Closed cell foam spa blanket

[Drew80]

I have a 7x11 1,250-gallon in-ground spa, and it takes FOREVER to warm up. I've read on some other forums about people who have had closed-cell foam blankets custom made that float on top of the water and add insulation. That would theoretically save gas and, more importantly, reduce the time to heat up my spa

I'm not so worried about the gas (it's cheap in Texas) but the warmup time completely eliminates any spontaneity with the spa. I have to plan ahead by about two hours, and that kinda defeats the purpose of having a hot tub.

Anyone have experience with these? Recommendations for where to buy one?

 

[mknauss]

You can use a normal solar blanket, but, I am not sure it will do anything as your spa water is being exchanged with your pool water when you circulate your pool to keep it clean and chlorinated.

 

[Drew80]

I set my pool and spa to circulate separately in the winter for this reason, and I have a solar blanket but it doesn't seem to do much. I was thinking that closed cell foam pads would have a higher R-value, so they might do a better job

 

[mknauss]

Might.

If you circulate the spa water at all without the heater running you will lower the water temperature quickly.

 

[Drew80]

Hmm. With the blanket over it, is it ok to not circulate the spa water? The chlorine shouldn't evaporate off since the blanket will keep it in

 

[mknauss]

Chlorine does not evaporate. It is consumed chemically by either organics or UV.

You do need to circulate some each day. But when you do, if you want to keep it warmer, you need to have the heater on when you are circulating. How are chlorinating it during the winter?

 

[Drew80]

I have a salt generator, so it just chlorinates when I circulate

 

[mknauss]

I saw that. But that is dependent on the water being warm enough for it to operate.

 

[Drew80]

True. I don't have an automated backup method. I have bleach if I need it, but I've only ever used it for shocking

 

[jseyfert3]

A blanket will help reduce energy to keep it hot, if kept above ambient, and will slow the rate it will cool off after use, but it won't speed up the heating process by much. The vast majority of the energy is required to heat water, which takes a lot of energy to heat. During that two hours you won't loose much (proportionally) because of a lack of cover.

You will loose some heat because of the lack of cover, but I'd be shocked if it reduced your warm up time by more than 15 minutes.

 

[jseyfert3]

I did a little math. A 1250 gallon spa, heated with a 350,000 BTU/hr heater at 83% efficency (290,500 BTU/hr output), with a desired temp of 104 °F, will take:

Starting Temp (°F)	Time to Reach 104 °F (hrs)
50	1.9
60	1.5
70	1.2
80	0.85
90	0.5

This does not include heat loss through pipes, walls of spa, or via air/evaporation. This is simply the time it will take your heater to heat that volume of water to 104 °F, based on the listed starting temps. You can never get faster than this due to the laws of physics. Your actual times will be longer than that listed in the table, as you will have heat loss in pipes, walls of the spa, and loss to the air via convection and evaporation. A blanket will help reduce loss to air via convection and evaporation, and help get you closer to the listed times, but it still won't be "spontaneous" with a 1250 gallon, in ground spa, unless you keep it hot all the time (expensive).

A 350 gallon, standalone spa, with that same heater, could heat from 70 °F to 104 °F in just a tad over 1/2 hour. Maintained at 80 °F, it would take a mere 15 minutes to heat up to 104 °F. They are much more suited to spontaneity, especially as they can be maintained at high temps with relatively low energy usage compared to big in-ground spas due to smaller surface area and lots of insulation.

 

[Drew80]

Wow, thanks for the info. Your times are about right. I've been using 1.5 hours as my go-to estimate lately