Heating: Natural Gas vs. Electric Heat Pump - Central Valley, CA

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J

JManteca

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Gold Supporter
Mar 4, 2014
13
California Central Valley
My wife and I are starting to review the proposed pool equipment from my PB. When it comes to heating, this is what I have come up with. What do you all think of this comparison:

Assumptions:

No pool cover so the pool will be cool in the morning. 78 degrees +/-?

We are on PG&E's E-6 Rate Schedule
https://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_E-6.pdf

Electric Rates:
9pm to 10am = $0.24362/kWh
1pm to 7pm = $0.43566/kWh

Gas Rate: $2.39/therm

Analysis:

Electric Heater – Off Peak - Morning
http://www.hayward-pool.com/pdf/manuals/Spa-Heaters-ISCSPAXI.pdf
Hayward 11kW Electric Heater
11kW = 37,532 btu/hr @ $0.24362/kWh = $2.68/hr or $1.00 buys 14,005 btu

Electric Heater – Peak – Mid Day
http://www.hayward-pool.com/pdf/manuals/Spa-Heaters-ISCSPAXI.pdf
Hayward 11kW Electric Heater
11kW = 37,532 btu/hr @ $0.43566/kWh = $4.79/hr or $1.00 buys 7,832 btu

Heat Pump – Off Peak - Morning
http://www.jandy.com/~/media/zodiac/global/downloads/sl/sl6408.pdf
Jandy JE3000T
Low Ambient Temp: 6.34kW = 86,000 btu = $1.54/hr or $1.00 buys 55,679 btu

Heat Pump – Peak – Mid Day
http://www.jandy.com/~/media/zodiac/global/downloads/sl/sl6408.pdf
Jandy JE3000T
Warm Ambient Temp: 6.34kW = 130,000 btu = $2.76/hr or $1.00 buys 47,065 btu

Gas Heater – Anytime
JXi Pool Spa Heater | Jandy Pro Series
Jandy JXi400
Assume 82% Efficiency: 4 therms of NG = 328,000 btu; $1.00 buys 34,310 btu

Thoughts?
 
Nice to meet you via TFP and good to have you here :)

I haven't checked your figures, cause it looks like you're doing the right analysis, and I like your method of "what does a buck buy me?"

Just my 2 cents, but it comes down to your expected use of the pool. It sounds like you're planning for seasonal use of the pool (April-September?).

If you want to keep the pool at a minimum usable temperature all the time, go with the heat pump, and make sure it has high enough output for the swim season you want. Be aware that its noise level is roughly similar to an air conditioner compressor with the same output.

If you're able to anticipate use, go with gas and heat only as needed. The cost per Btu is close enough to the same, and the gas heater will heat the pool faster. The gas heater will also work very fast for winter spa use. When the air is cold, the heat pump can probably still heat the spa, but your cost per Btu will be much higher due to its lower CoP in cold temperatures.

Be careful with energy cost analysis to use your marginal cost. Adding consumption usually doesn't add to daily service charges or monthly basic charges for residential customers. You'll probably also consider the installation cost difference between the two options.

I know covers are ugly, but I have to strongly suggest working out a way to cover the pool, even if just for a month in spring. Do you have any potential for rooftop solar heating in conjunction with the pool heater? That's the cheapest heat of all.

Have fun with your build!
 
needsajet,

Thanks for the tips. I did not know that a heat pump was as loud as an AC condenser, so thanks for that info.

I am just about 100% I will do some sort of solar. I don't have it totally figure out yet. But that will be the primary heating. So I am looking for supplemental heat when solar is just not quite enough.

I would love for the pool to be 85-88 degrees every morning to evening from June through August (when the kids are out of school). April, May, Sept. and Oct. I want weekday evenings and weekends morning to evening. There is nothing like a nice morning swim to get the day started.

So I would like the heater to start up in the early morning to get the pool to 85 by 9:00am on days when I want mornings. Then switch to solar to maintain the temp until evening. Then off at night.

I would like to have the spa heat up within an hour of turning it on. 1,003 gallons.

I would like to use the pool once or twice during the winter on a nice weekend.

With all that said, a heat pump would work best to heat the pool in the mornings. But a NG heater will work better for spa heating and winter heating... Right?

I hate trade offs.
 
Even if you assume the rosiest and perfectly insulated scenario, the biggest, baddest most powerful heater on the market will only get you a 1-2 degrees per hour.

24,000 gallons of water = 201,600 lbs of water

1 BTU --> energy required to heat 1 lb of water 1 degree F

Biggest nat gas heater possible - 400,000 BTUs per hour.

Biggest, baddest most rad heat pump ever! - 125,000 BTUs/hr

so if the pool was perfectly insulted and lost no heat to the environment and your heat pump was perfectly efficient (a totally unrealistic scenario), you could get 2 deg F per hour with a gas heater and about 0.5 deg F per hour with a heat pump. So, depending on how cold your water is in the morning, getting to 85F by 9am might not be possible. This is why you absolutely need a cover on the pool - if you do the analysis, you'll see that an 85F body of water will lose all of the heat input you added through the daytime from evaporative water loss.
 
A pool your size doesn't change temp as quickly as you are thinking it will. I'm assuming your pool will be in the 20,000 gallon range based on size if its deep that is probably a low guess. At 20k gallons you you have roughly 180,000 pounds of water (8.3 lbs of water per gallon). 1 BTU will raise 1 pound of water 1 degree F.

Based on that an electric 11kW heater at 37kBTU isn't even an option, they are really only intended for above ground well insulated stand alone spas.

