Remove Your Filter Cartridge To Check If It Needs To Be Replaced

Solar Pool Heater Not Working? Clean Your Filter!

In Education, Maintenance 9 Comments

I always get that sinking feeling in my stomach when a client reports that their solar pool heater is not working. What that usually means is the pool is not getting warm, or at least the solar pool heater is not performing as well as it once did. Once I get over the initial uneasy feeling, I get back to rational thought and go through the technical diagnostics. In the end, a large majority of the problems come down to one solution — clean your filter!

This is where I get that quizzical look of disbelief from people. In fact, some argue vehemently. “I have a great pool company. They clean my filter weekly. That does’t make sense. Yada, yada, yada.”

It’s hard telling people that their pool is not being maintained adequately. Some take it as if I have cast a serious aspersion upon them. And the last thing I want to do is throw pool service companies under the bus – we work with many of them. But in the end, a dirty pool filter restricts flow, reducing the effectiveness of solar pool heating systems (not to mention the cleaning effectiveness of your sanitation system). Much like a dirty air conditioning filter, a dirty pool filter needs to be replaced periodically. In fact, we recommend replacing the filter with a new one every 6-12 months depending on how much use you pool gets. If you don’t have a screened pool cage or have other vegetative debris issues, more frequent cleaning and replacement is required. Despite a regular and vigorous cleaning schedule, eventually small particles clog filter cartridges, and replacement is the only option.

The pump did not have a chance. With this kind of pressure, the pump isn't moving any water!

The pump did not have a chance. With this kind of pressure, the pump isn’t moving any water!

I recently went to a home where the pool was just 70ºF. That’s cool for this time of year, and the homeowner had a well sized solar pool heater. My attention went straight to the filter. When I arrived, as in many cases, the pressure gauge on the filter was broken. On this particular filter style, it is hard to remove the cover screw without breaking the gauge eventually. Fortunately, I always carry spare gauges with me, so my first order of business was to swap out the broken gauge.

When I started the 1.5HP pump back up, sure enough, the pressure rose immediately to 39 PSI! If you look at the performance curve for typical pool pumps, once the total system pressure (called total dynamic head) exceeds 30 PSI, the flow rate starts to drop dramatically. The filter pressure is only part of total dynamic head, ignoring the suction pressure. In other words, the total pressure in this system was well over 40 PSI, and at that pressure the flow rate had dropped to almost nothing. There were a few hints before measuring the pressure. For example, the pool had little deck fountains that had dropped to a trickle, and the check valve with clear cover showed a flapper that was barely moved out of the closed position. In truth, I knew we were looking at a flow restriction issue before I even replaced the gauge.

The next step was to check the system pressure with the solar on and bypassed. If there was restriction in the solar panels, bypassing the solar would have shown a dramatic drop in the pressure. In this case, it did not, which indicated that the flow restriction was unrelated to the solar panels. All of the valves were in the correct position, so the issue appeared to be the filter cartridge.

The all-telling diagnostic step is what we call “the filter test.” You can operate your pump with the filter cartridge removed and check the pressure. (Warning: when operating your pump without the filter in place for a significant length of time, bypass the solar to prevent debris from entering the panels). If there is more than a couple of PSI drop, you have a dirty filter. In this case, the pressure went from 39 PSI to 24 PSI with the solar in operation! The check valve flapper opened up wide, and the fountains started working again.

In this case you could plainly see that the filter was filthy and had been seriously neglected. It didn’t look like the pool company had cleaned it in months. The homeowner admitted that the cartridge had probably not been changed in over three years. I’m not in the business of getting pool service companies fired, so I gently explained how it is important to maintain the filter properly and replace it on a regular schedule. That’s just part of the cost of owning a pool, and a clean filter will result in an easier to maintain pool. It’s understandable that your pool service company, who is charging you good money to maintain the filter cartridge, doesn’t want to tell you that there is an additional expense of periodically changing your filter, but that is the reality of proper pool maintenance. The best thing to do is ask them if they can economically perform this service for you. Tell them you want your pool properly maintained, including regular cartridge replacement. They will be happy you asked, and will make a few extra bucks performing this service for you!

