Open Hot Water/Radiant System

Source: Radiant floor heating company: Solar water heating, The open system
The Open System

The Open Hot Water/Radiant System uses a heat exchanger to transfer thermal energy to a storage tank filled with potable water. This approach provides domestic hot water, radiant floor heat, and allows the system to be powered by a very low wattage pump.

A radiant heating system is considered "open" any time the same hot water is used for both radiant heating and domestic hot water. This type of system is very efficient because a single heat source (in this case, a hot water storage tank) provides for all the home's hot water needs. The homeowner doesn't need two completely separate systems, many times with overlapping mechanical components, performing separate heating tasks.

Note that cold water from the domestic supply enters the water heater via the floor tubing so that there's never any chance of stagnant water entering the domestic system. Fresh water enters the tubing every time hot water is used.

And although it looks at first glance as if cold water will be cooling down your floor, in reality that won't happen. The only cold water that can enter the tubing will be the "make up" water to your water heater. If no hot water valves are open in your domestic system, the radiant system is essentially "closed". In other words, cold water cannot enter the system unless there is an open hot water valve in the house somewhere. Normally, only the circulator pump supplying the radiant tubing can force water from the water heater into the tubing, and back, when your zone calls for heat.

Open system with solar tie-in

Solar water heaters interface well with radiantly heated floors because the large thermal mass common to radiant systems provides an excellent storage medium for the energy generated during the day. At night, this stored thermal energy is slowly released into the living space. The schematic below demonstrates how a solar thermal array can interface with an open radiant system.

The Stiebel-Eltron hot water tank has two heat exchangers into their storage tank. The lower (solar) coil heats the tank to a usable temperature, and the hot water is drawn off as needed for domestic and heating purposes. Fresh water enters the tank in direct proportion to the amount drawn off for domestic hot water. When hot water is taken from the tank for radiant heating purposes, it simply returns to the tank for re-heating.

Since solar panels can heat the tank to near boiling in the summer, a mixing valve tempers the potentially scalding water to a safe level. If cloud or winter limit the amount of sunlight , an on-demand (back-up) unit heats the upper coil so that hot water is always available for domestic or radiant heating purposes.