Pool and Hot Tub Auto Top-Up Systems: How They Work, Pros and Cons

Last updated: September 2026

A swimming pool or hot tub auto top-up system (sometimes called an auto-fill or water leveller) keeps your water level where it should be without you reaching for the hosepipe. When the level drops, a valve opens and lets in fresh water. When it’s back to the right level, it shuts off.

Are they worth having? Honestly, it depends on the pool, and I’m in two minds about them myself. So in this post I’ll explain how they work, the types, where they’re genuinely useful, where they cause problems, and the water regulations you need to know about, especially in the UK.

How does an auto top-up work?

Every system has the same three parts:

1. Something that senses the water level: a float, or an electronic sensor.
2. A valve that lets water in: a float-operated valve, or a solenoid valve switched by a controller.
3. A water supply, connected through the right backflow protection so pool water can never be drawn back into the drinking water supply.

The level it holds is normally set to about halfway up the skimmer mouth, which is where the skimmer works best. Set it too high and the skimmer stops skimming. Too low and the pump can suck air.

Types of auto top-up system

Type	How it works	Pros	Cons
Skimmer-mounted float valve	A float valve fitted through a knock-out on the skimmer body	Discreet, simple, cheap	Needs a compatible skimmer. The float lives in the skimmer
Separate leveller pot	A small chamber set in the deck, linked to the pool by an equaliser pipe, with a float valve inside	Out of the way, easy to service, works with any skimmer	More to build in, and the equaliser pipe has to go in during construction
Electronic sensor and solenoid	A sensor in the pool or skimmer switches a valve via a controller	Adjustable, and some can log or limit how much they add	Needs power, and more to go wrong
Smart or app-connected	An electronic system with remote alerts	Can warn you if it’s adding more water than normal, which is a leak alarm	The most expensive

In the UK, Certikin makes a widely sold automatic water leveller (HD108), which needs a 15mm (¾in) cold water feed.

My preference: skimmer-mounted

If you’re going to fit one, I like the type that fits into the skimmer. It’s discreet and simple. You do need a skimmer that’s compatible with an auto top-up, which generally means it has knock-out plates on the back.

Fitting is straightforward. Remove the knock-out plate, screw the leveller into place with a rubber washer against the inside of the skimmer to make it watertight, then connect the back of it to the water supply, through the right backflow protection (more on that below). If you have high water pressure, the better systems include a way to reduce the flow into the device. The better ones also have an anti-siphon feature, so the pool can’t slowly empty itself back through the device.

Do you need one? The pros and cons

This is where I’m in two minds. My DIY plunge pool guide says an auto top-up is even better than a fill pipe, and on a pool losing water every day to sun and wind, it is genuinely handy. But in What can we learn from the holiday pool? I said I wouldn’t pipe one into a pool like that myself. I find it easier to top up with a hosepipe, and it’s one more thing to go wrong. Both are true. It depends on the pool and on you.

Pros	Cons
The water level stays right, so the skimmer keeps working and the pump never runs dry	It can mask a leak. The pool quietly refills, and you only find out from the water bill or wet ground
Great when you’re away on holiday, or for a holiday let	One more thing to go wrong: sticking floats, dripping valves, blocked filters
Saves a job in hot, windy weather when evaporation is high	Constant fresh water dilutes and shifts your chemistry, and you may not notice
Neater than a hosepipe over the coping	Needs the correct backflow protection, which adds cost and space in the UK
	A stuck-open valve can overfill the pool if there’s no overflow

When it’s worth it

• A plunge pool, spool or swim spa in a sunny, windy spot that loses water every day.
• A pool you leave for weeks at a time, or a holiday let.
• Where you’re building a skimmer and plumbing in anyway, so adding it now costs little.

When I’d skip it

• Most hot tubs. With the cover on, they lose little water, and a hosepipe once in a while is fine.
• A pool you check every few days anyway.
• If you can’t fit proper backflow protection.

The leak problem

This is the big one. If your pool has a leak, an auto top-up will quietly keep it full, and the leak can run for months. Meanwhile, water is washing out the ground under your slab or behind your walls.

