A pressurized solar thermal installation is one of the most robust options available. Your standard two-minute monthly maintenance checks should suffice to keep the system in perfect working order for at least 10 years, though you should add a pressure check to the monthly routine just to keep an eye on things.

After the first 10 years have passed, you will need to check for three possible problems:

1. Solar fluid issues.
2. Pump problems.
3. Expansion tank troubles.

Pump problems are the easiest to spot, as they stop the system working. A broken pump needs to be replaced and is indicated by the storage tank being cold on a sunny day.

Expansion tank troubles can cause the system to fail. If your solar fluid is not circulating (the storage tank is cold), but the pump(s) and controller are working, the expansion tank's bladder may have failed. This is unfortunately inevitable and requires a new expansion tank.

To test the expansion tank, tap the bottom. If it sounds hollow, it's fine. If not, release a tiny bit of pressure from the Schrader valve on the bottom of the tank – if anything but air comes out, replace the tank.

Many solar thermal experts recommend changing the expansion tanks when you change the solar fluid, to ensure that everything is renewed at the same time. Most expansion tanks have a life of 15-20 years, especially those with a heavy duty bladder (recommended).

Checking the solar fluid

This is extremely important in pressurized systems as the solar fluid is an antifreeze mix of water and propylene glycol. Over time, that mix degrades and becomes less efficient, loses its ability to protect against freezing and becomes acidic (and potentially corrosive).

You should schedule the first fluid check at the 10 year mark, before the cold part of the year when freezes may occur. If the test comes out perfect, schedule the next test for the 15 year mark. If not, schedule accordingly (in one or two years, depending on the level of degradation).

To test the solar fluid, you will need a special acidity tester – you cannot use automotive testers for solar fluid.

This is most often a refractometer designed for propylene glycol, which is available from specialist solar stores, scientific instrument suppliers and some auto supply stores. Make sure it is a model designed specifically for propylene glycol or it is of no use.

Place a drop of your solar fluid on the refractometer, point it at something bright and look through the appropriate opening to see a scale to measure freeze protection. Each model is different, so check the manufacturer's instructions on how to test and read the results. Most are calibrated to show the "freeze point", though some show the "burst point" – you want the former.

You should also test the acidity level (pH) of your solar fluid, using either a pH tester or litmus paper. pH 7 is neutral. Solar fluid should never drop below pH 7.5 (a little bit alkaline).