Solar Water Heating System Troubleshooting & Repair

Part 3: Valves
A large, chrome-plated valve.
Schematic symbols for valves.
Coin vents are manually operated using a small flathead screwdriver.
Air-relief valves eliminate air automatically, but have a higher failure rate, especially when installed in places where they are subject to large temperature variations.
Ball valves are preferred by the pros to serve as shut-off valves.
A weak point in gate valves is the stem that connects the valve handle with the round gate, which moves up and down inside the valve. A stem failure results in the valve shutting incompletely or failing to open.
Check valves only allow flow in one direction and are used in forced-circulation and antifreeze systems to prevent thermosyphon losses at night.
Boiler drain valves, also known as fill-and-drain valves, are common on all types of solar heating systems.
Antiscald valves prevent scalding water temperatures by mixing cold water into hot water that’s above the valve’s setting. Upon cold water failure, such as from an upstream cold water valve being shut off, the valves shut off for safety.
Mixing or tempering valves are about half the price of antiscald valves but don’t have the safety shut-off feature and are not acceptable replacements for antiscald valves.
Vacuum breakers are still used in older drainback systems to introduce air into the piping and allow water to drain.
A T&P valve actuates upon a system pressure or a water temperature that’s higher than the valve’s setting.
A pressure-relief valve actuates upon a system pressure that’s higher than the valve setting.
Freeze valves are used in otherwise nonfreeze-protected systems, allowing water to dribble out of the valve when near-freezing temperatures are encountered. However, in solar heating systems, relying on valves for freeze protection has resulted in more collector damage than all other causes of failure combined.

Solar water heating system not working? Check the valves—those simple levers that are ubiquitous in SWH systems.

Gurgling at the top of the collector piping.

Cause: Air-relief valves are used in antifreeze systems to release air from the system. Air in pump impeller housings can stop fluid circulation, possibly causing the pressure-relief valve to open. Small amounts of air will normally gather at the topmost part of the system and air-relief valves are most effective when installed there. Manufacturers’ instructions usually specify an automatic air relief that has a Schrader valve, which is similar to a tire valve.

Solution: The Schrader valve stem can be pressed to ascertain if air is present in the system—if fluid comes out, the air has been purged. A leaking automatic air-relief valve can be replaced with a less expensive and more failure-resistant manual air-relief called a coin vent.

Small leaks

Cause: Gate and ball valves are used to stop the flow of fluids, isolate parts of the system, or to reroute fluids. Gate valves have higher failure rates than ball valves. A common failure of gate valves is failing to close. When valve replacement is required, use a ball valve, which is more reliable.

Gate and ball valves all have handles and a small “packing” nut, which seals the valve’s stem. This nut can loosen and cause a small leak.

Solution: Carefully inspect any leaking valve to ensure the packing nut isn’t the problem. A quick tightening can often remedy the need to replace an otherwise good valve.

Unusually high overnight heat loss from the SWH storage tank, and a difference in temperature between the supply and return pipes when the controller pump is off.

Cause: Check valves allow fluid to flow in one direction and are used in direct forced-circulation (DFC) and indirect forced-circulation (IFC) antifreeze systems to prevent the thermosyphon of the heat-transfer fluid when the pump is off. They are available as swing or spring models—which describes the mechanism that actuates the valve. Spring valves can fail due to the spring breaking, although this is rare. Most check valves fail due to an obstruction—perhaps a small piece of solder—that keeps it open. In some cases, the night thermosyphon results in the backup element actuating at night, resulting in extra energy use (and expense). If a vacation bypass valve has been accidentally left open, it has the same symptom as a failed check valve.

Solution: Valve failure requires a replacement unless an obstruction can be dislodged with what I call the “vigorous tapping procedure (VTP)”—that is, hitting it with a handy wrench.

Leaky drain valve.

Cause: Fill-and-drain valves are also known as boiler drain valves. They are manufactured with pipe thread on one end and garden-hose thread on the outlet. Fill-and-drain valves are normally used only a few times in the life of a SWH system and have very low failure rates. The most common problem is an older valve that won’t shut off completely.

Solution: The easiest solution to stop the leak is to use a bronze hose-thread cap with a hose washer.

Cool or tepid (never hot) water at the tap.

Cause: Tempering or antiscald valves limit the water temperature to prevent burns. The valves have cold, hot, and mix ports and can be either factory-set to a certain temperature or field-adjustable. When water entering the hot port is above the temperature setting, the cold water port opens to cool the hot water before it exits the mix port. When these valves fail, the symptom is cool or tepid water that never gets hot since the water is constantly being mixed regardless of the water temperature.

Solution: Replace failed valves with nickel-plated valves that have Teflon-coated inner components.

Excessive loss of drainback heat-transfer fluid; pump failure.

