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Hot tub normal heat loss


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Heat loss has too many variables to pin down a "normal". Different brands have different levels of insulation, different volumes/surface area will have different loss, ambient air temp has a huge effect (you lose more heat the greater the difference between air temp and water temp), location (on a deck, windward side of house, shaded or sunny, etc...), even thickness and density of the cover as well as how tightly it fits and if it has hinge seals. 

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Ok that's cool understood.

How about I flip the question..

What are the recommended options to prevent heat loss in a tub that is not filled with insulation, is outdoors, has what I consider a good cover (unlatched though).

For 6 degree warm up, from my elecric meter readings I estimate around 15kWh, which equates to about £2.20 per day, so over four summer months, that equates to around £250 heating costs alone. Which is also about £60 per month.

Would bubble wrap on the water surface provide a cheap fix?

I'm considering putting a large square garden furniture cover over the hot tub and cover but that will be £25 minimum. I'm interested in Costs vs savings.

We must all experience heat loss to a certain degree so what's the consensus?

Thanks

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12 hours ago, Coolruler said:

What are the recommended options to prevent heat loss in a tub that is not filled with insulation, is outdoors, has what I consider a good cover (unlatched though).

First, inspect the condition of your cover - Is it cracked/broken or ill-fitting?  Heat rises, so having a properly fitting cover in good condition is the most important thing.   If you have or know someone with a FLiR thermal imaging camera - this is a perfect opportunity to use it, keep the cover on and look for where heat is escaping around the cover.

Next up, the best thing you can do to increase energy efficiency besides the cover is to add insulation (if there is space available for more).  Grab some black contractor grade trash bags and then stuff them with stone Rockwool insulation (wear respirator - you don't want to breath in stone fiber dust).  Next place these insulated bags CAREFULLY between the cabinet walls and shell - be sure to leave plenty of space around the equipment and electronics area so they can vent heat sufficiently as needed.

You still won't get as energy efficient as a flagship spa which uses full spray-foam to fill *every* void between the cabinet and the shell walls, but stuffing rockwool into contractor bags should substantially increase heat retention and thus energy efficiency.

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Ha !!!  In Arizona summer, we have heat gain, not loss.  When the kids were little, this was our "swimming pool" in summer, and I'd have to leave the cover off twice a week to get the water to cool down !!!

So, being in Arizona desert, my heat loss in winter is way less than folks in colder areas.  My heat thermostat rarely comes on even in winter, under 1 hour a day for sure.

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Remove side panels and put bags of fiberglass insulation inside. Do not insulate around equipment (pumps, controls).

Cut 1" foamboard insulation to fit in the framework behind the side panels.

Buy spray foam and make it a full foam spa. Should cost less than $2k.

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7 hours ago, Cusser said:

Ha !!!  In Arizona summer, we have heat gain, not loss.  When the kids were little, this was our "swimming pool" in summer, and I'd have to leave the cover off twice a week to get the water to cool down !!!

So, being in Arizona desert, my heat loss in winter is way less than folks in colder areas.  My heat thermostat rarely comes on even in winter, under 1 hour a day for sure.

Same here on Florida's space coast - I only use the heater for about six weeks.  Otherwise this time of the year my spa heats up to over 103F (too hot for summer time for me).  So I make it a routine to throw open the cover for an hour or two every night - the spa's circulation pump helps bleed off heat fast for me.

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6 hours ago, ratchett said:

Same here on Florida's space coast - I only use the heater for about six weeks.  Otherwise this time of the year my spa heats up to over 103F (too hot for summer time for me).  So I make it a routine to throw open the cover for an hour or two every night - the spa's circulation pump helps bleed off heat fast for me.

What?? I'm 2 to 3 hours north of you (Gainesville) and I can relate to the OP.  I'm a first-time hot tub owner (a hand-me-down) and I'm having trouble keeping up the temp.  I actually just bought a cheap submersion heater to assist (my spa doesn't have an actual heating element)...  

The temps in daytime are only getting up to 92°F or so.   How is yours so warm?  Maybe the heating element is the primary difference... 

I'm sitting here thinking that using it multiple times a week is going to be super expensive... I'm even contemplating solar heating to help it. 

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2 hours ago, Sandlotje said:

spa doesn't have an actual heating element)... 

