Pool Heater advice for Central/N. California please

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stevet1

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May 28, 2007
38
Hello,

I was thinking of getting a heat pump to extend my swimming season for my planned pool. However, a local pool builder rep told me that heat pumps don't work well in my area because of the low humidity. Today is a fairly average summer day, and by the time the temp. drops to about 80 deg in the evening, the relative humidity probably rises to about 25%. Would a heat pump make sense in this case? It's not a very popular heating solution, most heaters are NG or solar in this area. If not a heat pump, then what would be the next best option?
 
stevet1,

I can give you names of pool builders in Northern California and even more NORTH of Northern California that say that Heat Pumps do work! If you would like references for homeowners and dealers, let me know.

Heat Pumps are not considered in Northern California in many cases because it's such a huge gas market for heaters. Aqua Cal's second largest area of heat pump installations in the North Eastern US. Much of the same types of climates you have.

Compare the heat pump that EVERYONE has in their own house already. Your refrigerator!
I know alot of homes that have a "second" refrigerator in their back patio, garage, tiki bar, or other outdoor location. Does it still work? Of course. A swimming pool heat pump uses the same principle as the refrigerator and will work, even in low humidity.
 
Humidity is not an issue at all. The main issue is the relative cost of electricty and gas. On the East cost electricty is usually way cheaper than gas, on the West coast electricty is typically more expensive than gas. Thus, on the West coast it is often better to get a natural gas or propane heater.
 
It all comes down to economics - both initial cost and operating cost. Look at the rates for gas and electric and the eff of a gas heater and the cop of a heat pump then make the decision based on cost.

I have a heat pump in NJ and would never use gas due to the higher operating cost. I also added two 2'x20' solar panels this year to minimize the use of the heat pump.
 
Okay,

I've been studying some of the Heat Output vs. Temperature curves for various heat pump manufacturers, and I've come up with a rough estimate that heat pumps will be about 25% less efficient in my area due to lower humidity and the lower average operating temp compared to published COP values. So yes, everyone here is right, heat pumps will definitely work and heat my pool in my area.

However, with my current local energy rates, calculation of energy costs for my specific situation shows that a heat pump with a COP of 4.3 produces heat at an equivalent cost of a 95% efficient gas heater (Again, specific to my situation only). Any heat pump with a COP of greater than 4.3 will start to show energy savings compared to gas. Since most heat pumps are in the COP range of 5-7, yes a heat pump does make sense compared to NG heater.

However, once I started to make calculations for solar, the cost calculations seem to skew in favor of solar. The problem with solar is that it will provide a fairly limited amount of heat, and cannot increase it's heat production based on demand. Also, it won't heat my spa, so I'll have to include the cost of a small gas heater anyway. I'm building a spreadsheet to help with my cost analysis, so maybe someone can help me with one of my variables that I can't seem to get a handle on.

I need to know how much heat it will take to maintain a temperature for my pool throughout the pool season.

So say for example, my pool is going to be 650 Sq Ft., 24k gallons, and I want to maintain a temp of 82deg.
Say it is 25% shaded, and I will not be using a pool cover. If the pool season will be from the beginning of April to the end of Oct (6 months), how do I determine how much heat (BTU's or therms) I will require for the entire season?

Without this critically important piece of information, it's kind of to make an informed decision. Can someone please help me with this variable?
 
I am in Northern CA and have solar with a bubble cover and the water temp has been 85-90 since the end of March. I also heat my spa with it up to 102 degrees, it just has to be during the day and it takes longer than the heater. CA electrical rates are outragous so I would stick with solar.

You could play around with this software which works fairly well. It also allows you to do a financial comparison between solar and the heat pump. Most of the inputs you can use the defaults but all dimensions are in meters.

A back of the envelope calculation is 1000 BTU/sq-ft/day of panel. So for me, that is 480,000 BTU/day which is about 3 degrees per day. That is pretty conservative since I can get about an 8 degree temp rise per day but that is both pool and panel heat gain. Without the panels, I can get about half of the that so I would use about 1450 BTU/sq-ft/day for reasonable estimate for our area. I think you will find that you can get a fairly good heat gain with solar.

The cover is key as well. I lose about 3 degrees at night with the cover and without about twice that. We get cool nights which with the solar heat gain makes the heat loss even worse.
 
mas985,

Thanks for the link. The city locales are all Canadian cities, so which one did you choose to make your calculations?

I found a formula for estimated heat loss from a pool:

Heat loss (BTU/hr) = Area (sq.ft.) * Temp. difference (pool - ambient) * R value (which is around 5 for mild conditions)

I then took average mean air temperatures for each month in my city and used 82 deg. desired water temp. I converted to Therms units. I then subtracted estimated solar Heat gain from the pool itself (1000BTU/sqft/day), and modified this by up to 20-25% on the fringe months of April/May/Sept/Oct.

Using my spreadsheet I got some interesting #'s for a 650 SF pool, no cover, target water temp 82 deg.


Therms needed per month
April 404
May 238
June 70
July -42
Aug -28
Sept 66
Oct 275

This model tells me that the vast majority of my energy need will be concentrated in 3 out of 7 months, April/May/Oct. It also tells me that I probably won't need any heating at all in July and Aug, and the avg pool temp will run higher than 82 deg during those months. Also excluding July/Aug, my estimated energy need will be about 1120 THERMS for the season.

