How to Fill a Hot Tub the Correct Way (and Avoid Air Locks)

Last updated: September 2026

Filling a hot tub looks like the easiest job there is. Stick the hose in, turn the tap on and wait. But fill it the wrong way and you can end up with an air lock: a bubble of air trapped in the pump or pipework that stops the jets working, stops the heater and, at worst, lets the pump or heater run dry.

Here’s the correct way to fill a hot tub, whether it’s a plastic shell tub or a DIY tub you’ve built yourself, plus what to do if you still get an air lock and what to do once it’s full.

Before you start filling

• Power off at the isolator. You don’t want a pump or heater starting before there’s water in it.
• Drain valve closed, all other valves open, and every union done up.
• Filter cartridge in. It catches anything that comes in with the water. See How to clean a hot tub filter.
• Tub clean. If you’ve just drained it, see How to drain and clean a hot tub.

First fill on a new DIY build

• Check the cure times. Waterproofing membranes, adhesives and grouts all have a minimum time before they can go underwater. Check the data sheets. There’s more in Waterproofing, grout and tile adhesive.
• Get the debris out. Building work leaves PVC shavings, grit and dust in the pipes. Expect to rinse the filter after the first few hours of running.
• Watch every joint as the water rises. It’s much better to spot a weeping union now, with the control room open. The flood test is covered in my in-ground build guide.

Hosepipe bans (UK)

If your area has a Temporary Use Ban, you generally can’t fill a hot tub with a hosepipe. Cambridge Water’s hosepipe ban FAQs, for example, list “filling or maintaining a domestic swimming, paddling pool, hot tub or cold-water plunge pool” as banned, with narrow exceptions such as medical use. Check your own water company’s rules before a summer refill. In the USA, drought restrictions are set locally, so check with your water provider.

What water should you use?

• Cold mains water is best. Very hot water from the house can trip the heater’s high limit, and most home hot water systems won’t hold enough to fill a tub anyway.
• Not fully softened water. It has almost no calcium, and water like that attacks grout, heater elements and pump seals. If you have a softener, fill from an unsoftened tap. More on this in my hot tub chemical levels guide.
• Well or borehole water can carry iron or other metals that stain. Test it first, and use a stain and metal remover if you need to.

Fill from the highest point

This is the key to avoiding an air lock. Filling from the highest point lets the water run down through the pipes and push the air out ahead of it, rather than trapping it.

Plastic shell tubs: fill through the filter

On a shell tub, the highest point is usually the filter housing. Take the lid off, put the hose into the filter compartment and fill from there. The water goes through the filter, into the pumps and the internal plumbing, and out into the tub, pushing the air out as it goes. Aqua Living’s air lock guide gives the same advice, and adds that you should bleed the pump before you power up.

DIY tubs: into the tub, and bleed the air

On a DIY tub, the highest point is often the tub itself, so that’s where the hose goes. But if your control room is lower than the waterline, which it often is, water rushes down the suction and return lines into the pumps, filter and heater from both ends at once. That can trap a big pocket of air in the kit. To stop it:

• Open the air bleed on the filter housing or the pump’s strainer lid as the water arrives. Close it when water spurts out.
• If there’s no bleed valve, crack open a pump union slightly until water runs, then nip it up again.
• Fill a bit slower once the water reaches the suction fittings, so the air has time to escape.

Where your control room sits makes a real difference to priming. See Hot tub control room position and, if your pumps sit above the water, Using a priming circuit above ground.

How full?

On a tub with a skimmer, fill to around the middle of the skimmer opening. Otherwise, fill to the level in your manual or plans, comfortably above the highest jet. Remember the level rises when people get in.

Purge the air with the jets

Once the water is above the inward suction drain (and ideally above the jets), power up and turn the jets on for 10–15 seconds, then off. Repeat a few times. This forces water through the jets and pushes any trapped air out into the tub.

Don’t run the pumps dry. Only run them when the water is above the suction, and keep the bursts short until you’re sure water is moving. A pump running dry for any length of time can burn out its seal, and a heater with air around it can burn out its element.

