heating in DIY indoor hot tub


  #1  
Old 04-01-05, 03:47 PM
jazer80
Visiting Guest
Posts: n/a
heating in DIY indoor hot tub

i am a total newb and just had a few quick questions. recently came across the concept of (well, actually just thought of it, but knew i'd find people doing it online) homemade hot tubs, and wanted to know a couple things about heating.

this is for an indoor hot tub in california, that will be covered when not in use(to prevent humidity/smell in my home). no matter what i decide on for holding the water, i will have it surrounded by a thick wall on all sides, which will be concrete. this should be very, very insulated, unless i'm missing something. can i get away with a weaker heating unit, and just keep the tub heated constantly, without having crazy energy bills? my thought is that it won't take much to keep it hot since the heat won't escape except for when i'm in it.
 
  #2  
Old 04-01-05, 04:07 PM
S
Member
Join Date: Dec 2004
Location: So. Calif.
Posts: 540
Likes: 0
Received 0 Likes on 0 Posts
the rule of thumb is simple, too make heat, then keep it in, hot springs spa, and demention one spa, are two of the better spa manufacturers out there that completely insulate there spas with polyurethane foam, and use a 3" cover to hold the heat in, most use a 6 kw 220V heater [ 6000 watts ]now , however hot springs used to use a 1.5 kw 120v [ 1500 watts ] heater to keep the water hot 24 / 7 when you consider most hair dryers are 1200 to 1500 watts, your not talking about a lot of energy.

hope this helps

steve
 
  #3  
Old 04-01-05, 07:07 PM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 28,351
Received 1,629 Likes on 1,462 Posts
I have a Hot Springs and can say that the little 1'500 watt heater can do the job. My spa is about 300 gallons and it will bring the water temp up about 2 degrees every hour. With all the insulating I have not noticed the spa on my electric bill.

I have had my spas inside the house for the past 10 years (man I'm getting old) and will never have a spa outside. The cover does a very good job containing the moisture when the spa is not in use, but if you have a wild party and have the cover off with jets running all night you can put 20 gallons of water into the air; so either be ready for the humidity when you use the spa or don't throw wild parties.

You mentioned that your spa was to be surrounded by thick walls of concrete. I assume that you know that concrete is not a very good insulator, it's a good thermal mass (sucks up heat & cold and is slow to change it's temperature). Rigid foam insulation inside your concrete walls would help considerably. I did a quick Google search and found that the average concrete has an R value of only .09-.3 for each inch of thickness, where fiberglass insulation is about 4 and Phenolic foam is around 8 (higher number is better).
 
  #4  
Old 04-02-05, 06:07 PM
jazer80
Visiting Guest
Posts: n/a
thanks a lot for this info guys. i can't stress that enough. i'm only 21 right now, and go to university in MA. me and my gf are moving to cali next year, and are beginning to work things out for our place. i'm all about DIY stuff, only things we know we wanna do for sure are the hot tub and a projection movie theater (viewable from the hot tub). any links you guys could give me to reputable/ reasonably priced heaters mentioned would be a big help.

also about that insulation. thanks, that'll help. i just assumed, wrongly, that very thick cement would be good. i'll definitely incorporate some of that stuff. where would i get those insulators (particularly the phenolic foam), home depot?
 
  #5  
Old 04-02-05, 06:08 PM
jazer80
Visiting Guest
Posts: n/a
Originally Posted by Pilot Dane
I have a Hot Springs and can say that the little 1'500 watt heater can do the job. My spa is about 300 gallons and it will bring the water temp up about 2 degrees every hour. With all the insulating I have not noticed the spa on my electric bill.

I have had my spas inside the house for the past 10 years (man I'm getting old) and will never have a spa outside. The cover does a very good job containing the moisture when the spa is not in use, but if you have a wild party and have the cover off with jets running all night you can put 20 gallons of water into the air; so either be ready for the humidity when you use the spa or don't throw wild parties.

You mentioned that your spa was to be surrounded by thick walls of concrete. I assume that you know that concrete is not a very good insulator, it's a good thermal mass (sucks up heat & cold and is slow to change it's temperature). Rigid foam insulation inside your concrete walls would help considerably. I did a quick Google search and found that the average concrete has an R value of only .09-.3 for each inch of thickness, where fiberglass insulation is about 4 and Phenolic foam is around 8 (higher number is better).
you said your heater only raises 2degrees/hr, so does that mean that you wait a day before using it, or do you just leave it at usable temperature at all times?
 
  #6  
Old 04-03-05, 08:01 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 28,351
Received 1,629 Likes on 1,462 Posts
The spa stays at temperature all the time.

When you fist fill the spa is takes a day for the heater to get it up to temperature. Then it stays at that temperature until you drain and clean the spa 3 months later.

In my new house I have the spa's room plumbed so I can fill it with hot/warm water. That way I can use the spa the same day I fill it.
 
  #7  
Old 04-03-05, 04:40 PM
jazer80
Visiting Guest
Posts: n/a
let me just make sure we're on teh same page here. you're saying you keep a 300gallon spa at operating temp constantly on a 1500watt heater, and that you don't even notice it on your bill (is that because your bill is typically high, or because you have $$$$, or because it is insignificant, like less than 30$ or something. i know my father doesn't even read lots of bills so he would never notice it, even if it were something significant to me).
 
  #8  
Old 04-03-05, 05:47 PM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 28,351
Received 1,629 Likes on 1,462 Posts
Well, 1'500 watts is half (or less) of what an electric water heater uses. There is a hit on the power bill when you first heat the water but unlike your water heater at home, the spa heater only has to maintain the temperature. You are not constantly asking it to heat up cold water (showers, washing clothes & dishes...). The walls and floor of my spa are filled solid with foam insulation and the top is covered when not in use by a 3" thick foam insulating cover so once the water is heated it takes very little to keep it warm. By comparison, most home water heaters have 1" of fiberglass insulation and premium ones may have 2" of insulation, which is still not very much.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: