Rainwater harvesting sounds simple. Harvest rainwater from your roof and redirect it into your toilet and washing machine until there’s none left. But once you start researching how to do it the options become bewilderingly technical.
Gravity fed or pumped? Which control system should I use? What happens during a power cut? Why do toilets need higher pressure than washing machines? Before you know it you’re deep into theories of fluid dynamics and lost in a labyrinth of fancy pump controls.
But it doesn’t have to be like this.
If you’re wondering how rainwater harvesting works but feel out of your depth in a sea of technical details then read this. There are two basic approaches to rainwater harvesting – gravity fed and pumped. Figuring out which option suits your home best is the place to start.
People often default to one approach or the other based on misconceptions – pumped systems are more high-tech so they must be better; gravity fed is naturally better because it’s how we used to do things.
Wrong on both counts.
Your choice of rainwater distribution method should be driven by the constraints of your property and your budget, plus the fixtures and appliances you want to use with your rainwater. Pick the wrong system for your situation and you’ll either struggle with an overly complicated system that constantly needs repairs or an overly simplistic one that can’t even provide enough pressure to flush your toilet properly.
Rainwater harvesting systems are typically used to flush toilets, supply washing machines and irrigate gardens (Grundfos). But if you switch to rainwater in the average home running the pump will only save you a few tens of pounds worth of electricity per year (Better Housing Better Health)
When savings are marginal, you need a rainwater harvesting system that just works. Don’t spend more money than you need to on unnecessary complexity, but don’t scrimp on the things that matter.
## The Science of Pressure
Everything about rainwater distribution comes back to pressure. Do you have enough pressure for your taps to deliver adequately flow rates, or will your toilet only dribble into the pan?
Gravity fed systems derive pressure exclusively from the height of your header tank above your outlets. Pumped systems use mechanical force to generate pressure.
Both systems achieve pressure through different means, so let’s look at how each works and what that means for your rainwater harvesting system.
### Pressure Explained – Where Does it Come From?
**Gravity Pressure – How High is High Enough? **
The higher the tank, the more pressure you have at the outlet. Simple maths. Pressure expressed in bar equals height expressed in metres multiplied by 0.098(CalculatorShub). For everyday tasks we usually use the domestic rule of thumb of metres x 0.1 = bar(MBD Bathrooms). This means 5 metres of head will provide 0.5 bar of pressure at your outlet.
Compare that with pumped systems. This pump example can deliver up to 3.5 bar pressure (Rainwater-Harvesting.co.uk). Enough pressure to run a power shower, more than enough for your toilets and washing machines.
Think about the electricity consumption difference between these systems. Rainwater harvesting is not without energy impact and high energy consumption has been raised as an issue during reviews (Renewable and Sustainable Energy Reviews). Pumped systems use electricity every time you flush a toilet or turn on a tap. Gravity fed systems only use electricity to refill the header tank once it reaches a predefined minimum level.
### Gravity Fed Rainwater Systems
Using a gravity fed rainwater system is basically the same as having old style cold water storage.
Water is pumped from the collection tank to an elevated header tank, traditionally located in your loft. Water then flows down to your outlets under gravity alone. Easy!
Installing a gravity fed rainwater harvesting system means placing a header tank upstairs high enough that you get useful pressure at your outlets.
If you’ve got downstairs toilets you’ll probably need your header tank at least 3-4 metres higher than the cistern to get reasonable pressure. That means up in your loft.
Loft installations work well on most homes in the UK. But if you do go this route you’ll need to make sure your header tank is properly supported –remember they weigh over 600kg when full – and adequately protected from freezing (RainwaterHarvesting.co.uk).
The system cycles between pump on and pump off states. When the level of your header tank drops below a defined level the boiler automatically switches on and pumps water back up to the header tank. It then runs on gravity until your tank empties enough for the pump to switch back on.
