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Radiantec’s Open Direct System

The Open Direct System is one single system that operates in two distinct ways:

  • Heating Mode Operation – In this mode it heats your home.
  • Domestic Hot Water Mode Operation – In this mode it provides hot water.

This ingenious system is entirely automatic. It seamlessy heats your home AND provides domestic hot water without any input from the homeowner. You don’t have to make any separate actions or adjustment at all to use it. This is how the two different modes work, starting with heating mode:


When the thermostat senses the need for heat, it turns on the pump. Warm water is pumped from the water heater through the tubing within the floor, radiating heat through your underfloor radiant system and into your living spaces. The warm water reaches the end of the tubing loop considerably cooler than it started and makes its way back to the heater to be reheated.

Note: Cold water does not enter the system during the heating mode unless someone draws hot water out because the system is already full.

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Open Direct Legend

Open Direct System Example

FLOW DESCRIPTION – Heating Mode Operation

  • When heat is called for, the pump comes on and water flows out of the top of the tank, through the heating zone and back to the bottom of the tank to be heated again.
  • This system is incredibly efficient. During the summer, limited free cooling is enjoyed, which saves on your air conditioning bill. Heat is absorbed from the building as the water in your radiant underfloor heating system warms up to room temperature. That room temperature water gets drawn into the hot water tank as hot water is used in the home, and gets replaced in the underfloor radiant tubing with cold water. The room temperature water pulled into the domestic hot water tank from your radiant system also costs less to heat, which saves again on your gas bill!


Warm water comes out of the tank and flows into your fixtures such as sink, shower etc. Cold water from the well or from the town water main must replace the water that was used. This cold water goes through the underfloor radiant tubing on its way to the water heater. This prevents stagnation of the water within the tubes during the summer and provides some free cooling.

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Open Direct Legend

Open Direct System Example

FLOW DESCRIPTION – Domestic hot water

  • When domestic hot water is used (at a sink or shower etc), water flows out of the top of the tank to the point of use. Cold water replaces the volume of hot water that was used by flowing in at the top of the system, through the heating system, and then into the bottom of the tank. The reasons for this are as follows:
  • The possibility of stagnation, particularly in the summer when the heat is not on, is absolutely eliminated.
  • In the winter, this flow pattern gives priority to the domestic hot water use if both heat and hot water are called for at the same time (so the person in the shower doesn’t run out). There is no efficiency loss in the winter; any heat taken out of the floor is put back into the tank so there is no net energy loss to the system.

The Radiantec Company played a leading role in the development of these systems and they are accepted by most of the major building codes. As with any building project, you should check to make sure that a Radiantec system of any type meets your local building codes.

Check with your local codes and if necessary ask Radiantec Company for assistance. Your code official is welcome to call us with any questions.


  • This system is very low cost.
  • It’s mechanically very simple, elegant and reliable.
  • Outstanding energy efficiency, particularly if the Polaris water heater is used.
  • Oxygen diffusion issues are not a factor because all of the materials are potable rated.
  • Using the outside faucet for watering lawns, washing cars, etc will further enhance cooling.
  • You can install a solar hot water supplement from the start or you can add one at any time in the future.


  • You must keep the water in all parts of the system potable and fit for consumption. The flow detail in our instructions and in the animation here must be executed. All materials used in the system must be potable rated.
  • You may need an expansion tank if check valves or backflow preventers prevent expansion water from backing up to the rest of the water system.
  • You must be sure that the system pressure is compatible with tubing temperature and pressure limits.


We believe this system is the best alternative. It is simple in design, efficient in operation, has modest initial costs and can make use of environmentally friendly solar technology. When and where local codes permit the delivery of space heating and domestic hot water through a single system, we recommend the Open Direct System. Here’s why:


The Open Direct System uses a single domestic water heater for space heating and domestic hot water, eliminating the need for a boiler. The low operating temperatures of radiant heating allow the use of state-of-the-art “condensing” domestic water heaters which are 95 percent efficient. The summertime cooling feature is free and actually lowers the costs of heating domestic hot water (see diagram).

Initial cost

Our Open Direct System is extremely simple and simplicity costs less. Consider, for example, a 2,500 sq. ft. house with two zones. Underfloor materials, pumps and controls will cost you approximately $4,300. Add a high-efficiency Polaris water heater (no boiler is needed) and the cost totals $7,500, an unbeatable price for a high-quality system.


Besides keeping initial costs low, a simpler system translates into greater reliability.


Major components are all made of stainless steel or polyethylene plastic and should last a lifetime. Additionally, our components are backed by exceptionally strong warranties.

Environmentally Friendly

This type of system can be designed to use solar energy initially or it can be converted to solar at any time.
Click here to see how the open direct system can make use of solar energy.