A few ways to protect yourself:

• Fit a water meter on the top-up supply and note the reading each week. You’ll soon learn what normal looks like for your pool.
• Turn it off once a month and mark the water level. If it drops much more than you’d expect from evaporation, investigate.
• Choose a smart system that alerts you if it adds more water than usual.
• If you’re topping up every week, check for a leak before you blame evaporation. See how to detect and fix a hot tub leak and how to fix a leak.

A cover helps here too. The US Department of Energy says a pool cover can reduce the amount of make-up water needed by 30–50%, because it cuts evaporation. Less evaporation also makes a leak easier to spot.

Water regulations and backflow protection

UK

This is the part most people, and a lot of product listings, skip. Under the Water Supply (Water Fittings) Regulations, anything connected to the mains must stop contaminated water being drawn back into the supply. Swimming pool water is treated as a Fluid Category 5 risk, the highest category. A backflow risk guide from water hygiene consultants Waterman says pool top-ups need Category 5 backflow protection, usually a break cistern supplied through a Type AA or AB air gap. A simple check valve or a hose-tap vacuum breaker is not enough. It’s sensible to treat hot tub water the same way.

In practice, that means:

• An air gap between the mains outlet and the pool water, either built into the leveller (the fill discharges above the highest possible water level) or via a break tank. Ask the manufacturer how their product achieves Category 5 protection, and use WRAS-approved fittings.
• Notification for bigger pools. Regulation 5 says you must notify your water supplier before building a pond or swimming pool over 10,000 litres that’s designed to be replenished automatically from the mains. Many plunge pools and swim spas are over that size.
• If in doubt, ask your water supplier or an approved plumber. It costs nothing to ask.

USA

Plumbing codes also require the drinking water supply to be protected from pool water, usually by an air gap or an approved backflow preventer on the fill line. Requirements vary between states, counties and water utilities. One practical point from the pool community: if the auto-fill valve is downstream of a backflow preventer, that preventer is under constant pressure, so it needs to be a quality device rated for that, and ideally testable. Check with your plumbing inspector or water utility before you connect anything.

Plan the overflow too

If you’re adding water automatically, give the pool a way to lose it safely. The holiday pool I wrote about has an overflow piped into the skimmer. If the water gets too high after a storm, or a valve sticks open, it drains away rather than flooding everything. Just make sure it goes somewhere sensible. Don’t send chlorinated water into a stream or a surface water drain.

Installation tips

• Decide during the build. A skimmer with knock-outs, or an equaliser pipe to a leveller pot, is easy to add before the concrete goes in and very hard afterwards. See how to install a skimmer.
• Fit an isolating valve on the supply so you can turn it off for testing, holidays in winter, or when you drain the pool.
• Protect the supply pipe from frost. Bury it below frost depth or insulate it, and drain it down if you shut the pool for winter.
• Reduce the pressure if your mains pressure is high. A slow fill is kinder to the valve.
• Keep testing the water. Topped-up water changes your chemistry, especially in hard-water areas. See the ultimate guide to balancing chemicals.

Filling a hot tub from empty is a different job. See how to fill a hot tub with water the correct way.

Planning a pool or tub?

If you’re building a plunge pool or cocktail pool, my plunge pool / cocktail pool plans include the plumbing layout and skimmer position, so you can decide on a top-up before you pour. For a hot tub, see my DIY hot tub plans. For the parts themselves, visit parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

Do I need an auto top-up on a hot tub?

Rarely. With a cover on, a hot tub loses little water, and a hosepipe once in a while is fine. They make more sense on pools that lose water every day.

Can an auto top-up hide a leak?

Yes, and that’s the biggest downside. Fit a water meter on the supply, or turn it off now and then and watch the level.

Can I connect an auto top-up straight to the mains in the UK?

Not without Category 5 backflow protection, which in practice means an air gap, because pool water is treated as a Fluid Category 5 risk. Pools over 10,000 litres with automatic top-up must also be notified to your water supplier.

Where should the water level be?

About halfway up the skimmer mouth.

Why is my pool losing so much water?

Evaporation and splashing account for a lot, especially in hot, windy weather without a cover. If you’re topping up every week, check for a leak.

Have you fitted an auto top-up, or decided against one? Let me know in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi