Cost to Heat Hot Tub Year Round - Builder Worry

[rockymountainhigh]

I am building a new custom hot tub and want to make sure what the builder is telling me is accurate. I greatly appreciate any help and I am willing to donate to the forum for assistance.

I live in Colorado and would like to be able to use the hot tub year round at 104 degrees. The average high in December is 40 degrees F and the low is 16 degrees F. The dimensions are roughly 16ftx28ft and 4ft depth, which I estimate at 13,500 gallons. The hot tub will have a good cover to retain heat.

The builder suggests that I use a propane heater (no natural gas available and the only other option is electric).

The builder states as long as the water is circulating, I can turn off the heat as the water won't freeze. So for cost efficiency, would I turn off the gas heater and 1 hour before using the tub be able to turn it on? Theoretically the water could be around 30 degrees F. Is it possible in the cold winter to bring the temperature back up to 104 degree F in a 13,500 gallon spa?

My final question.. what would be an estimate to use this hot tub, lets say I want to use it for 60 hours a month at 104 degree F and the rest of the time it is covered and not being heated. I am assuming I would need 4 therm per hour, and let's estimate the gas is $2 per therm, 60 hours x 4 therm per hour x $2 equals $480 per month. I know its probably more complex then this but just a rough estimate of my monthly heating costs.

Am I looking at this correctly? Is this possible or am I going in the wrong direction. So far the pad of the hot tub is all that has been laid. I really appreciate any help, I am a complete novice and just want to make sure this builder isn't selling me a pipe dream. $500 per month is a lot but that is something I would be able to afford for this luxury. I also have the option to install solar panels and use an electric heater instead if that is smarter. The propane has to be delivered as I live in a remote area.

 

[Ahultin]

Im going to go out on a limb and say those numbers are a pipe dream. One Btu is the amount of heat required to raise the temperature of 1 lb of water by 1°F.
13500 gallons x 8.3454 (lbs per gallon) = 112662 lbs of water
if water is 60 and you want to heat to 104 that is a 44 degree rise
44 x 112662 = 4,957,167 BTU's
1 gallon of propane is ~100,000 btu
4957167/100000 = 49.57 gallons of propane
in my area, 1 gallon of propane delivered is ~$4
49.57 x 4 = $198.28 to heat it ONCE
largest propane pool heater i have seen is 400,000 BTU which means it will take ~12 hrs to heat 13500 gallons from 60 to 104

For reference, our spa is ~900 gallons, I figure everytime we heat it is a min of ~$20 in propane

 

[DrewLG]

I am building a new custom hot tub .... The dimensions are roughly 16ftx28ft and 4ft depth, which I estimate at 13,500 gallons.

Wow, your hot tub is bigger than my pool.

Theoretically the water could be around 30 degrees F. Is it possible in the cold winter to bring the temperature back up to 104 degree F in a 13,500 gallon spa?

The biggest heater you're likely to find will have an input rating of 400,000 BTU/hour. Propane will produce 91500 BTUs per gallon, so that heater uses 4.4 gallons of propane per hour. At, say, $3 per gallon, it'll cost you $13/hour to run the heater.

Heaters are approximately 80% efficient, so that one will put out 320K BTUs per hour. One BTU will raise the temperature of one pound of water by one degree F. You have 13500 * 8.3 = 112050 pounds of water, so your heater will raise the hot tub's temperature 320000/112050 = 2.8 degrees F per hour.

At 2.8 degrees/hour, heating from 30F to 104F will take over 25 hours. At $13/hour, that will cost over $325.

what would be an estimate to use this hot tub, lets say I want to use it for 60 hours a month at 104 degree F and the rest of the time it is covered and not being heated. I am assuming I would need 4 therm per hour, and let's estimate the gas is $2 per therm, 60 hours x 4 therm per hour x $2 equals $480 per month. I know its probably more complex then this but just a rough estimate of my monthly heating costs.

Am I looking at this correctly?

No.

