Efficient Pool Heating

Summary

Have Technical Pool Repair:

1. program your control system for maximum efficiency
2. upgrade your eyeball fittings to venturi return fittings, and
3. use a liquid solar blanket.

Here are the details and explanations:

Inefficient Design Features

Negative Edges

Some pools are more efficient to heat than others. Pools that have negative edges (also called infinity pools or vanishing edges) are not efficient to heat. The reason is that as the pool water flows over the negative edge it cools. Thus, if you are trying to heat a pool like this, turn off the negative edge pump when heating the pool.

Waterfalls

Waterfalls also cause heat loss. Turn them off when heating.

Pool and Spa Combos

The most common type of pool in the Inland Empire of Southern California is the pool & spa combo. Both are part of the same system and when filtering the pool, water is returned to both the pool and spa and you have a spillover from the spa to the pool. While not as wasteful as the negative edge or a large waterfall, this spillover does cause some heat loss. To maximize the retention of heat, set the valves so that you minimize the time that the spillover operates. For instance, if you have automation, you can set up the system so that when heating the pool, the valves turn to isolate the pool. To keep the spa clean, you can program a separate filtration cycle just for the spa. Finally, to prevent the spa from slowly draining, you will also need to program a very short spillover time to ensure that the spa stays full.

Return fittings in the walls

The ideal way to heat a pool is from the bottom up. A few pools have deep heat returns where the heated water is returned to the bottom of the pool. Most, however, have returns in the sides. Since warm water rises to the top (and since the water in most pools is returned via wall fittings near the top), this results in a foot or so of warm water at the top and colder water at the bottom. Since much of the water is being drawn into the filtration system from the surface via the skimmer and returned near the surface, the heater will only be measuring the temperature of the water in this top warm layer. It will therefore stop heating when this top layer has warmed up to your set point. The bottom, however, will still be cold. To counter this effect, have Technical Pool Repair replace your standard eyeball returns with venturi fittings. These direct the water downward and have slots in the top that draw the warmer surface water down into the pool. This combination mixes the water better and allows your pool heater to more accurately measure (and therefore more completely heat) the entire pool, not just the surface.

Covers

The biggest obstacle to heating a pool is heat loss. Whether you heat your pool with a gas pool heater, solar panels, or just rely on direct sunlight, 80% of whatever heat that goes in will be lost due to evaporation.

As water evaporates there is a phase change from liquid to vapor. This phase change is an endothermic reaction—meaning that heat is absorbed from the water as the water evaporates. If we can cut down on evaporation, we can stop the primary source of heat loss.

A cover is the best way to minimize evaporation and lessen heat loss.

Automatic Covers

Auto covers (vinyl covers that run on tracks and open and close via a motorized reel and pulley system) are the easiest to use. However, these covers work best on rectangular pools and very few pools in our area have them nor are they designed to accommodate one. If you were to design an efficient pool, it would have an auto cover.

Solar Covers

These are the bubble covers that float on the surface of the water. They are generally pulled off by hand onto the deck, but they can be used with a reel that makes removal a little easier. It helps to have two people when opening or closing these as they can be awkward. It is often discouraging to clean a pool with a solar cover as when you first look under the cover, the pool will be relatively clean, however, when you pull the cover off, the debris that has blown onto the cover will rinse off into the pool making it look worse than when your first looked. Second, since these covers are typically only used in the spring and fall, they often harbor algae when not in use. When first putting them on for the season, you often introduce an algae bloom into your pool.

Solar Rings

An easier to handle alternative to the solar blanket is a set of solar rings. These rings are about five feet in diameter. You add as many as needed to cover as much of your pool as practical. Since individually these are smaller than a solar cover, they are easier to remove for use or cleaning. It is, however, not recommended that these be stacked and left in the sun when off the pool, so they are still somewhat challenging to deal with.

Liquid Solar Cover

While the least effective, a liquid solar cover is by far the easiest to use and least expensive option. A liquid solar cover is a product that you add weekly. It is lighter than water so that it forms a thin film on the surface of the pool thereby cutting down on evaporation. If you use one, it is recommended that you leave the pump off on cool nights so as to not disturb the surface tension and maximize the sealing effect.

