The Variable Flow Renaissance

Over the last half decade plus there have been a number of pump manufacturers getting into the variable flow market here in North America. End-users, maintenance technicians, wholesalers and installers have to sift through: the various features and benefits; the reasons for installing variable flow versus not; and the when and where questions this variable flow renaissance has brought about.

What is variable flow?

Simply put, variable flow means “changing” flow, but becomes a more complicated concept depending on how, why and when the change is done. Most of the time this is accomplished with a circulating pump that changes speed depending on what the system or application requires. However, you could affect flow change by using a modulating two-way or three-way valve, but that is a topic for another day.

The following six basic concepts will help with the decision making process.

Static or Dynamic Applications

Variable flow products are best suited for dynamic applications. These are applications where the flow is required to change, as opposed to static systems where flows are constant. Hydronic heating or cooling systems are 99.9 per cent dynamic. Flows change as heating or cooling loads change, a function of the laws of thermodynamics (Btus are affected by T and circulated pounds of water or US GPM). I have been questioned about two applications that might not be dynamic – boiler primary and individual pumped zones.

Boiler Primary

The piping system that moves Btus from the boiler(s) between the boiler loop supply and return would only be a static, constant flow application if there was only one on/off boiler on this loop. If there was one boiler with a modulating gas valve, to maintain a specific T or to control the return water temperature, the flow would have to be modulated with the firing rate of the boiler – hence this is a dynamic, variable flow application.Individual Pumped Zones

Individual Pumped Zones

This is also know as zoning with circulators. Indeed, if this single zone did not see any load change it would be static. However, as the room temperature is satisfied wouldn’t it make sense to slow the circulator down and modulate its flow based on room load? This makes it dynamic. So, it is safe to say all hydronic HVAC applications would see a system efficiency benefit by using a variable flow circulating pump. The big question is: What is the best way to control (T, P, Auto or External) the system. P is nice as it does not require the installation of external sensors but this does not work in some applications such as boiler protection, zoning circulators and injection. T or set point temperature works in all applications but requires sensor(s). If you are in doubt ask a trusted expert.

Circulator or Pump

Pumps consume or convert energy to boost pressure – for example a submersible well pump or a boiler feed pump. The application requires pressure boosting. Circulators consume or convert energy to create flow by overcoming piping and fitting friction losses caused by the flow itself. The application requires flow. Hydronic HVAC systems require flow to distribute heating or cooling (Btus) throughout the building. Higher loads require higher flows (more pounds of Btu carrying water) and higher flows increase friction losses.

System Curves versus Constant Speed Pump Curves

Both curve types have head on the “Y” or vertical axis and flow on the “X” or horizontal axis. Flow could be expressed in GPM or Btus (think about it). The System Curve is a graphical representation showing the effects of changes in flow and subsequent changes in friction loss (more flow=more friction). This curve starts at “0/0” (zero flow means zero friction loss) and head loss increases as flow increases.

Constant speed pump curves have two distinct points on this graph. The differential head at zero flow is shut off head or the highest head this constant speed circulator can produce. All of the 80 watts of power this little guy is using is going into producing head. The other is maximum flow (run out) where the differential head is the lowest (or even zero). 

Let’s state the obvious:

• A circulator provides flow by overcoming friction loss in a closed loop application.

• A constant speed circulator increases head as flow decreases.

• Friction head decreases as flow (or load) decreases.

• As zones close (loads go down), Btus (flow) required goes down as does friction loss, but the constant speed circulator’s differential pressure on the pump curve goes up. This is bad and causes a loss of system efficiency (boiler cycling and low T) and loss of overall comfort (noise and poor room temperature control in shoulder heating seasons). 

So, if you have ever wondered why some zones are noisy than others, what causes zone valves to fail prematurely, what causes boilers to cycle it is a misapplication (or oversizing) of a constant speed circulator. The circulator really needs to lower its differential head during decreases in flow, as in the case of a T circ. As an analogy, the more closed a shower valve is the more noise it makes.

WORTH NOTING: Closed loop three-storey buildings do not require circulators sized with an extra 30′ of head (10′ per floor) since the loss going up is gained coming down. Size the circulator to provide flow to satisfy the coldest day and the highest friction loss at that flow. 

Closed Loop or Open Loop:

Closed loop systems are typical of hydronic HVAC systems where the closed and pressurized system requires thermal expansion tanks. They have very low or no system make-up system fluid. Open systems (cooling towers, domestic or sanitary water) are classified as open systems. These are typically high or 100 per cent make up water and quite often have an open discharge. This is important when selecting pumps – materials of construction, high or low head, flat or steep curves and so on.

In summary, do not overthink the variable flow stuff – it is really very simple. All circulating pumps have a goes inta, a goes outta and one moving part (impeller). The end game is to modulate flow with system demand for maximum system efficiency and comfort – like controlling the speed of your car with a gas pedal.

Steve Thompson, is director of residential product management with Taco Inc.