You want to use the hot tub 60 hours/month, which sounds to me like 2 hours/day. But you won't run the heater for just those two hours. Every day, you'll have to run the heater enough to keep the tub at 104F while it's uncovered and you're in it, and also enough to make up for the heat that the tub loses in the 22 hours that it's covered -- because tub covers help to reduce heat loss, but they don't eliminate loss completely, especially if the ambient temperature is 70 degrees below the water temperature, and also especially if the tub has an enormous 450 sq ft surface area. And speaking of which... What kind of cover is this, and how will you remove/replace it, and where will you keep it while it's off the tub? Insulated tub covers are heavy and awkward even when they're only covering 100 sq ft.

Anyway, can your pool builder give you an estimate of how far the tub's temperature would drop in one day (with the pump on but the heater off) if you started with the water at 104F with an ambient temperature of 30F, then left it uncovered for two hours, then covered it for 22 hours?

Let's say the temperature drops 20 degrees under those conditions. To compensate for that loss, you'd need to run the heater for seven hours every day. Seven hours at $13/hour is $91/day or $2730/month.

Is this possible or am I going in the wrong direction. .... $500 per month is a lot but that is something I would be able to afford for this luxury.

I made a lot of assumptions and guesses above. But if those numbers are approximately correct, you could get down to $500/month by cutting the tub size from 13,500 gallons to 2500 gallons.

I also have the option to install solar panels and use an electric heater instead if that is smarter. The propane has to be delivered as I live in a remote area.

Electric heating is probably a non-starter. Electric heat pumps want to run in warm ambient air, and they want to maintain pool water at temperatures not much higher than ambient. Solar panels want low latitudes, long summer days, clear skies, and no snow/ice buildup. Your conditions and requirements are exactly opposite all of that.

 

[Bperry]

I am building a new custom hot tub and want to make sure what the builder is telling me is accurate. I greatly appreciate any help and I am willing to donate to the forum for assistance.

I live in Colorado and would like to be able to use the hot tub year round at 104 degrees. The average high in December is 40 degrees F and the low is 16 degrees F. The dimensions are roughly 16ftx28ft and 4ft depth, which I estimate at 13,500 gallons. The hot tub will have a good cover to retain heat.

The builder suggests that I use a propane heater (no natural gas available and the only other option is electric).

The builder states as long as the water is circulating, I can turn off the heat as the water won't freeze. So for cost efficiency, would I turn off the gas heater and 1 hour before using the tub be able to turn it on? Theoretically the water could be around 30 degrees F. Is it possible in the cold winter to bring the temperature back up to 104 degree F in a 13,500 gallon spa?

My final question.. what would be an estimate to use this hot tub, lets say I want to use it for 60 hours a month at 104 degree F and the rest of the time it is covered and not being heated. I am assuming I would need 4 therm per hour, and let's estimate the gas is $2 per therm, 60 hours x 4 therm per hour x $2 equals $480 per month. I know its probably more complex then this but just a rough estimate of my monthly heating costs.

Am I looking at this correctly? Is this possible or am I going in the wrong direction. So far the pad of the hot tub is all that has been laid. I really appreciate any help, I am a complete novice and just want to make sure this builder isn't selling me a pipe dream. $500 per month is a lot but that is something I would be able to afford for this luxury. I also have the option to install solar panels and use an electric heater instead if that is smarter. The propane has to be delivered as I live in a remote area.

Couple things. The water can certainly freeze if the temp is below 32F even if the water is moving. It’s just a matter of how long it takes to do so. One power outage can also stop water circulation.

Lots of heaters say that you should not use them when ambient temperatures are very cold, or at least the water temperature being very cold. I don’t know the details. Assuming you wont let the water get to 30F from 104F in 24 hours (because your heating it to stay above freezing or just that it can’t cool off that fast from the previous days use), you’ll certainly be heating for more than the 2 hours per day you expect to use the hot tub. So you need to factor that into your cost estimate.

 

[RDspaguy]

First spa, huh?
For starters, that's a pool, not a spa. Why so big? You might consider a smaller spa with a separate pool maintained at lower temps or even closed during winter.
You will not cover it with any seriously insulated cover that I know of, your only option will be a fabric autocover. Expect your heat loss to be significant. I'd at least double whatever estimates are made here.
Who's your builder? Has he tried to talk you out of this incredibly bad idea?

