Answers to the questions you've been asking.

PM's popular columnist John Siegenthaler gets numerous inquiries from readers seeking his help with thorny problems in the field of radiant heat. We have created the "Radiant Forum" as a special question-and-answer section to share with you some of the exchanges between John and his fans.

Our intent is to promote wide dissemination of ideas and technical information. Opinions expressed by either Siegenthaler or the correspondents about specific products and techniques are theirs alone, and should not be construed as a PM endorsement.

If you have a question regarding radiant heating systems, please send it to "Radiant Forum," c/o Plumbing & Mechanical, 3150 River Road, Suite 101, Des Plaines, IL 60018; or e-mail us at

How much variation in supply water temperature is acceptable when several circuits are connected to a single manifold?

John replies: Most radiant panel design procedures determine the supply water temperature for each circuit at design load conditions. It's common for projects involving multiple circuits to have several different supply water temperatures. These temperatures can vary considerably depending on load, floor coverings, and other factors.

Although it's possible to configure hardware that will provide every circuit with a specific water temperature, it's usually impractical from a cost standpoint. Instead, circuits with similar supply temperatures will be grouped together on the same manifold.

Opinions vary on how much variation is acceptable between the theoretical circuit supply temperature and the actual temperature the circuit receives when connected to a manifold. I feel the variation between the highest and lowest supply water temperature should be limited to 15 degrees.

This would, for example, allow circuits with design supply temperatures of 100 degrees F and 115 degrees F to be connected to the same manifold. The water temperature supplied to such a manifold should be equal to or slightly greater than the average of all the supply temperatures of the circuits.

Any slight overheating in the lower temperature circuits can be corrected by reducing the flow rate using balancing valves or by cycling flow on and off using valve actuators.

Remember that tube spacing can be adjusted during the design process to bring circuits that initially have widely different supply water temperature requirements within the suggested 15-degree range of variation. This will usually prove less expensive than creating systems with the necessary hardware to supply many different water temperatures.

Figure 1

How should I pipe a buffer tank into a residential system with many small space heating zones?

John replies: If the system provides both space heating and domestic hot water, I like the approach of a single tank that buffers both loads. The concept is shown in Figure 1.

The heat source in this system keeps the water in the tank between preset limits at all times. For a typical application, the boiler is turned on when the tanks gets down to 130 degrees F and remains on until the tank reaches 190 degrees F to 200 degrees F. The space heating loads simply draw heat from the tank when needed.

Small loads, such as a towel warmer radiator, operated for a few minutes on a cool fall morning may not even draw enough heat from the tank to fire the boiler.

Because the temperature in the tank varies over a wide range, it's important to size the heat emitters in a system based on the average temperature of the buffer tank. Mixing devices used to supply low temperature loads should also be sized to provide design load heat transfer when the tank is near the lower end of the discharge cycle.

Domestic hot water is provided by the immersion coil(s) inside the tank. It's really an extension of the old tankless coil concept, except that the heat is stored in a well-insulated tank rather than the heat exchanger of the boiler -- the latter being much more susceptible to off-cycles losses.

Since the water in the buffer tank is often much hotter than the DHW delivery temperature, It's imperative to provide an anti-scald tempering valve to limit the water temperature supplied to fixtures.

It's also imperative to use a very well insulated tank, and provide protection against heat migration at all piping connections. You're trying to turn the buffer tank into a thermos bottle.

Should I always adjust the balancing valves so each radiant panel circuit operates at the same temperature drop, say 15 or 20 degrees?

John replies: Balancing is about getting the right amount of heat into each area of the building. It's not about having the same flow rate or temperature drop in every circuit.

In many cases a circuit will operate at a higher flow rate than anticipated. This raises the average circuit temperature, and decreases the temperature drop. The result is often higher heat output. If the area it serves is comfortable, don't bother to reduce the flow rate. If the area is overheating relative to other areas that's when it's time to partially close the balancing valve.

The longer circuits on a manifold station will usually have higher temperature drops than the short circuits. That's fine if the appropriate amount of heat is being delivered to each area.

Can I operate a radiant floor circuit at a high inlet temperature as long as the flow rate is very low?

John replies: From the standpoint of thermodynamics, it's possible that a high temperature drop and low flow rate will yield the proper heat output. However, this approach creates very noticeable differences in floor surface temperature from one area of the room to another.

Most owners don't like this asymmetry, even if the room is maintaining the proper air temperature. High water temperatures near the beginning of the circuit may also cause problems with flooring, such as embrittlement of vinyl or discoloration in carpet.

I suggest 15 degrees as the maximum allowable circuit temperature drop in situation where people might walk barefoot on the floor. On larger commercial or industrial projects, the temperature drop can be increased to 20 or even 25 degrees. This will decrease the flow rate requirement and quite possibly reduce the size and operating cost of the distribution circulator(s).

Can manifold stations be mounted lower than the tubing circuits they serve?

John replies: In my experience, this has not lead to problems. Manifold stations for radiant wall and ceiling systems are almost always mounted lower than the tubing circuits they serve. The same can be done for floor circuits.

We've also mounted manifold stations horizontally on the underside of wood framed floors without air entrapment problems. The latter allows manifold stations to be accessed from the basement eliminating the need for manifold enclosures and access panels in the main living areas.

Be sure to power purge the system, circuit by circuit, at start-up. Also, be sure to design the system so tubing circuits have a flow velocity of at least 2 ft./sec. while operating. For 1/2-inch nominal tubing, this corresponds to a flow rate of about 1.1 gallons per minute. This minimum flow velocity ensures that any residual air bubbles will be entrained and delivered back to the air separator.