Your opinion on this statement...

[Chas]

Taking the devils advocate...I would be a little more likely to agree.  Look at the way we build houses.  The attic is insulated and the walls are insulated...but not the basement floor.

No comparison. Houses are not filled with water, and the whole house is the living space, the walls are all we have to keep in the heat. By contrast, the spa is a container of water, and the air space, if there is one, is designed as part of the heat retention system or insulation scheme. My house has no insulation in the walls, and very little in the attic becuase I live in paradise. The house I'm building up in the mountains will have insulated floors. Your next statement;

Heat rises, otherwise hot air balloons would be less enjoyable.  IMO.

Is incorrect. Heat doesn't rise: it goes in any direction from hot to cold. Any direction. The reason a hot air balloon rises is because HOT AIR is less dense than cold air, and yes, HOT AIR will rise above cold air. But that is not synonymous with heat rising. In fact, if you looked at a hot air balloon with thermographic imaging equipment, you would see it 'bleeding' heat in all directions. And you would see huge convective currents moving about inside of the envelope.

A tub keeps its heat best if it keeps all forms of heat transfer to a minimum - in all directions. I will say that in my experience 4 to 6 inches of foam insulation is about all that is needed. Going beyond that will not get you a spa that will keep the heat in better. Insulating the bottom of a spa is important: if you don't believe that try to keep an in-ground tub warm for a week with the pumps/heat off and the lid on.

[stuart]

Houses are not filled with water, and the whole house is the living space,

unless you live in FL......

[Steve]

Ah, you guys are great! And may I add somewhat predictable too! (said respectfully)

The lovely Lori brings up a valid point. In attempting to recall his exact words, I changed that word a number of times prior to posting. I'm almost certain, but he might have said most as well. Let's go with that instead maybe...

As my friend Bill (careful, I'm filming you over your left shoulder as you're reading this ) mentioned, there is a reason. Now, I have to be careful as most know me as this FF advocate that excepts no other alternative but it opens up dialog if nothing else.

As the intelligent ZZ mentioned, heat does travel through acrylic ( and fiberglass as well) quite easily as it's a poor insulator in itself. What if there was a small area underneath the acrylic that utilized the heat transferred from the tub itself and back when the pumps were running? Pumps do generate about 120 degrees on high speed and would this area not help with proper heat induction?

I have always disagreed strongly with companies like Arctic that want to take a design and try to heat a very large area with pumps that run usually 4 hours a day generating 112 degrees on low speed. It's far too large an area to trap heat effectively in cold climates. But what if the majority of the spa was foamed with this small area directly under the entire shell allowing the induction of heat? It makes great sense to me though I have nothing to back up any words of superiority at this point. I’ll letcha know if that changes!

Both acrylic and fiberglass allow the induction of heat effectively. This is an instance where allowing the transfer/induction of heat when the motors are running on a high speed and generating 120+ degrees, to help in the efficiency of the spa once the cover is off. (practicing my sales presentation on you guys!   )

Understanding that the majority of the heat is lost  in surface area once the cover (hat) is removed, this application in a small area under the acrylic is functional. It's also the area where the plumbing is for easy access and heat transfer. In the Hydropool spas, this design is standard and for one, I really like it. It's not a real stretch from Beachcomber with the slight variation of not allowing wasted heat when the pumps are running on high speed and the cover is off.

Getting back to my question though, I do believe that if you took the top 10 manufacturers in the spa industry and ran them all side by side, the operating costs won't vary more than a few bucks a month with the covers left on the entire time in the same conditions. I do believe that to be quite accurate and have always felt that way.

Just pondering benefits of alternate methods that we as salespeople are trained to disclaim without sometimes taking the time to consider. I'm as guilty as anyone in that regard though Arctic still sucks! lol...  

Steve      

[Vinny]

I think you have to make assumptions with Steve's statement. Two assumptions that should be made IMO are: 1) your talking about tubs of the same quality and 2) the tubs' sides are insulated to some extent.

As Bobb stated there are laws in thermodynamics (unfortunately I didn't study it so I can't elaborate) and all things must follow these laws just as they do in physics (I did study this).

Heat does radiate (radiant heat) in all directions but hot water does rise as hot water is lighter than cold water (oceans and lakes) and some 2 story houses are warmer on the 2nd story than the 1st, this property of water and air is called stratification.

As huh? stated we insulate attics and walls. The recommendation has always been to add insulation in the attic to slow the flow of heat upward.

Also, there is a law of diminishing returns when it comes to insulation. A blanket statement that if Y" of insulation is good then 2 x Y is better is wrong. Without knowing at what point the insulation isn't being efficiently used could end up wasting more money insulating than saved in heat retention.

So after all that said I guess my answer to that comment would be - Yes, this is a fair statement.

[Vinny]

Steve,

I thought I knew where you were going when you started this but now you brought in the top 10 manufactures.

Please stick to one debate at a time!  

Vinny

[Steve]

Rule #1 Vinny... Never follow me cause one never knows where I'm going...

Steve

[Shut_Down_Stranger]

I can answer this question, hey that degree in chemical engineering comes in handy once in a while.

heat loss is calculated using the following equation

q= K(eff) A dT

q= heat loss
K(eff) = Thermal Conductivity
A = surface area
dT = Temperature differential

simple enough. The trick of minimizing heat loss is to minimize the K(eff) or the surface area.  Temperature is generaly beyond our control, unless the tub is in a house.