 

[ajw22]

Welcome to TFP, the voice of reason over builders who are willing to take your money for any project.

The biggest heater you're likely to find will have an input rating of 400,000 BTU/hour.

Hayward has 500,000 BTU H-Series heater...

Universal H-Series Propane Gas 500,000 BTU Low NOx

The Universal H-Series is an energy efficient, high performance, installation adaptable commitment to the pool service professional.

hayward.com

Raypak has commercial heaters up to 4,000,000 BTUs...

Hi Delta – Raypak

Raypak Pool & Spa, Residential and Commercial Hydronic Products -

www.raypak.com

XTherm – Raypak

Raypak Pool & Spa, Residential and Commercial Hydronic Products -

www.raypak.com

In addition to the issues discussed will be how well the heater handles corrosive condensation, how long the heater lasts, and how well the heater operates in freezing conditions unless the heater is installed in a heated enclosure.

Heater Condensation - Further Reading

www.troublefreepool.com

Overall I think the "large spa" design as described is impracticable..

 

[Pauls234]

You've got plenty of the math evidence above so I'll just add my anecdote that with my 400k BTU natural gas heater and 21k gallon pool I get about 2 degree increase per hour when it's 85 out.

 

[Mdragger88]

What you’re describing is a pool. Perhaps with many jets.
The cover will likely just be a normal automatic cover which might help retain some heat but I can’t see it being able to have an insulated cover like a normal spa would to really retain heat with those dimensions. No one would be able to move it.

 

[rockymountainhigh]

Just wanted to provide an update in case someone reads this in the future. Also - huge thank you to everyone who took the time to comment.

After reading the responses here, despite my pool builder continuing to claim this was possible, I decided to turn this project into a heated pool that I will most likely close in the winter. Next spring I plan to add a seperate spa that is large but much smaller in size then the original project (maybe 10x10).

Its sad there are companies claiming to build pools who have no idea what they are talking about. I ended up pouring the pad, running all the plumbing and lighting myself, and hiring a crew for the shotcrete because I didn't trust the builder after reading all your responses. Will add an autocover and the plaster in the spring. Thank you again.

 

[Mdragger88]

Don’t forget to post pics! We love a cool build

 

[trivetman]

I for one sincerely hope you win the powerball tomorrow and feel the financial freedom to call your builder back and give him the green light to go back to the original design with the 4M BTU Raypack to ensure you have an abundance of heating capacity.

It would be a hot tub to behold and may have by itself had a measurable impact on environmental change and ozone layer depletion.

 

[ColoradoPool2022]

I'm here in Denver and had a directionally similar conversation with my builder's sales guy. I have a pool/spa combo and I was told I could leave the pool open in the winter and allow the water to drop as low as 50 and then heat it up to 90 in a couple hours when I wanted to use it. That was a big selling point for us (and a clear indicator of my naivety). Then, after contract signed, pool built and the start-up crew shows up, I learn that I cannot/should not (??) run my heater in the winter if the water is below 70 degrees (I have the 400k BTU several others referenced above). Something about condensation collecting in the heater and possibly freezing. So in the end we've decided to close the pool portion in the winter (honestly, a much more sensible decision than wasting all that gas to use it maybe twice a month). Oh, and I also agree with the comment that we get about 2 degrees of temp increase per hour with the pool open- total of 11K gallons so not too different from your set up. Glad you aren't wasting all that gas!!

 

[DrewLG]

I learn that I cannot/should not (??) run my heater in the winter if the water is below 70 degrees (I have the 400k BTU several others referenced above). Something about condensation collecting in the heater and possibly freezing.

Not exactly true. The heater manuals don't express this very clearly, though, so the misunderstanding is common.

The actual rule is:

Whenever water is heated from a temperature below 68 degrees F, make sure that the heater operates continuously until the water temperature reaches at least 68 degrees F.​

This also means that you must not use the heater to maintain the water at a temperature below 68 degrees F -- for example, by keeping the water just above 32 degrees F in order to prevent freezing.