Does this seem in the ballpark based on actual experience?
Also, if this is in the ballpark, then the economics favor solar in a big way.

Mas985, in your case, your system generates 4.8 Therms/day. For an average month, this will come out to 144 Therms/month if it's sunny every day. Since your pool is close to 650sq ft., according to my model, if you didn't have a cover, you would need additional heating in April/may/oct. , even with solar. Does this sound about right? Or do I need to tinker with my model more?
 
Why are you worried about the cost to heat the pool? I am not joking, as you say you care but also say you won't use a cover. It will cost you roughly 5 times as much to heat an uncovered pool versus one that you cover.
 

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stevet1 said:
mas985,

Thanks for the link. The city locales are all Canadian cities, so which one did you choose to make your calculations?

There are two packages, one for the US and one for Canada. There are also add on cities for the US but I cannot seem to find where it is downloaded. I will keep looking. Also, I used Sacramento since it is about the same climate.


I found a formula for estimated heat loss from a pool:

Heat loss (BTU/hr) = Area (sq.ft.) * Temp. difference (pool - ambient) * R value (which is around 5 for mild conditions)

I then took average mean air temperatures for each month in my city and used 82 deg. desired water temp. I converted to Therms units. I then subtracted estimated solar Heat gain from the pool itself (1000BTU/sqft/day), and modified this by up to 20-25% on the fringe months of April/May/Sept/Oct.

This is probably a good estimate but there are many other factors that come into play.

Using my spreadsheet I got some interesting #'s for a 650 SF pool, no cover, target water temp 82 deg.


Therms needed per month
April 404
May 238
June 70
July -42
Aug -28
Sept 66
Oct 275

This model tells me that the vast majority of my energy need will be concentrated in 3 out of 7 months, April/May/Oct. It also tells me that I probably won't need any heating at all in July and Aug, and the avg pool temp will run higher than 82 deg during those months. Also excluding July/Aug, my estimated energy need will be about 1120 THERMS for the season.

Sounds about right

Does this seem in the ballpark based on actual experience?
Also, if this is in the ballpark, then the economics favor solar in a big way.

Mas985, in your case, your system generates 4.8 Therms/day. For an average month, this will come out to 144 Therms/month if it's sunny every day. Since your pool is close to 650sq ft., according to my model, if you didn't have a cover, you would need additional heating in April/may/oct. , even with solar. Does this sound about right? Or do I need to tinker with my model more?
Click to expand...

Actual, my surface area is closer to 560 sq-ft since it is a freeform pool but your estimates are not too far off. First year I had solar, I did not use a cover. However, that year May was pretty hot so I didn't need one either. What I found was a problem without the cover was temp swings. I have my panels set to turn off at 90 degrees so most sunny days >75 degrees air temp, the pool will hit 90. Without a cover, it will drop down to 82 the next morning if the night is < 65 degrees. So it is not uncommon for the water to swing between 90 and 82.

If your target is 82, it will not be so bad for you and you may be able to get away without a cover.
 
Steve,

Electricity
PG&E 14-26 cents
Roseville Electric 9-18 cents
SMUD 9-18
Use the high end during the swim season. More electricity being used to keep houses cool. More being used=the higher the tier.

Gas $1.25 per therm

Does this sound about right? Which electrical company are you getting power from?
 
kirbinster said:
Why are you worried about the cost to heat the pool? I am not joking, as you say you care but also say you won't use a cover. It will cost you roughly 5 times as much to heat an uncovered pool versus one that you cover.
Click to expand...

I never said I wouldn't use a cover. But I don't want to make my decisions based on the presumption of using a cover. This is why I don't want any data that goes into my model based on a cover. A cover can be used with any heating source, be it solar, electric or gas. The energy savings will be the same. Also, I'm trying to be realistic, because I don't think the wife will like the look of the yard with a cover on, and more often than not, I'll probably forget to use it.
 
Poolsean said:
Steve,

Electricity
PG&E 14-26 cents
Roseville Electric 9-18 cents
SMUD 9-18
Use the high end during the swim season. More electricity being used to keep houses cool. More being used=the higher the tier.

Gas $1.25 per therm

Does this sound about right? Which electrical company are you getting power from?
Click to expand...


Sean,

Thanks for digging this up. It's way more than you need to do, and it's appreciated. Yes I have SMUD at 17.8 cents /kwh and PG&E gas which varies monthly, but more often $1.40-1.60 per therm. My house is large, so I'm always at the highest tier. I've already put these numbers into my spreadsheet, but like I said earlier, the hardest variable to estimate is how much energy total I will need to heat my pool.

However, based on several different energy calculation scenarios, it seems like it's going to go down like this:

1. Solar first: No matter which way you look at it, appropriately sized solar provides approx 700 therms/season in my area, for free. It will pay for itself in 3-5 years and has a life expectancy of 15-20 years.

2. If I were completely diligent with a cover, solar + cover would probably be completely adequate for my pool for the entire season.

3. If I need additional heating, then depending on how much extra energy needed, it will come down to plain gas vs. an additional heat pump. I suspect that I will need a gas heater for the spa anyway, so I'll probably go a season with solar + gas and closely monitor how much energy I actually use. If the economics make sense, I can always add a heat pump the following season ( or learn to use a cover more consistently)
 
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