Thanks for that quick reply. And LOL, you are spot on with the test kit suspicion. I'm just thankful you still replied, knowing that I'm that bad.
One important note that I might not have been clear on. I have NOT tested the heater since I descaled it and added the flow meter on the output side. I still waiting for a new EGL sensor before I can do that. I'm also going to get all the insulation I destroyed reinstalled before testing again. It might all be good at this point but I'm trying to prep for if it trips the sensor again after all this.
I've never really had an algae problem so that never even occurred to me and I didn't know there was such a thing as unseen algae that would clog filter cartridges that badly/quickly. That said, I do have a flow meter on the heater output now and it's showing that I have very good flow rate. At the normally running bypass valve position, I'm currently getting 60GPM out of the heater. If needed, I could get that up to 100GPM by adjusting the bypass valve but I believe too much flow rate can be an issue as well.
Should I still be able to get those flow rates if I have algae clogging the filter?
The heater should only need about 30 GPM flow but I can question the calibration of your flow gauge. You don't normally get pool equipment flowing at 100 GPM. What pump do you have with what size pipes?
Post pics of your heater and equipment setup and valves so we can see your situation. Also please create your signature with details of your pool and equipment.
It is a lot less expensive ruling out the water chemistry issues before starting to replace hardware.
What is your water temperature now? What was your water temperature when this problem began? Algae is usually dormant as water temperature gets below 60F. But as you heat up your water the algae gets more active the warmer the water gets. So as your heater runs and warms the water the algae problem grows with the warm water.
If you had a pure mechanical problem you would not get the heater working for a while and then the progression of knocking and shutdown.
Remove your cartridges and run your system with no filter. If everything runs well then it will confirm your cartridge clogging is slowing your flow over time.
Sorry, I updated my signature with pertinent info about my setup.
The service manual for my model heater (purposely oversized for my size pool, long story) states a minimum flow requirement of 40GPM and a maximum of 125GPM. The pump is 1.5HP with 2.5in pipes. I don't have a current pic but I can go out and take one when it's daylight and the weather improves a bit.
The water temp, when I did the last test, was about 46F when I first turned on the heater. When the knocking first started, I don't know what the starting water temp was but I'm sure it was below 60F. We were getting ready to have an unseasonably warm weekend and I was going to heat the pool for a couple of days. During the last test, I never heard the knocking. It just shutdown when the EGL tripped. It's quite possible the fact that most of the insulation around the edges of the components is missing and allowed hot exhaust gases to enter areas where it shouldn't have. That's why I ordered all new insulation strips to install to rule that out as I'm sure they serve an important purpose.
I'll mention again, I cannot confirm that I even still have a problem as I'm still waiting for some parts so that I can get it all back together for a new round of tests. There's a good chance that I might solve the problem with the new insulation installed as it should be and freshly descaled heater as well as fresh filter cartridges. I'm sorry if I've confused the issue but I'm just trying to find out if there are any other items I should be looking at IF it happens again once I have all this put back together and test it on a nicer weather day.