Insulate Your Intex Spas!

[jseyfert3]

So I got an Intex spa for me and my wife for Christmas. I had wanted to get some extruded polystyrene (XPS) foam board to go under the spa, but I ordered the spa
too late and was running out of time to get it set up before Christmas, so I set it up directly on the concrete floor of our garage with the meager "bubble wrap" ground cover they supply (it seriously is bubble wrap with a slightly tougher skin on one side). I figured it wasn't ideal because of the heat loss, but I went with it. I set up the spa the Friday before Christmas (Dec 22nd). The spa has a meager 1300 W heater, and it took two days to get up to 100 °F from ~60 °F starting temp, becoming extremely slow the last few degrees. Once there, it could hold it, but with the cover off and no bubbles it could not hold temp, and would drop ~4 °F within an hour, at roughly 50 °F ambient air temp. I figured I'd add insulation the first time I changed the water...

Then I got my power bill. I was somewhat shocked to see it had jumped from 411 kWh to 797 kWh from the Dec to Jan bill, and was $106. And the spa was only running two weeks on that bill. I decided I was going to add insulation now. Unfortunately that means I have to go through the initial water setup only 3 weeks after I did it the first time, but it should really help. As the water heater was reheating after I dumped it into the spa (natural gas is roughly 2.25x cheaper than electric), I did some rough calculations on heat loss:

Rough Calcs by jseyfert3, on Flickr

If you can't follow my scribbles, basically I calculated the heat loss from just the bottom of the spa was ~565 W when it was sitting on the concrete, and would be mere ~43 W with the 1.5" layer of foam I added. That's a savings of ~521 W, or 375 kWh/month, which at my $0.11/kWh is about $41/month savings, or just a little bit less than the cost of the foam. Now, there's a lot of assumptions and simplifications I did in these calculations, but it should be roughly in the neighborhood. If I could have found my wattmeter I could have gotten a much better before/after comparison, but unfortunately I couldn't find it.

Here is the spa refilling on top of the foam. I'll probably trim off some of the corners at some point.

Spa on Insulation by jseyfert3, on Flickr

Bottom line: If you have an inflatable spa, buy some XPS foam and put it under the spa right away. You'll get your money back in short order.

 

[kimkats]

Well done! Now lets see if your power bill proves your equations out!

I would think another benefit of the foam would be comfort as it would be softer.

Do you find yourself using the spa every night? What do you use for a cover on top of it?

Kim

 

[jseyfert3]

Next power bill is in early February. I'll update wether or not it worked.

It's actually not softer that I can tell. XPS isn't particularly soft, and there was that "bubble wrap" ground cloth thing underneath. Putting out outside on dirt would probably be a different matter cause it's not usually perfectly flat.

So the lid that comes with it is a very thick vinyl sheet that wraps over the edge and has buckles to snap into place and meet child safety requirements. To support this there is an inflatable ring about 4” thick that goes in the middle of it. I thought the entire cover was overkill for an inside pool, and found the inflatable support works just fine by itself, and that's all I use. By accident I have it slightly underinflated in the cold (50 °F) ambient. When I drop it on the water it heats up, expands, and seals pretty good on the walls of the tub. I may experiment sometime later with a foam board DIY cover or simply placing foam or something over that inflatable thing to provide extra insulation.


Generated by http://tools.rackonly.com

We were using the spa nearly every till my wife got sick around the 9th or 10th, and tonight is our first night back in now that the drain/foam/fill/balance is over. I'm posting this from my phone while we soak.

 

[jseyfert3]

While I don't have power data yet, it's certainly made a difference. Where before I mentioned that it was dropping roughly 3-4 °F per hour, uncovered, last time we used it it dropped 1 °F in an hour, at around the same ambient 50 °F. My guess is once the ambient gets up to 60 °F or so it'll be able to hold temp without dropping at all.

 

[jseyfert3]

Update: My latest bill, for the period 1/5 - 2/6, is 1065 kWh. December was 411 kWh, January was 797 kWh with the spa running two weeks, and I'm guessing the majority of the power jump was the spa since we have gas heat and gas clothes dryer. This latest bill should be the worst it gets, as it has 4 weeks of the spa, and the first 9 days of the current bill I still hadn't insulated under the spa. So I expect it to drop back down somewhat on next month's bill.

So, did the insulation and my equations work out? If we say the jump from Dec to January was indeed purely due to the spa, then the spa used 400 kWh in two weeks. Extrapolating to 4 weeks, we'd expect it to use 800 kWh. Actual use was somewhere around 650 kWh. That's a difference of about 150 kWh, I estimated with my equations I'd save 375 kWh. Since it was uninsulated for 1/3 the bill, 1/3 of 375 is 125. 125+150 = 275 kWh, which isn't 375 but it's not terribly far off for all the simplifications and extrapolations. Bottom line is that yes, the insulation helps out a lot! I really need to find my power meter so I can get real, concrete numbers instead of estimating.
.
Oh, and I don't know if I mentioned but we keep it at temp all the time. Unfortunately, because of the meager heater we can't turn it down between uses and crank it up an hour or two before use since best case you might get 1 °F per hour rise. We've been keeping it at around 99 °F since that's where we like it.

 

[kimkats]

NICE follow up!! Glad you did this for sure!!

Kim