Heater for Spa

[DAB942]

I'm having the gas company remove the old propane heater, today. While it can heat the pool, based on valve position, I only need it for the spa. I don't need the horsepower for the whole pool. Could I use one designed for an above ground pool?

 

[mknauss]

You actually want a larger heater to heat the spa versus the pool. You want the spa to heat up quickly. Rarely do you decide 4 hours earlier that you want to use a spa.

 

[DAB942]

You actually want a larger heater to heat the spa versus the pool. You want the spa to heat up quickly. Rarely do you decide 4 hours earlier that you want to use a spa.

Are the inst-hots worth looking at?

 

[mknauss]

Heating is all about BTU. How much water does our spa hold? How much temperature rise are you typically needing? How fast do you want the water to be heated?

 

[DAB942]

Maybe 500 - that's certainly on the high side - gallons. Honestly, we've never used it much -- mostly kids hang out with the bubbles and generally don't ask for heat. I would imagine at the most a 25 degree rise would be fine - in 15 minutes-ish.

 

[mknauss]

500 gallons of water weighs 4200 pounds. A 25 degree rise will take 104,000 BTU's. A 400K BTU heater will take about 20 minutes. Only propane or natural gas will work.

 

[DAB942]

Thanks. I'll know what to shop for. It turns out the gas company only did part of what they were supposed to -- the were giving me an estimate on tank size and bury and run to the pergola for the grills. They did not disconnect the heater.

 

[JamesW]

I would consider a 200 to 250,000 btu/hr heater to reduce the output temperature.

For example, at 40 gpm for a 400,000 btu/hr heater results in a 16.8 degree temperature rise from inlet to outlet.

If the water going into the heater is 103 degrees, the water exiting the heater and coming out of the returns will be at 119.8 degrees.

Below is the flow in gpm (X-axis) vs. temperature rise (Y-axis).

You would need to get to 80 gpm to drop the temperature rise to 8.4 degrees.

A 200,000 btu/hr will have a temperature rise of 8.4 degrees at 40 gpm. 103 + 8.4 = 111.4 degrees coming out of the returns.

A 400,000 btu/hr heater will give you about 80 degrees per hour or about 1.3 degrees per minute. 25 degrees in 19 minutes.

A 200,000 btu/hr heater will give you about 40 degrees per hour or about 0.67 degrees per minute. 25 degrees in 38 minutes.

 

[jakeg]

I would consider a 200 to 250,000 btu/hr heater to reduce the output temperature.

Are you recommending this to prevent the chance of scalding from the returns? Or is it more of a concern for the plumbing? Don't most heaters max out at 104 degrees?

 

[JamesW]

Are you recommending this to prevent the chance of scalding from the returns?

Yes. There was a thread recently where the poster was saying that the water from the returns was too hot for the people in the spa.

Some references say that the human pain threshold is around 106-108° F.

I would try to limit the heater output temp to a maximum of 110 degrees.

104 is measured at the heater inlet, not the outlet.

The heater will not (should not) continue to heat if the water entering the heater hits 104 degrees.

 

[jakeg]

Very good to know - thank you!

I'm in a similar situation as the OP, but my Spa might be a little bigger ~600 gal, and I have been looking at a 300,000 btu Raypak. I was leaning toward something with slightly higher btu to 1. Heat the spa faster and 2. Be able to heat the pool if I ever wanted to. I didn't even think about the possibility of scalding - Spas are supposed to be relaxing and comfortable, not painful!

 

[DAB942]

I have an old Hayward 250, now. I think I'll replace it with the same. I appreciate the input.