ASK THE EXPERTS: Expansion Tank for a Thermosiphon System?


I read with great interest Vaughan Woodruff’s article on the use of expansion tanks with solar water heating (SWH) systems in HP162. My SWH system is a Chromagen thermosiphon—an 80-gallon electric heater/reservoir tank paired with a 33-square-foot flat-plate collector—located on the roof of my house. The tank is located above the collector, which circulates a glycol solution that passively transfers heat to the water in the reservoir tank. The piping between them is about 3 feet long.

The pipe coming from the street water main, which serves the reservoir tank, has a pressure-reducing valve that is creating pressure problems in the potable water system. The manufacturer has suggested using a thermostatic mixing valve between the hot and cold sides of the reservoir tank to eliminate the problems created by the existing configuration.

I am considering placing an expansion tank in the system, but I am unsure if this would require a pump. What do you think about locating an additional expansion tank immediately after the pressure-reducing valve to buffer any water-volume expansion issues in the house piping?

Eric Kosak • via email

There are some important differences between the indirect thermosiphon/solar portion of the system that you describe and the pressurized glycol system that I detailed in “Protecting Water Heating Systems with Expansion Tanks” in HP162.

Your system is passive—it doesn’t rely on a pump to circulate the glycol in the system because the storage tank is above the collector; the fluid circulates by convection as it is heated in the collector. The heat from the glycol is transferred to the potable water in the reservoir/storage tank through a heat exchanger. This cools the glycol, which sinks into the collector to be heated again.

The glycol-based system described in my article is an active system, which requires a circulator pump to move the glycol through the collectors and the heat exchanger. In many of these systems, the collectors are located above the water heater. Without a pump, the heated fluid would stay at the top of the system, failing to provide usable heat that can be transferred to potable water. This is exactly what happens during stagnation, which occurs when the circulator pump is unable to circulate fluid due to a power outage or an issue with the controls, or if the water in the water heater has reached its maximum temperature and additional heat from the SWH collector would damage it.

Thermosiphon systems do not require an expansion tank on the solar loop (the piping in the system that contains glycol). Fluid expansion is much more predictable in these systems and can be readily planned for during the design process. A pressure-relief valve, which will open and release some glycol, is used to protect the system components from pressures exceeding their rated capacity. In some cases, the system is designed to release some glycol as it is heated for the first few times until pressure equilibrium is reached.

In contrast, active indirect glycol systems are pressurized upon commissioning. Excess glycol is pumped into the solar loop during startup to increase the pressure to the system specifications. These systems are designed with a properly sized solar expansion tank that helps regulate the system pressure at a level below the rating of the pressure-relief valve. In these systems, if the pressure-relief valve opens and releases glycol, air may be introduced into the system. If this occurs, the circulator pump may become air-locked and unable to move heated glycol from the collectors to the heat exchanger.

Since you have a thermosiphon SWH system, you do not need a solar expansion tank (nor a prevessel) in the solar loop. The potable water system may be another story. Thermal expansion occurs in the potable water system as the water in the tank is heated by the solar loop. As discussed in my article, pressure issues will arise if there is a device between the water heater inlet and the city water supply that does not allow the water to expand as it is heated. If a pressure-reducing valve is preventing the public water system from absorbing the thermal expansion occurring in your water heater, an expansion tank is commonly required. Without one, the potable water pressure can vary significantly. The water heater’s temperature- and pressure-relief valve alone is not a safe means by which to control pressure in the system. There are significant safety hazards associated with pressurized hot water. If in doubt, it may be time to call a plumber.

Vaughan Woodruff • Insource Renewables

Comments (2)

David Schlottenmier's picture

I think his problem is on the potable water side. Their is a conflict between the pressure reducing valve feeding the storage tank, the tempering (mixing) valve on the discharge of the tank and the wide pressure variation caused by the solar thermal energy additions. There is no place for the expanded water to go when the tank is heated because the reducing valves stops flow back to into the municiple water feed. I think he needs an expansion tank on the potable water side so the spring loaded bimetallic tempering valve functions properly to moderate discharge water temperatures and to minimize overpressure events. That is my understanding of his question/problem. My experience is on the practical side. I try to figure the problem and then apply the code to properly address it.

Fred Golden's picture

You menntion the need for pressure expansion tank on every water heater when there is a pressure reduction valve on the city water line. This is required. The tank should be piped into any cold water line, normally within 3 feet of the hot water tank, and can fill with warm water as the tank heats up. Because the solar heated water can easily exceed 120F, a thermostic mixing valve is required to prevent scalding hot water from entering the home hot water lines. It should be set at 115 - 120F to prevent scalding.

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