Once you’ve used our calculator to find a rough capacity, learn what the figures mean here. Understanding how to size a sprinkler tank can avoid delays on site.
Table of Contents
How to size a sprinkler tank
Unlike our online calculator, sizing a tank isn’t a two step process. There are a number of factors in play which could mean that even a tank of the right capacity isn’t suitable for your project. The water in that tank may be unsuitable due to the tank’s design. It might be too heavy for the area that it was supposed to fit in. It might be the wrong shape and so unable be moved into the room which was planned for it.
A competent designer won’t simply consider the tank capacity. They’ll also consider the needs of the project itself. What resource is available, any access requirements, how will the system be drained down (should it ever need to be), is there a need to futureproof the tank for any future phases of the development?
If these aspects haven’t been properly considered then your designer hasn’t properly understood how to size a sprinkler tank and they may have put the development, your contractors, the occupants, and your property at risk.
Do you need a sprinkler tank?
Simply needing sprinklers is not a guarantee that you will need a tank. Unlike high pressure mist systems, sprinklers can operate directly from the public mains supply. If you have a suitable feed, you could find yourself not needing a pump or a tank at all.
If this option sounds good to you, then head on over to our guide on how to measure that pressure and flow of your infill, and start exploring this route. However, this guide is only for a preliminary test. We still advise understanding how to size a sprinkler tank in case something changes and you later find that you still need one.
Consider the location
In all likeliness, you’re reading this guide because you’re thinking about the location. Maybe you have a location in mind, or maybe you’re looking at your layout and thinking ‘where on Earth are these going to go??’ . The location is often as much of a driving factor as the sprinkler system itself.
The process can involve a lot of back and forth too. Where do you begin? You need to know the location before you can size the tank, but you can’t size the tank without the location. If you’re simply looking for an indicative size, so that you can begin choosing a location, then contacting us or checking out our sprinkler tank calculator could be your starting point before you consider how to size a sprinkler tank in detail.
If you already have an idea on location, then keep this in mind as you work through the guide and consider how to size a sprinkler tank.
Access and removal
Often, when developers think about how to size a sprinkler tank, they overlook this element. It isn’t simply a case of finding a tank which holds enough water. You also need to get it into your property. At some point in the future (ideally decades away), you may even need to take it back out.
What’s going to be involved in this process? How big is this tank? How wide? How tall? Are there stairs to consider? Will the tank fit through doorframes? Is lifting equipment going to be needed?
All of this should be considered by your installer. While much of this will be co-ordinated by the fitters and project manager, the designer should also have this point in mind as they develop your system. There’s a lead time to consider and if this hasn’t all been confirmed before fitters reach site, it may be too late to avoid delays.
Manual handling
The Health and Safety Executive links manual handling to several debilitating illnesses and conditions. In order to avoid this, the advise that developers assess the risks involved at an early stage and looking to avoid the lifting element altogether by using lifting equipment, where suitable. Rhino Safety have a similar view in their guide to risk assessments for manual handling.
When assessing whether or not manual handling is a suitable method of moving the tank, there should also be some consideration into how easy the tank is to hold. Is it an awkward size? Are there handles? Would the material be slippery when covered in water, rain, or dust?
When considering how to size a sprinkler tank, installers should be considering the effect on the fitters themselves. A large factor to consider is weight.
Weight
There are two key weight to consider for sprinkler tanks:
- The weight of the tank while it’s empty;
- The weight while its in use.
Both values need to be considered during selection. The tank will be empty while its being moved. When planning how to size a sprinkler tank, installers should also be considering the impact of the empty weight on access and removal strategies. The empty weight is not negligible, with a sprinkler tank for a house often being around 50kg whilst empty. Due to the size of the tanks, they often require a two person lift. UK Law refers to a two-thirds limit for two person lifts, so each person should be able to lift close to 40 kg. While this is achievable, the possibility of needing to lift this weight up stairways, or through access hatches, shouldn’t be overlooked.
