How much does the hot tub heat up per hour, cover on vs. cover off?

[bolds18b]

Hi - would appreciate some help: how many degrees should a 320 gallon hot tub warm up per hour with the cover off?  Cover on?  If it matters, imagine it's a 45 degree day, the water starts at 99 degrees, and the jets are off. 

I think the answer is it should warm up 1-2 degrees per hour with cover off, and 5-7 degrees per hour with cover on, but I'd love an expert opinion! 

Thanks

[Dr. Spa™ Ret.]

Too many variable to determine. How big is the heater? How much surface area of water is there? How windy is it?

[bolds18b]

Shoot - I don't know how big the heater is.  It's a 2007 Sundance Marin.  I'm looking at a used one - the owner doesn't know anything, and the user manual was written for a few different models so I'm not sure which one this is.  The tub is about 6'x7' of surface area, and it was not that windy.

[Tman122]

Wind will play a huge part wicking away heat from the waters surface. Your in the ball park though.

[goneaway]

My 5 year old artisan heats up about 1°C per hour, with the covers on.
And will maintain heat with the covers off and in use.

We've not done a winter yet though

[wmccall]

My 5 year old artisan heats up about 1°C per hour, with the covers on.
And will maintain heat with the covers off and in use.

We've not done a winter yet though

Winter does has a whole other element.  As Spatech said, so many elements. My Dynasty tub had a larger surface area.  Most winter nights if we were in the tub 30 minutes it might lose a degree or two with the cover open, but on really cold nights with wind that would be much worse, but we wouldn't stay out that long.  We had a mild winter last year with may smaller newer tub. We will see.

[goneaway]

I can't wait for the snow

[bob snader]

but on really cold nights with wind that would be much worse,

And in a few different ways too.

We have sort of a unique situation in that we can bath with the cover on or off, but when it starts hitting -15c (5f) we just plain stop opening the lid because we get too much ice build up on the edges of the tub. This in turn presents big problems in attaining a positive seal between the lid and the tub upon closing.

Now I have opened the lid on a few -30c (-22f) days with the wind up just to see what it's like... and it's NOT real fun. The water also cools faster than can be heated (4800 watt heater), and if your hair is a bit wet.... it freezes pretty fast!

[Spoiledrotten]

Where are you? Arctic Circle? Wow, I can't imagine living in extremes like that.

[zroger73]

My '08 HS Sov II holds 355 gallons and has a 6 kW heater. The calculated temperature rise is 6.9°F per hour with the cover on. Real-world observations have confirmed this. Very cold ambient temps will lower this rate by a tenth or two of a degree per hour. I've never had a reason or desire to calculate/measure rate of rise with the cover off since that would be very inefficient. As has been stated, there are many other variables that come into play when the cover is off such as...

- Shell color and heat load from the sun (the slowest rise will result with no direct sun or a light color shell, the fastest rise will result with direct sun and a dark color shell)
- Ambient temperature (this is pretty obvious - the colder it is, the longer it will take to heat the water. If the heater is too small, it won't heat at ALL with the cover off if the heat loss is equal to or greater than the heater capacity. Most 110V units have this problem because their heater wattage is limited to no more than 1.5 kW.)
- Wind speed (wind speeds up evaporation, which absorbs heat)
- Humidity (lower humidity = faster evaporation = more cooling, higher humidity = slower evaporation = less cooling)
- Jet/bubbler usage (injecting air into the water will cool it off - some brands like HS slightly preheat the air stream using waste heat from the jet pumps for slightly better efficiency)
- Occupancy (more heat-producing bodies will help raise the temperature of the water...IF it is less than 98.6°F)

[bob snader]

Where are you? Arctic Circle? Wow, I can't imagine living in extremes like that.

Manitoba (Canada). We get it all, from +40c (104F) in the Summer to about -42C (-44F) plus wind chill in the Winter.

[Tman122]

Where are you? Arctic Circle? Wow, I can't imagine living in extremes like that.

Manitoba (Canada). We get it all, from +40c (104F) in the Summer to about -42C (-44F) plus wind chill in the Winter.

Bout the same here in Northern Minnesota. 40 below and anything over about 95 is pretty rare here. But not 20 below.

[wmccall]

10 years of owning a hot tub in Central Ohio, the coldest I have soaked in was 4F.  No wind that night.  At 104 we were out there 30 minutes quite comfortably.  They sky was really clear and with snow on the ground the neighborhood was super quiet. (No gun shots). The next night it was 15F, but with a 40 mph wind, we didn't go out.   During the 10 years I have owned the tub, it has only been below 0f a few times and each night was super windy. 

In the summer our tub never goes below 101.  Even in August with the temps in the 90s during the day it drops at least into the 80s overnight and coming outside from the air conditioning  101 is nice for up to 30 minutes.