Signs you’ve still got an air lock:

• The pump hums or whines, but little or no water comes out of the jets.
• The jets surge and splutter.
• The heater won’t come on, or the pack shows a flow or “dry” error.

What to do about a stubborn air lock

1. Lower the water to below the jets and hot water returns, but keep it above the suction.
2. Run the pump. With the jets out of the water, air can escape through them easily.
3. Refill while it’s running in short bursts, keeping the level above the suction the whole time.
4. Still stuck? Bleed at the pump union or filter as above.

My full troubleshooting guide is How to deal with a hot tub air lock. If your pump sits above the water and never seems to prime, self-priming pump options are worth a look.

After the fill: water chemistry

Different areas have very different water, so test the fill water before you add anything, using test strips, a drop kit or a digital tester. Then adjust in this order:

1. Total alkalinity to 80–120 ppm. Around 100 is a good target. It buffers the pH, so do it first.
2. pH to 7.2–7.6. Add small doses of increaser or decreaser with the jets on, and wait 15–20 minutes between doses before retesting.
3. Calcium hardness, if needed. If your tap water is very hard, use a scale-prevention product from the first fill. If you see scale on your showerheads and taps at home, that’s a good clue.
4. Sanitiser: free chlorine at 3–5 ppm, or bromine at 4–6 ppm. Shocking on a fresh fill deals with anything that came in with the water.

The full ranges and the reasons behind them are in my hot tub chemical levels guide, and shocking is covered in Hot tub shock.

How long will it take to heat?

Longer than most people expect, so plan for it. A good rule of thumb: degrees per hour ≈ heater kW × 860 ÷ litres. That’s with no heat loss, so real life is a bit slower. Keep the lid on while it heats.

Heater, 1,500 litres (about 400 US gal)	Best-case rise	From 10°C to 38°C (50°F to 100°F)
3kW electric (typical UK pack)	~1.7°C (3°F) an hour	~16 hours or more
5.5kW electric (typical US pack)	~3.2°C (5.7°F) an hour	~9 hours or more

So anything from 8 to 24 hours is normal, depending on the size of your tub and heater, and the starting water temperature. An air source heat pump will depend on the air temperature. See my guide to air source heat pumps for hot tubs. There’s more on first start-up in the hot tub setup guide for new owners.

Common mistakes

• Leaving the power on while filling, so a pump starts dry.
• Dropping the hose straight into the tub when the control room is below the waterline, with no air bleed open.
• Running the jets before the water is above the suction.
• Filling from a water softener.
• Adding a pile of chemicals before testing the fill water.
• Filling with a hosepipe during a hosepipe ban.

Building your own?

Air locks are far less of a headache when the control room and plumbing are laid out with filling and priming in mind. My DIY hot tub plans include the plumbing layout, and The Ultimate Hot Tub Control Room Guide covers where the pumps, filter and bleed points go. Filters, valves and pump parts are available from parts4tubs.co.uk in the UK and parts4tubs.com in the USA.

FAQs

Where do you put the hose when filling a hot tub?

On a shell tub, in the filter housing. On a DIY tub, usually in the tub itself, with the air bleed on the filter or pump open if the control room is below the waterline.

How do you stop an air lock when filling a hot tub?

Fill from the highest point, bleed air from the filter or pump as the water arrives, and purge with short bursts on the jets once the water is above the suction.

Can I fill my hot tub with hot water?

It’s best not to. Very hot water can trip the heater’s high-limit safety, and a home hot water system rarely holds enough. Fill with cold water and let the tub heat it.

Can I fill a hot tub with softened water?

Not fully softened water. It has almost no calcium and can attack grout, heater elements and seals. Fill from an unsoftened tap.

Can I fill my hot tub during a hosepipe ban?

In the UK, usually not with a hosepipe. Temporary Use Bans typically include filling or maintaining a hot tub. Check your water company’s rules.

How long does a hot tub take to heat after filling?

Typically 8–24 hours, depending on the heater, the volume and the starting temperature. A 1,500-litre tub with a 3kW heater takes around 16 hours from cold.

Got a filling or air lock question? Drop it in the comments below. I read them all.

Cheers,
Andi

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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