Would a heat pump work for my pool?

 Summary

While heat pumps are "efficient" they are not practical for the dry and sunny climate of the Inland Empire of Southern California. Solar pool heating is a better option for our area. (If we lived in Florida or on the coast, by recommendation would change.)  

Let’s look at some examples:

Case 1: A small add-on heat pump

Let’s say we want to add a heat pump to an existing equipment pad and we already have a standard electrical supply and subpanel that was originally sized to be used for pumps, blowers, lights, and a gas heater. This means we can likely only add a small heat pump that can be put on a 20-amp breaker.

RayPak makes a CrossWind 500-I that will use a 20-amp breaker and draws 16 amps and uses 1,280 watts of power. (This is similar to what an old single speed 1.5 hp pool pump would use.)

Now, let’s look at what we can expect from it. At 80-degrees with 80 percent humidity and trying to heat the pool to 80 degrees (this is known in the industry as 80, 80, 80,) this heat pump would have an output of 44,750 BTUs. Compare that to a typical residential gas heater that is rated at somewhere between 260,000 and 400,000 BTUs. It is not uncommon for a typical gas heater to heat a pool at a rate of about a degree an hour. With this heat pump we would expect about a tenth of a degree per hour. From this calculation we see that this heat pump wouldn’t heat the pool so much as maintain the heat in the pool.

Now, the above example was a best-case scenario for the heat pump (80, 80, 80 – see previous paragraph). While you might get 80% humidity in Florida, in California it is more likely to get 63% humidity. At this lower humidity, that same heat pump that we looked at in the above example goes from producing 44,750 BTUs down to 42,500 BTUs, at the same 80-degree temperature. But what if you try to heat at night when the temperature drops to 50 degrees? Then, the output of that same heat pump drops to less than half of its ideal and only delivers 21,000 BTUs. (It should also be noted that heat pumps do not work below 48 degrees and most have a built it cut off at 50 degrees.) 

But here is the further downside – that heat pump still consumes 1,280 watts regardless of whether it is supplying 44k BTUs into your pool or only 21k BTUs. Unlike gas, you pay for the time that the compressor is running, not the heat that is delivered.   

 

Case 2: A large heat pump

Having seen that the small heat pump does not have the capacity to heat an average pool / spa combo like a gas heater did, perhaps we want to look at a larger heat pump. The largest heat pump that RayPak makes is the TWPH-8550EHT08.

If we want to install this one, we will need a 60 to 70-amp breaker. But the breaker is only the beginning, we also need larger wires run from the main breaker panel to the pool subpanel. We would likely need 4 AWG wires. This is larger than the wiring run to most pool subpanels so we would need an electrician to upgrade the conduit and wires between the house and the pool panel and likely upgrade both of those panels as well.

When running, this large heat pump would consume 6,600 watts of power. (Contrast this to a modern variable speed pool pump that typically only consumes between 100 and 600 watts.) The only power use comparable to this is the power used by a portable spa when it is heating. Portable spas typically have a 6kw heater. When the heater and pump are both running together they use about as much power as this heat pump. So it sounds like you can heat a pool for the same cost as heating a spa? No. With the spa, you heat it once for about 8 hours and it stays warm due to being insulated. Using a heat pump to heat a pool requires that you use that much power eight hours a day for every day that you are heating. 

Let's do some cost calculations: 6.600 kwh * 8 hours a day * 30 days a month * $0.38/kwh = $601.92 per month to maintain the heat in your pool with a large heat pump. (Contrast that to heating your portable spa: 6.600 kwh 8 * hours * $0.38 = $20.06.)  

Aren't heat pumps supposed to be more efficient? Yes they are. Let's compare this large heat pump to the 6kw heating element in a portable spa. The element in the spa is 100% efficient. It uses 6kw of power to produce 19k BTUs. So what can we expect from the heat pump at the same power usage. It depends on the following factors. 

