3-way solenoid valve gets hot
If this is definitely an on/off valve, which is designed to be either fully operated or fully released - see note below - then you can reduce the power it dissipates by reducing the drive power after you initially operate it. Solenoid-operated devices like valves and relays can normally maintain their 'on' state with a considerably lower coil current (holding current) than is necessary to switch them from 'off' to 'on' (operating current) though. After an initial 'hit' of the rated supply voltage to operate the valve, you should find you can reduce the voltage/current to a lower level and it will stay open quite happily with a lot less power.

You could devise a clever circuit to do this but unless you're short of space the easiest way is to use one resistor and one capacitor:







simulate this circuit &ndash; Schematic created using CircuitLab

L1 is the valve coil. Start by measuring its DC resistance, by connecting a multimeter on resistance range to the two coil terminals. You'll need to experiment with the values of R1 and C1 but I would start with R1 about the same as the resistance of the coil and C1 chosen for a time constant of about a second with that resistance, i.e. if L1 measures 100 ohms then try 10 000 µF for C1, if 50 ohms try about 20 000 µF, and so on. D1 is there to protect C1 from a voltage spike when you switch the power off; a 1N4000 type rectifier diode should be fine.

Obviously C1 must be rated for at least your drive voltage, and R1 must be rated for the power that it will dissipate. For example if the coil has a DC resistance of 60 ohms and R1 is also 60 ohms then once C1 has charged the current through R1 and the coil will be 0.1 A, and R1 and the coil will dissipate 0.6 W each.

One catch with this circuit is that if you try to switch the valve off then on again less than a few seconds afterwards, C1 will not have time to fully discharge through R1 and so the valve may not operate properly. Another is that if the coil resistance is really low, the size and cost of C1 may become impractical. If either of those is a possibility you may have to use something more complex e.g. a monostable timer to generate the 'hit' pulse - if so, ask and someone can help you further.

Note: if you had a proportioning valve i.e. one that partially opens one port and closes the other in proportion to the drive current, then this technique would not work well. In that case you could experiment with putting a resistor in series to reduce the current, as perhaps you wouldn't need the full 12 volts to fully close the 'normally open' port.