Expansion Drip Explained

Cobra Explains Geysers: which of these should drip, when should they drip, and when to call a plumber? Read more…
18 May 2021 Technical Info
Expansion Drip Explained

Most houses in South Africa have geysers installed in the roof space, with typically 3 pipes sticking out under the eaves. Cobra Explains which of these should drip, when should they drip, and when to call a plumber?

Have you ever noticed a permanently damp patch, or moss growing on the paving under the eaves, and then looked up only to see the dripping end of a pipe? There may only be one pipe, perhaps two, but in a correct installation, there should be three separate pipes.  Typically, one would be a small diameter (1/2”) plastic or “poly-cop” pipe, another would be a slightly larger diameter (3/4”) copper pipe, and the third, a much bigger diameter (1 ½”) black plastic (LDPE) pipe.

Each of the three pipes is there for particularly good reasons, but if they are dripping onto an area that is an annoyance, this is something that ought to have been considered when doing the Hot Water Cylinder installation, and perhaps something that could be rectified when next you are using a plumber. 

It is best to roughly understand what “goes on” around the HWC installation, and then the function of each pipe will make a bit more sense to the homeowner. Although there are numerous water heating alternatives and types of geysers installed these days, the typical Hot Water Cylinder installed in our modern South African home is a high-pressure, closed system, cylinder. Because the system is in a closed vessel, the incoming mains water pressure first needs to be reduced to within the rating the geyser can cope with. HWC’s are very often 400 or 600 kPa rated, and this will be labelled on a little metal tag on the geyser. For this to happen, a Pressure Control Valve is installed in line with the incoming supply pipe. There are many brands and types of pressure control valves. The second thing which needs to happen is that as the water heats up, as with anything, expansion takes place, and the expanded water needs to be allowed to “get out”, or else it would rupture the closed cylinder, because water, unlike air, cannot be compressed. The relieving of this expansion is taken care of by an expansion relief valve. This function is very often incorporated into the pressure control valve. For the expansion function, we learn about the first pipe. It is typically a thin pipe, most often plastic, because the water which drips out of it will be cold water. This is the pipe that should drip whenever the geyser is heating up, which would be after hot water has been drawn off. There is a precise formula to calculate the amount of expansion that takes place, using all the exact parameters, but a simpler rule of thumb is that the average 150l domestic geyser, being used in a typical family household, would cause roughly 6l of expansion per day. It is for that reason that the expansion outlet pipe should be positioned where it will not cause an annoyance – above a gulley, or planter, etc.

Now that we understand the role of the first pipe, what about the other two? Because an electric geyser is a potentially hazardous appliance there are many safety factors that need to be applied in a compliant installation (SANS -10254). One is that there needs to be a separate safety, Temperature, and Pressure Valve (TPV) installed on the geyser. This valve would open and allow water to gush out if either the pressure control valve or the thermostat failed for any reason. It is installed at a high point on the HWC, and if it opened, the water flowing through it would most probably be extremely hot. It is for that reason that the pipe needs to be metal, and of a larger diameter. Lastly, the HWC and the valves around it need to be installed above a drip tray. This drip tray would have an outlet which usually has a 1 ½” plastic pipe coupled to it and led to the outside as well.

Now that we understand all of that, let us look up again and see which pipe is dripping and decide whether we need to get hold of a plumber or not. If the little ½” pipe is dripping, that is probably normal, unless it never stops. If it is dripping and the power to the geyser is switched off, the drip should stop after a few minutes. If not, this points to either a faulty valve or a faulty thermostat – call a plumber. If the bigger metal pipe has water coming out of it, there definitely is a problem – call a plumber. If the big plastic pipe is dripping, here too there is definitely a problem*, the most common of which, is that the geyser has finally corroded through, and water is leaking out of the casing into the drip tray – the best thing to do here would be to check if you can, by getting into the ceiling and looking if the geyser itself is leaking, and if it is, contact either your plumber or your insurance company.

*If any of the pipes are led into the drip tray, which is sadly a common shortcut, and an illegal one at that, then the expansion drip would be coming out via the drip tray. Although this is not a faulty HWC or valve, it is an incorrect installation and should ideally be rectified.

Hot Water Cylinder installations are a very specialized part of the plumbing and should only be done by registered and qualified plumbers, and certainly not left to the “handyman” – there are simply too many potentially fatal mistakes to be made. The above guide is intended purely for you to be able to recognize the possible cause of a drip that annoys you.

With national after-sales service support by qualified plumbers for on-site repair or warranty replacement of Cobra and Apex Pressure control valves within 48 hours, Cobra encourages plumbing professionals, plumbing merchants, and end-users to make use of our local call centre by dialling 0861 21 21 21 Or sending an email to service.africa@lixil.com.