Just for a moment think about the comfort of hydronic floor heating in every room of every house you deal with as a heating professional. Imagine every client “sees the light,” promptly alters the original flooring choices, ups the construction budget and tells you to install the ultimate comfort system you have just described throughout their new home.

Now go splash some cold water on your face and come back to reality. Complete hydronic floor heating in every part of every house you deal with is literally a “pipe dream” (except for you guys out in Utah).

But even so, we shouldn’t overlook the opportunities to provide floor heating in select areas of houses with existing or planned hydronic systems. Often it’s the tile floor of a bathroom or kitchen that beckons to be transformed from a thermal liability into a source of comfort. Every time your client’s bare feet land on that toasty tile floor they’ll appreciate your design expertise. That small heated floor can be just as effective in getting future referrals as would be a whole-house floor heating system ... maybe even more so because other, unheated floors in the same house provide a quick comparison. That client will undoubtedly tell many others about “the nicest floor in the house.”

Here are some ways to do it.

Independent Vs. Slave Zoning: When designing a small zone of floor heating into a hydronic system a decision has to be made whether the floor will operate as an independent zone, or be “slaved” to other parts of the distribution system. As the name implies, an independent floor heating zone can call for boiler operation, water circulation and mixing control regardless of whether other portions of the system are operating.

A slave zone, on the other hand, can only provide floor heating if other parts of the hydronic distribution system are also on. Figure 1 shows one possible piping schematic for a slave floor heating zone tied into a baseboard heating circuit. In this system, injection mixing is used to reduce the water temperature from the baseboard circuit to a suitable supply temperature for the floor circuits.

When zone valve (V1) opens, hot water from the baseboard circuit mixes with cooler floor circuit return water at tee #1. The temperature of this mix depends on the flow rates and temperatures of both the hot and cool water streams, (see “Hydronics Workshop,” May 1997 for the calculations). The partially closed balancing valve creates a pressure differential between the risers providing an “incentive” for water to flow whenever the zone valve is open. The more the balancing valve is closed, the greater the amount of hot water admitted to the mix. Ideally the balancing valve should be closed just enough to provide design water temperature to the floor circuits when the baseboard circuit is also operating at design conditions.

A possible wiring schematic for this system is shown in Figure 2. Notice that a 120 VAC — rather than 24 VAC — zone valve is used to eliminate the need for additional control hardware. Upon a call for heat both circulators turn on as does the zone valve. The high limit aquastat protects against excessively high supply temperature to the floor circuits by interrupting power to the zone valve if its set point is reached. The aquastat should be set so that its normally closed contacts open a few degrees higher than the desired supply temperature to the floor circuits. Its differential should be wide enough so that the floor circuits will cool a few degrees below the desired floor supply temperature before the contacts reset. If the balancing valve is properly adjusted, the high limit aquastat should not cycle on and off frequently. If it does, the balancing ball valve is closed too far.

When a slave zone is used, it’s important to account for the temperature drop it creates within the baseboard circuit. This drop depends on the heating load of the floor zone as well as the flow rate in the baseboard circuit. It can be estimated as follows:

Formula 1


³T = the temperature drop created in the baseboard circuit by the floor heating zone (in degrees F).

Qfloor zone = The design heating load of the floor zone (in Btu/hour).

f = the flow rate in the baseboard circuit (in gpm).

490 = a constant based on the properties of water. Note this constant will be different for other fluids.

Ideally, all heat emitters located downstream of the floor heating zone should be sized based on the reduced water temperature. This may not always be possible in retrofit applications. In such cases it is preferable to tie the floor heating zone in near the end of the higher temperature distribution circuit.

Give Me Independence: I personally like being able to provide floor heating to certain floor areas without having to heat up the rest of the house. This would let you, for example, provide a cozy warm bathroom on a spring morning after bedroom windows have been open overnight and the rest of the house is still cool.

Independent zoning requires a bit more piping and electrical hardware. Figures 3 and 4 show possible piping and wiring schematics.

This design uses a separate hot water supply circuit (rather than the baseboard circuit) to transport hot water out to the floor heating zone. Since this new circuit is in parallel with the baseboard circuit, and can operate independently, it’s critical to install a flow check valve in both circuits to prevent reverse flow.

The piping from circulator (P3) out to the location of the floor heating zone could be copper, PEX or aluminum/PEX composite tubing. The flexibility and long continuous lengths of the latter two could be significant advantages in retrofit applications.

This piping design also uses injection mixing. However, a non-electric thermostatic valve is shown instead of a zone valve. The thermostatic valve is set to maintain design water temperature to the floor circuits. It senses the supply temperature to the floor circuit downstream of the floor circulator and adjusts hot water input to maintain this temperature within a narrow band. A high limit aquastat is also shown as a secondary means of protecting the floor circuits against excessively high water temperature. Other control arrangements are also possible.

Protecting The Boiler: Both the independent and slave floor heating zones provide a degree of protection against flue gas condensation in the boiler. However, this protection is predicated on proper piping design and circulator selection. The designer must estimate the temperature of the water returning to the boiler based on flow rates and heating loads in the two zones. It’s entirely possible to design both types of systems so that boiler return temperature is in the range of 15 to 20 degrees F below the boiler supply temperature. Given the relatively high settings of most boilers supplying baseboard systems, this ³T allows for a comfortable safety margin in preventing sustained flue gas condensation. Remember, however, return water temperature is not being sensed nor are any corrective actions offered by these relatively simple controls.

The systems discussed are by no means the only way one might add a small area of floor heating to an existing hydronic system. For example, if tubing is installed between floor joists in a so-called “suspended tube” configuration, it may be possible to eliminate the mixing controls all together, and route the high temperature water directly through the floor circuits. In such a case the floor circuits might be tied into the main circuit in a primary/secondary configuration. Still another possibility is to use diverter tees in combination with a zone valve. Whatever the concept it’s important to “do the math” associated with proper design thus ensuring proper flow, temperature drop, and the resulting heat output.