Spa temp and energy usage

[windsurfdog]

Interesting where our minds wander during a peaceful, solitary early morning soak.........

My spa water temp is set at 97 and many times creeps to 98 or 99 which is fine.  I am amazed at how many users keep their spas at 102-104 and share that their sessions are rather long time-wise.  Living in Florida and not having enjoyed my spa through it's first winter definitely puts me at a distinct disadvantage when trying to relate to a tub that warm but 97-99 feels plenty warm to me.

None-the-less, I think many threads/discussions regarding energy consumption fail to consider both usage trends and water temperature settings.  Obviously, more time soaking means more pump run time plus more "cover off" time which translates into more energy consumption.  But what about temp setting?  Is it cut and dried that a higher temp setting will result in higher energy consumption beyond the initial heating of the water and the re-heating based upon usage?  If so, is it linear or possibly logarithmic as the temp increases?

Please DO NOT let this thread digress into another TP v. FF disaster--think of this as it is related to a single tub--your tub, if you will.  To anyone who may venture there:  May the goddess of Spatopia curse you with leaky pumps, green, foamy water and waterlogged covers--DON'T TEMPT FATE!
8)

 

[Chas]

I think the equation would be so convoluted that even the amazing Mr. Sappy would have a hard time figuring it out.

I say that because I can think of several variables which change from use to use: wind being the biggest factor I can think of.  But the use of jets is not always consistant: sometimes you inject air into half, all or none. Some tubs inject heated air into the jets, others pull it in from the great outdoors.

If your tub is equipped with diverters you may run all, half, or some selection in between.

Then - and I'm treading lightly here - you have to consider if the insulation allows for heat loss or heat gain when the jets are running. Also, some tubs rely on an air-tight seal from the siding, and they may not be all that air-tight as they age.

Blower? Some may pull heat into the water for the first few minutes, others don't add heat at all - most will cool the tub quickly after any hot air has been transfered.

Water falls and other waterfeatures cool the water, though not by much.

Foam density.

Cover thickness is a large factor in the non-use time. Also - some folks leave the cover on halfway to expose less surface area to the breeze during use.

So to simplify the whole thing, I think we can safely say that a well-built tub will cost you less to run: YMMV.

 

[Vinny]

Windsurfdog,

The answer is all tubs will use more energy when the difference in temps (water and outside air) is greater (this doesn't include wind).  It's a law of thermodynamics.

If tub "A" is running at 97 and the outside temp (O.T.) is 97 (and no other factors involved) the heater won't kick on. Tub "A" will kick on somewhat if O.T. is 90 and will kick on even more at 60 and so on. It works whether the water temp is higher or O.T. is lower.  Also, when the outside air is 105 and the water is 97 the water temp will rise - the insulation is working in reverse.

Now with that said, the amount of energy used may not be noticable if tub "A" is very well insulated (cover included).  

As for the if it's linear or logarithmic, I believe it's linear - water needs a certain amount of energy to raise 1 degree regardless of temp. It's been many years since I studied this stuff so I'm REALLY not familiar with the calculations on how to figure heat loss or heat gain.

Windsurfdog, I stayed with in your guidelines so please, Please, PLEASE keep the Spatopia goddess curse away from me. ;D

Vinny

 

[Brewman]

I think Vinny is right on.  All things being equal, it will take more energy to keep water at 104 than keeping it at 97.  How much more is open to a whole lot of variables.  
Brewman

 

[Chas]

As for the if it's linear or logarithmic, I believe it's linear - water needs a certain amount of energy to raise 1 degree regardless of temp.

True, but the difference between water temp and ambient air temp has a huge impact on the heat loss out the top during use. It's intuitive that if there is a 50 degree difference between water and air temp, the heat loss (cover open) will be quite a bit more and quite a bit faster than if there is only a few degrees of difference. And - That differential is effectively increased by causing the water to move around and the surface to be turbulant, via jets or blower systems.

 

[windsurfdog]

Chas and Vinny,
Based upon your responses, I didn't make the scenario very clear though Vinny addressed more of what I was looking.  Taking the same spa with the same usage trends under the same conditions (understanding that a poorly insulated tub/cover would amplify any energy needs), what would the difference in energy consumption be if operated at an outside temp of 70 and a tub temp of 97 and again at 104?  Starting at 97 as a baseline, would the difference at 104 be 7 times greater than the difference at 98 or would it be greater or less than 7 times?  Would the energy used to maintain the heat be greater expotentially/logarithmically as the ambient temp dropped?  Or do I just not have enough to think about?  

I guess the real reason for venturing here is to bring to light the notion that the temp chosen for a spa does have an influence on the energy consumption with the question of "to what degree" in debate.  When someone complains that their energy usage is "out the roof", how much of an influence is the temp to which they have set their tub?

Thanks to both of you, Chas and Vinny.  I'll whisper VERY nice things about each of you to the goddess.....

