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.

Dumb Valves

Can I save money by installing a four-way mixing valve without an actuator/controller?

John responds: Of course you can! Be advised however that the four-way valve will control water temperature about as well as you can drive a car with The ClubR permanently installed. Neither system can react to changing input conditions, so neither can provide a stable output.

The only thing a manually set four-way valve can do is maintain a fixed proportion between the hot boiler water and cooler system return water that mix together in the valve. If the temperature of both the boiler water and system return water remained steady, a manually adjusted mixing valve would work just fine. Unfortunately, such conditions almost never occur. Boilers heat up and cool down, as do heated floors. The manually set four-way valve can no better adjust to such conditions than can a simple shower valve adjust to changes in hot and cold water temperature. Both are "blind" to what's happening with temperature.

If you're of the opinion that a manually set mixing valve is all your system needs, save yourself a few bucks by installing a couple of simple globe valves instead of a more costly four-way valve. The resulting control accuracy will be the same. Besides, you can use the money you save to partially cover the callback that's almost certain to come.


My injection pump never seems to operate above 30 percent speed, even on cold days. What's wrong?

John responds: Most injection pumps used in residential and light commercial radiant systems are garden-variety 1/25-horsepower wet rotor circulators. These pumps are often capable of injecting all the hot water needed by the system without ever revving up to more than a fraction of their full speed, even at design load conditions.

Although most floor heating systems will usually perform "adequately" with a limited-range injection pump, control accuracy is needlessly wasted.

Imagine for a moment that the gas pedal in your car only had to be pressed down 1/4 inch to make the car go 65 mph. What would it be like trying to accurately control the car's speed while driving in slower city traffic? Possible, maybe, but certainly a bit touchy. Your ability to accurately control the car's speed would improve if the gas pedal movement needed to achieve 65 mph was, say, 2 inches.

Likewise a variable-speed pump control can fine-tune supply water temperature better when its full speed corresponds to design load conditions. Control specialists would say the "rangeability" of the controlled device has been improved. Likewise a properly applied, two-way valve should be fully open at design (injection) flow rate.

The most common method of forcing the injection pump to run at full speed under design load conditions is to increase the flow resistance it operates against. A globe valve installed in the return injection riser allows additional flow resistance to be dialed in as needed. The installer partially closes this valve to force the pump to higher speeds. Techniques for determining the proper valve setting vary. Some assume the balancing valve can be set to a specific CV value. Another method uses temperature measurements at start-up.

And then there's the trial and error approach. Look for these to be discussed in future PM Hydronic Workshops columns.

Two-Way Floors

If I install tubing on a suspended floor deck can I use it to heat the rooms below as well as the rooms above?

John Responds: Heat always flows from areas of higher temperature to areas of lower temperature. If the air and surface temperatures of both spaces are lower than the surface temperatures of the floor deck, heat will flow in both directions.

However, the rate of heat flow in each direction depends on the upward vs. downward thermal resistances of the floor assembly, as well as the air and surface temperatures of the two spaces. Establishing thermal resistances that allow the proper proportion of heat flow in each direction at the same time is no easy task. The floor and ceiling finishes desired by the owner may not even come close to providing the proper resistances.

Even if you could provide the right proportions of upward to downward heat flow for a given load condition, it's very possible these proportions would change, hour by hour, depending on the use and internal heat gains of each space.

For example, suppose the space above the floor was a living room with plenty of south-facing windows, and the space below was a heated basement. There will likely be times during cold weather when solar heat gain eliminates the need to add further heat to the living room. The basement, however, doesn't experience these solar heat gains and hence still needs heat. Under such circumstances it's a no-win choice between overheating the living room or underheating the basement.

Because suspended radiant floors cannot adapt to changing load conditions both above and below at the same time, they should not be expected to perform well as two-way heat emitters. Accurate temperature control requires good backside insulation and a separate radiant panel (floor, wall, or ceiling) in each space.

Robin Hood For Btus

What are the advantages of constant circulation in radiant slab systems?

John responds: There are several. One is comfort. Imagine a tubing circuit in a slab covered in some areas by tile and in other areas by carpet (see Figure 2). When the circuit is operating, heat is distributed to all areas of the slab. But what happens when the thermostat is satisfied and circulation stops? Because of its lower upward resistance to heat flow, the tiled floor area releases heat from the slab faster than the carpeted area. Without water circulation, heat stored in the slab beneath the carpet cannot relocate to where the tile is steadily cooling. Eventually the tile will feel cooler than a bare foot would prefer.

If water circulation continues after the thermostat is satisfied, some of the heat stored under the carpet would be transported to the slab area under the tile, and thus slow floor-surface cooling. Occupants will not notice pronounced differences in comfort while walking across different floor areas.

A similar argument can be made for transporting heat that's "stuck" under thick area rugs, furniture with floor-length skirting, or other poorly conducting objects on the floor.

Now imagine a garage building with large overhead doors and a heated floor. Slab-edge insulation details are often skimpy or nonexistent under such doors. The concrete under the door acts as a thermal wick, pulling heat away from the slab just inside the doors. To make matters worse, there's often substantial air infiltration under such doors and directly across the floor surface beneath them.

Assuming water circulation has been off for a few hours (such as at the beginning of a temperature setback), it's pretty obvious where the circuit is likely to freeze. (By the way, ever notice how your ability to contemplate such conditions improves dramatically as outside temperatures drop well below zero?)

Enter constant circulation. The flowing water borrows heat from the interior core areas of the slab and carries it out to the chilled area just inside the doors. In the event the boiler is down, constant circulation could likely prevent freeze-ups at such cold spots for several days.

Floor drying is also enhanced by constant circulation. Again, it's the ability of water circulating through the floor circuits to borrow heat from warm areas and drop off that heat at local heat sinks (in this case caused by melting snow and evaporating water).

Finally, constant circulation in combination with reset control of supply water temperature minimizes thermal stresses in the floor. This is especially important when solid sawn wood flooring will be used. Temperature changes take place over hours rather than minutes. Expansion noises are also minimized with this strategy.