One advantage of header tank style systems is that they can keep functioning during power cuts using whats stored in the header tank (Rainwater-Harvesting.co.uk). If you’ve sized the tank properly this could give you 12-48 hours reserve during an outage. Not something every system can boast.
You are limited by how high you can go. Unless you put your header tank in the loft roof you’re unlikely to get more than 0.8 bar pressure on ground floor outlets. Which is fine for flushing toilets and filling up washing machines…but useless for high pressure demands.
Less moving parts equals less maintenance. The pump only runs while the tank is being refilled. Every other cycle it’s not being used. Compare that with a fully pumped system where the pump could theoretically run every time you open a tap.
You also get easy access to all the critical components for servicing. Just remember that header tanks can be very heavy when full.
Cheaper to install. Header tank rainwater harvesting systems are described as low energy when compared to systems that constantly pump water on demand(RainwaterHarvesting.co.uk). They also require less sophisticated controls.
You won’t need a pressured vessel tank, pressure pump and ongoing electricity supply to run the boiler. The pump only kicks in when the tank needs refilling.
### Direct Pumped Rainwater Distribution Systems
Direct pumped systems don’t use a header tank. Water goes straight from the collection tank to your outlets. When you turn on a tap, or flush a toilet the boiler automatically switches on. As soon as you switch the tap off the boiler turns off too.
Your pump and control system can live anywhere you want. Utility room, garage or basement. You don’t have to worry about installing a massive header tank in your loft.
You get mains pressure (Rainwater Harvesting Community). Or at least as much pressure as you want. Domestic rainwater pumps are typically rated at between 2 and 4 bar. Enough pressure for power showers and high pressure appliances.
All the action happens instantly at the point of demand. As soon as you turn on a tap the boiler fires up. You’ll get consistent pressure no matter how many outlets you have running at once assuming your boiler is appropriately sized to deal with your houses peak demand.
Pumped rainwater systems use more electricity. Traditional (RainwaterHarvesting.co.uk) direct pumped systems could cycle their pump 15 to 20 times per day due to toilet flushing alone. This frequent cycling uses more energy than just leaving the pump on and it wears pumps out faster.
They’re also more complex. You need pressure switches, flow sensors and more sophisticated controls. Many pumps will use variable speed drives to modulate pressure. More moving parts and electronics means more things that can go wrong.
If you read through some of the installation guides online you’ll also notice these systems have far more bulky and expensive components. The pumps, controllers and pressurised accumulator tanks aren’t cheap.
### Hybrid Rainwater Distribution Systems
Trying to pick which system is better? Many installers recommend a hybrid approach.
UK guidance documents actually list three distribution system types (Environment Agency). One of those types is ‘ pumped to elevated cistern then gravity’. It’s a hybrid system that takes elements of both approaches.
Water is lifted from your collection tank and pumped up to a header tank like a standard gravity system. But the pump and control package is much smarter than you’d get with traditional gravity fed systems.
You get all the energy efficiency of gravity fed distribution with better pressure control during pump up cycle.
Header tank sizing gives you reserve during power cuts. And because you’re using a pressure pump refilling should take around the same amount of time no matter where your collection tank is positioned or how full it is. Winter-summer. Peak-trough. The pump automatically adjusts to ensure a reliable refill.
The disadvantage is you get the cost and installation overheads of both systems.
### How to choose between Gravity Fed or Pumped Systems
Pick pumped systems because you think you need higher pressure.
This is the number one reason why people choose pumped systems and it’s wrong. Most people massively overestimate how much pressure they need and underestimate how much electricity frequent cycling will cost them.
Think carefully before automatically ruling out gravity systems based on pressure alone. You might be surprised how little you actually need.
Install a gravity system without ensuring you have enough height.
Another common error. Remember your header tank needs to be at least 3 metres above the toilet for decent pressure. And that’s just toilets. Whats stops you putting a washing machine on the ground floor too? Ideally you want 4-5 metres head height to give yourself enough playingroom.