At peak efficiency of 80-degree air temperature, 80 percent humidity, and 80-degree pool temperature, this heat pump would produce 138k BTUs (while this is much more than the 19k BTU heating element in a portable spa, it is small compared to a gas pool heater which is typically 260k to 400k BTUs). Thus, we see that the largest heat pump is half the output of a small gas heater at optimum conditions.  But, if we look at more realistic conditions for Southern California (80-degree air temperature, 63 percent humidity, and 80-degree water temperature) we see that the output drops down to 130k BTUs and at night when the temperature drops to 50-degrees, then the output would only be 87k BTUs. But remember, the heat pump still consumes the same 6,600 watts of power regardless of whether it is giving out 130k BTUs during the day or only 87k BTUs at night.

If 6kw electric heater is 100% efficient (and it nearly is) and since it produces 19k BTU of heat, when you compare it to a heat pump that can produce between 87k and 138k BTUs of heat for the same power consumption, it looks like a heat pump is much more efficient and thus appealing. However, there is an even better option: 

Since the Inland Empire of Southern California has a lot of sunny days and low humidity, a better alternative is to use solar for heating. Solar pool heaters simply route pool water up onto your roof and pass the water through black (heat absorbing) solar panels that absorb free heat from the sun. This is much more economical than a heat pump as no extra electricity is required for heating (the pump may need to run at a higher RPM, but the same would be true if using a heat pump.) Since you are not paying for the heat (as it comes from the sun) solar is much more economical than the heat pump. So why are heat pumps popular in other locations? Florida has high humidity so a heat pump there will be more efficient than the same one in the drier climate of Southern California. Closer to the coast, there is more cloud cover, so a heat pump has an advantage over solar, but here in the Inland Empire we have low humidity and mostly sunny days making solar pool heating more ideal than a heat pump.

Case 3: So maybe a heat pump isn’t best for heating the pool, but what about just the spa?

You notice that the above calculations all assume that we want 80-degree water. That is because the heat pump is most efficient at generating 80-degree water. But, just as the efficiency dropped when the humidity dropped to 60 percent and when the air temperature dropped to 50 degrees, so too does efficiency drop when trying to heat the water over 80 degrees. Thus, for every degree over 80, it will take more time to heat than the previous degree.

Thus, because we want a spa to be 101–104 degrees, heat pumps are not very good at achieving the higher temperature desired in a spas.

Case 4: So maybe an electric heater for the spa?

If you have a self-contained, insulated, and covered spa, an electric heater is a great option. If you have typical pool and spa combo and only want to heat the spa up for occasional use, electric heat is still not a good option as these heaters are simply too small to be practical. Raypak makes an E3T-27 electric heater. It is rated at 92k BTUs. So again, this electric heater is significantly lower output compared to a small gas heater that is typically 260k BTUs (and much less than a 400k BTU pool heater).

As for power requirements, this heater alone calls for three 40-amp 220v breakers. That is more capacity than most pool subpanels (and more than the wiring going to them can accommodate) so you will have to have an electrician upgrade your conduit, wires, and likely both panels as well.

As for heat up time, if it typically took 30 minutes to an hour to heat your spa, plan on several hours.

A better way requires different designs

So, how are homeowners who live in new developments that are not installing gas lines going to heat pools and spas without gas? Pools and spas will have to be designed differently.  

Heating the pool can be accomplished with either solar panels or a heat pump. Neither are good at heating quickly, but both do a decent job maintaining temperature throughout the season—but neither are ideal for year-round use.

The key to making them work is to minimize heat loss by keeping the pool covered. Approximately 80 percent of heat is lost through evaporation. Thus, to minimize the cost of using a heat pump, the pool should be covered at night when the temperature drops.

The typical Southern California pool is a free form shape and often has raised walls on some sides and typically has a spa that spills over into the pool. This design does not work well with automatic covers. Automatic covers work best on simple rectangles.

Many pools also have negative edges where the water flows over a vanishing edge into a catch basin. This design is not practical for use with any heater because the water going over the edge loses heat to the air.  If you want to heat such a pool, turn the edge pump off when heating and then realize that the pool will cool as soon as you resume using the edge pump.

