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Open Direct System

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

  • Heating Mode Operation
  • Domestic Hot Water Mode Operation

This ingenious system is entirely automatic. It needs no separate actions or adjustments by the homeowner at all. Explanations of the two operation modes are detailed below


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 and back to the water heater.

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.
  • Incredible efficiency. During the summer, limited free cooling is enjoyed. Heat is taken out of the building which saves on the air conditioning bill. The heat is then put in the domestic hot water tank saving again on the gas bill!


Warm water comes out of the tank and into the fixtures. Cold water from the well or from the street must replace the water that was used. This cold water goes through the floor 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 consumed, water flows out of the top of the tank to the point of use. Cold water replaces the hot water 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 used, 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 has played a leading role in the development of these systems and they are accepted by most of the major codes. As with any building project, you should check to make sure that a Radiantec system of any type meets your local building codes.


  • Very low cost.
  • Mechanically very simple, elegant and reliable.
  • Outstanding energy efficiency, particularly if the Polaris water heater is used.
  • Oxygen diffusion issues are not relevant because all of the materials are potable rated.
  • Use of the outside faucet for watering lawns, washing cars, etc will further enhance cooling.
  • A solar hot water supplement can be added initially or at any time in the future.


  • The water must be kept potable and fit for consumption. The flow detail in our instructions and in the animation here must be executed and all materials used in the system must be potable rated.
  • Check with your local codes and if necessary ask Radiantec Company for assistance. Your code official is welcome to call us with any questions.
  • An expansion tank may be needed 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

The Open Direct System is extremely simple and simplicity costs less. Consider, for example, a 2,500 sq. ft. house with two zones. The cost for underfloor materials, pumps and controls is approximately $4,300. Add to this the price of 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, this translates into greater reliability.


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

Environmentally Friendly

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