Trying to heat a spa with residual heat from the pump motor is not effective, as you have seen.

2 hours ago, Sandlotje said:

bought a cheap submersion heater

Nooooooooo!!!! Danger, Will Robinson! Make sure, and I mean absolutely SURE, it is plugged in to a functional gfci. And don't let it near the spa shell.

Better yet, put in a control system with an actual, real life heater. Cheap ones can be found for $300 or so. 

Actually, start your own thread with pics of the equipment area. We'll get you squared away.

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12 hours ago, Sandlotje said:

What?? I'm 2 to 3 hours north of you (Gainesville) and I can relate to the OP.  I'm a first-time hot tub owner (a hand-me-down) and I'm having trouble keeping up the temp. 

Lol, well my three seater isn't exactly a budget grade spa - my Jetsetter has a multi-density spray foam insulation and a properly fitting cover to retain heat - so any heat generated is retained and not bled off.  Also my spa is out on the back patio in direct sunlight all day long so that helps the water heat up.  From May to October I set my water temp to 96F and my water temp will never drop below 98F unless I throw open the cover for half the night to let heat bleed off.

 

12 hours ago, Sandlotje said:

  I actually just bought a cheap submersion heater to assist (my spa doesn't have an actual heating element)...  

The temps in daytime are only getting up to 92°F or so.   How is yours so warm?  Maybe the heating element is the primary difference... 

Well my Jetsetter is wired for 110V so the heater cannot run concurrently with the jet pump (instead there's a low wattage circulation pump) - but my spa does have a true 1000watt heater.  With the full foam insulation, my spa heats up at 4 degrees per hour on a fresh fill (measured/tested/confirmed personally) .

Think of it like this - buy two bags of ice, put one in a nice Yeti cooler, and the other in a cardboard box.  Which one is going to melt first?   Same principal applies to hottubs.   

 

12 hours ago, Sandlotje said:

I'm sitting here thinking that using it multiple times a week is going to be super expensive... I'm even contemplating solar heating to help it. 

I've said before and I'll say it again - a well insulated energy efficient flagship spa from the 90's is going to be more energy than a brand new construction budget grade spa without adequate insulation.  

 

We use our spa nearly every evening and I've only seen around a $10 increase in monthly electrical bills since delivery.

20200814_084730.thumb.jpg.c9dde127b5ed26da98189b3fe74941bf.jpg

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On 5/20/2021 at 2:27 AM, RDspaguy said:

Trying to heat a spa with residual heat from the pump motor is not effective, as you have seen.

Nooooooooo!!!! Danger, Will Robinson! Make sure, and I mean absolutely SURE, it is plugged in to a functional gfci. And don't let it near the spa shell.

Better yet, put in a control system with an actual, real life heater. Cheap ones can be found for $300 or so. 

Actually, start your own thread with pics of the equipment area. We'll get you squared away.

Lol, I love your reaction, and I greatly appreciate it.  Mine also uses red friction "heaters," but is not very effective, either.  

I will get the submersion heater hooked up to a GFCI receptacle before I use it again, I promise. :) 

Regarding the heater being in close proximity to the shell, don't you worry!  That is a top concern of mine also.  So what I was doing is I was leaving the cover on the hot tub in a half-open/half-closed position, and suspending the heater in the center.  I do not trust it very much, and the "English" instructions only lowered my confidence.   

I don't intend on using this as a primary means to heat the hot tub, only something to use in a pinch (after I install the GFCI receptacle, which I've been meaning to swap out an exterior outlet anyway, and have the replacement ready to go).  

 I'm actually interested in doing a DIY solar water heater that captures the heat radiation of the sun and heats up black tubing (using perhaps a simple solar-powered tiny pump that runs when the sun is shining to pass the water through the tubing).  However, my bank account says I need to work more hours to pay my bills this month, so this is going to have to wait a while before I can commence on such a project (that may or may not be functional in the end).

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21 hours ago, ratchett said:

Lol, well my three seater isn't exactly a budget grade spa - my Jetsetter has a multi-density spray foam insulation and a properly fitting cover to retain heat - so any heat generated is retained and not bled off.  Also my spa is out on the back patio in direct sunlight all day long so that helps the water heat up.  From May to October I set my water temp to 96F and my water temp will never drop below 98F unless I throw open the cover for half the night to let heat bleed off.