Cause: Vacuum breakers were used on early drainback systems to prevent freezing by facilitating the draining of collectors. Placed at the highest point of the system, they introduced air into the piping when a vacuum was produced as the collector-loop pump shut off. In areas where hard water is prevalent, these bronze valves are prone to sticking. Their failure was the cause of freeze-breaks in thousands of collectors, resulting in the poor reputation of drainback systems.

Solution: If it isn’t possible to alter the piping or system design to eliminate the vacuum breaker, the valve should be removed every year and soaked in vinegar or another mild, acidic solution to neutralize the hard water deposits. Another fix for systems that cannot be modified (for example, piping that cannot be sloped to the drainback tank) is to install two vacuum breakers to provide some redundancy. Even then, they should be descaled annually.

Leaks at the relief-valve pipe.

Cause: Temperature and pressure (T&P) relief valves are required by plumbing and solar codes on all water heaters, including SWH storage tanks. The valves are available with different specifications; the most common relieve excessive pressure above 150 psi or at temperatures above 210°F. These can develop small leaks.

Solution: T&P relief valves should never be plugged or capped like the leak solution for a drain valve. They all have a manual lever that also actuates the valve. Moving this lever repeatedly while applying the VTP can sometimes fix a leak. But never move the lever unless you are trying to stop a leak—actuating it can often cause the valve to leak. If the leak can’t be stopped, the valve needs to be replaced.

Excessive pressure causes fluid to be released by the pressure-relief valve.

Cause: Pressure-relief valves are actuated as a result of excessive pressure buildup in SWH and hydronic heating systems. Common causes of excessive pressure are pump and control failures and undersized or failed expansion tanks. The pressure gauge on an antifreeze system will read zero if the pressure relief has been actuated. Pressure reliefs should not be mistaken for T&P valves—they lack the white temperature-sensing stem that T&P valves have.

Solution: Troubleshoot the pump, expansion tank, or control to find the cause of the excessive pressure, and repair or replace the component as necessary.

Burst collector tubing (water running out of collector)

Cause: Freeze-protection (aka dribble) valves are designed to dribble water out of a collector to prevent the tubes inside from freezing. This strategy is akin to letting a kitchen or bathroom faucet trickle to prevent household pipes from freezing. The most common freeze valve is plastic; more expensive valves are made of brass or stainless steel. Freeze-protection valves are available in two temperature ratings. A 45°F valve is recommended for DFC systems to protect the collector’s small riser tubes. A 35°F valve is used for integral collector storage (ICS) systems, which don’t have the small tubes. The valves require no energy to actuate. The small flow of water starts at 35°F to 45°F and allows warmer (hopefully) tank or ground water to enter the collector to keep it from freezing.

In DFC systems, the valve is normally between the collector’s hot return outlet and a check valve installed below it on the same pipe. This allows the pressurized water to flow through the entire collector prior to dribbling out of the freeze valve, and replaces the about-to-freeze water in the collector with storage-tank water. Freeze valves have high failure rates, so are seldom used outside of southern Florida. Even if a hard freeze occurs only every 10 years or so, a freeze valve failure will cause numerous burst tubes, usually rendering the collector too expensive to repair.

Since the symptom of a clogged freeze-protection valve is its failure to drain at outside temperatures near freezing, they usually are discovered only after they fail—and collectors have already been damaged. Freeze-valve failures have ruined thousands of collectors and some state, municipal, and utility incentive programs exclude systems using them for freeze protection.

Solution: Modify the system to a drainback or antifreeze system.

Comments (5)

Tiethilo's picture

How do the simple gravity powered tube systems prevent overflow? I've got a suntask solar water heater filed by a higher-up tank but when the hot water tank is full water comes out of the little assistant tank until I close the supply valve. Consequently I've been filling manually by opening and shutting this valve, but there should be a way to make this automatic, no?

Michael Welch's picture
Hi Tiethilo. I cannot find the answer to this, but I suspect that there is a float valve in that little tank that is supposed to shut off the water when it reaches a certain level. Can you see into that little tank?
Tiethilo's picture

It's the most curious thing, it's got three in or outlets with half-inch threads, but there is nothing inside at all. The manual just tells me i have to use this assistant tank on top of the hot water tank if i supply from a main tank (660 gal rotoplas in my case) which cannot be more than 3 feet higher up than the hot water system. i guess mine is a little higher than that which might explain the overflow... did i just answer my own question? :)

Charles Marken's picture

Hello Tietilo
Many of these systems do not have a float valve to automatically regulate the water level since they assumed they would be filled upon need manually. As Michael gives above, a float valve that will withstand hot water will solve this. Float valves are used in many stock tanks, evaporative coolers. older toilets and many other applications where a certain water level needs to be maintained. They can be found at many home centers and plumbing supply houses.

Tiethilo's picture

I'll have a look if i can find a float valve to fit into that tank, but i guess that would involve opening the tank which is tricky as it's factory sealed... Thanks!

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