A Y-strainer is an in-line filter for hydronic systems. As the strainer starts trapping dirt, resistance to flow (pressure drop) will increase. Measuring the pressure drop across the strainer while the system pumps are running, will indicate how plugged the strainer is.
The Caleffi 120 Series Y-strainer has two 1/4" NPT female pressure tap connections - one upstream and one downstream - which are used to measure the pressure drop across the strainer. To determine when the strainer needs to be cleaned, first take a differential pressure (DP) measurement between the pressure tap connections when the strainer is new and the system is running at design flow. Use this DP as your "clean strainer" reference. In general, when the pressure drop across a y-strainer gets to about 5 or 6 psi differential, it's time to clean it.
Never let it get too plugged; for example, more than 10 psi. Since strainers are inline devices, increased differential pressure means increased system head loss and increased pump power, all which lead to lower system performance and higher operating costs.Take DP measurements frequently during startup of the system and then stretch out the frequency as part of a normal maintenance program.
A convenient way to temporarily measure the pressure drop across the Caleffi 120 Series is to buy the models with PT taps pre-installed on the pressure ports. Use a portable, wet-wet differential pressure measuring device with gauge adapter plugs to take readings. Permanent indication is nice; there are companies that make wall-mount wet-wet differential pressure gauges that you can connect directly to the 1/4" NPT ports on the Caleffi 120 models without PT plugs.
If connection to a building automation system is required, find a wet-wet differential pressure transmitter that will connect directly to the 1/4" NPT ports and has the analog output signal the BAS needs.
Cleaning the 120 Series is easy, especially if there is a 538 Series blow-down valve installed on the drain port (recommended). Usually, a blow down through the 538 will sufficiently clean the strainer. The strainer can also easily be removed for inspection and cleaning if necessary. Check out the 120 Series Installation Instructions for all the details.