 

Well my Jetsetter is wired for 110V so the heater cannot run concurrently with the jet pump (instead there's a low wattage circulation pump) - but my spa does have a true 1000watt heater.  With the full foam insulation, my spa heats up at 4 degrees per hour on a fresh fill (measured/tested/confirmed personally) .

Think of it like this - buy two bags of ice, put one in a nice Yeti cooler, and the other in a cardboard box.  Which one is going to melt first?   Same principal applies to hottubs.   

 

I've said before and I'll say it again - a well insulated energy efficient flagship spa from the 90's is going to be more energy than a brand new construction budget grade spa without adequate insulation.  

 

We use our spa nearly every evening and I've only seen around a $10 increase in monthly electrical bills since delivery.

20200814_084730.thumb.jpg.c9dde127b5ed26da98189b3fe74941bf.jpg

A cardboard box, eh?  I think you're onto something there regarding insulation, and heat retention.  I think the large concrete slab also helps significantly.  Mine is in a corner of the yard, and it is sitting on a section of deck that is only slightly wider than the hot tub itself and is in the shade mid-late afternoon; so your picture definitely helps show the major differences in setup.  

I'm extremely impressed that your hot tub only adds a ~$10 increase to your monthly bill.   I would've guessed it to be much higher.  I love your setup btw.  All of the blue lighting is of the same shade it appears.  And then you've got the Gator-themed sunset in the background to really make it pop! (just kidding!)  Thanks for sharing the setup.  

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4 hours ago, Sandlotje said:

uses red friction "heaters,

Not a real thing. You cannot create energy (heat) from nothing, it defies the laws of physics. If it is making heat it is also making your pump work harder by restricting flow. A pump running constantly will reach temperatures well over 100 under normal use, and will heat the water some by simple proximity.

4 hours ago, Sandlotje said:

actually interested in doing a DIY solar water heater that captures the heat radiation of the sun and heats up black tubing (using perhaps a simple solar-powered tiny pump that runs when the sun is shining to pass the water through the tubing). 

It would be cheaper to get a small inline heater or spa pack and install it in the spa. I get the DIY urge, but I guarantee it will cost much more and heat much less than just doing it right. 

Post some pics of the equipment area and existing circuit board/ control pack and we can offer some suggestions.

Dang @ratchett, that's a nice setup you've got there. 👍

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On 5/20/2021 at 12:19 PM, ratchett said:

Lol, well my three seater isn't exactly a budget grade spa - my Jetsetter has a multi-density spray foam insulation and a properly fitting cover to retain heat - so any heat generated is retained and not bled off.  Also my spa is out on the back patio in direct sunlight all day long so that helps the water heat up.  From May to October I set my water temp to 96F and my water temp will never drop below 98F unless I throw open the cover for half the night to let heat bleed off.

 

Well my Jetsetter is wired for 110V so the heater cannot run concurrently with the jet pump (instead there's a low wattage circulation pump) - but my spa does have a true 1000watt heater.  With the full foam insulation, my spa heats up at 4 degrees per hour on a fresh fill (measured/tested/confirmed personally) .

Think of it like this - buy two bags of ice, put one in a nice Yeti cooler, and the other in a cardboard box.  Which one is going to melt first?   Same principal applies to hottubs.   

 

I've said before and I'll say it again - a well insulated energy efficient flagship spa from the 90's is going to be more energy than a brand new construction budget grade spa without adequate insulation.  

 

We use our spa nearly every evening and I've only seen around a $10 increase in monthly electrical bills since delivery.

20200814_084730.thumb.jpg.c9dde127b5ed26da98189b3fe74941bf.jpg

Well, upon further investigation it appears you're $10 estimate might not be far off!  I discovered my local library has Kill A Watt devices available to borrow for one-week stretches (which was quite a surprise!).  Even it's only been plugged in for ~30 hours, the KAW is estimating monthly cost to be around $15.  

Obviously there are countless variables, but the net result looks to be way less than I would've guessed! 

I'll be posting my results at the end of the week somewhere around here.   It's at 5.00kWh so far after 32 hours.  

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