Solar/hydronic pool heating with PEX

One of the most popular and practical uses for “extra” solar heat in summer is to heat a swimming pool without the use of fossil fuel. This is especially easy to accomplish when the hydronic solar heating system is constructed using a “New Standard” primary-loop configuration as described many times in this column. (Archives and links to past articles can be found on the websites of Plumbing Engineer, PHC  News and SolarLogic LLC.)  

A pool or spa can be treated much like any other heating zone when tied into a standard primary loop. In fact, in a number of systems built in recent years, the pools have been connected the same way as any other radiant heated floor. Figure 71-2 shows a block diagram of our typical standard primary loop configuration. You can see at the bottom of the diagram that a pool or spa heating zone is often included on a pair of ‘closely spaced tees’ either indoors or outdoors as required by the job.

‘Radiant’ pool examples

Take a look at the pools shown in the photos in Figure 50-1 (A, B and C). These were installed by professional pool builders using standard in-ground concrete shells that were site-built. They are located near Pecos, Taos and Galisteo, New Mexico, respectively. What you cannot tell from the photos, is that they all contain PEX tubing in the floors and walls of their concrete shells. The tubing was installed by wire-tying it to the remesh in the pool shell just before the concrete was poured. This allows the concrete shell to be heated hydronically the same way radiant concrete floors are done. It just takes a little extra planning and coordination with the pool construction people.

In all three of these examples, the heat distribution to the pool floors was designed in much the same way as the other solar heated concrete floors in the nearby buildings. Zone valves, zone pumps and two stage thermostat controls were employed to allow solar heat or boiler heat to warm the shell of the pools, under the control of the owners, in much the same way as the other radiant floors are controlled by room thermostats.  

All of these pools are attached to larger heating systems with similar design features often described in this column.They all use large multiple banks of flat plate solar collectors as their primary heat source. They all use primary-loop heating system configurations that include domestic hot water tanks, heat storage tanks, backup gas boilers (propane), and multiple zone valves and circulators for space heating in addition to the ‘radiant’ pool heat zones.  The systems in photos A and C are seasonal pools in off-grid locations, so they are connected to the glycol side of the solar combi-system primary loop, in the same way an ice-melt zone is handled (e.g Fig. 71-2 bottom left). Photo B shows a year-round indoor pool which is connected to the indoor boiler loop similar to any indoor radiant heated floor (e.g Fig. 71-2 bottom center).

A word about ‘radiant’

It seems natural and convenient to call these ‘radiant’ heated pools. After all, the same construction technique is used in hydronic concrete floors, and they are known as “radiant floors.” But, while warm floors really do transfer most of their heat by thermal radiation to the room, the same is not actually true for pools. The heat from the warm concrete surface in a pool is transferred to the adjacent pool water mostly by natural convection. Strictly speaking, this is convection and not thermal radiation or radiant heating. But, this is not the first misnomer of its kind. The fin-tube hot water baseboard is commonly called a radiator, when it, too, is really working mostly by natural convection of the room air.  So, in that spirit, I suppose the term ‘radiant’ pool is allowable.

Side benefits of ‘radiant’ pool tubing

As the solar heating designer or installer, it is a good idea to keep your equipment separate from the pool mechanical equipment. In conventional solar pool heating systems this is not possible since it is common practice to use the filter pump to provide flow for both the conventional pool boiler and the external solar heat exchanger, as well as the filter system. This presents a gray area of responsibility when something requires maintenance. The pool guy may attempt to shut down or restart the solar heat after servicing the filter or the solar guy might alter the filter system or its valves or controls when servicing the solar heating equipment.  

When PEX tubing is embedded in the shell of the concrete pool, the pool equipment is positively separated, literally by a wall of concrete, from the hydronic heating equipment. The solar guy has his hydronic equipment, and the pool guy has his filter system for the pool water. The only coordination needed is when the pool filter system has its own boiler. The filter system boiler must be set to a minimum temperature that is compatible with a (higher) temperature range provided by the warm shell of the solar heating system.

Case study: indoor pool upgrade

Let’s take a closer look at one of these pool systems. Figure 50-2 shows an indoor and outdoor photo of a solar combi-system near Santa Fe installed around 2008. In the summer of 2012, we had the opportunity to design the modification and upgrade of the piping and control system to a New Standard configuration. The new control system is a SolarLogic Integrated Control (SLIC) that includes continuous data-logging and Internet connectivity as standard features. This has allowed us to observe and record the performance of this existing ‘radiant’ heated pool continuously in real time over the past four years.  

Heating system description

The solar heat collectors seen in Figure 50-2B are connected with a glycol loop to: (1) a heat dissipation zone (similar to an ice-melt zone) and (2) a flat plate heat exchanger similar to components seen in Figure 71-2. (The heat dissipation zone is pumped by photovoltaic circulators that cool the solar heat collectors even during a grid power failure.) The heat exchanger allows solar heat to pass into the building where a primary loop (full of water) connects all the heating equipment together inside. This includes a Lochinvar Mod/Con boiler, a DHW heat exchanger tank, two radiant floor zones and the pool floor zone. The primary-loop piping is configured to allow any heating source (solar or boiler) to heat any heating load (pool, floors and DHW) directly, under the control of the SLIC control system. (This is similar to the combi 101 system configuration often mentioned in this column.) 

The owner of the pool requested that the water temperature never drop below 81 F. The target temperature was set in the control system in a range of 82 to 84 F. The controls only allow the boiler to fire from 82.0 to 82.2 F, to maintain a comfortable low-limit in the pool.  Solar heat is allowed to heat the pool as high as 84 F in summer. A pool cover is kept in place most of the time that helps to cut down on heat loss and evaporation when the pool is standing by. The two other floor heat zones are not used in summer.

There is a flow meter and numerous thermistor sensors built into the control system that allows direct measurement of heat flow (in BTU’s) in or out of anything connected to the primary loop. Since the control system is internet enabled, we can verify the performance of this solar heating system at any time, and it has proven its worth over the years with reliable performance and high solar heating efficiency.

Long term results

The examples presented here embody some of the most important design strategies for successful long term solar-hydronic heating operation. All of these systems were designed around the New Standard primary loop configuration seen in Figure 71-2 which allows efficient use of multiple heat sources to serve multiple heat loads. The swimming pools seen above all have PEX tubing embedded in their concrete shells to allow trouble-free solar-hydronic heating to be used as their primary heat source. All of these systems were designed to have integrated control systems for intelligent control, compatible with the capabilities of internet monitoring, data-logging and remote control. This has proven to be a successful combination with reliable and effective performance over a significant number of years.

Final Notes

The solar heated ‘radiant’ pool combi-system seen in Figure 50-2 was originally designed, installed and upgraded by AMEnergy Inc. in Santa Fe, New Mexico. Thanks to Peter Page and AMEnergy for the successful performance of this building over the years. 

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, New Mexico, where he is involved in development of solar heating control systems and design tools for solar heating professionals.

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