Hot water reticulation loop (Ring-main) basics by the ITS Heat Pumps & Solar technical team.

Do you hate waiting for your shower to warm up in the morning?

A common way to avoid this is to install a hot water ring-main. This type of system works by circulating hot water around your house to minimise the amount of waiting time and water wasted when you open a tap.

Consider an ordinary hot water system. You’ll have a hot water cylinder somewhere in your house. If you have a large home, the distance between your water heater and your bathroom might be quite a long way.

When you open a hot tap, water has to run from your water heater all the way down to the tap. If the pipes are cold, it will cause the water to cool down along the way so you’ll have cold water coming out of the hot tap until all the pipes have warmed up between your water heater and your tap.

The ring-main solution has a small return pipe installed with allow water to flow back to the water heater. A circulating pump keeps the hot water moving slowly around the system, so that hot water is always available within a couple of metres of each hot water fixture.

The pump is controlled by a thermostat attached to the hot water return pipe. This turns the circulating pump on when the pipe cools down.

The only drawback with a hot water ring-main system is that the pipe thermal losses will increase. This increases your water heating costs. But, if the pipes are properly insulated and you are using a very efficient heat source like a heat pump, this increase is minimal.

If properly designed and installed, a hot water ring-main will provide you with hot water on tap almost instantly, which is essential especially in larger and more up-market homes.

Key components of a ring-main system

The below diagram shows a basic hot water storage tanks system with a ring-main.

diagram shows a basic hot water storage tanks system with a ring-main

The key components are:

  • Hot water storage tank with a ring return port – There are hot water storage tanks that are specifically designed for ring-main systems. The ring return port position on the tank is very important in order to prevent the disturbance of the stratification in the tank.

  • Return pipe – The return pipe is the pipe that completes the circuit from the further point of the hot water supply back to the tank. Since minimal flow is generally needed, this pipe can usually be a smaller diameter pipe than the hot water supply pipe.

  • Ring-main pump – This is the pump that will circulate the water through the ring to keep it warm. Typically it would be a fairly small pump since relatively low flow speed and volume is needed.

  • Ring-main non-return valve – A non-return valve is required on the ring-main return pipe (usually between the pump and the ring return port on the hot water storage tank.) This valve prevents water from being drawn from the ring return port instead of the hot water outlet port of the tank when a hot water tap is opened in the circuit.

  • Ring-main pipe insulation material – The complete ring-main from the hot water outlet port on the tank to the ring return port on the tank must be properly insulated. A 50m total length ring that is well insulated and inside a roof could, for example, have a thermal loss during winter of around 0.5kW. Without insulation this thermal loss can easily be around 2.5kW. If this pipe is then built into the wall or concrete floor, the losses will get even worst as the ring is now trying to heat the structure around it. We unfortunately often get phone calls where installers thought they would not need to use pipe insulation material if they use composite plastic type piping instead of copper. Usually by then these pipes are already build into the walls and the thermal losses are so high that they even struggle to get the water at the furthest tap to be warm enough. It is critical to properly insulate every last centimeter of the ring-main loop no matter what type of hot water piping is used.

  • Ring-main pump controller – In a well-insulated ring-main the water only needs to be circulated once the pipe has cooled down by a few degrees. A ring-main controller function like the one found in the SR81 solar thermal controller allows the user to set up time windows for the ring but also has a ring return temperature sensor. The controller will only allow the pump to switch on when the ring has cooled down by the set value, and then pump until the ring is at the desired temperature again. This results in lower thermal losses in the ring-main as well as very low total power consumption by the pump. It also reduces the disturbance of the stratification in the tank, since the pump is not running the whole time.

Using a heat pump to heat your hot water storage tank will typically result in an average electrical heating cost saving of more than 60%. With the higher piping thermal losses associated with a ring-main, it makes even more sense to use a heat pump to do the tank heating.

A “poor man’s” ring-main

In most cases ring-mains are in bigger houses with higher water consumption and so the typical system starts at 300L or more. The smallest proper ring-main tank we supply is the Reflex 500L form Winkelmann in Germany. We often get clients that want a ring-main but do not want to replace their hot water storage tank with the correct type of tank. In most cases this can be made to work but it will always give inferior performance compared to using the correct equipment for the job. Below is an example of how two standard horizontal 200L geysers with a heat pump as the heat source can be plumbed to connect a ring-main.

ring-main diagram

A “poor man’s” ring-main

As mentioned, the above is a compromise. It is not the correct tanks for the job and so the following must be noted:

  1. The hot water stratification in tank 2 will be disturbed by the ring-main. So, if for example the above tanks are 200L each, you will only be able to draw about 250L out of the system at a time under certain draw-off profiles, and not 400L.
  2.  The ring pump must be temperature controller by a controller like the SR81. This will greatly reduce the stratification disturbance/ heat pump loading disturbance and it will reduce energy consumption. The pump must be set to the lowest speed required.
  3. Variants of the above that we have seen includes bringing the ring back on tank 2, or on the cold inlet of tank 1. This will introduce more problems and should be avoided.
  4. The heat pump must be installed using the ITS patented loading valve technique. Due to the flow restriction caused by the tanks in series etc., the loading valve must be an LK821 of the correct size, and not a LK550.
  5. The heat pump size will be based not only on the number of liters of hot water required per day, but also on the length of the ring main and quality of the ring insulation.
  6. Any difference on the actual port positions compared to the above example can have a drastic impact
    on performance.

For more information on this topic and help with system design, please feel free to contact our technical department and we will gladly assist.