Size your pump too small.
Direct systems are only as strong as their weakest link. Dont just size your pump to cope with your average demand. Size it for peak draw too. A pump that can easily cope with one toilet keeping someone else occupied might well struggle if you decide to run the washing machine at the same time.
If your pump cant easily cope with everyone in the house doing everything at once then its too small.
Forget about freeze protection on your gravity tank.
No-one likes thinking about UK winters but trust us when we say you need to do something about that header tank if its going to live in an unheated loft.
Insulation and trace heating is the accepted solution (RainwaterHarvesting.co.uk). Allow £200-£400 for proper freeze protection. Not an option. A necessity.
Go into your project expecting drastic electricity savings.
Gravity vs pumped is only really going to save you £20-60 per year in reduced electricity consumption. So unless you plan on using gallons upon gallons of rainwater every year its not the overriding factor in decision making you should be making it out to be.
This guide is based on published evidence from industry trade bodies, academic research studies, product manufacturers, installation guides and personal experience of installing rainwater harvesting systems in a wide variety of properties. Unfortunately there are no academic papers directly comparing rainwater harvesting distribution systems.
As far as we can tell the decision between gravity fed vs pumped is not documented anywhere so we had to compile information from multiple sources. Where we’ve linked to specific websites we’ve tried to use English language resources that are freely available online.
We’d love to hear your experiences of rainwater harvesting systems in the comments below. Dont forget you can also share this page to spread the word about rainwater harvesting.
**Rainwater Harvesting for Beginners (& Avoiding Costly Mistakes)**
In under 5 minutes we’ll tell you everything you need to know to get started.
You’ll discover which rainwater harvesting systems are best suited to flat-dwellers, Victorian homes, new builds and much more.
PLUS: you’ll get exclusive access to our checklist to avoid costly mistakes when installing rainwater harvesting.
## Gravity fed vs Pumped Rainwater Systems – Which is Better?
It’s tempting to look at these two approaches and think there must be a clear winner.
Pumped systems just seem high-tech and expensive. And gravity fed rainwater systems are old school, musty and ineffective.
In reality both approaches can provide trouble free rainwater distribution if they’re designed and installed correctly. But neither system is ideal in every situation.
Lets look at who these systems work best for.
### Renters living in Flats
Flat owners are pretty much limited to direct pumped systems. Unless you’re allowed to install a header tank in your loft by your condo association you’ll struggle to make gravity distribution work. Even if you get permission its unlikely you’ll share loft access with your neighbours so boosting your rainwater tank 5 metres into the air becomes a real challenge.
Compact direct pumped systems can often be installed in your utility room without too much upheaval. No roofing required.
Gravity fed systems are overkill. You have limited storage, hate using your rainwater and don’t mind the electricity consumption of running a pump.
Installers love fitting you indirect pumped rainwater harvesting systems with massive pressure tanks. You’re better off without one.
Purchased a new build home? You might have struggled to find roof space for your storage tank. Gravity fed systems avoid the space requirements and structural loading issues of loft installations. If you have some usable space left over install a hybrid system.
Owning an old Victorian or Edwardian house? Chances are you’ve got loft access and your house is tall enough inside to give you ample height for a gravity fed system. If your old house already comes with cold water storage you could even save money by swapping your tanks out for rainwater capable alternatives.
Modern house with limited/no loft space. Again direct pumped gives you all the space savings of having no header tank.
Lucky enough to live in the countryside on-off power? Don’t underestimate how much continuous supply means to us city dwellers but if you do lose power gravity tanks keep on going using whats stored in the tank. Pumped systems stop.
You need high pressure. To run power showers or high pressure appliances you’ll need something that can match mains pressure and direct pumped systems can.
You want enough head size to give yourself reserve during power cuts. And a pressure pump that wont leave you waiting an hour every time your rainwater tank needs topping up.
Consider hybrid systems that combine the best of both setups.
