# Under-sized pool heater

#### Raytownloc

##### Member
I have been getting a lot of conflicting information about how big a pool heater I need. The guy at Leslie's pools told me that for a 16,000 gallon pool, I need at least a 250,000 BTU gas pool heater and he would recommend a 400,000 gas pool heater. The problem is that my plumber is telling me that based on the existing gas lines, the best that I can do affordably from that side of the house is about 150,000 BTU or maybe 200,000 at the most. He recommends the smaller heater to be on the safe side.

Based on my calculations, my pool is about 500 sq ft. and about 16,000 gallons. I live in a warm climate. Currently (in early March), the pool is at 66 degrees and I would like to bring it to 80 degrees. I'm willing to buy a solar blanket. Do you think that 150,000 BTU heater will do the job?

From the energy.gov website
:
-To calculate the size of a heater you need: Pool Area x Temperature Rise x 12 -This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0

So I would need 500 sq x 14º x 12 x 1.5 = 130,000BTUs to get the pool heated in between 9-14 hours. Is something wrong with my math? Seems like the small heater should be enough no?

#### Jimrahbe

TFP Expert
Ray,

Welcome to TFP... A Great resource for all your pool questions...

We have some great heater experts who I'm sure will be along shortly..

Thanks for posting,

Jim R.

TFP Expert
Heat pump or solar an option? If you are limited on you gas line listen to your gas guy and don't push the gas supply. If you want a bigger gas heater you are going to have to upgrade your gas line.

#### Raytownloc

##### Member
Solar and heat pumps are not an option... but I don't mind an undersized pool heater as long as it will work. Long times to heat up dont really bother me. Its just that I was led to believe that it was not even worth it.

#### JamesW

TFP Expert
The specific heat of water is 1 btu per pound per degree Fahrenheit. 16,000 gallons weighs 132,800 lbs. So you need 132,800 btu per hour to get 1 degree Fahrenheit per hour. Assuming an 82% efficiency, you need a heater rated at about 162,000 btu per hour.

That's the gain rate. The loss rate depends on multiple factors. I would estimate that the loss rate at a 14 degree temperature difference would be about 30,000 btu/hr.

Heat at 82% efficiency 30,000/0.82= 36,585.

So, I would suggest getting a heater rated at a minimum of 162,000 + 36,585 = 198,585 btu per hour.

In other words, a 200,000 btu/hr heater will give you a heat up rate of about 1 degree per hour.

#### Raytownloc

##### Member
Thank you for the advice - So does that mean a 150K BTU heater would not work? Or does it just mean I might have to wait another month to start heating the pool (to a 10 degree temp difference)? Are you factoring in the use of a solar blanket and would that help any? I know that Pentair makes a 175K heater... I have the plumber coming tomorrow... perhaps I can get his opinion on putting in a 175K heater instead. I'm not trying to be stubborn I just have limited options for expanding the gas output.

TFP Expert
It will work. But it will be expensive to run in the long term. If a heat pump is out of the question for you than I don't think you will be happy with the performance of a gas heater that size.

#### JamesW

TFP Expert
150,000 btu will work. You just have a longer wait for heatup. A solar cover helps a lot.

A 150,000 btu/hr at 82% efficiency will add 123,000 btu per hour. That gives 0.926 degrees per hour or 15.1 hours to heat 14 degrees. That's assuming no loss.

If we assume 15,000 btu/hr heat loss with a cover, that gives a net heat gain of 108,000 btu/hr. That's 0.81 degrees per hour or 14 degrees in 17.3 hours.

If we assume a 30,000 btu/hr loss, that gives a heat up time of about 20 hours.

Once the set temperature is reached, the heater should be able to keep up.

#### Raytownloc

##### Member
150,000 btu will work. You just have a longer wait for heatup. A solar cover helps a lot.

A 150,000 btu/hr at 82% efficiency will add 123,000 btu per hour. That gives 0.926 degrees per hour or 15.1 hours to heat 14 degrees. That's assuming no loss.

If we assume 15,000 btu/hr heat loss with a cover, that gives a net heat gain of 108,000 btu/hr. That's 0.81 degrees per hour or 14 degrees in 17.3 hours.

If we assume a 30,000 btu/hr loss, that gives a heat up time of about 20 hours.

Once the set temperature is reached, the heater should be able to keep up.
Everything I've read says that a good pool cover reduces night time heat loss and therefore raises the temp of the pool by 5 degrees or so. If that's the case then when I put one on my pool will be about 71 degrees... So does that make it a 9 degree differential and change the math? I get 11 hours when I use .81 degrees/hour net gain.

