, applied by someone competent for your specific building. Treat a heatwave or a cold snap as a prompt to test those assumptions, never as licence to move the goalposts.
Your next step
Pull last year’s monitoring records and lay summer against winter for each sentinel outlet. The outlets that drifted with the season, the cold ones in the heat and the hot ones in the cold, are where this year’s attention belongs. Do that before the next quiet period arrives, then brief whoever covers flushing on the closure plan so the building is not left to sit still while everyone is away. If the records make that comparison hard to do, the fix starts further back, with how the assessment and scheme are written, covered in Writing a Legionella risk assessment report.
Sources
[1] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm