Understanding water pressure, output, and how to get the best from your AquaTru Under Sink.
Slow output from an Under Sink system is rarely a sign of a faulty unit. More often, it comes down to a single factor that is easy to overlook: the pressure of the mains supply feeding it. Understanding how that pressure affects performance is usually the key to putting the problem right.
This guide explains how the Under Sink system works, why water pressure matters so much, and what you can do if your output is slower than expected. It applies specifically to the AquaTru Under Sink, which connects to your mains water supply, rather than the countertop models.
How the Under Sink System Works
Unlike the AquaTru countertop models, which use an internal pump and a tank you fill by hand, the Under Sink connects directly to the mains water supply that feeds your kitchen tap. It relies entirely on the pressure of that supply to drive the filtration process, as there is no built-in pump to assist it.
Water pressure pushes the incoming water through each stage in turn. First through the pre-filter, then the reverse osmosis membrane, then the carbon VOC stage, and finally into the holding tank ready for use. Each of these stages needs adequate pressure behind it to keep the water moving through the system.
The 40 psi (2.76 bar) Minimum
The AquaTru Under Sink is designed to operate within a pressure range of roughly 2.76 to 5.86 bar, which means it requires a minimum incoming water pressure of around 40 psi (approximately 2.76 bar) for the reverse osmosis membrane to function properly. This is in line with reverse osmosis systems generally. Industry guidance from Watts Water Technologies notes that pressure below 40 psi is generally considered insufficient, and that the ideal operating pressure is around 60 psi.
When incoming pressure falls below 40 psi, the reverse osmosis membrane cannot work as it should. The result is that filtration slows down, and output can reduce to a trickle or stop altogether. The holding tank then fills very slowly, or does not fill completely, because there is not enough pressure behind the water to push it through the system.
Water temperature can also play a part. As the Water Quality Association explains, colder water is more viscous and harder to push through the RO membrane, so output in winter can be naturally slower than in summer, even when your pressure is within the normal range.
What To Do If Your Output Is Slow
The AquaTru Under Sink should be installed by a qualified local plumber, following the steps shown in our visual step by step installation guide. It is important that the system is set up as advised, because a variation in the installation can affect how it performs. The storage tank is a good example. It is designed to stand upright, so that the weight of the water inside presses down on the internal bladder and valve as intended. If the tank is laid on its side to save space, that weight bears sideways instead, which can damage the bladder and stop the tank working correctly. It is easy to see why someone might position the tank differently to fit it into the space under the sink, but a small change like this can reduce filtration performance or shorten the lifetime of the unit.
Having a professional fit the system means that if any adjustment is needed for your particular plumbing, they can assess it correctly at the time. Please also note that fitting the unit yourself, rather than having it installed by a qualified plumber, may void your warranty.
If you suspect that slow output is a pressure issue, a plumber can measure the water pressure coming into your home and confirm whether it meets the 40 psi minimum.
If your pressure is confirmed to be below 40 psi, the recommended solution from our technical team is to switch to an AquaTru countertop model. The countertop models are not affected by your water supply pressure at all. They use an internal pump and a reservoir that you fill by hand, which makes them the right choice for households with lower pressure.
If your pressure is at or above 40 psi but output has still slowed over time, the cause may lie elsewhere, such as an ageing filter. In that case, please contact our customer support team for troubleshooting.
It is worth remembering that slow output caused by low pressure is not a fault with the unit. It is simply a question of whether your water supply provides the pressure the system needs. The unit is doing exactly what it is designed to do.
Choosing the Right AquaTru
Whichever AquaTru model you have, the water it produces is the same. The four stage reverse osmosis process removes up to 99% of contaminants, including lead, chlorine, PFAS, fluoride, and nitrates, with performance certified to NSF/ANSI standards. The difference between the models is not the quality of the water, but how each one fits into your home.
If you do not yet own an AquaTru, it is worth understanding that difference before you choose, as it comes down to setup and convenience rather than performance.
Countertop models are ideal for those who value portability and easy setup. They are designed for plug and play use, requiring no plumbing alterations, which allows for quick installation and relocation. This makes them well suited to renters, or to anyone who changes their kitchen layout from time to time. They offer flexibility and convenience, and because they are not connected to the mains, they are unaffected by your water pressure.
The Under Sink connects directly to your plumbing, keeping your worktop clear and giving you purified water on demand through a dedicated tap. It is particularly suited to homeowners who prefer a streamlined kitchen and are settled in their home for the longer term. With a 7-litre holding capacity and an automatic refill from the mains, it provides a steady supply for busier households, provided your water pressure meets the 40 psi minimum.
You can read more about how the two compare in our blog, which can help you decide which is right for your home.
Not sure which option is right for you? If you would like help choosing between the Under Sink and countertop models, our team is always glad to help. You can also explore the full AquaTru range to find the system that best suits your home.
