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.