The most efficient way to keep a pool warm is to design it as a rectangle, install an auto cover, and keep it regularly closed at night.

Additionally, water features like vanishing edges, spa spillovers, sheer descents, and waterfalls are all sources of heat loss. These should be minimized, or they should be separate bodies of water from the heated pool.  

Spas need to change too.

Or, rather, Southern Californians need to get away from the idea that a pool and spa combo is ideal and, instead, use detached self-contained and covered spas. These spas will typically need a 50-amp, 220v power supply and have a 6 kw heater that takes around 8 hours to heat. This heater draws about the same amount of power as does the large heat pump in the above case #2, but whereas the heat pump runs all day every day to maintain the temperature of the pool, the cost to run the heater in a self-contained spa is really only noticeable during the initial heat up cycle (about $20). Thereafter, since the spa is insulated, you rarely notice a difference in your electrical bill.

But rather than heating them before every use, as is typical with a gas heater, people that have these type of “portable spas” leave them hot, covered, and ready to use all the time. The insulation around the shell and in the cover makes this quite economical and very convenient.  

The biggest change will be aesthetics. Here in Southern California, we are accustomed to looking out the back door and seeing pools and spas with sparkling flowing water. The more typical view in these communities will be the sight of closed covers.  

Basic Spa Care

This advice is  intended for personal backyard spas that are only used by a couple of people and stay covered all the time. 

Get a test kit that can measure Bromine, pH, and Alkalinity. I prefer the ones that use drops, but you can also use strips. (I just have trouble discerning the colors on the strips and can see the color change with the drops better.) Test about once a week. 

If the pH or Alkalinity is low -- add Baking Soda (a couple of tablespoons at a time)
If the pH or Alkalinity is high -- add acid (dry acid is safer for spas) (a couple of tablespoons at a time) 

For sanitizing, I recommend using 1" bromine tablets in a floater. (If your skimmer has a dedicated place for tablets, you can put them there.) Two or three at a time should be fine. They stay in all the time and provide slow and constant sanitizer.

Every couple of weeks you can add an oxidizer shock. Use potassium monopersulfate (maybe 2-3 Tbs at a time). Do not use chlorine shock. 
On alternate weeks from the oxidizer I recommend using an enzyme called Spa Perfect. It gets rid of odors and body oils.

There will be many weeks that you won't need to do anything except maybe add another bromine tablet.

The spa needs to filter every day, but doesn't need much time--maybe an hour. 

Taking care of a spa is pretty easy. Less is better. You can run the sanitizer on the low end of ideal since the spa has few people using it. 

Algaecides and Mustard Algae

 Chlorine will kill most algae, but has a hard time killing mustard algae. Mustard algae is a yellow algae that will grow on the sides of a pool. It generally brushes off easily, but if you don't kill it, it will come back. 

In addition to showing up in the pool, it will also coat the grid inside the filter and cause the filter pressure to build up quickly (sometimes within a day after cleaning). 

The only way to deal with mustard algae is to kill it with one of the following types of algaecide:

• Copper based algaecide -- this is my first weapon of choice for mustard algae. You can buy it as a liquid or in a more easy to use canister form (Pool Rx). 
• Silver algaecide -- this is another good tool for killing mustard algae.
• Sodium Bromide -- this also works to kill black algae by turning free chlorine into chloramines. While this is not normally a good idea, it works for mustard algae.

Each of these is further explained in other posts on this same blog. Look there for further information and be sure to follow package labels for dosing amounts. 

With all of these, brushing is essential. As long as the algae is on the side of the pool, it is somewhat protected from the chemicals in the water. Brush daily to put the algae in the water so that the chlorine and algaecide can kill it.

Finally, if you have mustard algae, you will need to clean your filter often until you kill the algae--and then once after you do kill the algae to remove the dead algae from the filter. As long as there is algae in the pool, your filter pressure will go up and your circulation will be poor. Kill it first, then filter out the dead algae.   

