Size of heater???

[ddoggphx]

I am trying to get the right heater size for our 30x15 foot playpool with a 7x7 inset spa. The people who designed it say it needs 400KBTU but that seems excessive given what I've seen online.

What SHOULD I have, and links to purchase would be appreciated. I am doing a By Owner build.

 

[ajw22]

There is no substitute for BTUs.

The more BTUs the faster your spa or pool will heat.

If you don’t mind waiting any size will do.

 

[PoolBrews]

Get the biggest they offer.

 

[JoyfulNoise]

As @ajw22 alluded to, it all depends on how long you want to wait. Let’s say your spa is approx 1000 gallons of water. A 400kBTU/hr gas heater operating at peak efficiency will heat that water from 85F to 100F in roughly 20mins. Any heater smaller than that will take longer. You live in Phoenix and so you will rarely, if ever, use the heater to heat the pool. Maybe you might use it in the shoulder months to warm it up but definitely not in the summer months. So it’s only consistent use will be for the attached spa.

As with most every other piece of pool equipment - get the biggest one you can afford and do it now during the build when the incremental cost of upgrades is tiny compared to the overall budget.

 

[Bill1974]

Being that you are in Phoenix, would a heat pump be a better choice? Yes they are slower than a gas heater. But you have the option to cool the pool when it gets to hot, if you get a heat pump can can also cool. Generally a heat pump does cost less to operate, but you should check you utility rates to make sure. Also using a cover to hold the heat in and reduce evaporation will greatly save on the heating costs. Your really dry climate and bit of breeze will suck heat out of the pool because of evaporation.

 

[JamesW]

The problem with 400,000 btu/hr is that it will increase the temperature by about 17 degrees at 40 gpm, which means that at an input temperature of 103 degrees, the output is 120 degrees, which might be too hot for some people.

You can run the pump at a higher speed to get a lower temperature rise or you can go to a lower btu/hr heater.

A 250,000 btu/hr might be a better choice if the wait time for heat up is not a big problem.

 

[JoyfulNoise]

The problem with 400,000 btu/hr is that it will increase the temperature by about 17 degrees at 40 gpm, which means that at an input temperature of 103 degrees, the output is 120 degrees, which might be too hot for some people.

You can run the pump at a higher speed to get a lower temperature rise or you can go to a lower btu/hr heater.

A 250,000 btu/hr might be a better choice if the wait time for heat up is not a big problem.

I have a 400kBTU heater on a 600 gallon attached spa and this is never the case. When I set the spa temp to whatever I want it to be (90F, 98F, 104F), it gets there and the EasyTouch turns off the heater. Yes, the water coming out of the jet is "hot" but it's not any warmer than what one might get in a shower. The heater has a high limit switch anyway that cuts off at what, 140F?? That's about the same output temp as a residential water heater. Most people can stand that pretty easily for a short duration of time and, if the jet temperature feels too hot, a person can simply skooch over away from the jet and it will not hurt.

Being that you are in Phoenix, would a heat pump be a better choice? Yes they are slower than a gas heater. But you have the option to cool the pool when it gets to hot, if you get a heat pump can can also cool. Generally a heat pump does cost less to operate, but you should check you utility rates to make sure. Also using a cover to hold the heat in and reduce evaporation will greatly save on the heating costs. Your really dry climate and bit of breeze will suck heat out of the pool because of evaporation.

Heatpumps are terrible here in the desert southwest. The relative humidity can drop down easily into the teens here making a heat pump much less efficient. And since the majority of the use will be for the spa, it will take a lot longer for the spa to heat up with a heat pump. Electrical rates aren't as bad here as they are in the neighboring state of CA, and no one is as bad off as you Long Islanders with LIPA's history of electrical rate gouging (I grew up on LI ), but it ain't cheap here either. The most expensive energy tiers occur during the time-of-use scheme when everyone comes home - 3pm to 9pm. So, if you dare to run your AC or your expensive spa heater when any normal human being would want to use it, you'll be financially punished accordingly ...

 

[JamesW]

I have a 400kBTU heater on a 600 gallon attached spa and this is never the case.

It depends on the flow rate and to some degree what temperature you find to be too hot.

At 40 gpm, the temperature rise is 17 degrees.

At 80 gpm, the temperature rise is about 8.5 degrees from inlet to outlet.

If the water going into the heater is at 103 degrees, the output is 120 (at 40 gpm) or less depending on the gpm.

What is your flow rate?

I remember one thread where the person was complaining about the jets being too hot with a 400,000 btu/hr heater.

If someone wants to use the jets as therapy and the water is 120 degrees, then they might find it unpleasant.

In any case, I am not saying that a 400,000 btu/hr heater will not work, but the temperature rise should be considered.

 

[JoyfulNoise]

It depends on the flow rate and to some degree what temperature you find to be too hot.

At 40 gpm, the temperature rise is 17 degrees.

At 80 gpm, the temperature rise is about 8.5 degrees from inlet to outlet.

If the water going into the heater is at 103 degrees, the output is 120 (at 40 gpm) or less depending on the gpm.

What is your flow rate?

I remember one thread where the person was complaining about the jets being too hot with a 400,000 btu/hr heater.

If someone wants to use the jets as therapy and the water is 120 degrees, then they might find it unpleasant.

In any case, I am not saying that a 400,000 btu/hr heater will not work, but the temperature rise should be considered.

I don't have a flow meter on my plumbing so I really don't what my flow rate is. But I suspect it's in the range of 40-60 GPM for most use-cases unless I'm running the pump at nearly full speed on the spa to get faster jets. But I rarely do that.

I would say this is more of an issue when I'm running the automation system in "AERATOR" mode. As eluded to above, the AERATOR feature sets the intake valve to the pool side plumbing and the return valve to the spa jets (water returns to the pool through the spa overflow). With the heater running in that configuration the water entering the heater will always be lower than the actual spa temperature because the pool volume is much, much larger than the spa volume. So if you take that out to the extreme and let the heater run indefinitely, eventually the pool water could get up into the high 90's with the spa being a lot hotter than the typical 104F cut-off because the temp sensor that the automation uses to control the fireman's switch will always be seeing pool water. But that would take an awfully long time to happen.

Even with all that said, the convenience of having the extra BTU's is worth it ... just like no one ever really says, "Gee, I really wish I had built a smaller pool ...", I suspect no one with a pool will ever say, "Gosh, I wish my pool heater would work slower ..."

 

[JamesW]

In Spillover mode, you are not sending 103 degree water into the heater.

If the water is going into the heater at 83 degrees, the heater output is only 100 degrees, which is not going to feel too hot.

In Spa Mode, you will send 102 to 103 degree water into the heater and the output can get to about 119 to 120 degrees.

 

[ddoggphx]

This is a great discussion, thank you!!