Turning Down The Temp Vs Leaving It Set

[DiverDownAZ]

Very reasonable analogy, except for one thing. If you apply Neutons law to it, when the bucket is full, it will be leaking out the bottom faster, than when it's less than full. Think of it like, as the water level drops, and there's less pressure on the bottom of the bucket, it will be leaking slower. The further the water level drops, the slow the water will leak out.

I agree

If you use basic high school thermodynamics q = UA(T1-T2) where

U = overall heat transfer coefficient

A = heat transfer area

q = overall heat transfer rate

T1 = spa temp

T2 = ambient temp

U, A, and T2 will stay constant as T1 goes down (lowering temp from 100 to 80) q will also go down.

if T2 is assumed to be 50 deg. then q will go down by 40%.

Also a motor will consume more power when it is turned on then when it is running in steady state. So allowing the spa to heat up in one shot would be more efficient than periodically cycling the heater and pumps on thru out the day.

[TinyBubbles]

I agree that it's not worth worrying about too much. A spa is for relaxation. If I can't afford to heat it, then I would rather not have it. On the other hand, it's still fun and interesting to think about. I can't believe a power company is dead wrong. I looked online, there are several energy conservation websites that list the exact same thing. Dr. spa, you are completely right in your thinking, you are just leaving one thing off. Let's say your spa is 90 degrees and you warm it up 2 times a day to 100. Your other option is to leave it set on 100 and when it drops to 99 it will heat up to 100. It is true that the water will heat at a faster rate when it is cooler, but at the point it reaches 99 degrees in the first case, it will then take the exact same amount of time and energy to reach 100 as it will in the second case. I honestly don't mention this to be arguementative or to try to prove anyone wrong. It's just interesting discussing things with knowledgeable people with similiar interests. Fortunately, none of us has to pay anyone elses powerbill, so we can all do what we think is "right". Roger hit the nail on the head, without a useage meter hooked to our spa, I think it's really impossible to know how much it's costing us. This month will be my first bill with my spa. It's also the first month that temperatures have dropped to the freezing point. Last month with mild weather, my powerbill was extremely low. This month the heat as been running and it will be much higher.

[Dr. Spa]

???? Water will heat at the same rate (degrees per hour) if you put the same kW into it, regardless of the water temp.

Confusion seems to be that peopel think, "well if you let the water cool to from 100 to 90 it's going to take 2 hours of heating to bring it back up to 100". What people don't realize, is that if you maintained the temp at 100, the heater will need to run MORE than 2 hours to do so.

[Whoneedsavacation]

If you use basic high school thermodynamics q = UA(T1-T2)

sure, BUT:

U, A, and T2 will stay constant as T1 goes down (lowering temp from 100 to 80) q will also go down.

if T2 is assumed to be 50 deg. then q will go down by 40%.

Is not true because the difference between 80 degrees and 50 degrees Farenheit is not 40% in absolute units. dq is a much smaller fraction of q than that.

And, even if the numbers were right, it would change by the same amount going down and up. You still don't win unless you "bottom out" for a while.

[Whoneedsavacation]

???? Water will heat at the same rate (degrees per hour) if you put the same kW into it, regardless of the water temp.

If this is true....

Confusion seems to be that peopel think, "well if you let the water cool to from 100 to 90 it's going to take 2 hours of heating to bring it back up to 100". What people don't realize, is that if you maintained the temp at 100, the heater will need to run MORE than 2 hours to do so.

This would not be true, modulo ineffciencies.

It's not the physics but the engineering that makes the biggest difference here.

[Brock]

you guys with 15lb brains are making my pea sized brain hurt......I think I will just soak and not buy another case of adult beverages to make up the difference in costs. LOL

[Roger]

you guys with 15lb brains are making my pea sized brain hurt......I think I will just soak and not buy another case of adult beverages to make up the difference in costs. LOL

Just buy an 18 pac instead of the case of 24. Because the difference is very very minumal.

[mannfilm]

I'm sorry but you all are way over thinking this. Due to weather, insulation, wind breaks, whatever, everyone's situation is unique. Your electric meter tells you how much energy you are using. Write down your 24 hr usage while changing the Tub settings. Call up your provider and see how to transcode those numbers into dollars charged to you.  The beauty of this, is that you can now start experimenting with solutions, including physical. Put up a temporary wind break, throw a tarp over the tub at night, ect. Tells you how much you are paying if you go on vacation for a week vs draining it. No more guessing.

[Cusser]

OK, have a 1988 model CalSpa, since new, 220 VAC.  The first 2 decades, due to a demand electric rate surcharge for entire month based upon highest HOUR of usage, the spa was on load controller, which wouldn't deliver electricity for heating unless the demand was below our set limit.  Then the load controller died,over $1000 to fix, and electric company was due to change rates to demand/higher costs in certain afternoon - early evening hours.  So I started just leaving it "set" to 104F 24/7 so any "demand" during those peak hours would be low.

To my surprise, the spa only actually heats less than 30 minutes a day in winter, and winter here can be 35-40F. So now I just leave it 104F all the time, except in summer, when we don't heat it because it gets that hot anyway (Arizona).

[Scotty the Scot]

What a load of guff is written here about heat loss and economy.

Far too many people with “ notions “ about heat loss and thermodynamics based on their emotional approach rather than a scientific one. The question is which is the most economic use, not what brings the biggest savings or usage benefits so let’s not cloud the waters. 
it s a matter of scientific fact that not heating something uses less energy than heating something.
 

The economics are basic no mater what your source of heat is. There are currently 4 laws of thermodynamics, but the one concerning us here is regarding heat loss of an object at a given temperature. This is proportional to the temperature. Simply put, the hotter the item, the faster the heat will be lost (the insulation values always remain the same in the case of a hot tub) so it is simple to show that constantly maintaining a higher heat will cause more heat to be lost over the same given time as maintaining a lower temperature. This equates to “ more expensive” 

The energy required in raising an object to a set temperature must also be compared to the energy used in maintaining a set higher temperature and the savings made during the cool down period.

As a wise Scotsman once said “Ye canny beat the laws of physics”

So please ignore all the anecdotal and notional evidence promoted here.

If the opposite was true, then we would have the Holly Grail of physics “Free Energy” which would mean that perpetual motion machines could exist.

If you still disagree, I would be seriously looking at your education system.

 

 

[Cusser]

It's been over 45 years since I last had physics and calculus, but I tend to agree with Scotty the Scot from Scotland.

As to my spa, it's a 1988 model so the temperature setting is by a dial that one turns, then repeats over a period of time by trial and error to reach temperature, so no way would I turn the temperature down periodically, as I'd never readily get back to my desired temperature.

Plus, my spa is always at temperature when I want to use it.  Sure, I'm in Arizona, but lows here can reach 35F, and my spa/heater (240 VAC) doesn't run that many minutes per day. 

Everywhere: a decent cover is a must.

[Jay G]

On 11/8/2007 at 10:57 PM, DiverDownAZ said:

I agree

If you use basic high school thermodynamics

Basic what-now?  I must have missed that day in HS.