Even a fully functional solar hydronic heating system may overheat, and this is most likely to happen when there is plenty of sun, but the heat cannot be used. This can happen for several reasons, but most commonly: 

1) when the heat is not needed because all the heating loads are satisfied, or 
2) because of a power failure, pump failure or control failure in the heat collection, storage or distribution systems. 

Heat begins to build up in the solar collector loop when it is not carried away to a useful heating job, and if not arrested, can reach the boiling point of the hydronic fluid. Overheating is often accompanied by the sound of steam hammering in the solar heat collector; propylene glycol may start to cook and may begin to turn brown in color and then becomes increasingly acidic. A plume of steam may appear at any open float vent, and the pressure relief valve may begin to drip or spurt fluid while the pressure and temperature (P&T) relief valve on the heat storage water tanks may begin to release hot water.

The conditions that initiate overheating may happen only once a year, or even less often, but when it does happen the results can range from annoying inconvenience at best, to major heating system failure at worst. A properly designed system must always employ controls and strategies that can dissipate excess heat safely and reliably, and also provide temperature protection during a power failure on a sunny day.

Top four ‘fail-safe’ solar overheat strategies

Overheat prevention is included in virtually every solar heating system we design these days, and both passive and active multiple strategies are usually included together to provide a “belt and suspenders” approach. The four most reliable and fail-safe methods we use today are as follows: 

1. Thermosiphon self-cooling (TSC) fin system. TSC fin-tubes can be added to any bank of flat-plate solar heat collectors, as long as the piping inside the collectors meets some simple requirements. That is, the collectors must be “harp” configurations with the internal headers (top and bottom) running horizontal with straight parallel risers running vertical. 

Figure 98-1 shows a photo of a passive self-cooling fin-tube system installed on the back of a bank of eight solar heat collectors.

Thermosiphoning can be defined as the movement of fluid around a plumbing loop driven only by a temperature difference across the loop (fluid is “pumped” only by heat). Hot fluid is less dense than cool fluid, so when it is contained in a loop, the cool fluid tends to fall downhill, and the hot fluid tends to float uphill. This principle can be used to dissipate solar heat by including cooling fins in the loop. 

Figure 98-2 shows how simple the plumbing details can be when connecting the TSC loop to a bank of flat-plate collectors. The normal panel tilt allows the hot fluid to rise by natural convection to the top, and the slope of the fin-tubes in the back allows cool fluid to drain downhill and re-enter at the bottom of the collectors. On a sunny day, if the solar circulator pump stops, the swing check valve at the bottom opens easily in response to thermal flow and cooling occurs by natural convection. When the circulator pump turns on, the cooling loop snaps shut, using the passive swing-check valve that closes in response to the relatively high flow and pressure provided by the circulator pump. In this way, thermosiphon cooling flow continues as long as the sun provides heat and stops when the circulation pump turns back on.

2. Photovoltaic powered self-cooling (PVSC) fin system. Some solar heat collectors cannot be cooled properly by thermosiphon flow. For example, flat-plate collectors that use a serpentine flow path or other “non-harp” internal piping arrangement cannot be used with the TSC system described above. A cooling fin loop may still be added to the collector array, but it must be pumped with a solar circulator to produce the proper flow for cooling. In these cases, we employ a PV solar circulator and a small solar-electric panel so that the cooling system keeps running off solar power even during a grid power failure.

Figure 98-3 shows a photo of a PV self-cooling system using a Caleffi solar pump module with the PV pump option installed at an elementary school in Albuquerque.

3. Drain-back solar collector configuration. Drain-back solar heating systems will also survive overheating and power failures just fine, because the collectors empty themselves when the solar pump loses power. Water is most commonly used as the collector fluid and drains by gravity down the supply pipes to a drain-back holding tank indoors whenever the system shuts off. Air from the drain-back tank takes the place of the water, which protects the panels and pipes outdoors from freezing or boiling. Panels and supply pipes must be sized and tilted properly to drain quickly and completely to prevent freeze breakage. Serpentine collectors and some other types of flat-plate and evacuated tube collectors may not be used in a drain-back configuration, so follow the manufacturer’s recommendations.

