Every metre your water has to climb costs you roughly 10 kPa of pressure. A typical Sydney storey is about three metres, so your upstairs bathroom is working with around 30 kPa less than your kitchen tap. On a healthy mains-pressure system nobody notices. On a system that was already marginal, that’s the difference between a decent shower and a dribble.
That’s why hot water can be perfectly fine downstairs and useless upstairs. The problem usually isn’t your hot water system. It’s the climb.
Why upstairs behaves differently
Water loses pressure going up. It’s the same physics as a header tank, just working against you. Cold water is subject to the same loss, but the cold side usually starts with more pressure to spare because it comes straight off the mains without passing through a hot water system, a tempering valve and a longer pipe run first.
So the hot side arrives upstairs having already given up pressure at every restriction along the way, and then has to climb. Anything that narrows or slows that path shows up on the top floor first.
If the hot water is weak everywhere in the house, not just upstairs, the cause is different and we’ve covered it separately in low hot water pressure.
What causes it on the top floor
A gravity-fed or low-pressure storage system
Some older North Shore and Eastern Suburbs homes still run a low-pressure or gravity-fed storage system, where the pressure at the tap depends on how far the water falls from the tank. If the tank sits in the roof space and the bathroom is on the first floor, that fall might only be a metre or two.
Downstairs taps get a reasonable head. Upstairs taps get almost nothing. There’s no adjustment that fixes this, because the height difference is the whole system. The options are a pressure pump or converting to a mains-pressure system, and which one makes sense depends on the tank, the pipework and where the system sits.
Air trapped at the high point
Air rises. In a two-storey house, the highest point in the hot water pipework is almost always the upstairs run, which is exactly where an airlock will sit.
Airlocks usually turn up after the water has been off, so if this started right after a mains repair, a new hot water system, or work anywhere on the plumbing, that timing is your clue. The symptom is spluttering, inconsistent flow that comes and goes rather than a steady weak stream.
Clearing an airlock properly means bleeding the line at the right point. Do not connect a hose between your hot and cold taps to force it out, which is advice that circulates online. That creates a cross-connection between two supplies at different pressures and it’s the exact scenario backflow prevention exists to stop.
The pipe run to upstairs is too long or too narrow
Hot water loses heat as it travels, and pressure as it squeezes. A long run to an upstairs ensuite, particularly one that was added during a renovation and teed off an existing line rather than run properly, gives you both problems at once.
Undersized pipe is common in extensions. A 15mm line that was fine feeding one tap struggles when it’s feeding a shower, a basin and a toilet on the floor above. If your upstairs pressure drops away the moment someone runs a tap downstairs, that points at pipe sizing rather than the hot water system.
Where the run is simply long, the water also arrives cooler than it left. If your upstairs shower takes an age to warm up, that’s the same cause and we’ve written about it in why your shower heats up slowly.
A continuous flow unit that won’t fire upstairs
Continuous flow systems need a minimum flow rate before the burner ignites. Downstairs, where pressure is highest, the unit fires without trouble. Upstairs, after the climb and any restriction along the way, the flow can fall just below that threshold.
The giveaway is that you get cold water rather than weak hot water. The unit isn’t firing at all. Turning the tap on harder sometimes gets it going, which confirms it. This often appears suddenly with no change inside the house, because Sydney Water adjusts network pressure in some areas and a unit that was marginal stops coping.
A tempering valve struggling on the upstairs branch
Tempering valves cap delivered temperature at 50°C to prevent scalding, and they restrict flow as they age and collect sediment. On the ground floor there’s enough pressure behind it that you don’t notice. Upstairs there often isn’t.
These are not a DIY job. Opening one on a live system risks scalding and flooding, and in NSW it’s licensed plumbing work. There’s more on how they work in our guide to the hot water tempering valve.
What to check before you call
| What you’re seeing upstairs | What it usually points to |
|---|---|
| Steady but weak hot water, cold is fine | Pressure loss over the climb, or a restriction on the hot branch |
| Spluttering, on and off flow | Airlock in the upstairs run |
| Cold water only, no heat at all | Continuous flow unit not firing at that flow rate |
| Weak upstairs only when a downstairs tap runs | Undersized pipework on the upstairs branch |
| Warm rather than hot, takes ages to arrive | Long pipe run and heat loss |
| Hot and cold both weak upstairs | Whole-house pressure, not a hot water fault |
Two things worth doing yourself first. Check the isolation valve on your hot water system is fully open, with the handle parallel to the pipe. And take the aerator off the upstairs tap, soak it in white vinegar and retest, because a partly blocked aerator on an already low-pressure branch will finish the job.
If the water arriving upstairs is warm rather than hot, that’s a temperature problem rather than a pressure one, and our guide on why your hot water is only warm covers it.
When you need a licensed plumber
Book someone in if the upstairs hot water has never worked properly since the house was built or renovated, if it’s a gravity-fed system, if the unit isn’t firing on upstairs taps, or if you’ve cleared the obvious things and nothing has changed.
Sizing a pressure pump, converting a low-pressure system, resizing pipework and replacing tempering valves are all licensed work in NSW, and getting the pump sizing wrong on a two-storey home either does nothing or gives you water hammer.
Get it sorted today
James and Mitch have 30 years of experience between them and know the older two-storey housing stock across the North Shore and Eastern Suburbs well. We’re Master Plumbers NSW members, fully licensed under Licence #364116C, and rated 4.9 stars across 300+ Google reviews.
You get a fixed price for the job before we start, not an hourly rate. Our onsite inspection is $90 and it’s waived if you go ahead with the quoted repair, and every repair is backed by our Lifetime Labour Guarantee.
Contact us for a fast quote, or read more about our hot water repairs.
