What BTU heater recommendation for in ground spa only?

[Restomod]

I currently have a gas Hayward Pool and spa heater that is 20yrs old and is giving me maintenance required errors. I'm looking to replace it with a smaller heater for the spa only as we never heat the pool in Texas. What BTU should I be looking for?

Spa is roughly 825 gal.

Thanks

 

[mknauss]

How fast do you want to heat the spa in when the water temperature is 40F?

 

[Restomod]

What is the typical time,, say 1 hour?

I do not see us getting in when it's that cold out, more like later Summer /early fall in Dallas , so say 65 degrees air temp.

 

[mknauss]

OK -- so 825 gallons of water weighs 6200 pounds. Thus you need 6200 BTU to raise the water by 1degF.
To go from 65 to 105F is 40 deg. That means you need 250k BTU. At 80% efficiency it will take 75 minutes using a 250k BTU heater.

Pretty slow really. Best would be to get a 400k BTU heater for your spa.

 

[Restomod]

Excellent, thanks so much!

 

[1poolman1]

I currently have a gas Hayward Pool and spa heater that is 20yrs old and is giving me maintenance required errors. I'm looking to replace it with a smaller heater for the spa only as we never heat the pool in Texas. What BTU should I be looking for?

Spa is roughly 825 gal.

Thanks

When I was very new in the industry, I allowed two customers to talk me into putting in smaller heaters in an installation like yours. All to save a couple hundred dollars. I believe they both still hate me for it. Like them, you will regret going smaller if you have a 400k now. You're buying convenience with as large a heater as you can get. In the dead of winter, a 400k should heat your spa from 40 to 102 in about 15 -20 minutes, time it takes to get changed.