PS--and thanks to you too, Brewman--you snuck one in there while I was composing and being interupted as well.

 

[stuart]

Blowers and air induction will cool the water no matter where you’re drawing it from or if you have a heated blower. I think that a setting at 97 vs. 103 would be negligible from month to month for power consumption and would be more concerned with, pump run times, cover, wind and many other factors.

Saturday morning I too was having an early morning soak in 103-degree water, feeling the fall chill in the air, when a bug of some kind landed in the spa. As I watched closely, he struggled between life and death in the hot water and I wondered if he was more frightened by the hot water or the fact that he was drowning.... Suddenly I had what most of you would call an "epiphany" and I knew clearly what I had to do........

STOP DRINKING BEFORE NOON! ;D 8)

 

[Chas]

Chas and Vinny,
Based upon your responses, I didn't make the scenario very clear though Vinny addressed more of what I was looking.  Taking the same spa with the same usage trends under the same conditions (understanding that a poorly insulated tub/cover would amplify any energy needs), what would the difference in energy consumption be if operated at an outside temp of 70 and a tub temp of 97 and again at 104?  Starting at 97 as a baseline, would the difference at 104 be 7 times greater than the difference at 98 or would it be greater or less than 7 times?

Ah. OK, with those parameters set, then my answer becomes much easier to arrive at: I don't know.

But I will look into it. I may even ask one of my brainy kids - and get back to you. This is a very good question, and very apropos to this forum!

 

[windsurfdog]

Saturday morning I too was having an early morning soak in 103-degree water, feeling the fall chill in the air, when a bug of some kind landed in the spa. As I watched closely, he struggled between life and death in the hot water and I wondered if he was more frightened by the hot water or the fact that he was drowning.... Suddenly I had what most of you would call an "epiphany" and I knew clearly what I had to do........

STOP DRINKING BEFORE NOON! ;D 8)

Now let's not get radical!  ;D ;D

 

[windsurfdog]

Ah. OK, with those parameters set, then my answer becomes much easier to arrive at: I don't know.

But I will look into it. I may even ask one of my brainy kids - and get back to you. This is a very good question, and very apropos to this forum!

Thanks, Chas...Let me add another scenario by changing just 1 parameter--what would the differences be if we were to take the same parameters but put in an ambient temp of, say, 45 degrees as opposed to 70?
8)

 

[salesdvl]

I didn't do well with word problems in school.  Shouldn't one of these scenarios involve a train going east??

 

[windsurfdog]

I didn't do well with word problems in school.  Shouldn't one of these scenarios involve a train going east??

LOL!!
Kinda like, "Is it further to Chicago than by bus?"

 

[salesdvl]

I dunno, I got that one wrong.    ???

;D

 

[Vinny]

Windsurfdog,

If you want an exact answer then it's going to take some time to research. Those books are long gone from my possession - replaced by hot tub brochures.

    ASSUMING ALL THINGS ARE EQUAL IN THE TUB

For heat loss > it's generally an equation of temp difference and insulation factor (I believe that someone posted the formula on this site a while back). All things being equal the closer the two temps are the slower the heat loss.

For heating water > it's generally an equation of (temp desired - actual temp ) x kw/ degree (F or C). Example: if the water temp is 50 F and you want 97 F and ASSUME each degree of temp rise takes 13kw then (97-50) = 47 x 13kw = 611kw, using the same info except the desired water temp is now 104 (104 - 50) = 54 x 13 = 702 kw.

Now once the water gets to the desired temp,  the other equation takes place to keep the water warm. Actually the heat loss calculation comes in while heating the water and as the water's temp moves further away from the outside air temp the tub loses more heat (I believe Chas mentioned that before)

These are over simplifications and the actual calculations are probably more complex (I forget what they are).

Windsurfdog, if you really want the calculations, I will try to come up with them but and this is a BIG BUT - YOU will start turning into a Engineering Techno GEEK and won't be happy with the simpler pleasures in life. We are an elite group of people that will ask questions like "How do you really know that your pump is putting out the correct HP" and "how many lumens does that 7 LED light put out vs a halogen bulb vs the 22 LED bulb".  You will buy a UV light meter so that you know how much output your ozonator bulb is producing. Friends might shun you but you'll have a wealth of useless knowledge! ???  ;D

Please take my advice ponder on more important things in life - Beer or Wine in the tub!
Anyway, thanks for the good word to the spatopian goddess!

 

[Tman122]

Windsurfdog,

If you want an exact answer then it's going to take some time to research. Those books are long gone from my possession - replaced by hot tub brochures.

    ASSUMING ALL THINGS ARE EQUAL IN THE TUB

For heat loss > it's generally an equation of temp difference and insulation factor (I believe that someone posted the formula on this site a while back). All things being equal the closer the two temps are the slower the heat loss.

