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Q: Will a manually set four-way mixing valve used as part of a radiant floor system adequately protect a boiler from low return water temperature?
A: Four-way mixing valves have long been the standard mixing device used to interface conventional (non-condensing) boilers to lower temperature distribution systems. They are intended to accomplish two tasks at once:
- Regulate the supply water temperature to the distribution system, and
- Maintain the temperature of the return flow to the boiler high enough to prevent sustained flue gas condensation.
The valve accomplishes this by creating two mixing points, one on the supply side, the other on the return side as shown in Fig. 1. The temperature of the water returning to the boiler is boosted by blending a portion of the hot boiler water with the cool water returning from the distribution system.
However, 4-way mixing valves are intended to be modulated by motorized actuators. The controller operating the actuator monitors the temperature near the return connection to the boiler. If the controller detects a low return water condition it begins closing the hot water port of the mixing valve such that the distribution system cannot dissipate heat faster than the boiler can produce it. A four-way valve without an automatic control is essentially blind to what's happening at the boiler return (and for that matter at the supply side as well).
Under quasi steady-state loading in a system where the boiler output equals or exceeds heating load, the valve will provide a boost in return water temperature. But most heating systems seldom tarry at such steady conditions. Instead they go through repeated recoveries from temperature setbacks, restarts after power outages, or other transient situations. During these times a manually set 4-way valve cannot protect the boiler from low return water temperatures. The distribution system with its large cool thermal mass will extract heat from the water flowing through it much faster than the boiler can replace this heat. Under such conditions thermodynamics dictates that the water temperature in the system goes down, in many cases low enough to hold the boiler in sustained flue gas condensing mode for several hours. This holds true regardless of what type of manually-adjusted mixing hardware is present between the two sub-systems.
Only a controller that senses low return temperature and then reacts to it by limiting hot water input through its associated mixing system offers protection under such conditions. If you're going to use a four-way mixing valve equip it with a motorized actuator and controller having return temperature protection capability, and let it do what it's designers intended.
Q: What's the best way to pipe a multiple boiler system?
A: Options vary on this issue. The foremost objective is to prevent heated water from flowing through boilers that are not being fired. Not doing so lets the system use the boiler jacket and flue piping as a heat dissipator.
One way to prevent flow through unfired boilers is to pipe them in a series primary/secondary arrangement as shown in Fig. 2a. However, the series arrangement also creates higher entering water temperatures in the downstream boiler(s) when more than one boiler is firing. The higher water temperature tend to increase stack temperatures as well as jacket heat loss, and hence reduce the efficiency of the downstream boilers.
Piping the boilers in parallel as shown in Fig. 2b allows each to receive the same inlet water temperature, and hence overall efficiency is improved. The piping for each boiler is merged into manifold piping that forms a primary/secondary interface with the system piping. The latter prevents the system pump from interfering with flow through the boilers. Assuming each boiler's circulator only operates when the boiler is firing, this method holds a slight efficiency advantage, albeit at the expense of slightly more involved piping.
In either case it's imperative to install a flow-check valve in the piping circuit of each boiler. This prevents forward heat migration or, in the case of parallel piping, reverse flow through an inactive boiler.
Q: How should expansion tanks be sized when a high temperature boiler is matched to a low temperature distribution system?
A: This is a question that often arises during the design of larger radiant floor heating systems. The objective is to accommodate the total expansion of the system fluid without popping the relief valve.
Think of such systems as two sub-systems. One consisting of the boiler, boiler piping and other components on the "hot" side of the mixing assembly. The other consisting of the floor tubing, manifold piping and other components on the low temperature side of the system. Total expansion volume is determined by calculating the individual expansion volumes of both sub-systems (given the maximum temperature extremes each "sees") and then adding the expansion volumes together.
For example: The high temperature sub-system may see water temperature vary from say 60Ý to 200ÝF, while the low temperature sub-system may swing from 60Ý to say 110ÝF at design conditions. Obviously the expansion volume per gallon of fluid in the low temperature side is significantly less than that on the high temperature side.
This sizing method is still conservative. It assumes all the water on the low temperature side of the system sees the maximum mixed supply temperature. This is not true given the 15Ý to 20ÝF temperature drop along most floor heating circuits. The same argument can be made for the piping between the mixing assembly and the manifolds. Likewise there's a temperature drop on the high temperature side of the system.
In large systems where the majority of the system's water resides on the low temperature side of the mixing assembly this method significantly reduces oversizing relative to the classic sizing procedures that assume the entire system volume reaches the maximum boiler temperature.
Q: How should I select the two-way valve and flow restrictor valve for an injection mixing system?
A: The typical piping used for a two-way modulating valve controlling heat input to a radiant floor systems is shown in Fig. 3. The controller operating the valve's actuator senses the supply water temperature and then adjusts the injection valve as necessary to steer the system toward the target supply temperature.
The flow restrictor valve creates a pressure differential that forces flow through the injection risers as the injection valve opens. It's setting depends on the Cv of the injection valve.
First, let's size the injection valve. Doing so requires us to determine the injection flow rate the valve must pass at design load conditions. This can be calculated using Formula 1:
fi = load ÷ 490 x (TI - TR) where:
fI = required (design) injection flow rate (gpm)
Q= design heat input rate to the distribution system (Btu/hr)
TI = temperature of hot injection water (ÝF)
TR = temperature of water returning from the floor circuits (ÝF)
490 = a constant based on water as the system fluid.
Now select an injection valve having a Cv equal to or slightly greater than the required injection flow rate calculated. Next we need to determine how much "restriction" the flow restrictor valve must produce to let the injection valve operate properly.
Most references on control valve sizing state that the pressure drop across a fully open two-way control valve (operating at its full design flow rate) should be at least 50% of the pressure differential imposed across the valve when it's fully closed. The ratio of these pressure differentials is called valve authority. Unlike a Cv value which pertains only to the valve itself, valve authority describes how the pressure drop characteristics of a valve relate to those of the piping it becomes a part of.
The Cv of the flow restrictor valve can be determined using Formula 2 which is derived based on the assumption of giving the injection valve an authority of 50%. It also assumes that the injection riser piping is relatively short and hence contributes very little pressure drop relative to the valves:
CVbypass valve = 0.707 x (fs ÷ fi) x Cvinj. valve where:
CVbypass valve = the Cv setting on the bypass valve
Cvinj. Valve = the Cv of the selected injection valve
fs = the flow rate in the distribution system (gpm)
fI = design injection flow rate determined from Formula 1.
Several manufacturers offer balancing valves with calibrated scales that can be set to the necessary Cv value.