Is that correct or is it wishful thinking.

#### pooldv

TFP Expert
Platinum Supporter
Yes, a solar cover will reduce over night heat loss quite a bit. It doesn't even need to be a good one. Get a thin, cheap one that is easier to handle. An 8mil clear or blue bubble cover is all you need. Mine reduces overnight heat loss down to 1-3 degrees generally when nighttime temps are around 20-25 degrees below pool temp.

#### JamesW

TFP Expert
11 hours for 9 degrees is possible assuming low loss.

You might have higher loss rates, so possibly slower heat times.

#### Rollercoastr

Gold Supporter
I have a 200k BTU heater for 12,000 gallons. I don't mind the scheduling/planning that's required for a big heat-up, but lack of planning has caused me to miss some swimming opportunities. My friends have learned to ask me what the pool temp is as we plan wkds /parties etc.

My bigger issue is with the pump cycle necessary for a heat-up. My pool is happy with a short interval of slow circulation to stay clean, but the heater requires a long duration of high-speed circulation for a significant heat-up. That means that I get a double energy hit for heating: gas & electricity. I have a newly-installed VS pump, so I'm now collecting data on the lowest pump speed that will satisfy the heater.

When the time comes to replace the heater, I'll go bigger.

#### JamesW

TFP Expert
Rollercoastr, what is your typical heat up time?

How many degrees do you heat and do you use a cover?

#### Rollercoastr

Gold Supporter
"Resting temp" for my pool is about 68°. My preferred swimming temp is 88-90°. I haven't done a full heat-up since getting my VS pump, but at full-bore with the old single-speed, I could see 1° per hour. I've noticed lately that the heater has been cycling, so aside from having the thermostat looked at, I'll be experimenting to find the best compromise of pump and heater efficiency. This week I watched the heater while the pump was running at 3,000 RPM. It would cycle-off 15 seconds about every 90 seconds. At lower speeds, the heating cycle shortened and the off cycle lengthened.

The pool structure and deck will get warm after a few days of the pool being warm, so loss is difficult to average. I keep the pool covered whenever it's heated and I'm not in it. The cover makes a big difference in evaporation and heat loss.

#### ps0303

TFP Expert
In The Industry
I wouldn't go any smaller than a 250K BTU unit.

You also need to see what your meter is rated for, assuming natural gas.

Look into a Raypak heater.

#### JamesW

TFP Expert
I agree that 150 would be undersized. It can work assuming low loss. At moderate to higher loss, the pool will seem to take forever to heat up.

If 150 is a legitimate constraint and it's the biggest you can do, I think that it will work, but be prepared for potentially long heat times.

If possible try to get the gas guy to make 250 happen. I don't think that you would be satisfied with a 150.

Maybe get a second opinion on the gas supply. Maybe the first guy doesn't know what he's doing.

What gas supply do you have now?

What is your meter rated at?

#### JamesW

TFP Expert
"Resting temp" for my pool is about 68°. My preferred swimming temp is 88-90°. I haven't done a full heat-up since getting my VS pump, but at full-bore with the old single-speed, I could see 1° per hour. I've noticed lately that the heater has been cycling, so aside from having the thermostat looked at, I'll be experimenting to find the best compromise of pump and heater efficiency. This week I watched the heater while the pump was running at 3,000 RPM. It would cycle-off 15 seconds about every 90 seconds. At lower speeds, the heating cycle shortened and the off cycle lengthened.

The pool structure and deck will get warm after a few days of the pool being warm, so loss is difficult to average. I keep the pool covered whenever it's heated and I'm not in it. The cover makes a big difference in evaporation and heat loss.
Ok, thanks. Always good to get real world data to compare to estimates.

#### Raytownloc

##### Member
I agree that 150 would be undersized. It can work assuming low loss. At moderate to higher loss, the pool will seem to take forever to heat up.

If 150 is a legitimate constraint and it's the biggest you can do, I think that it will work, but be prepared for potentially long heat times.

If possible try to get the gas guy to make 250 happen. I don't think that you would be satisfied with a 150.

Maybe get a second opinion on the gas supply. Maybe the first guy doesn't know what he's doing.

What gas supply do you have now?

What is your meter rated at?
I'm not sure - the plumber is coming again on Tuesday and he did say something about getting the power company to upgrade the meter. I'll give an update after that. I appreciate all of yalls wisdom it makes it a lot easier to make a decision. Thanks!