4. Steam-back collector overheat configuration. Another common passive strategy used in closed-loop glycol systems is the “steam-back” expansion tank method. This does not prevent high-temperature steam in the solar heat collectors during a power failure, but rather allows steam to fill the panels without the loss of any collector fluid. The volume of liquid glycol that is displaced by steam as it builds up inside the hot collectors will try to seek refuge in the glycol expansion tank. If the expansion tank is large enough and was installed with the proper air pressure, this can prevent the glycol from leaking out the pressure relief valve. After sunset when the steam condenses inside the collectors, and the air pressure (in the expansion tank) forces the glycol back into the solar loop, the system will continue to run normally, so long as the electric power, pumps, valves and controls are not damaged and the glycol pressure has not dropped too low. 

Steam-back systems work best when the collectors and connective plumbing are installed to drain downhill to the expansion tanks, much like how drain-back plumbing is done. The liquid acceptance volume in the expansion tank should be at least equal to the fluid volume of the solar collectors themselves.

Other common solar overheat strategies (less fail-safe)

Some of the most common methods used today to control solar overheating are not entirely fail-safe. This is because they typically depend upon active electrical controls or circulator pumps to provide cooling for the solar heat collectors. In our installations, we combine the fail-safe methods seen above with as many of the common controls listed below to provide the most comprehensive and redundant overheat control possible. So, for example, we have commonly combined numbers 1 and 4 above with A, B, C and E below in the majority of our recent installations.

A. Tilting or fixed shading of the collector. Collector tilt must be considered carefully on any solar combisystem to maximize heat collection seasonally when it is needed, and minimize it when it is not needed. A steep tilt between 65 degrees to vertical, for example, will favor winter collection and reject much of the mid-summer heat in most U.S. locations. A steep tilt can also be augmented with a carefully designed, fixed roof overhang for summer shading, (typically on wall mounted panels), to further reject summer heat gain if required.

B. Night circulation tank-cooling through collector. Flat-plate panels can be used at night for cooling. This is known as night sky radiant cooling (NSRC). NSRC cooling can be accomplished using glazed flat-plate solar heat panels or, even better, using unglazed flat panels (often used to heat swimming pools). In many recent installations we have included control settings that allow the warm floors to be cooled at night in summer by running the solar collectors backwards at night. Similar control functions can be programmed to dissipate heat at night from overheating water tanks when the stored heat is not being consumed.

C. Active heat dissipation (to ground, fan coil or zone). It is common practice to program the control system to dissipate heat using the heat storage capacity of existing hot water tanks, garage floor, ice-melt sidewalk (or other normal masonry heating zones) to cool the collectors in a controlled way. In some cases, this can be used as heat banking to preheat a garage floor for winter or accomplish some other useful heat-buildup strategy. 

Existing fin-tube convectors or fan coils are also sometimes employed for intermittent cooling, too. When controlled properly, human comfort is not compromised and steam is prevented in the collectors, using existing in-floor or in-ground zone loops. The use of existing heat distribution equipment for overheat control can eliminate the need for more complex cooling system add-ons. This approach can extend the life of the solar heating equipment by keeping it within a more moderate temperature range during normal operation. It will not, however, offer any temperature protection during a power failure on a sunny day unless automatic emergency electric power is included.

D. OEM thermal disconnect, venting or dissipation. Ask your preferred original equipment manufacturer (OEM) solar equipment supplier what is new in cooling. Solar manufacturers have been thinking about this for a while now, and along with new controls, some have come up with other interesting products. For instance, Apricus and Butler Sun Solutions each provide heat dissipation equipment that works by thermal expansion fluid diversion into a cooling system. 

Also, collector manufacturers have put some thought into cooling. Some vacuum tube collectors (e.g. Thermomax) have a high limit temperature shut-off built into each tube, and EnerWorks provides a flat-plate collector model that includes a heat activated ventilation system built into the frame. These OEM cooling strategies are very different from one another and designed for their own packaged systems, most commonly available with smaller domestic water heating designs.

E. Hot water P&T blow-off. Every pressurized hot water tank must have a P&T valve for safety reasons required by the plumbing code. When a solar heated water tank gets too hot, the P&T blow-off will act to cool it with make-up water as the overheated water blows off. The P&T valve is not intended to be an operating control, so when this happens, cross your fingers and hope the P&T valve stops leaking later when things cool down. This is a cooling system of last resort, and is not recommended for normal operation.

Disclaimer: These articles are targeted toward residential and small commercial buildings smaller than ten thousand 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. Back issues of this column can be found in the archives at the TMB Publishing and SolarLogic LLC websites.

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. Visit www.solarlogicllc.com for more information.