P.S. If treating mustard algae don't bother using an Algae Preventative (it is much too late to use a preventative once you already have it). Also polymer algaecides like algaecide 60, while good for green algae are not going to work for mustard algae. 

Sodium Bromide

Sodium Bromide works similar to an algaecide, but what it is really doing in the water is combining with free chlorine to produce chloramines. Now, normally, you don't want chloramines in your water. Also known as combined chlorine, chloramines are largely responsible for the objectionable odor of chlorine. Additionally, they are not as good at killing bacteria as chlorine is. However, chloramines are better suited to kill mustard algae than is free chlorine. 

To use sodium bromide, add an initial dose according to label instructions. At the same time, add extra chlorine. A good rule of thumb is that whenever you add  sodium bromide, add a gallon of chlorine at the same time. You need the extra chlorine so that the sodium bromide can use it to form chloramines. Continue adding a smaller maintenance dose for about a month following the initial treatment to ensure that the algae does not return. 

While sodium bromide is effective, try not to use more than recommended on the label and try not to use it for more than a month. Using too much sodium bromide will eventually make it hard for the pool to hold chlorine. 

Sodium bromide is generally packaged as Yellow Out or Yellow Treat or something like that. 

Phosphates

 Phosphates are food for algae--and any other plants

Most phosphate removers are liquids that are added to the pool water. They bind with the phosphates and the resultant slime is collected in the filter. This phosphate slime is a slimy silvery coating that causes filter pressure to climb and circulation to rapidly diminish. If using one of these types of phosphate removers, plan on cleaning your filter a couple of times shortly after adding it. 

A major pool retailer pushes a "Perfect weekly treatment" product that contains a phosphate remover. When a pool owner uses this, I am often called out to troubleshoot a poor circulation issue only to discover that this product is being used. These pool owners will often be quite adamant that  the circulation problem can't be a clogged filter because they just cleaned it. And yet, when I take apart the filter I find the tell-tale silver slime coating their grids. Once they stop using the Perfect product, their circulation problem goes away.    

There is a better way to deal with phosphates

It is called PhosLocker. It comes in a pouch that you place in your skimmer. As pool water washes over this pouch, the granules inside the pouch absorb the phosphates. You leave the pouch in the skimmer for a month to collect the phosphates. Then, and this is the cool part, you take the pouch out, cut it open and spread in your garden where the trapped phosphates fertilize your plants. After all, phosphates are plant food. So stop feeding your algae and use them to feed your plants. 

Here is a link where you can order PhosLocker. 

Using Alum as a flocculant

 When a pool is really green and has so much algae that you can't even see the bottom, it is either time to drain and refill the entire pool or floc the pool and vacuum to waste. A flocculant drops everything to the bottom of the pool so that it can be vacuumed to waste. 

One of the best flocculants is Alum (i.e. Aluminum Sulfate). Alum is Al₂(SO₄)₃

Here is the reaction when added to water: 

Al₂(SO₄)₃ + 6H₂O ® 2Al³⁺ 3SO₄^2- + 6H⁺ + 6OH^- ® 2Al(OH)₃ + 3H₂SO₄

The resulting product is Al(OH)₃  which is an insoluble floc that will drop everything to the bottom of the pool where it can be vacuumed to waste. This including a good portion of cyanuric acid (stabilizer), carbonate alkalinity, and calcium hardness. (Adjust as needed after vacuuming and refilling.) 

Besides the floc, the remaining by-product is 3H₂SO₄ (Sulfuric acid) meaning that adding Alum will lower the pH. You can deal with this in one of two ways. You can raise the pH to a higher than normal level prior to adding Alum so that the pH doesn’t drop too low after adding it, however, the Alum will work better if you add the Alum when the pH is normal (near 7.2) and then raise back as needed after it drops. 

Temperature is also important as the water needs to be 70 degrees Fahrenheit or higher for a good chemical reaction to take place. 

Dosage isn't precise: dissolve and broadcast 2-6 lbs alum per 10k gallons.

Where to find it. Alum can be purchased from pool stores, but you will find better prices on larger quantities at garden centers as alum is also used in flower gardens.