Faulty Solar Anti Frost Protection Valve (FPV) Leaking? Why? And how to Prevent It?

A faulty solar hot water anti frost protection valve (FPV) will leak water from one corner of the solar collectors. The actual panels are usually fine with no visual cracks or wet patches underneath. This can be a common fault for many Australian solar hot water heater systems as the winter sets in each year.

So why and what is causing the solar anti frost protection valve FPV failures? How to prevent this? Today we will dive in this topic.


1. What is a solar anti frost valve and how does it work?

Below photos show a typical solar anti frost valve made by AVG and Caleffi that are most common in Australian markets.

AVG FPV anti frost protectoin valveCaleffi FPV anti frost protection valve

These anti frost protection valve design is quite genius in my book, as it can open and shut without using any electronic solenoid or vacuum like in most mechanical machinery these day. The FPV employs a wax filled piston inside coupled with a spring pushing against a sealing seat towards the solar panels. The wax will contract or expand according to the ambient air temperature in order to move the piston. When a frost condition is present, the wax contracts sufficiently enough to compress the spring and open the frost protection valve FPV seat causing a water leakage from the solar hot water panels into the valve itself. Majority of Australian solar hot water systems are direct flow type, meaning the solar collectors are directly connected with the water tanks and  pressurized from the mains supply. This pressure will continuously push water out of the frost protection valve and the hot water from the storage tank will move into the solar panels to fill the void from the leakage. This hot water will in turn heat up the solar panels and protect the panels from freezing. At the same time frost protection valve FPV heats up itself from the hot water leaking through it, until the FPV wax expands and pushes piston to shut off the leakage by closing the valve sealing seat. The cycle will then repeat when the solar panels slowly lose heat and the anti frost valve FPV opens again. 

From above, the key points are the frost protection valve senses the ambient air temperature and does not care of the actual water temperature. The FPV operation relies on a mechanical wax piston mechanism and won’t work if this mechanism is damaged. Finally, the seating seal is very important at opening and shutting the water leakage.


2. Why does FPV fail/leak? What is the solution?

The anti frost protection valve can fail in 2 ways. Firstly, the debris in the water can accumulate at the valves’ sealing seat seals. This is largely due to the position of the FPV at the right angle end of the collectors in a dead corner. It is only made worse by the heat, as the sludge and calcium will build around the valve. As one can imagine, after the months’ build up in the summer periods, the gunk will rush into the valve when it opens for the first time in winter and won’t be able to shut off again. This can be easily sorted by taking the valve off the solar panels and give it a good flush with a water hose. The water pressure in the panels should push out any debris in the water passage way inside the panels. A re-installation can test out of this problem is solved. This should be considered as part of the periodic maintenance in solar hot water systems with such valve.

The second reason for the FPV failure may be more troublesome. The frost protection valve is attached to the solar panels which can reach up to 150 deg plus in summer for a prolonged period. This can over time damage the piston wax mechanism. The wax may get cooked and change its physical property and no longer expand/contract at the designed temperature range. This is when some panels may leak prematurely before even hitting the usual 3 deg frost air temperature. In other scenario, the heat can cause volatility in the wax and some may even escapee via the piston assembly. When less wax is present, the insufficient piston expansion may not even compress the spring enough to shut the valve and will leak all the times.

To address this heat problem, a “dead leg” may be used. A dead leg is basically a length of copper pipe with fitting that keeps the frost protection valve away from the panels and reduce the heat exposures in summer. Because the valve still opens and shuts based on the ambient air temperature, this dead leg solution does not affect the valve’s operation and the water can still travel through the dead leg when the intended water leakage takes place. 

There are downsides to the dead leg solution. One is aesthetics, as the panels won’t look nice on the roof with a leg, especially for new homes. Other reasons include cost and labor. Many plumbers may not understand or use this concept. It is obviously much simpler to screw a FPV directly onto the panel fitting, when no specific instructions are given by most manufacturers. Here at www.allplumbingonlien.com.au, we sell a 100mm extension fitting as shown in the picture below. It is polished chrome finish to please most eyes. Being one piece fitting, it means fast installation and eliminates unions/joints otherwise used with a copper pipe to save time and less future leaking points. 

AVG Frost Protectoin Valve with dead legAVG Frost Protectoin Valve with dead leg

I must a make note that the FPV should always be installed at the opposite side to the cold water inlet at the bottom of the solar collectors. e.g. if the cold water enters the collector from the bottom left and leaves at the top right, then the FPV should be installed at the bottom right dead end corner. This position is the coolest point in a collector. 