Your heat pump at its peak will get you about 1 degree of temp rise in your pool every 2 hours, which as you noted will vary with ambient outdoor temps. For your spa the heat pump will likely only get you about 10 degrees an hour temp rise.

The gas heater will get you about 2 degrees an hour on you pool and probably 20 an hour in your spa.

A heat pump will maintain your pool at a set temp efficiently they are a slow but steady wins the race kind of animal. Combined with a good solar panel setup can be a great way to extend your season while keeping costs to operate down.

The gas heater is a cheetah like animal it will sprint your pool to a temp but used long term will exhaust your wallet like an Olympic swimmer eating all the pancakes at a breakfast buffet. Again like the heat pump if you couple the gas heater with a good solar panel system you will lower your operating cost. Solar and natural gas might be a better fit for your desired pool usage by the sounds of it.

When it comes to pool heating technologies there is nothing but trade offs when looking at 1 vs the other. More often than not combining two different heaters is what really needed to make people happy and lower the utility bill. Unfortunately the trade off there is the significantly increased up front cost of installation.
 
I didn't see the cover bit..... yea you are just spinning your wheels and wasting tons of energy trying to heat a pool that isn't covered. The difference between covered and un-covered pools is nothing less than shocking when compared side by side for heat retention.

Like joyfullnoise pointed out all of my numbers are best case ideal numbers. Real world you won't quite get there do to convection losses to the air and conductive losses to the ground.
 
I'll chime in with the evaporation rate. I don't have numbers to back this up, but I'd venture to guess the evaporation rate may be a bit higher in Arizona than California :)

that at being said, we keep our pool at 88-90 degrees toward the end of the day and most of the time wake up to a 85 degree pool. Admittedly our pool is reasonably sheltered by the wind so our situation may be optimal.
 
To clarify, a heat pump will have roughly the same noise as an air conditioner compressor of similar output. Your house aircon is probably bigger, so just don't want to throw you off. There are small pool heat pumps that are quieter than a whole-house air con, closer to the noise level of a single room system.

But overall it sounds to me like gas is the best option for meeting your needs. A heat pump wouldn't give you quick heating in the morning and would need to be running through the night to have the pool warm for 9am in spring and autumn.

A combination of solar pool heat and a good-sized heat pump would get you pretty close, but probably not into late September or October. Hopefully someone from California jumps on the thread cause they'd be able to pin it down better than I can.
 
CaptainCannonball said:
I'll chime in with the evaporation rate. I don't have numbers to back this up, but I'd venture to guess the evaporation rate may be a bit higher in Arizona than California :)

that at being said, we keep our pool at 88-90 degrees toward the end of the day and most of the time wake up to a 85 degree pool. Admittedly our pool is reasonably sheltered by the wind so our situation may be optimal.
Click to expand...

The OP lives in California's Central Valley. That's pretty close to Tucson weather. Relative humidity and wind speed play a part. But even the mildest wind speed will be enough to enhance evaporative losses.

If you want to go and play with numbers, just Google "engineers tool box" and look up evaporative heat loss.
 
JoyfulNoise said:
The OP lives in California's Central Valley. That's pretty close to Tucson weather. Relative humidity and wind speed play a part. But even the mildest wind speed will be enough to enhance evaporative losses.

If you want to go and play with numbers, just Google "engineers tool box" and look up evaporative heat loss.
Click to expand...
Uh, yeah... I live here too and can confirm that humidity exists here :)
 

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CJadamec said:
You want humidity you should visit here in august 90+ RH and 90+ degree heat.
Click to expand...

I grew up on Long Island, so I remember those days well. That's why we had the south and north shore beaches...no need for a pool when you're less than 30 minutes away from cool ocean waters....

We do get some humidity here in Tucson right after it rains (monsoon season is mid-July through August). But the humidity has the exact opposite effect - it drastically cools the air just before it rains. I've seen my dash board temperature reading on the car go from 115F to 75F in a matter of 15 minutes...then WHOOOSH, rain pours like crazy and the air is cool and moist. SO when people around here complain about humidity, I just roll my eyes :crazy:
 
There's a practical side to heating worth mentioning. In the spring, everyone's gung-ho by mid-April because the days are getting nicely longer and lots of warm temps for when you get out of the water. By May they're keen, and we all put up with a bit cooler water than we'd love. After Labour Day, or depending on people, maybe the latter September equinox, the interest in the pool goes down pretty fast.

That's the season people often end up with, and of course equipment choices are lower cost vs. sizing for all of October. By all means, go for it, it's your pool, so don't get me wrong, only commenting so you're aware.

Pool heat can be a deep dark hole but If you're rich enough, go for it without a cover. If you're like most of us, and you're gonna heat, design back from the cover, so it's easy to do.

There are approaches that use the cover just in spring, and it can be taken away and stored through the summer. Mine is a dead-easy rectangle, which might be too extreme for you. Having a wide spot in the middle of a free-form pool's long axis makes mas985's DIY method easy. But both these allow you to only use the cover for a while in spring, or through cold spells.

Reels can be hidden in benching. Avoiding L-shapes sometimes helps. etc. etc etc. There are lots of options right up to a hidden cover under your decking, automated covers, covers that double as safety covers, and so on. The dollar savings allow for some of the extra investment, including savings on chemicals.
 
JoyfulNoise said:
Well, good to know someone has some RH....
Click to expand...

I didn't know what low humidity was until I moved to Reno... was such a shock that 100+ heat wasn't bad when your sweat evaporates before it leaves the pores :) I guess I always imagined Arizona to be that way.

When I was researching a heatpump for our area, I noticed our humidity is high in spring and fall but low in the summer. A nice fit for its efficiency. I was pretty close to pulling the trigger until I found that I just didn't need it.
 
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