1) all materials have thermal conductivity (TC) coefficients. If the hot tub was made of aluminium (very high TC) for example there would be very little to restrict the flow from hot to cold. The comment about the shell being a realtively poor insulator (e.g. high TC) is true, relatively.  

2) most insulation materials attempt to trap air which has a very low TC coefficient. This is called for lack of a better word creating "dead-air".  Thus increasing the thickness of foam or using foam made from a material that has a low thermal conductivity coeffiecient is important.  As anyone with an attic knows, increasing the thickness of the insulation decreases the Thermal Conductivity.

3) in order to calculate the effective Thermal Conductivity it is related to the inverse sum of the indivdual layers

1/Keff = 1/K1 + 1/K2 + 1/K3 +++ 1/Kn

where K1 and K2 represent the thermal conductivity of each layer of insulation.

if i remember my thermo correctly.

1/Keff is basically the R value of insulation.

4) so the base of the tub gets insulated but is is also the space to the outside and the wood or plastic that is in part of the tub, and the base.

5) A hot tub that is molded with minimal nooks and crannies is therotiacally a bit better as the surface area is minimized when the insulation is applied.

6) The top is another consideration. Although we have an air pocket above the water this air is not truly "dead air" so its insulation effects are less. The cover and thickness of the insulation are what minimzes heat-loss.

7) so the key to minimizing heat loss is placement of the tub in a wind shielded area, and minimizing the losses from the bottom. So there will be a difference if you locate the tub on a deck or ground.

that stated, I need to determine the allowable heat loss from a can of beer using a foam insulator.

     

[r100rs]

God I hated math!!!  

I have enough problems balancing my checkbook.  Maybe because I don't have any money in it.  Somedays I can't afford to pay attention.  

When I get my tub, I just want it to work. Period

r100rs

[Steve]

When I get my tub, I just want it to work. Period

r100rs

You are 100% correct  r100rs and my post wasn't meant to cloud the real benefits of ownership, but rather open up a debate for those of us hardheads who take it to a level most don't care the least bit about (nor need to)!

Steve

[UnderTheStars]

Well, FWIW I was doing ok with this thread, (enjoyed the hot air ballon ride - nice explanation Chas!) until Shut_Down told me to calculate heat loss thus:  q= K(eff) A dT.

After I thought about it a while, yeah I can follow that.  But the next step was a little tougher ("the effective thermal conductivity is related to the inverse sum of the indivdual layers thus:  1/Keff = 1/K1 + 1/K2 + 1/K3 +++ 1/Kn.")   Now that made my eyes kinda glaze over and they still aren't focusing quite right.

But you landed me safely again with ". . .to determine the allowable heat loss from a can of beer using a foam insulator.")  Now THAT I can work with and I've decided to design & market a beer cozie using the thermo-pane design.  I figure I'll capture the "other half" of the can cozie market.  (and I think that's pretty broad minded of me considering I've always been a full-foam cozie kind of guy.)  

[Steve]

LMAO!!   Great post UTS! See...we can all change.

[Chas]

[Lori]

Rule #1 Vinny... Never follow me cause one never knows where I'm going...

Steve

That is why I replied to the statement, not the content, so to speak!!!

wmccall, I knew the other shoe would drop, but I figured I would bait Steve a little!!!  Hey, why not, he baited the other professionals on here!!!

LOL!!!  Gotta love ya, Steve!!!  

[doodoo]

Taking the devils advocate...I would be a little more likely to agree.  Look at the way we build houses.  The attic is insulated and the walls are insulated...but not the basement floor.  Heat rises, otherwise hot air balloons would be less enjoyable.  IMO.

Well speaking from years of experience within the housing industry. Chas said it correctly, warm (hot) air rises whereas heat goes from hot to cold.

In North America we have strong building science but terrible application of that science. We fill attics (typically R-40) and have OK walls (typically R-20) but nothing in the basement.  Over 20% of the heat loss occurs through the foundation. And so it is very important to have full height insulation and even more important to have underslab insulation.  The same principle applies to hot tubs. Although a cover is extremely important in that it provides a barrier to heat loss, insulation in a hot tub in the wall structures and underneath is equally important.  Thermodymanics 101

Now in re-reading Steve's original post ''all spas are comparitavely efficienct with the cover on'' I tend to dissagree. Because of the heat loss that can occur due to poor insulation in the walls or the underside of a hot tub.

But then again, we can all get in on the beer cozie thing and make millions.  UTS can we buy shares?

[marmac]

I am a neophyte spa'er, tubber' darn it, I don't even know what you call folks who like to hang out in hot tubs.

As a  high school science teacher, I am impressed with the professional analysis of thermal coeffiecients and more so with the discussion on air pockets.  I am waiting for the heat vector analysis and I'll be set.

I must say though as an avid home brewer.....that after all of this discussion,  I most strongly concur with those of you who just want to relax w/ a good beer or a great wine and a warm spot w/ a great view.

This is a great forum that has helped me a lot, I have learned a ton and most definately saved myself from hot tub hysteria brought on by the purchase of a poor system.  That being said, I am still looking for the best tub.....

Keep that info comin'
Mac