Remove Your Filter Cartridge To Check If It Needs To Be Replaced

Remove your pool filter cartridge to see if it needs to be replaced. After removing this cartridge, the pressure dropped by 15 PSI. A filter cartridge should only add 1 or 2 PSI at typical pool circulation flow rates.

If you solar pool heater is not working, or not working like it used to, your first step should be to investigate your filter cleanliness, clean it, and possibly replace it. Low flow reduces the effectiveness of solar pool heating systems, and once your flow rate approaches zero, solar pool heaters become totally ineffective.

If you are having other performance issues with your solar pool heating system, call your solar dealer’s service department and ask for additional tips!



  1. This has been very helpful! I thought my pool guy would know this trick.. guess not.. our pool is only registering 80 degrees.

  2. I have a helicoil system this works but I set the temp on the controller it does not maintain the temp I can set it on manual it will go on and heat up pass-the 91 Degrees there 3 parts the sensor the controller and the other part I don’t know whats it called there is a switch on it you put it in manual and then the value turns to run hot water in the pool how can I test the parts thank you for your help gary thomson I live in az if it matters thanks

    1. Author

      Heliocol is a brand of solar pool heating panel, not a solar controller. I don’t know what controller you have so it is hard to provide advice. The “other” part is a motorized valve actuator, or actuator for short. Apparently that works if the toggle switch makes it move. It sounds like you do not have a digital display on your controller, so you probably need to test the two sensors manually. You need to remove the two leads from each sensor and put an ohm meter on it. At 77 degrees Fahrenheit they should read 10,000 ohms for most controllers’ sensors out there today. If the temperature is higher the ohms will be lower. If the temperature is lower (unlikely in AZ this time of the year) the ohms will be higher. To check accuracy you would need to consult the thermistor chart for your sensor, or a general 10k ohm thermistor chart. If both sensors check out fine you probably have a faulty controller. It’s pretty difficult and rare to change a component in a solar controller to repair it. Usually it requires replacement. You might have a bad circuit board. Best bet? Hire a pro. Unfortunately, I’m two time zones away!

  3. Can you only use a filter cartridge with solar or is d.e. or a sand filter an option for filtration?

    1. Author

      DE And sand filters work, but both produce much more back pressure which reduces flow. Reduced flow equals less solar performance and/or more energy use (if using a variable speed pump). We do not recommend DE and sand filters when heating your pool with solar. In Florida, these are not used much in residential settings. The energy use is exponentially higher to achieve good flow and turnover.

  4. If my filter is clean, as are the skimmer baskets and pump basket, and my pump is working fine when I’m bypassing the solar panels, what would be causing the pressure to drop significantly with very little water flow returning to The pool when I switch the water flow up to the panels?
    Within 20 seconds of the water going out to the roof, the water level in the pump basket drops below the input pipe and the pump starts cavitating. . Tony
    The guy who installed the panel came out for well over an hour and could find nothing wrong. He went up on the roof and tightened all the connectors in the headers and also checked fro leaks. Nothing was wrong. HELP!

    1. Author

      You should also check your water level (in your pool). If the water level is low, when water is sent to your solar panels the pool level might drop below your skimmer inlet, causing the pump to suck in air.

  5. Good article, thank you. Unfortunately I am having the opposite problem – my filter has just been cleaned (by me) and when I bypass the solar system the pressure comes way down but, when I run the water through the panels the pressure spikes to 36+ psi. Do you have any suggestions for clearing a blockage in the solar panel system? Thanks in advance.

    1. Author

      Hi Alan,

      What you are describing rarely indicates a blockage. Even the most restrictive panels should not produce a large spike in pressure. Are you sure there is not a closed valve on the return line coming down from the roof? Some of our competitors (wrongly) put an isolation valve in that position. It should be a check valve, but for some reason they do it wrong, leading to unintentional deadheading of the pump and pressurization of the solar panels.

      If you panels are truly plugged, it is probably hopeless. There is one brand of solar pool heating panel with very small holes that could theoretically get plugged with small particles if you had your pool resurfaced or if it was full of sand or something like that, but it would be very rare. That’s not to say it can’t happen, but usually these issues come down to user error – a valve in the wrong position.

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