The weight of the tank while it’s in use should be communicated to your structural engineer to ensure that the preferred location can hold the tank, it’s ancillary equipment, and the water it will contain. We provide guidance on the anticipated weights of our pumps, tanks, and pipework, but we aren’t structural experts. Tanks to be placed in attics need particular consideration. While these areas are often used for storage, the weight of a tank will far outstrip your Christmas decorations. 1,000 litres is the same as 1,000 kg. It shouldn’t be assumed that coffin tanks can be used. Structural limits should also be confirmed when planning how to size a sprinkler tank.
Available height
The height of the chosen location will naturally have an impact on the most appropriate method of how to size a sprinkler tank. BS 9251: states that tanks should have a lid, to prevent things from falling in, and that lid will need to be opened at some point. There should sufficient space above and around the tank to allow for visual inspections during servicing and maintenance.
When planning how to size a sprinkler tank, the tank should be considered in it’s working configuration. Understanding how to size a sprinkler tank doesn’t only require understanding how to size a sprinkler tank itself. Installers also need to consider inlets, outlets, lagging, valves, and proximity to the roof above. It should be noted that systems should be maintained above 4 degrees Celsius at all times. When this is not achievable, trace heating, lagging, and tank heaters are advised.
Inlets and outlets
Water tanks in the UK are regulated. One of the most important pieces of regulation is the The Water Supply (Water Fittings) Regulations 1999 (aka the Water Regs), which dictates much of what goes on with your system, including how to size a sprinkler tank.
One of the most crucial regulations within the Water Regs for tanks is the need to include a warning pipe (as well as an overflow pipe for tanks over 1,000 litres). This works just like the overflow in your bath. If the infill is activated, but fails to switch off, water will only go so high before flowing through the overflow (and / or warning pipe). The infill is sometimes activated as the system settles, which can lead to trapped air working it’s way out through automatic air vents.
This overflow then redirects the extra water into a safe space for drainage. It should also happen in a space which will be spotted by the occupant or other person in the area. This is absolutely a great innovation and it keeps your home safe and dry. However, as you start to learn more about how to size a sprinkler tank, you’ll no doubt begin to realise that this takes space away from the usable portion of the tank.
Drainage (where to place drain off cocks)
The warning and overflow pipes are at the top of the tank (their exact locations are dictated by the Water Regs) taking away usuable space from the top of your tank. At the bottom, there’s also the suction pipe (or outlet). This needs to be spaced in a location which allows space for the connecting components (such as drain off cocks) to operate as they need to. BS 9251: 2021 also dictates how low the outlet can be (if placed in the side of the tank).
All water below, level with, and for a portion above the outlet is considered unusable. This is often referred to as dead water.
Brim v actual v effective v working capacities
When considering how to size a sprinkler tank, it’s important to consider which value you’re aiming to achieve.
| Capacity type | Used in sprinkler design | Calculated as |
| Brim | No | The tank, without considering modifications, full to the brim with water. |
| Nominal | No | Similar to brim capacity. Although some suppliers include a datum inlet / outlet which is unlikely to reflect the final design. |
| Actual | Yes | The water in the tank, considering the effect of modifications (such as outlets and infill), but not the dead water. |
| Effective / usable | Yes | The water in the tank, considering the effect of modifications (such as outlets and infill), and disregarding the volume of dead water. |
| Working | Yes | The same as effective capacity (this is just another name for the same thing) |
When developers want to understand how to size a sprinkler tank, they’re generally interested in weight and external dimensions of the tank. For installers, things aren’t so simple. As can be seen from the above.
The effective capacity is the water that we’re really interested in. This is the amount of water that is required to run the system, in the way that it was designed to be run, for as long as the standard requires.
What is Dead water?
Dead water is the unusable water at the bottom of a sprinkler tank. This is considered unusable because it’s below the outlet which leads to the pump. It’s a little bit like holding a straw above the bottom of your drink. As the level of your drink drops below your straw, you need to move your straw downwards to get the rest.
Pumps and tanks aren’t able to move their outlets in this way. Partially to keep the costs down and design simple, but also because vortices can form in the bottom of the tank.
