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Is an air to water heat pump appropriate to keep a hot tub at temperature throughout the year? The hot tub will run 24 hours a day, seven days a week, covered when not in use.

Presumably if the efficiency is greater than 1 even at the lowest temperatures (say -5 °C worst case in the South of England) and the heat pump can generate 3 kW at that temperature, then it can maintain 1,500 litres of water at 39 °C more efficiently than a traditional electric heater. Is this assumption correct?

We do not currently have solar or energy storage, if that matters.

Anything I should be wary of when contacting suppliers?

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    Hello. Welcome to Sustainable Living. Is keeping a hot tub running 24/7/365 ever going to be sustainable (before we have clean, practical, large scale fusion, I mean)? What is your sustainability goal here? – Jean-Paul Calderone Apr 16 '19 at 14:06
  • Hi Jean-Paul, I was thinking that if a heat pump is used then that could address a big chunk of the energy required in the short term. Medium term, we're waiting on the new gov consultation to determine how to best use renewable energies. We just missed the boat on the solar panels as we moved into our house after they stopped the solar grants. Longer term, we're keeping an eye on the energy storage options, it seems like an expensive and not widely used option at the mo, but longer term may be a welcome addition to our house. – Cath Apr 16 '19 at 19:43
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tl;dr: heat pumps are widely used for swimming pools, can probably be used for hot tubs, and should work reasonably well in your climate.


Swimming pool with cover

I couldn't find good information on hot tubs, but I did find that the U.S. Department of Energy has this page on heat pump water heaters for swimming pools. Then, a quick search suggests that Seattle is the city with the climate most similar to London, which is in the south of England.

For a pool with a cover, a heat pump (HP) pool heater will use 1700 to 3300 kWh per year. This assumes that you're using it for the months of June to August, and that the HP has an average COP of 5.0.

Depending on what the HP replaces, this usage will represent savings of 40 to 80%.

Hot tub vs water heater

Three things change for your situation with a hot tub:

  1. Year-round operation
  2. Hotter temperature
  3. Less water

In this case, it starts to look more like a storage water heater, so I started looking at some of the data that impacts COP of heat pump water heaters (HPWH).

[+] Advantages of a hot tub vs HPWH

  • Bigger is better. According to an NRDC study, HPWHs with a larger tank have a higher annual COP, especially in cold climates -- in North Dakota (site of the coldest average installation in the study), an 80 gallon (300 L) tank was about 15% more efficient overall than a 50 gal (190 L) tank. A hot tub is obviously bigger than a water heater.

  • Lower setpoints are better. See chart below from a NIST study. For a hot tub, the "tank" doesn't need to be as hot as for a water heater.

COP vs tank temperature

[-] Disadvantages of a hot tub vs HPWH

  • Cold climates are worse. From the NRDC study cited above, The HPWHs studied were in basements, whereas yours will likely be outdoors. However, they found that the colder the basement, the worse the COP -- see chart below. So for a hot tub that's outdoors, you'll probably get a worse COP than for a water heater.

Annual COP for cold, Intermediate, and Hot Climates (50-gal)

  • Warmer inlet temperature is worse. A water heater is heating up cold tap water, whereas a hot tub will be re-heating the water already in the tub, which means the COP will be reduced -- see chart below from a study by the Universidad Politécnica de Madrid. However, you don't need to add as much heat since the water is already closer to the setpoint temperature, so the total impact on energy usage may be minimal.

COP vs tap water inlet temp

[=] Conclusion

Since heat pump water heaters are already marketed for swimming pools, it appears they would already be available and reasonable for a hot tub as well. I couldn't find any research data on heat pump hot tub heaters, but there's a lot of literature on heat pump water heaters for domestic use. A hot tub heater will have some advantages and disadvantages compared to these, but it's probably safe to assume that the COP will never drop below 2.0, and could reach as high as 5.0 in the summertime.

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