3. Do we even need FPV and how many do I need?

After reading the above, many may feel the troubles associated with a frost protection valve. Do we need it in the first place? It depends. The main job for the FPV is to prevent water freezing in your solar collectors and cracking the copper pipe passage ways inside. If your area are in the major metro cities around Australia (except Canberra), you are probably fine and don’t need this at all. In frost areas such as in land or high altitude areas, the frost valve may be a good investment, considering replacing the panels can cost thousands after the labor and equipment to raise the heavy panels to a high roof especially.

In these frost areas, the customers may consider using evacuated tubes instead of flat panels. There is no water inside the glass tube to freeze (it is a vacuum heat pipe inside a vacuum double walled glass tube) and the only water passage way inside the manifold is well insulated (flat plate VS evacuated tube deserve another article of its own, which I am an expert of. Leave me a comment if you like to hear more). In this case, the FPV may not be required even in snow condition.

Roof top systems (water tanks on the roof) are easier to freeze, because it relies on natural thermosyphon and no active protection device. In contrast, a pumped system often has a roof sensor and controller and the controller often has an active anti frost function and can start pumping water when the roof temperature drops to 3 degrees. A FPV may not be required in a pumped system. 

Luckily, when a FPV is required, you will really only need 1 such valve in a system. I used to work a solar hot water system consultant for many big manufacturers in Australia as such as Apricus, Aestiva and Consolidated energy. Manufacturers only specify one at the manufacturers’ level.

Some myth form some plumbers suggests 1 valve each panel, thinking it provides better coverage. However, this is really overkill. The valve opens by ambient air temperature, not by the water temperature inside the panels. There is no difference in the accuracy by having 2 valves, as they all sense the same surrounding air. 2 valves will give better water flow, but in most Australia frost condition, the temperature is simply not low enough to cause rapid freezing (think Australian winter VS Europe/US). More valves means more potential failures during its service and added costs.


4. How do solar hot water systems cope with freezing conditions in Europe or US? Why don’t we use them here in Australia.

In really cold regions around the world, the solar hot water system used is known as a drain back system. It literally means the water in the panels will drain back and no fluid there to freeze. But how does this happen and why not in Australia?

The plumbing standards and ways of doing things are vastly different in other countries to Australia. In a drain back system, the water tank is not pressurized by the mains supply, but vented to the atmospheric pressure. There is filler valve automatically adding water to the tank when hot water is drawn (just relate to how your toilet water tank works, it always fills up automatically after flushes, but not under any pressure). The system has a big powerful pump that can actually pump heads to the roof when in work to allow the solar circulation. As soon as the pump stops such as in cold winter times, the water from the roof will simply drain back down due to gravity, as there is a pressure from the pump or mains supply to keep them there.

In Australia, almost all of our systems are direct flow and the collector will always have pressurized water inside (water will piss out if loosening a fitting on the collector, even if no pump running or in a roof top system without a pump), so a drain back mechanism is not possible.

Another method is to use double jacket tank, also known as a mantle tank system. The outer tank cavity is filled with food grade anti freeze and this anti freeze is what circulates to the solar collectors instead of water and never will freeze. The inner tank is filled with portable water and heated up by the antifreeze from the outer tank.

Similarly, a heat exchange in the forms of wrap around coils or in water coils or external brazed plate heat exchanger can be used too and filled with anti freeze for the solar circulation loop. However, all such anti freeze types are not popular in Australia, due to costs and anti freeze servicing. Furthermore, the heat exchanger type systems are always less efficient than direct flow and less STC government rebates in Australia, further increasing its cost at point of sales.

The author of this article is Siwei Tom Li, who was a senior solar hot water specialist and consultant to many Australian manufacturers such as Apricus, Aestiva, Solargain, Neopower and Astivita. This article is of my original work and contents. Anyone is welcome to share or repost this article to spread the knowledge. Please just quote and reference the source to my website www.allplumbingonline.com.au


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1 comment
  • This is a good article. I have a solar system and am off grid. The annoying thing about the valve failing (one is failing at the moment and this is the second. The last one was replaced last winter) is the fact that it means my water pump cycles too often, even on hot days, draining my battery, so I always have to be mindful of this.

    Anne on

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