Best Practices for Hydronic Systems Part 2: Selecting and Installing Pressure Gauges

Pressure gauges are required for hydronic heating and cooling systems, steam systems, domestic hot water and plumbing systems. Pressure readings, like temperature readings, are needed to verify proper operation and assist with troubleshooting at the inlet and outlets of pumps and equipment. Pressure gauges should also be installed at the water make-up to ascertain that the system has adequate fill.

Selecting the Proper Pressure Gauge

Dry Pressure Guage

Selecting the right type of pressure gauge for a given application is pretty simple, as there are only a couple of different types to choose from (liquid filled or dry) and the selection is very straightforward. Dry pressure gauges can be used on any of the above-mentioned applications, as they have a temperature rating of up to 190°F. If your application operates at 140°F or below, you may consider a liquid fill gauge because the liquid fill helps dampen vibration and pulsation, and they withstand pressure spikes better than dry gauges. The internal fluid (glycerin) helps lubricate delicate parts of the gauge and prevents the formation of condensation behind the glass, which can obscure readings and shorten the life of the gauge.

Liquid-Filled Pressure Gauge

Liquid-filled pressure gauges should be avoided is in hot water and steam applications because they are not designed for the associated high temperatures. However, in most HVAC systems, a liquid-filled or dry pressure gauge with a 4.5 in diameter dial and ±1.0% full-scale accuracy will do just fine. Both types of gauges are available with a compound scale, meaning that they can register both negative (vacuum) and positive pressures. Compound pressure gauges should always be used on cooling tower suction piping because negative pressures can occur when the cooling tower pump is running. Compound gauges are also indicated on steam systems as when steam condenses, a vacuum can result unless devices are present to break the vacuum.

Pressure gauges are available with a variety of pressure ranges. Pick the pressure range in which the normal operating pressure of the system falls close to the middle of the scale. For example, if the normal operating pressure is 50 PSI then use a 0 to 100 PSI gauge.

Installation and Operation Tips

Below are some additional tips for installing and using pressure gauges.

Pressure fluctuations and vibration. Severe pressure fluctuations, vibrations, and/or pulsation are the primary sources of wear and tear on pressure gauges. Always avoid installing gauges in areas of high vibration. Consider a liquid-filled gauge if the operating temperature of the system isn't too high as the fluid will lubricate the movement and reduce friction. It’s also a good idea to install a snubber to suppress transient pressure fluctuations sensed by the gauge.

Special precautions for steam. When installing pressure gauges on steam systems a pigtail syphon must be installed between the gauge and the steam pipe. This protects the gauge from steam temperature and pressure fluctuations by providing a condensate trap between the steam pipe and gauge.

Differential pressure readings. When taking differential pressure readings across a centrifugal pump or other pieces of equipment always use the same pressure gauge. This ensures an accurate differential reading because it cancels out an erroneous reading if the gauge isn’t calibrated properly. We recommend mounting a single gauge with tubes connected to the suction and discharge of the pump as shown below for accurate differential pressure readings on pumps. Differential pressure between a suction diffuser or strainer inlet, and the pump suction is often also measured to determine when service is required for the straining element. Some manufacturers offer gauge kits which are supplied with valves, nipples, fittings and tubing required for differential pressure readings of up to three process locations These kits, similar to the one depicted below, are ideal for pump gauge installations.

Recommended mounting of differential pressure gauge on pump.

Gauge replacement. Despite all your best efforts, eventually, every pressure gauge will one day need to be replaced assuming the system remains in operation. Plan ahead for this day by installing a shut-off valve between the system and the gauge so it can be replaced without draining the system. Consider using a needle valve because its small orifice opens slowly, gradually bleeding the system pressure to the gauge, thus protecting it from a sudden jolt of pressure. The owner and/or operator will appreciate your foresight.

Note: Our gauge kits use ball valves in lieu of the above suggested needle valves.