Reviving a Hot Tub: A Complete Heat Pump Installation and Upgrade

Last updated: September 2026

A client of mine, Chris, set out to upgrade his seven-year-old hot tub, which had been suffering from several problems, including water damage, poor insulation and internal leaks.

The main goal was to fit a new air source heat pump, but the process uncovered deeper problems that took real time and effort to fix. In this post I’ll walk you through Chris’s step-by-step approach, his solutions and the impressive results he got.

Diagnosing the issues

When Chris took the hot tub apart, he found a series of problems that had been hurting its performance. Rainwater had got into the bottom skirt, causing structural damage and wood rot. The hot tub also had almost no insulation, so it was losing a lot of heat and, obviously, wasting money on energy.

On closer inspection, Chris found four internal leaks coming from speakers that had completely failed. The leaks had damaged internal components and added to the water damage in the base.

So the project was no longer just about fitting a heat pump. It was about rebuilding key parts of the hot tub. That’s common with older tubs, and my checklist for restoring or repairing an old hot tub covers what to look for.

Repairing the internal leaks

The first job was the leaks. All four speakers were beyond repair, so Chris removed them, along with the sound system, wiring, bass speaker and power supply unit.

To seal the holes the speakers left behind, Chris cut uPVC sheeting to the exact size of each hole. That stopped the leaks and left the hot tub watertight. If you’re chasing a leak of your own, see how to find and fix a hot tub leak.

Dealing with wood rot

The wooden base had rotted badly after long exposure to rainwater. Chris carefully cut out all the rotten wood and treated the remaining areas with a wood hardener. He filled the gaps with expanding foam, sanded it down, and painted all the visible external surfaces with Owatrol Clear decking paint.

To stop it happening again, Chris fitted a damp-proof course (DPC) around the bottom of the hot tub. It acts as a splash guard, stopping rainwater soaking into the wood and extending the life of the structure.

Improving the insulation

One of the most important upgrades Chris made was the insulation. Poor insulation was a big part of the heat loss, so he took a layered approach:

• Base: 25mm Celotex boards on the inside floor, for a solid, insulated foundation.
• Walls: waterproof silver bubble insulation from top to bottom along the fibreglass walls, followed by 100mm of fibreglass wool.
• Outer panels: 25mm Celotex boards on the outer panels before the covers went back on.

These measures cut heat loss considerably. Chris estimated that the new insulation reduced the temperature drop by two-thirds, even in cold weather. On a heat pump especially, insulation pays for itself, because it lets a small unit keep up. There’s more in my DIY hot tub insulation guide and hot tub energy efficiency.

Installing the heat pump

With the repairs and insulation done, Chris moved on to the new heat pump. This stage needed careful planning to get a good flow of water with as little resistance as possible.

He had trouble fitting the flapper valve and T-pieces into the existing filter pipework, because the flexible pipe was particularly stiff. To solve it, Chris ordered an extra T-piece and made precise cuts to the original setup.

The heat pump pipework was laid out deliberately: the outlet pipe slopes upwards and the inlet pipe slopes downwards. That way, when the heat pump valves are isolated, water drains away from the heat pump rather than sitting inside it. Chris used only four 90° bends to keep flow resistance to a minimum.

For the full process, see how to add an air source heat pump to your hot tub, and for getting the heat pump and the spa pack to work together, how to control a heat pump with any spa pack.

Testing the results

After priming the system and connecting the heat pump to WiFi, Chris ran a series of tests. With an air temperature of 12°C (54°F) and 90% humidity, he reported the heat pump running at 90% efficiency. It raised the water temperature by 3°C (about 5.4°F) an hour and reached a comfortable 37°C (99°F) with ease.

By comparison, the old 2kW heater only managed 1.5°C an hour. The new heat pump was drawing about 900W (0.9kW), so it was delivering twice the heating rate on less than half the power. That makes it roughly four times more efficient than the old system.

Chris’s results	Old electric heater	New heat pump
Power drawn	2kW	about 0.9kW
Heating rate	1.5°C an hour	3°C an hour (at 12°C air)
Heat per kWh used	about 0.75°C	about 3.3°C (roughly 4x)

Heat pump efficiency is normally quoted as a COP (coefficient of performance) rather than a percentage, and it changes with the air temperature. The figure that really matters for your bills is the one above: how much heat you get for each kWh you pay for. There’s more on this in the ultimate guide to air source heat pumps for hot tubs, and on how I cut my own running costs in how I cut my hot tub running costs by 66%.

Adjusting for cold weather

During a cold spell, with temperatures down to -7°C (19°F), Chris decided to shut down the heat pump temporarily to protect it. He isolated the valves and drained the heat pump so it stayed safe. Thanks to the extra insulation, the hot tub held its temperature far better than before, even in those conditions.

Chris noted that the hot tub was originally designed for tropical climates, but his upgrades had adapted it well for colder weather.

Every heat pump has its own minimum operating temperature, so check your model’s range before winter. Mine kept going in a -4°C freeze, and I’ve shared the results in I ran my hot tub heat pump at -4°C. If you do shut yours down in a hard frost, isolate and drain it as Chris did, so there’s no water left inside to freeze. See winterizing a hot tub for more.

Closing thoughts

Despite the unexpected problems and the time the project took, Chris is thrilled with the result. His hot tub is now more energy efficient, better insulated and structurally sound. The combination of a modern heat pump and targeted repairs has turned it into a reliable, comfortable place to relax.

Chris’s experience is a useful example for anyone upgrading a hot tub, and it shows how important it is to fix the underlying issues if you want the upgrade to last.

Thinking of adding a heat pump to your hot tub?

Start with how to choose the right air source heat pump for your hot tub or pool and 5 things to consider before fitting one. You can find air source heat pumps and connection kits at parts4tubs.co.uk in the UK and parts4tubs.com in the USA. For another real-world install, see Upgrading to an air source heat pump: Chris’s journey.

And if you’d rather build a properly insulated hot tub from scratch, my DIY hot tub plans have everything worked out for you.

FAQs

Can you add a heat pump to an existing hot tub?

Yes. Chris added one to his seven-year-old tub by fitting T-pieces and a flapper valve into the existing filter pipework. It’s worth checking the tub’s condition and insulation at the same time, as he did.

How much faster does a heat pump heat a hot tub?

It depends on the heat pump and the air temperature. At 12°C air, Chris’s raised the water by 3°C an hour, against 1.5°C an hour from his old 2kW heater.

Is it worth insulating an old hot tub?

Chris estimated his new insulation reduced the temperature drop by two-thirds, so yes. It also helps a heat pump keep up in colder weather.

Should I turn my heat pump off in freezing weather?

Check your model’s minimum operating temperature. If you do turn it off in a hard frost, isolate and drain it so no water is left inside to freeze.

Have you upgraded an older hot tub? Let me know how you got on 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