tub heating time

[buzzsaw90]

So i tried doing the math to see how long to heat my tub when it arrives next week.

550 G water(or 2,000,000 grams)  with a 4300 W heater. To go from 50F to 100F would take about 242 million joules or 230,000 BTUs


I worked it out as 15.5 hours to raise the temp 50 degrees F, or the temp should go up about 3 degrees/hour.

at 8 cents per KwH i got an initial heating cost of a little over $5

do these sound like reasonable expectations?

and once up to temp, how many hours/day will the heater be going, i'm figuring 1-2 depending on temp outside and how much we're using it.

[Dr. Spa™ Ret.]

Probably off, maybe a lot, depending on the spa and how well it's insulated. You didn't calculate in the heat loss while it's heating up. During the time it's heating, it will also be losing heat to the atmosphere , which will have to be accounted for. There's also the electricity to operate the pump while it's heating.

[Spoiledrotten]

Are you saying that the water comes out of your faucet at 50*? I would think it would be a little higher than that. What part of the country are you in, Alaska?

[buzzsaw90]

Are you saying that the water comes out of your faucet at 50? I would think it would be a little higher than that. What part of the country are you in, Alaska?

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that was just a guess on my part. I'd never bothered to measure it. Hopefully warmer here in Pennsylvania. Think i'll check that out tonight.

[buzzsaw90]

Probably off, maybe a lot, depending on the spa and how well it's insulated. You didn't calculate in the heat loss while it's heating up. During the time it's heating, it will also be losing heat to the atmosphere , which will have to be accounted for. There's also the electricity to operate the pump while it's heating.
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Probably not much heat loss. At first the air temp will likely be warmer than the water (at least in September) so i may get some free heating, especially on a sunny day.

Pump power i did skip on as i don't know the power consumption from the pumps. Then again that process might also generate some heat along the way. But this exercise was just to get a ballpark estimate of time/cost before the first soak.

[Tman122]

I see 55 degree water coming out of my well. Northern Minnesota.

[buzzsaw90]

OK, i got 70 degrees from the tap. So that means only +30 rather than +50 and put the heating time down to 10 hours

[Spoiledrotten]

OK, i got 70 degrees from the tap. So that means only +30 rather than +50 and put the heating time down to 10 hours

Except for well water, I thought 50* sounded a little chilly for piped in water.

[bimmerdog]

I got my Sundance Optima in February. Outside air temp 41. Water filling the tub was 68. It took 8:30 to get to 101. 

[buzzsaw90]

three days to go until delivery/install.We thought it would be today, but the truck took longer then planned. 68 hours to go.

Like waiting for Christmas morning when you're a kid. Just hoping that the water is warm enough that i can be in the tub when the kids get home from school.

[bob snader]

Except for well water, I thought 50* sounded a little chilly for piped in water.

I live in Manitoba (Canada). The tap water is about 35 degrees in the Winter. It takes about 2 full days to heat an 1100 gallon tub.

[wmccall]

Are you saying that the water comes out of your faucet at 50? I would think it would be a little higher than that. What part of the country are you in, Alaska?

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that was just a guess on my part. I'd never bothered to measure it. Hopefully warmer here in Pennsylvania. Think i'll check that out tonight.

50 isn't a horrible guess for this time of year in PA.  I'll guess 58.   Mine comes out around 40 in January and 72 in August.

[Spoiledrotten]

I guess you guys don't make the mistake more than once of turning on just the cold water in the morning for the shower. That's c-c-c-c-c-c-old!   

[zroger73]

That sounds reasonable. Years ago, I created a spreadsheet to calculate the cost of heating a water fill for my 2008 HS Sovereign II. Using some of your data:

50°F to 100°F
355 gallons
6,000 watt heater
$0.05 (winter) to $0.08 (summer) per kWh
=
6.9°F per hour
7.2 hours
$2.17 (winter) to $3.47 (summer)

As previously mentioned, the amount of heat lost/gained during the fill due to ambient temperature and the spa's R-value has some effect, but it is trivial during the time it takes to fill a typical spa unless you're filling in -50°F during a blizzard.

50°F is very cold water - for me. I live in Texas very close to my municipal water supply that gets its water from a deep well. The water averages about 81°F. With the cover in place and the heater completely off, I'll drop about 1°F per day during the winter and gain about that during the summer. In fact, I have to leave the cover off every few nights (and/or activate Summer Mode) during the hottest part of the summer to keep it from overheating.

[Jamester]

Calculations also assume 100% efficiency of heat transfer to the water, so probably need to adjust a little for that.

Can't believe you electricity is so much cheaper, we will be paying around 18c / kwH here in the UK...