What kind of head is it (make and model)? It isn't usually recommended that a head that was designed for raw water flushing, be converted to pressurized fresh water - because it wasn't designed for that application, and if you don't do it right, you can wind up contaminating your fresh water tanks. The following is my recommendation on how to convert a Raritan Crown Head; others may (or may not) be similar:
Converting a Raritan Crown Head to flush by using onboard pressurized fresh water:
While it is generally discouraged, there is a safe way to convert a Crown Head from seawater flushing to pressurized fresh water flushing from your onboard pressure water system. It is always better to use the head only in the application it was originally designed for. However the conversion can be done. It isn't as simple as adding a check valve but if you insist that you want to do it, it isn't terribly complicated if you carefully think about how the system will work when completed, and take every proper precaution to avoid contamination of your onboard fresh water tanks through backflow from the toilet. Note that converting a Crown Head from its original configuration, to a different one after it has left the factory, will invalidate any product warranty on the head, and will also absolve Raritan Engineering Company, Inc. from any liability caused by undesired results from converting and/or modifying the toilet. Once the toilet has been modified from its original factory configuration, Raritan Engineering Company, Inc. assumes no responsibility or liability whatsoever for any operational problems with the toilet, any damage to the boat, or any health hazards to personnel that may result from its having been modified.
The proper way to have a Crown Head flush with onboard pressurized fresh water is to add a tee to the existing pressure water lines in a convenient location, then add a shutoff valve to the output from the tee, add an electrically activated solenoid valve to the output from the shutoff valve, pipe from there to an atmospheric vacuum breaker (anti-siphon device), and from there pipe to the rear of the toilet bowl. When you push the button to flush the head, the solenoid valve opens to allow flush water to the bowl. At the same time the macerator and discharge pump are running to carry the contents of the toilet bowl away to your holding tank, treatment system or other ultimate destination.
How to accomplish this:
1) The seawater pump on the Crown Head must first be disabled. Go to the through-hull that supplies the water to the seawater pump and turn off the seacock, securing it with wire, etc., so that it won't be accidentally opened at some point in the future. Then go to the Crown Head and remove the short hose that runs up from the seawater pump to the elbow at the rear of the toilet bowl. After doing this, remove the 4 screws that hold the metal plate on the rear of the seawater pump, remove the cover, and take out the rubber impeller from inside the pump. Replace the pump cover and 4 screws. The intake pump is now disabled.
2) Turn off your pressure pump and relieve the pressure in your water lines by opening faucets. Find a convenient spot and add a tee to your existing pressure water line, with the output from the tee going to a shutoff valve, either a globe valve or a ball valve, either is O.K., preferably in 1/2" NPT size.
3) Next, you'll need a 1/2" NPT electrically-activated solenoid valve in the same DC voltage as the Crown Head - usually 12 volt (Raritan part # CWPS12A or equivalent part). This is the next component after the shutoff valve. Be sure to note on the solenoid valve which port is marked IN and which port is marked OUT - don't install it backwards. The two wires from the solenoid valve are lengthened and connected to the two lugs on the Crown Head where the large battery wires connect. It doesn't make any difference which wire from the solenoid valve goes to which terminal lug on the Crown Head - you can't get it wrong, it'll work properly either way. So there will now be two wires on each terminal lug on the Crown Head - the power supply wires and the two wires going to and from the solenoid valve.
4) From the output of the solenoid valve, plumb to a nominal 1/2" NPT atmospheric vacuum breaker (anti-siphon device) - Raritan part # PLAVB0-5, or equivalent (P L A V B [zero] - 5). This fitting MUST be the last mechanical device before the water goes into the rear of the toilet bowl. It also MUST BE mounted a minimum of 4" higher than the top of the toilet bowl. Where to mount it? - that's the problem. Perhaps inside the wall behind the head; perhaps in an adjacent hanging locker. It doesn't have to be in the immediate proximity of the toilet, but it must be the last mechanical device before the water enters the toilet bowl, and must be a minimum of 4" higher than the top of the toilet bowl. This component seals tightly when water is going through it, but vents to atmosphere in both directions when there's nothing going through it. It is a safety device to stop any possibility of contaminating the drinking water in your tank(s) due to possible backflow from the head. It is a requirement of the National Plumbing Code for residential applications - and would be best used on a marine application, as well.
5) From the output of the vacuum breaker, run hose to the elbow at the rear of the toilet bowl. Be sure all plumbing connections are secure, that piping or hoses are supported at proper intervals, and that all joints are well-clamped. Turn off faucets, turn on pressure pump and check the system for leaks. When testing the system after completion, add about a gallon of water to the toilet bowl, then press and hold the button in (to flush the toilet). The discharge pump and macerator should run in the bottom of the head (much quieter operation than before) and after a few seconds’ time delay, rinse water should come into the bowl.
Converting the Crown Head macerating toilet to pressurized fresh water flushing adds three benefits: 1) you don't have the dank, dirty, smelly, polluted "green" marina water laying in the bottom of your toilet. 2) The sound level of the toilet, when running, will be remarkably lower - closer to the sound level of your toilet at home, often quieter. No more "747's" taking off in the middle of the night... 3) The battery power consumed by the head will be substantially less than before, as well.
Warning: Pressurized water must never be run directly into the seawater pump of any marine toilet - they aren't built to withstand pressure - they're built to move water only. -- Unless, of course, you're looking to have an unusual "fountain" in the head of the boat... Further, there's no provision to preclude contaminating your tanks when you pipe pressure water directly to the head without installing the necessary safety device(s).
