Case study: solar combi with “radiant” swimming pool

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

One of the most popular uses for “extra” solar heat in summer is heating a swimming pool. This is especially easy to accomplish when the hydronic solar heating system is constructed using a primary-loop configuration as recommended many times in this column. (Archives and links to past articles can be found on the websites of TMB Publishing and SolarLogic LLC.) A pool or spa can be treated much like any other heating zone and, in a number of systems in recent years, the pools have been connected the same way as any other radiant-heated floor.

“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 Galesteo, 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 re-mesh in the pool shell just before the concrete was poured. This allows the concrete shell to be heated hydronically, the same way radiant concrete warm floors are heated. It just takes a little extra planning and coordination with the pool construction people.

In all three of these examples, I designed the heat distribution to the pool floors in much the same way as the 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 combisystem primary loop, in the same way that an ice-melt zone is connected.

A word about “radiant”

It seems natural and convenient to call these “radiant” heated pools. After all, the same construction technique is used on 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 wall of a pool is transferred to the adjacent pool water mostly by natural convection. Strictly speaking, this is not thermal radiation or radiant heating. 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 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 to have a filter pump that provides flow for the conventional pool boiler and the solar heat, as well as the filter system. This presents a grey 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. The only coordination needed is when the pool filter has its own boiler. The filter-boiler must be set to a (low) temperature that is compatible with the (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 combisystem near Santa Fe, installed around 2008. This summer (2012) we had the opportunity to design the modification and upgrade of the piping and control system. The new control system is a SolarLogic Integrated Control (SLIC) that includes continuous data-logging and Internet connectivity as standard features. This allows us, for the first time, to observe and record the performance of an existing radiant-heated pool in real time.

Heating system description

The solar heat collectors seen in Figure 50-2B are connected with a glycol loop to a heat dump zone (similar to an ice-melt zone) and a flat plate heat exchanger. The heat exchanger allows solar heat to pass into the building, where a primary loop full of water connects all the heating equipment 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° 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 that helps to cut down on heat loss and evaporation when the pool is standing by is kept in place most of the time. The two other floor heat zones are turned off for summer.

A flow meter and numerous thermistor sensors built into the control system allow direct measurement of heat flow (in Btu) in or out of anything connected to the primary loop. A sample display can be seen in Figure 50-3 showing Btu usage of the propane boiler during the first two weeks of normal operation.

Propane gas usage

Around the middle of July, the upgrade was complete, and normal pool operation began. Fig. 50-3 shows the gas heat used to raise the pool one degree on July 13 and 14, as we dial-in the desired comfort temperature in the pool. This adjustment was made remotely over the Internet connection to the SLIC control box. The Btu data shows that a one- degree rise in the pool (from 81.2 to 82.2 F) consumed 473k Btu over a 6.5 hour period. This is the rough equivalent of 5.5 gallons of propane. The average heat delivery rate into the shell of the pool was 73k Btu/hr from the boiler during this period. This defines the size of the fuel savings that we are trying to offset with solar heat.

After the pool temperature is set, the only boiler heat seen on the Btu display is less than the 50k Btu/day required to make up heat to the DHW that is mostly being drawn out by the instant hot water recirculator. The low fuel consumption day after day throughout the rest of the month indicates that all of the pool and much of the DHW are being heated by the solar collectors.

Solar heat contribution

The solar heat delivered to the building is measured and recorded the same way that gas heat is recorded. Figure 50-4 shows the daily total solar heat recorded during the same period in July. These records show that typical solar heat (going mostly to the shell of the pool) amounts to around 175k Btu/day and can be seen to jump up above 325k Btu on the most sunny day during that time. The temperature data (not shown) for the pool confirms that this is enough solar heat to maintain the desired temperature range in the pool as the temperature increases from day to day, slowly but steadily. This pool appears to be able to function on “solar only” for weeks at a time, and we expect substantial and consistent propane savings as a result.

Final notes

The solar heated radiant pool combisystem case study described above was originally designed and installed (and was recently upgraded) by AM Energy Inc. in Santa Fe. Thanks to Peter Page and AM Energy for a successful upgrade in this building.

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, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.