Hot Tub Forum
Original => Hot Tub Forum => Topic started by: Steve on March 21, 2005, 11:29:29 am
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As the debate on covers and direct relation to efficiency continues on other threads, I wanted to pose a question.
How thick does a cover need to be to be considered energy efficient?
It was suggested that because some manufacturers use thicker covers, that the possibility of a misleading study could result.
Here's my experience in the climate of northern Alberta.
On my previous spa, I had a tapered 3-2" cover. Thin by most standards. That said, at -35 and snow on the cover, at no point did I see melting or any indication of heat loss. Was I missing something? Can there be no melting or steam escaping from the perimeter and the cover still be sub standard? ???
If someone with far more knowledge than I on this subject could explain this to me I would be grateful.
The obvious and major drawback is that I couldn't invite my friends and family over for a party and have all of us do'n the jig on the cover itself. It was a real issue on a number of occasions but we managed to find room on the deck for that sort of behavior eventually. ;)
Steve
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Well I know that my cover seldom sees frost, let alone snow. But when it does get covered in frost, it will stay that way until the sun comes up and melts the stuff off.
So that tells me it is doing a good job of keeping the heat inside the spa.
3 1/2" to 2" standard HotSpring cover.
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We got a lot of snow and cold weather this winter here in Ontario.
Our spa has a standard 4" - 2" cover. Standard on Dimension One Spas
Did not see any snow melting, not even at the hinge part.
I would consider it energy efficient.
So, your question is and interesting one, Steve.
Are thicker covers just over kill? or...?
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Most people forget the seem that runs down the middle. Most spa covers have no insulation in this part. Some manufactures have added an extra strip of foam down the middle of the seam. I am not sure how much this helps, but for you folks that see the snow when it does the only place on your cover that does not have snow is the middle seem so heat is getting out. Every little bit helps.
Gary
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Heat transfer is the relation of R value and thickness to the difference in temps.
A R value of 1 per inch with a 1/2" thick cover won't insulate too well. Where a R value of 5 per inch with a 3 inch cover will insulate much better.
The higher the R value and thicker the cover, the more it insulates up to a certain point. After that point the law of dimishing returns applies and yes a higher R value and or a thicker cover will insulate better but is it worth it. So if a 70 degree difference (see below) needs a total of R 18 to be the most efficient, going to R 20 will work but not as good as going form R 16 to R 18.
Also, keep in mind that since your talking about a lot of insulation your not going to have snow melt since all the temp difference is in the cover and not at the surface. The snow is at 32 F and the spa is at 102 F so the cover will transition the 70 degree difference in the cover, the cover will be cold to the touch outside and warm to the touch spa side. This is the major problem with house insulation and moisture problems - the insulation gets wet from the inside out during winter season and high humidity.
Actually in FF spas the cover is the weakest link (THIS IS NOT A FF VS THERMOPANE DEBATE) in the insulation IF the amount of insulation in a cover isn't at the point of dimishing returns.
Vinny
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If your spa cover is melting off snow you had better look into a new one, the real test is how the snow comes off. Since I service all types of tubs I see alot of different brands. When you clear the snow off most covers with snow on the top there is not ice formed under the snow on the cover. Other covers will have ice around the edge of the tub, but not on the area covering the water. A cover that is retaining heat really well, will have ice formed over the whole surface of the cover under the snow.
( I don't mean a sheet of ice, but that the snow is frozen or sticking to the cover, it doesn't just come off clean from the surface)
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A cover that is retaining heat will have ice formed over the whole surface of the cover under the snow.
Really? Can you further explain this? Thx,
Steve
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What do you want explained. Heat loss through the cover will keep ice from attaching to the cover under snow.
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What do you want explained. Heat loss through the cover will keep ice from attaching to the cover under snow.
Forgive me If I am mis reading something but that seems to contradict what you said.... that a cover that retains should have no ice under the snow...
