Electric toilets and joker valves
The vented loop in the intake line for an electric toilet has a solenoid valve that's connected to the flush button Pushing the flush button closes the air valve so that the pump can prime, releasing the flush button opens it again to break the siphon
You said, " I strongly disagree with this idea of rebuilding toilets on some schedule, replacing parts on some schedule - like replacing a joker valve "at least every other year" - which implies preferably annually. This is a waste of time and money."
I hope you don't apply the same thinking to the preventive maintenance for the engines in your boat and vehicles! Unlike household toilets, marine toilets have moving parts that require PREVENTIVE maintenance. It's called "preventive" maintenance because it PREVENTS problems that have to be fixed. You get to do PREVENTIVE maintenance on YOUR terms, when it's convenient for YOU...unlike repairs, which NEVER are needed at a convenient time.
Since you're unaware of electric solenoid valves in vented loops, you've only had manual toilets...and it's time you learned how a manual toilet works:
JOKER VALVE 101
Most people think that the only thing the joker valve does is acts as a check valve to stop backflow from returning to the toilet or odor from the tank from escaping through the toilet. But that's not a joker valve's most important function...in fact, the joker valve is THE single most important replaceable part in a manual toilet. Have you even bothered to LOOK at yours in the last 10 years??
Here’s how the discharge half of the pump works: On the upstroke of the piston, a vacuum is created in the area beneath the piston. This causes the joker valve to close tightly, and the flapper valve beneath the pump to open, allowing some of the contents of the toilet bowl to be drawn into the bottom half of the pump. Then, on the down stroke of the piston, the flapper valve is slammed shut, and the effluent is forced out of the bottom of the pump, through the joker valve, and off down the line. But when the joker valve becomes worn and/or there's a buildup of sea water minerals on it, it can no longer seal tightly on the upstroke of the piston...less vacuum is generated when you pump it. And as it becomes more worn--as the slit gradually becomes just an open hole, creating less and less vacuum--finally the bowl contents simply move up and down a bit, but don't go anywhere. Sometimes the flapper valve needs to be replaced too, which is why toilets should also be rebuilt at least every 5-6 years as PREVENTIVE MAINTENANCE.
You prob'ly won't notice the loss of efficiency at first because it's so gradual...same as we don't see that we've gotten a little older than we were yesterday when we look in the mirror each morning. But I guarantee you that if it's been more than a couple of years since you replaced the joker valve, you need to pump the toilet at least 50% more times to move the bowl contents to the tank...or, they just aren't getting there any more.
You said, "The best way to keep a marine head working is to flush with lots of water when you use it."
Really?? You must have VERY large holding tank, or you don't bother to use it. Because the right way to keep a marine toilet working to spec is by keeping the pump well lubricated and by doing preventive maintenance on schedule. When a toilet is working anywhere NEAR specs, it can move bowl contents at least 6 linear feet and up to 4 vertical feet in the dry mode...more than far enough clear the top of any vented loop. Maintaining your toilet and learning to use the dry mode to do more than just empty the last of the water out of the bowl can double the number of flushes your tank can hold.
You also said, "On the other hand, if your system is plumbed to that it's uphill to the holding tank, then it's just designed wrong."
Really??? Please tell where to put a vented loop that isn't uphill from the toilet. However, even without a vented loop in the system almost all tanks on sailboats are at least slightly higher than the toilet discharge. The bilges aren't deep enough to put them below it.
However, it's obvious from the tone of your post that your approach to maintenance is "only fix what breaks" and nothing I say will convince you that preventive maintenance keeps things working better longer and costs less in the long run. But it's your boat...whatever floats it for you is fine with me.
