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Q&A - Back pressure and a bubbling head

Q: Why do bubbles sometimes come up into the bowl of your head?

A: A head usually bubbles because there is back pressure and the device designed to stop it isn’t working properly. A certain amount of back pressure is normal in a head discharge line, but too much is not.

Normal back pressure can be caused simply by the column of water standing in the vented loop. This is the loop in the discharge hose typically found on many boats, particularly those with heads below the waterline. The purpose of the loop is to prevent water outside the hull from flooding into the head bowl. The hose rises well above the level of the bowl, and as a result there is a column of water between the top of the loop and the head pump’s discharge port. This column has weight and thus causes a small amount of back pressure.

Another cause of back pressure can be a restricted discharge hose. This isn’t unusual, particularly in older installations where the inside diameter of the hose is reduced because of the gradual buildup of calcium-like deposits. Back pressure also can come from holding tanks if the holding tank vent isn’t large enough or is obstructed. You also can get back pressure from ambient water pressure generated while under way if the through-hull is installed in a poor location.

Heads deal with back pressure in various ways. Many use a flapper or joker valve at the head’s discharge port. Both of these are made of rubber or similar material and will tear or otherwise fail eventually. Also, with time, a calcium-like deposit can build up on the rubber lips, resulting in a poor seal. And if you’ve used a cleaning product not recommended for the head (perhaps an oil- or chlorine-based product), this may have damaged these valves and impaired the seal. Any of these events will let in back pressure, and you will see bubbles and/or water coming back up into the head. This water can be smelly because it comes from the discharge line.

Assuming the hose isn’t constricted, usually the best thing to do is replace the valves. They aren’t expensive and often are relatively easy to replace if you plan the job, rather than doing it in an emergency with a head that’s just been used.

If the discharge hose is constricted, you should be able to see the buildup when you remove the hose from the nipple of the head discharge, holding tank, or through-hull fitting. The best way to remove this crusty deposit is to remove the hose and beat it against something — or, far better, replace the hose with new hose manufactured for the purpose. Always be sure a holding tank vent is clear. The process of decay in holding tanks creates gasses, not to mention the pressure you introduce simply by pumping the head. Some of these gasses can be dangerous.

Some heads use techniques for pumping and back pressure prevention that differ from what I’ve discussed here. For example, some systems utilize vents. Be sure to check your head user manual and, if needed, call the manufacturer to get help specific to your application. Take care of the problem, because if you don’t the results could range from a smelly inconvenience to something much worse.