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I just read my post, a cover that is retaining more heat will have ice attached to the cover. I don't know what is so confusing about.
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Let me explain the confusion Stabby.
The development of ice from snow suggests melting and refreezing. Melting is associated with heat loss through the surface of the cover due to the transfer of heat. Heat loss is associated to lack of efficiency. Seemed like a simple question to me? ???
I might add that you are the first person in the many years I've been doing this to suggest that ice on a cover is a sign of efficiency. That is the reason I was looking for an explanation as your post doesn't seem to make sense to more than just I obviously. ???
Steve
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I can't even believe someone with the knowledge of the industry like you Steve, needs this explained.
Put a spa cover on the ground, and one on a spa during the next snow storm, wait a few days before you clear the covers off. You will see what I mean.
On a surface that does not have heat coming up through it, (parking lots, roads, etc) ice or snow will freeze to that surface. Example, When you plow or shovel a driveway that has had snow setting on it for a couple days the snow is frozen to the concrete, you have to scrape the bottom layer or put ice melt down to get the drive perfectly clear (down to the concrete).
Have you ever seen a heated driveway, They have heat coming up through the concrete much like heat coming up through a spa cover.) a heavy snow fall will cover the drive, but when you shovel it the snow is not frozen to the surface, it comes off clean.
The steady heat coming up through the cover is not enough to melt off large amounts of snow but keeps it from freezing to the surface.
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I think I see what the stabbed one (or is it sta bone? :))is saying....Seems he has experienced ice under snow on covers with 2 different scenarios....ice that is only around the edges of the tub where there is no warm water underneath and those that have a sheet of ice over the entire cover, underneath the snow. If he has indeed experienced this (I don't doubt his word, but I live in Florida and it's hard for me to see for myself, thank goodness ;)) then his rationale makes sense--ice will form over the warm water area of the cover and be of the same thickness as the area of the cover that is not directly over warm water for a cover that has better insulation....and, consequently, ice may not form over the well heated areas but may form over the edges of a cover that is not as efficient. Apparently, the ice forming is independent of heat being transferred through the cover in his scenario. Just an observation from a snowless party....... 8)
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The edges of a spa cover go out past the water, and over hang the shell. These are the areas you will usually find ice on the cover surface. I have broomed spa covers off, that the section over the water comes off perfectly clean, yet the part of the cover from where the shell starts to the end of the cover I have to scrape the snow or ice off because it is frozen to the cover surface.
There is little or no heat coming up through the portions of the cover that are not over the water. Thats why snow and ice stick to the surface on the outsides of covers.
A spa cover that snow and ice are sticking to over the whole cover is retaining more heat than one that the snow just comes off clean. The amount of heat coming up through the cover is a constant, spa covers have moisture on the surface from the snow, once the snow starts accumulating on the cover, the moisture trapped under the snow will freeze and stick to a surface that does not have heat coming up through.
Heat coming up through the cover keeps the top surface of the spa cover to warm for ice or snow to stick. Which is a sign of heat loss
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Let me explain the confusion Stabby.
The development of ice from snow suggests melting and refreezing. Melting is associated with heat loss through the surface of the cover due to the transfer of heat. Heat loss is associated to lack of efficiency. Seemed like a simple question to me? ???
I might add that you are the first person in the many years I've been doing this to suggest that ice on a cover is a sign of efficiency. That is the reason I was looking for an explanation as your post doesn't seem to make sense to more than just I obviously. ???
Steve
Sorry Steve, I did not make myself clear. What I mean is not the formation of ice, but whether the ice and snow are sticking or frozen to the cover surface.
A less effecient cover can cause melting from the cover surface and the snow on top is causing freezing, in that siuation the ice would not be frozen to the cover surface, it would just slide off the top of the cover. My point is a cover retaining heat will have ice and snow sticking to the surface.