For heating water > it's generally an equation of (temp desired - actual temp ) x kw/ degree (F or C). Example: if the water temp is 50 F and you want 97 F and ASSUME each degree of temp rise takes 13kw then (97-50) = 47 x 13kw = 611kw, using the same info except the desired water temp is now 104 (104 - 50) = 54 x 13 = 702 kw.

Now once the water gets to the desired temp,  the other equation takes place to keep the water warm. Actually the heat loss calculation comes in while heating the water and as the water's temp moves further away from the outside air temp the tub loses more heat (I believe Chas mentioned that before)

These are over simplifications and the actual calculations are probably more complex (I forget what they are).

Windsurfdog, if you really want the calculations, I will try to come up with them but and this is a BIG BUT - YOU will start turning into a Engineering Techno GEEK and won't be happy with the simpler pleasures in life. We are an elite group of people that will ask questions like "How do you really know that your pump is putting out the correct HP" and "how many lumens does that 7 LED light put out vs a halogen bulb vs the 22 LED bulb".  You will buy a UV light meter so that you know how much output your ozonator bulb is producing. Friends might shun you but you'll have a wealth of useless knowledge! ???  ;D

Please take my advice ponder on more important things in life - Beer or Wine in the tub!
Anyway, thanks for the good word to the spatopian goddess!

Which means, to answer your question, the difference will be so little if I were to throw a penny off the bridge every day I go for a walk 3-4 times a week, oh wait, I do do this, then we would have the difference in operating cost.

To much time on my hands, to, to, to,to. Remember the song?

 

[Chas]

I'm still working on a simple equation to show the rate of heat loss off still water with a different delta t. But in the meantime, a thought struck me. Don't worry, it happens several times a year and I wasn't hurt this time....

A practical way of looking at this would be to do a simple calc of the cost of running the heater. For the sake of this flimsy example, let's use a one-hour soak.

If you have a 5000 watt heater, it will cost you a little over 5KWH (kilowatthours) to run for one hour. If you are paying $.25 per KWH, then your heater will cost $1.25 to run for an hour. I think a Quarter is a high rate, most non-California residents pay less.

Just for fun, imagine a spa with very low temp difference between water and ambient air. The heater may run for as little as ten minutes out of the hour.

That would be 1.25 / 6 or $.21 to run the heater for that one-hour soak.

So, the higher the temp differential between water and air, the longer the heater will run. Keep in mind that in extremely low air temp situations, the heater may run for more than an hour for a one-hour soak - taking a few minutes AFTER the lid is closed to get back to the heat setting - but I think that would only happen in extremely cold climates, and nobody with any sense would live there.  

 

[HotTubMan]

I'm still working on a simple equation to show the rate of heat loss off still water with a different delta t. But in the meantime, a thought struck me. Don't worry, it happens several times a year and I wasn't hurt this time....

A practical way of looking at this would be to do a simple calc of the cost of running the heater. For the sake of this flimsy example, let's use a one-hour soak.

If you have a 5000 watt heater, it will cost you a little over 5KWH (kilowatthours) to run for one hour. If you are paying $.25 per KWH, then your heater will cost $1.25 to run for an hour. I think a Quarter is a high rate, most non-California residents pay less.

Just for fun, imagine a spa with very low temp difference between water and ambient air. The heater may run for as little as ten minutes out of the hour.

That would be 1.25 / 6 or $.21 to run the heater for that one-hour soak.

So, the higher the temp differential between water and air, the longer the heater will run. Keep in mind that in extremely low air temp situations, the heater may run for more than an hour for a one-hour soak - taking a few minutes AFTER the lid is closed to get back to the heat setting - but I think that would only happen in extremely cold climates, and nobody with any sense would live there.  

A cheap shot at your Canadian friends, eh!?

At $.25/KW hour USD, you'd have to be crazy to live where you do! I pay $.085 CDN 8)

 

[windsurfdog]

A huge thanks from both the goddess and myself for squeezing your brains and wringing out such knowledge.  I agree with Vinny--I certainly don't want to add "Engineering Techno Geek" to my other claims of geekdom but I just wanted to very loosely quantify my thoughts re: all parameters involved in explaining to someone, especially those who are exploring their first tub purchase, what factors are involved when considering tub energy usage.  It sounds as if the concensus is that keeping one's tub at a higher temp does indeed use more energy but not at a great differential from lower tub temps.  I would guess most keep their tubs anywhere from 95-104 degrees.  Using the thoughts we've discussed, would it be fair to guess that keeping one's tub at 104 would be < $10/month more than keeping it at 97 or $10-$20 more or even more?  From the consensus, it sounds like the first option to me.

Certainly, the other 2 main factors (are there more?) of tub design/construction and usage trends would have a much greater effect but I believe temp setting would have some effect, allbeit to a lesser degree.

Thanks again for the gray matter!
8)

 

[Chas]

At $.25/KW hour USD, you'd have to be crazy to live where you do! I pay $.085 CDN 8)

The day my power bill arrives each month, I would agree!

 

[salesdvl]

I still say there should have been a moving train in the formula to make it more challenging.