Cold air, wind chill, and lid leaks: the three real reasons your tub is heating at half its summer rate, and what actually fixes them.
Hot tubs do not get less powerful in winter. The heat is leaking out as fast as the heater can put it in. Once the air drops below 5C and the wind picks up, the rate at which a UK garden tub loses warmth roughly doubles. Here is what to do about it.
About 60% of the loss in winter is through the cover. The next 30% is through the shell where the cabinet panels meet the floor, especially if the foam insulation has shifted. The last 10% is through the plumbing and the equipment bay.
If any of those three are compromised, you are heating the garden as much as the tub. The cover is the easiest to inspect and the easiest to fix.
Press down on the centre of the cover. If it sags more than 2cm, or you can hear water moving inside the foam, the cover is waterlogged. Replace it before you spend another month feeding the heater.
Walk around the cabinet. Any panel that feels noticeably colder than the others may have lost its insulation jacket, especially after a service.
Look at the equipment bay. Pipes should not be sweating or icing. If they are, the bay heater (a small unit that keeps the plumbing above freezing) may have failed.
A floating thermal blanket on the water surface, under the main cover. Five to ten pounds, cuts evaporative loss by half. Evaporation is the biggest single heat loss mechanism even in winter.
A wind break upwind of the tub. A solid panel or a tall planter on the cold side cuts the rate at which air pulls heat off the cover. You can feel the difference within 24 hours.
If those do not move the dial, get the cabinet panels off and check the foam insulation. Older tubs benefit hugely from a 25-pound aftermarket insulation kit.
Yes, by one or two degrees. Going from 38 to 36 saves more in January than it does in July because the gap to the air temperature is bigger.
Only if you are not going to use it for at least three months. The drain-down, refill, and full chemical reset is its own cost, in time and in product.