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This is a confusing issue! Not sure if this is a good comparision, but I do know that when I get ice or snow on my windshield at night, when the interior of the car is not heated, there is little or no thermal insulation from the glass. Nothing to insulate! Sometimes the ice or snow is frozen to the windshield and sometimes it is not, but when the car is running and the heater is on, the ice and snow that may be stuck to the glass start melting from the glass side of the windshield and the ice and snow start sliding off the windshield.
With that said, I would think that a non-insulating barrier, in this case the glass, with heat on one side, causes the ice and snow to NOT stick to the other side of the barrier. BUT...that's where the ice or snow is already stuck to the windshield!
All barriers are going to have some residual heat because it's mass. That doesn't necessarily mean that the heat is from a source on the other side of the barrier. Could that residual heat, if above 32F, cause the initial snow to melt and then refreeze as the ambient temperature drops? I think that's what happens to driveways when ice forms from snowfall. Same with windshields. If the barrier is more than the freezing point, the moisture on the barrier becomes liquid. When it gets less than the freezing point, the moisture will freeze.
It seems to me, if the logic is valid, if frozen moisture, (snow or ice), is not liquid on the cover surface, the insulative properties are substantial enough to keep the outside of the barrier less than the freezing point! If ice is frozen to the cover, I think there is minimal heat getting all the way through the cover or else that heat would keep the ice from freezing to the cover. If the outside of the cover is below freezing when the snow starts, the snow should slide right off as there is no heat to melt the snow and, thus, no liquid moisture to freeze and adhere to the cover! This what happens when it starts snowing on a windshield that is already below freezing. The snow slides right off when you turn on the wipers or brush it off. It's not heat coming through the glass that keeps it from sticking, it's the lack of a moisture bond, (liquid freezing), to the glass.
Hope that makes sense. Remember, too, that snow is a very good insulator!
Just my two cents worth of logic 8)
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Steve this is a great question. In addition to thickness there is also a specification for foam density usually expressed as pounds per square foot in USA and probably kilograms per square meter in the civilized world. ;-) The higher the density presumably, the more insulative the cover.
In terms of overall effectiveness, too, it is my understanding that the seal around the perimeter is extremely important. So with that said here would be my criteria for selecting a cover in any cold climate: Thicker is better, higher density is better, and the seal of the cover skirt to the tub wall ideally should be very tight with no air leaks. I am sure there is some point of diminishing return in each of these points but suspect that the limit is set by what is commercially available.
For a point of reference, HotSpring uses a tapered foam cover 3.5-to 2.0 inches and the density is specified as 2-lbs per square foot. The seal around the perimeter is good, but there are minor gaps.
Regards,
Bill
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Thank you that was very well put, the outside cover surface maybe warmer than the freezing point due to the temperature, sun during the day, etc before or at the time snow begins to fall causing the snow to melt as it hits the cover surface. After snow accumulates on the cover and temperatures remain below freezing for long periods of time a cover that is losing little to no heat through it, will allow the top of the cover surface to fall below the freezing point (snow or ice firmly attached to the cover)
In the same situation a cover lossing more heat keeps the top surface above the freezing point, not allowing the snow and Ice to attach itself to the cover surface. In this situation when you would broom the snow and Ice off the cover it would come off clean.
There are alot of variables in this discussion, temperature, wind chill, period of time temps below freezing.
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I'll have to go back and search, but I could of swore Stabone was looking for a spa about 6-8 months ago and there was a debate regarding why he wanted to purchase a particular brand. But at that time you were a shopper and now you say your a service guy for many brands? I'm confused. Let me search.
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Here's my experience in the climate of northern Alberta.
On my previous spa, I had a tapered 3-2" cover. Thin by most standards. That said, at -35 and snow on the cover, at no point did I see melting or any indication of heat loss. Was I missing something? Can there be no melting or steam escaping from the perimeter and the cover still be sub standard? ???
yep yer missing something... snow is an increadible insulator - the Eskimos learned that quickly... now just how much R factor the snow adds is the question.