It’s the vortices which make the space slightly above the pipe unusable too. The water effectively begins to roll around, forming waves which will sometimes go below the level of the suction pipe. When the suction pipe isn’t able to collect any water, air is pumped through instead. Of course, air isn’t of any use to the sprinklers. Once the water reaches this level, the rest is considered unusable. This is the dead water and is defined under BS 9251: 2021 using the diagram below, which specified how to size a sprinkler tank.
Allowing Space for Internal components
Tanks are more than just empty shells which store water. Safeguards are in place to ensure that appropriate action can be taken if ever the system was to be activated or failed. The largest internal component is usually the ballcock (also known as a float vale), which is used to refill the system, if ever it decreased in volume. Volume decrease isn’t necessarily a sign of leaks, and can be due to other factors (such as deoxygenation). The ballcock in your tank works in much the same way as those used in older toilets. As explained by Plumbing Force in their article, and Tomkat Gas Training in the video below.
Don’t forget your sprinkler pump (QMax)
The sprinkler pump also impacts the size of the tank. This isn’t obvious at first but makes a lot of sense once you understand why systems are designed the way that that they are. As you begin to explore how to size a sprinkler tank, you’ll come across these factors more and more often.
The sprinkler heads themselves will require a specific amount of water to work. This require the pump to work at a certain speed, which acts as our minimum. We can’t go lower than this. However, we can and do go higher. Partly, this is to allow a margin of error which inevitably happens during manufacture of any product (the Government regulate this margin of error for some industries, such as food because they cannot be avoided, but they can be limited). However, the actual reason is even simpler.
Pumps, particularly in residential environments, are rarely made to order. They’re usually pre-sized and chosen out of a catalogue. This often leaves a gap between the pump speed needed, and what is actually delivered. The result being that additional water is discharged than is required.
We refer to the actual flow from the pump as the QMax and use it to size the tank. It can be seen from the example below that the Most Favourable Area (MFA) and Most Remote Area (MRA) are beneath the pump curve. This ensures that at least the minimum is delivered. However if we want to truly understand how to size a sprinkler tank, we need to account for the QMax when considering our capacity.
Off the shelf pumps and tanks v custom builds
We live in a world where it seems as though everything is now mass produced. Although each sprinkler system that we produce is bespoke and purpose built for the property which it is installed in, the components are generally mass produced. This brings costs and lead times down, while increasing reliability and opening the door to product approvals. CSL Automation has written an excellent summary of the advantages and disadvantages of mass production which is worth a read. Fundamentally, the method of how to size a sprinkler tank remains the same.
The range of configuration for pumps and tanks is flexible enough for a competent designer to find an efficient pairing for any residential property. Although some of the larger residential developments may benefit from a custom tank, it’s not until projects reach higher levels of risk (usually industrial applications) that bespoke designs for pumps are required. At these scales, stakeholders are generally more familiar with the process of how to size a sprinkler tank, but even then its never a guarantee.
Calculating the effective capacity of your sprinkler tank
So, after all that, you ask: ‘..but when will you tell me how to size a sprinkler tank??’
We need to consider all of the above. To start, let’s go back to Figure 4, from the standard itself.
It can be seen that not all of this water is usable. What we need to focus on to ensure a working system is the effective capacity, and we get that from the QMax, which is based on the MFA.
In truth, understanding how to confirm each of these elements is what makes sprinkler installation a specialist field and why it’s so important to contract the work to a certified installer. Certified installers are audited to ensure that they understand the best practice for how to size a sprinkler tank. The final figure will depend on your property and requirements. That’s not to say that you can’t find an approximate figure at an early stage.
If you would like us to provide you with an approximate size and weight, or even a confirmed size and weight, please book a CPD on tank sizing, check out our sprinkler tank calculator, or contact us today.
How can I reduce the size of my sprinkler tank?
Have you been given a tank size that you’re hoping to improve upon? Try our CPD on tank sizing. This covers how to size a sprinkler tank in more detail and with a wider range of configurations.
Find out more
If you found this post useful, feel free to share it and take a look at our other resources over at the Sprinkler Academy for Developers and Regulators.
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