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How to Avoid Problems With Your Hydronic System Pumps

Some hydronic systems continuously have trouble. The owner of such a troubled system is paying service bills to repair or 

replace various components that are constantly failing. Service technicias such as unheatable circuits, noise, air binding, excessive

component failure, especially pumps, etc., needs to be analyzed to discover the reasons for the constant trons find the failed

component, replace it, and tell the owner the system has been “fixed.” Any system that has continuous problemuble. Properlydesigned, installed, and started hydronic systems will be trouble-free for many years.

Hydronic engineers who have plans and specifications usually design large hydronicsystems. As long as the installing contractor follows the plan and spec, no systemproblems should be encountered. Smaller systems, residential and commercial systems,are usually “designed” by the installing contractor. These systems can exhibit ongoingproblems, and instead of just replacing parts, need an analysis to discover the realproblems.

Many mistakes are made locating circulating pumps in relation to the expansiontank. When pumps were first used, they were always located on the return piping, pumpinginto the boiler. This was where the water was coolest, having circulated through the system

and given up its heat. Manufacturing tolerances could not be as closely held as they aretoday, so where the water was coolest was the norm for locating circulating pumps. As wewill see, this “standard” is obsolete and not necessarily the best location for the booster pump. Manufacturing processes have improved so that the pump can be located in theboiler discharge water with no adverse effect on the pump. The pump location isdetermined by where the expansion tank is connected to the system.

When the pump is off, the only pressure existing is static pressure (see Info-Tec26, Hot Water Heating Systems). Starting the pump will change the system pressure to anew set of conditions. The pump head will appear across the pump. The pressure at thepump discharge will be higher than the pressure at the pump suction by an amount equal to

the pump head. Pressure drop ('P) will gradually decrease from the discharge to the pump

suction.By specifying a point of no pressure change, system pressures can be governed when

the pump is on. The point of no pressure change is where the expansion tank connects intothe system. This is because the air in the compression tank must follow gas laws: a changein air pressure must be followed by a change in air volume. A change in air volume resultsin a change in water volume in the tank. A change in water volume in the tank must causea change of water volume in the system. Pump operation cannot increase or decreasesystem water volume, since water is incompressible. Therefore, pump operation cannotchange tank pressure. Since tank pressure cannot change because of pump operation, the

 junction of the tank with the system must be a point of no pressure change.Based on this fact, if the compression tank is placed at the suction side of the pump,

the pump suction pressure will not change, regardless of whether the pump is on or 

off. Since the pump suction cannot change, the pump discharge must change when thepump is on. All the pump head must show up as a positive increase at the pumpdischarge. The pressure increase will decrease through the system to the original staticpressure at the pump suction. (This is called hydronic gradient.) This is graphicallyrepresented in Figure 1. Note the line representing the pump head or hydraulic gradient. Itis above the original status pressure line throughout most of the system.