Many people who research rainwater harvesting assume that a bigger tank means more rainwater collected. What they may not realise is that the way many direct systems manage their mains water backup can quietly undermine that logic, using tank volume for mains water during dry spells and leaving less room for the rainwater you actually wanted to collect.
It’s an aspect of rainwater harvesting system design that’s worth understanding before you specify a system, because it can make a significant difference to how much your system actually delivers over its lifetime.
How does a typical rainwater harvesting system manage mains water backup?
In many direct rainwater harvesting systems available in the UK, the process works like this. Rainwater collected from your roof passes through a filter and fills the underground storage tank. The system monitors the water level in the tank and supplies your toilets, washing machine, and garden from that stored rainwater. So far, so good.
The problem arrives during dry spells. When the tank level drops below a set threshold, the system switches to a mains water supply to keep your home running. In direct systems, that mains backup water is introduced into the underground tank itself, topping it up to maintain a usable level. Not all systems work this way. Indirect systems and those designed around a separate internal cistern handle mains backup differently, which is a distinction worth understanding before you choose.
For direct systems, though, the result is the same. The tank stays reasonably full, your household doesn’t notice any interruption, and the system keeps running. What’s also happening is that your available tank capacity is being occupied by mains water during the periods when rainfall is at its lowest.
Why does mains top-up into the tank cause a problem with rainwater harvesting tank space?
Consider what happens when it rains again after a dry spell. If your tank has been topped up with mains water during that dry period, it’s no longer empty and waiting to receive rainwater. It can only collect up to its remaining unfilled capacity. The more mains water that went in during the dry spell, the less rainwater the tank can absorb when rain arrives.
Over the course of a year, this cycle repeats itself every time there’s a prolonged dry spell, which in the UK is a regular occurrence. According to the Met Office, England tends to see lower rainfall during the summer months than in winter across many regions, with extended dry spells a common feature of the growing season. Every time a direct system tops up the tank with mains water during those periods, it reduces the amount of rainwater collected when conditions improve.
The result is a system that consistently underperforms its theoretical capacity. You’ve paid for a tank of a certain size, but you’re not consistently getting the benefit of that full storage capacity for rainwater collection. A portion of that tank volume is permanently in competition with your mains supply.
Does the size of the tank affect how much this problem matters?
Tank size does not solve the problem, it just changes the scale of it. A larger tank topped up with mains water during a dry spell still loses proportional capacity to rainwater collection. The issue is not how much storage you have, it’s what that storage is filled with when rain arrives.
The UK’s variable rainfall pattern makes this particularly relevant for British self-builders and homeowners. The Environment Agency’s National Framework for Water Resources 2025 highlights that water stress is a growing concern across large parts of England and Wales, with demand regularly outstripping supply during summer months. A rainwater harvesting system that maximises what it collects during wetter periods is more valuable in this context than one that spends dry periods reducing its own available capacity with mains water.
What is the alternative approach to mains backup in a rainwater harvesting system?
The solution is to keep mains water out of the underground tank entirely. Rather than routing mains backup into the tank, a different design routes it into a separate, smaller integrated cistern inside the property. When the rainwater level in the underground tank drops below the threshold, the system draws from that internal cistern and feeds mains water directly into the plumbing. The underground tank remains reserved for rainwater.
When rain arrives, the full storage capacity of the underground tank is available to collect it. None of that tank volume has been occupied by mains water. The tank remains available for rainwater storage, and the cycle of collection is as efficient as the system’s capacity allows.
This approach also has a significant compliance advantage. Because mains water never enters the underground tank, the two supplies remain physically separated at all times. This provides what the Water Supply (Water Fittings) Regulations 1999 describe as a Type AA air gap, a compliance route that helps achieve Category 5 Protection. For self-builders, this approach tends to make the Building Control approval process more straightforward, though compliance still depends on correct installation and local approval requirements.
How do GRAF UK’s AA systems solve the tank space problem?
GRAF UK’s AA Eco-Plus and AA Silentio rainwater harvesting systems are designed around this principle from the ground up. Mains water never enters the underground tank. When the system needs to switch to mains backup, it draws from a separate integrated cistern inside the property and feeds directly into the plumbing. The underground tank remains reserved for rainwater storage.
Both systems supply toilets, washing machines, garden irrigation, and outdoor cleaning. Tanks are available from 1,500 to 7,500 litres, manufactured from 100 percent recycled plastic, with a shallow installation profile that requires gravel backfill rather than concrete. Both systems carry a Type AA air gap as standard, providing Category 5 Protection and simplifying compliance for self-build projects.
The AA Silentio adds three features on top of the Eco-Plus: a digital display showing the live rainwater level in the tank, a reversible flow filter for additional filtration, and an automatic self-cleaning function on the filter controlled by a timer.
For self-builders who have spent time understanding what a rainwater harvesting system should deliver, the distinction is worth considering. A system that keeps its tank volume reserved for rainwater preserves more capacity for rainwater collection, especially after dry spells, compared to a system of the same size that introduces mains water into the tank during dry periods. Over the lifetime of the home, that can make a real difference.
How do you work out which AA system and tank size is right for your project?
The right tank size depends on two things: how much rainwater your roof is likely to collect, and how much your household is likely to use for non-potable purposes. A larger roof catchment area and higher household demand both point towards a larger tank, but the relationship between the two is not always intuitive.
GRAF UK’s tank size calculator gives you a system recommendation based on your specific roof area and household usage. It takes around two minutes and gives you a concrete starting point for specifying the right system for your project.
Posted by Callum Vallance-Poole, on April 27, 2026.