Changing Direction

I spend a lot of time looking at piping schematics. Many of them show arrows next to the lines representing piping, to indicate the proper direction of flow. The “quip” about such arrows is that the installer needs to draw them on the inside of the piping as it is installed, using an indelible marker, so that the water can see them, and thus know which way to flow. Let me say that with proper design, this is not necessary.

I AM STARVING

Occasionally, you may come across a hydronic system, which, for various reasons, is starved for proper flow rate. The symptoms typically include complaints of inadequate heat delivery and large temperature drops from the beginning to the end of the circuit.

One example of such a situation would be a radiant floor circuit that contains 1000 feet of ½” PEX tubing. Why, you may ask, would anyone install a circuit that long? The answer is simple: Because ½” PEX tubing is sold in coils up to 1000-feet long. Given this, an installer with little understanding of flow dynamics, simply keeps uncoiling and fastening tubing to a floor area until the 1000-foot coil is used up. I have heard of this happening on several occasions. The realization that it is a problem usually occurs after the concrete has been placed over the tubing, thus sealing the error for posterity.

I used Hydronics Design Studio software to model what would happen with a 1000-foot long circuit in a bare concrete slab, assuming the tubing was placed at 12″ spacings and supplied with 105F water. I assumed a typical 1/25 horsepower circulator was used to drive flow through the tubing circuit and that the room air temperature above the slab was 70F. The results are as follows:

Circuit flow rate = 0.67 gpm

Temperature drop = 32.1F (supply temp. = 105F, return temp. = 72.9F)

Total heat output of circuit = 10 641 Btuh

Average heat output of circuit = 10.6 Btuh/ft²

Compare this to a situation where the same 1000 feet of tubing was divided up into four 250-foot long circuits. Use the same circulator and the same supply water temperature. Here are the results:

Circuit flow rate = 1.26 gpm

Temperature drop = 9.76F (supply temp. = 105F, return temp. = 95.2F)

Total heat output of circuit = 24 449 Btuh

Average heat output of circuit = 24.5 Btuh/ft²

The total heat output of the four 250-foot circuits is almost 2.5 times greater than that of the single 1000-foot circuit. Furthermore, the heat output near the end of the 1000-foot circuit is a paltry 5.5 Btuh/ft² and is very likely well under the value needed to maintain proper comfort at design load conditions. To make matters worse, it is all cast into concrete and any correction may seem impossible.

It would be very expensive to tear out the slab and replace it with proper proportioned circuits. Fortunately, there is another solution that can be much less expensive and invasive. The fix is to periodically reverse the flow through the long circuits. Over time, this allows all areas of the floor to emit approximately equal amounts of heat, based on the average water temperature in the circuit.

One way to do this is to use a 4-way motorized valve, one that is usually intended for mixing applications, as a “reversing valve.” The concept is shown in Figure 1.

The shaft of the 4-way valve is controlled by a two-position spring return actuator, such as the one shown in Figure 2.

When supplied with 24VAC, this actuator rotates the valve shaft about 90 degrees. Flow through the system would be as shown in the upper portion of Figure 1. When the 24VAC signal is removed, an internal spring causes the actuator to rotate the valve’s shaft back to its original position. Flow now moves through the system as shown in the lower portion of Figure 1. The periodic on/off switching of the 24VAC signal to the actuator can be handled by a commonly available repeat cycle time delay relay. Although, I have not studied the effects of cycling time, I suggest setting the periodic repeat cycle to something between 30 and 60 minutes.

OTHER USES

This method of periodic flow reversal can also be used in snowmelting systems. One characteristic of such systems is that snow tends to melt faster in pavement areas near the supply side of the embedded tubing circuit. This happens because the circulating fluid is warmer near the supply side of the circuit. Periodic flow reversal tends to even out the amount of heat delivered by each unit of area served by the circuit. Over an operating period of several hours, this will average out the heat delivered to each square foot of pavement. 

If you plan to use this approach, use manifolds with either no valves, or valves that are tolerant of flow through them in either direction. Avoid globe type valves. Reversing flow through such valves, especially when the valve’s disc is close to its seat, can cause cavitation. Besides noise, this situation can eventually damage the valve’s seat and disc.

Also, be sure that there are no check valves in the piping through which flow will be reversing. Hopefully, the reason for this precaution is obvious. If not, you are probably better off not using flow reversal. 

STRATIFICATION SUPERVISOR

Yet another application of flow reversal is to maintain proper stratification in larger thermal storage tanks that contain heated water during heating season operation and chilled water during cooling operation. Figure 3 shows an example of such a system, which uses an air-to-water heat pump, operated during periods of warmer outdoor temperature, to heat a large and very well insulated thermal storage tank during the heating season. 

The heat pump also operates on low cost off peak electrical energy during summer nights to chill water in the storage tank. This water provides the next day’s cooling through zoned air handlers with chilled water coils.

This system uses two 4-way “reversing valves,” one on the energy input side and the other on the load side. These valves operate at the same time. In heating mode, the valve on the left side of the tank provides a flow direction that delivers the warm water from the heat pump to the top of the storage tank. The valve on the right side of the tank allows the warmest water at the top of the tank to be delivered to the load.

In cooling mode both valves change the flow direction to allow chilled water from the heat pump to be delivered into the lower portion of the tank. Likewise, chilled water for the air handlers is extracted from the lower portion of the tank. Figure 4 shows a simplified concept of these operating modes.

Granted, the situations and applications for hydronic flow reversal do not come along every day. Still, knowing how and when to apply it is another addition to your professional repertoire. Use it where it is appropriate.