HUGE Hot Tub taking 1.5 hours to heat 20 degrees??


New member
Jun 10, 2016
Wilton, CA
We have a brand new in-ground pool and hot-tub. Big pool- 50x20, big spa, 10x9. We have a big heater too- 400 btu propane heater. However, even when the starting temp of the spa is 81 or so, it's taking over 1.5 hours to get up to 99-100 degrees! I think that's WAY too long. We worked with a "pool consultant" who was incredibly flaky and incompetent so I have a feeling he probably has never installed a spa this large. I'm wondering what kind of heater is used at resorts or public pools/spas? Do they use two heaters? I'd rather have gotten a more expensive/bigger heater that would heat it quicker (30 mins???) so we didn't have to wait HOURS (and I'm worried about winter now!) to enjoy our hot tub.

Any ideas or suggestions would be greatly appreciated. Or is it too late at this point to change/add equipment?

Thank you in advance for any help!! :)


Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
What is the volume of water in your spa?

VOL * 8.345pounds/gallon * 20degrees = BTU required / 320000 BTU/hr(assumed 80% efficiency) = hours to add 20 degrees not accounting for environmental losses

For example, if you have 2000 gallons using the above would estimate about 1 hour to add 20 degrees not accounting for environmental losses.


Well-known member
Jun 5, 2016
Martinez, CA
We have a very large spa and 400k heater. As you stated, it takes considerable time to get to temp. What has helped me is the iaqualink controller so I could turn on the spa remotely ahead of use on my phone.

as to adding an additional heater, you may have to replace the existing gas piping or used a divorced pressure regulator in order to provide enough gas to the burners.


LifeTime Supporter
Jun 6, 2013
I think a 400kbtu heater is as big as you can get for a residential pool/spa. I have a 900 gallon 9 foot octagon and it takes around 45 to 60 minutes to heat it from a 75 degree starting temp. Are you sure when it is heating that the spa is totally isolated so all of the heat is going to the spa?


Jun 14, 2016
Sarasota FL
Our spa takes over an hour in the summer and sometimes 2 or more hours in the winter. We use an app on the phone to turn it on because we usually decide to use the spa while we are out to dinner.

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