Two losses, two different mechanisms
A covered tub loses heat by conduction: warm water on one side of an insulating layer, cold air on the other, and a rate proportional to the area, the temperature difference and the reciprocal of the R-value. That is the familiar arithmetic, area times delta T divided by R, and it gives a number in BTU per hour. For a seven foot square tub at a fifty degree difference under an R-9 cover, the cover accounts for around 270 BTU per hour.
An uncovered tub loses heat mostly by evaporation, and evaporation is not a conduction problem at all. Molecules leave the surface carrying the latent heat of vaporisation with them, which is roughly a thousand BTU per pound of water that departs. The rate depends on the vapour pressure difference between the water surface and the air, which means it depends on humidity, and it depends heavily on air movement across the surface, which means it depends on wind. A single coefficient in BTU per hour per square foot per degree, which is what this page uses, is a crude stand-in for all of that. It is transparent and it is roughly right in still, moderately dry air, and it is badly wrong in wind. Where the losses matter enough to model properly, the pool heat loss and cover savings page separates humidity and wind speed and does the psychrometrics rather than lumping them.
The cover is the whole game
The ratio between open and covered loss on a typical tub is somewhere between five and fifteen to one. That is the reason a cover is described as the single largest factor in what a tub costs to run, and it is also why cover condition matters more than cover specification. A cover is foam in a vinyl skin. The vinyl eventually splits or the vapour barrier around the foam fails, the foam takes on water, and waterlogged foam is close to worthless as insulation while being much heavier than dry foam.
| Cover state | Rough R | How you can tell |
|---|---|---|
| New, tight, dry | 11-13 | One person lifts it comfortably; the fold seals flat |
| A few years old | 8-10 | Still a one person lift; the hinge fold has softened |
| Sagging and heavy | 4-6 | Two hands and effort; visible dip in the middle; steam escapes at the fold |
| Waterlogged or split | 2-4 | A two person lift; water runs out when tilted |
Weight is the diagnostic worth trusting, because it changes long before anything is visible. A cover that has doubled in weight has taken on many pounds of water, and it is both insulating badly and loading the hinge and the lifter.
What the calculator leaves out
Three real losses do not appear above. Pump and blower motors add heat to the water, which offsets some of the loss and is why a tub run on a long filtration cycle can sit warmer than the arithmetic suggests. Plumbing and equipment bay losses are separate from the shell and vary enormously between a full-foam tub and one with an open cavity. And any water that leaves, whether through evaporation or a splash-out, is replaced by cold water that has to be heated from tap temperature, which is a real cost that scales with how much use the tub gets.
The shell R-value input is doing a lot of averaging as a result. Full-foam construction, where the whole cavity is filled, behaves quite differently from a tub with a foil blanket on the inside of the cabinet, and the floor sits against a slab or a deck rather than against air. Treat the standby figure as an order of magnitude with a defensible structure behind it rather than as a meter reading.
Health, stated once and plainly
Immersion in hot water and time in a hot room both put load on the cardiovascular system, and they interact with pregnancy, with heart and blood pressure conditions, with a long list of medications, and with alcohol. Whether any of that applies to you is a question for a clinician who knows your history, and it is not a question a calculator can help with. Nothing on this page states a water temperature or a length of time as safe, and nothing on it should be read as a recommendation to sit in water at any particular temperature for any particular period. The temperature field is there because the heat loss depends on it, and the figure in it is yours.
Questions people ask
How much does a hot tub cost to run per month?
The honest answer is that it varies by a factor of three or more between tubs, and this page exists to show why rather than to hand over a figure. The variables that dominate are the temperature you hold, the outdoor temperature, the condition of the cover and how many hours a day it comes off. Put your own numbers in and the monthly figure at the bottom is arithmetic on your electricity rate. Expect the real bill to run somewhat above it, because pump energy, equipment bay losses and heating cold makeup water are not in the model.
Is it cheaper to turn the tub down between uses?
It depends on the gap. Standby loss is proportional to the temperature difference between water and air, so holding the water ten degrees lower cuts the standby loss by roughly the ratio of the two differences, which on a cold day is a fifth or so. Against that, the water has to be brought back up before use, and that recovery costs the same energy the drop saved, minus whatever less-hot standby you gained in the meantime. The break-even is a matter of days rather than hours: for a tub used every evening, dropping it is usually a loss once you count recovery time; for a tub used at weekends, holding it lower during the week is usually a saving.
Why does leaving the cover off cost so much more than the R-value suggests?
Because the loss changes mechanism. Under the cover, heat leaves by conduction, which the R-value describes. With the cover off, the dominant path is evaporation: water leaves the surface as vapour and takes the latent heat of vaporisation with it, roughly a thousand BTU for every pound of water that goes. That is a much larger term than conduction through anything, which is why the open figure on this page is often ten times the covered one. It is also why you can see the loss happening as steam on a cold evening.
What R-value should a cover be?
This page takes it as an input rather than telling you, because the specification a cover was sold with and the R-value it currently has are usually two different numbers. Covers are commonly marketed somewhere between R-10 and R-16 when new, with tapered ones quoting an average across the taper. What matters after a couple of years is whether the foam is still dry, and the cheapest test is lifting it. If your cover is heavy, the value it delivers is nothing like the value on the label, and the arithmetic here will follow whatever you enter.
Does a floating blanket under the cover help?
It does, and it works on the evaporation term rather than the conduction one, which is why the effect is larger than its thickness suggests. A thin sheet of closed cell foam sitting on the water suppresses evaporation from the surface it covers, and it also keeps the underside of the main cover out of the steam, which is one of the routes water gets into cover foam in the first place. The trade is handling: it has to come out and go back every time, and one left crumpled in a corner is doing nothing.