Guide to Heating System Pressure & Temperature Gauges

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Hydronic (hot water) Heater Pressure Gauge and Normal Pressure Ranges

Pressure and Temperature gauge on hot water or hydronic heating boilers: this gauge displays the heating boiler internal pressure and temperature. Typical pressure for a residential boiler serving a two story home would show 12 psi cold, and less than 30 psi hot. Over 30 psi boiler pressure will cause the pressure relief valve to open. Typical operating temperature settings on a boiler call for a Low temperature (boiler cut-in) between 120 and 160 degF. Typical operating temperatures on a hydronic boiler call for a high temperature (boiler cuts off) of 180-200 degF. Over 200 degrees F. we're at risk of spilling at the pressure temperature relief valve.

Typical operating temperature observed at the gauge will be below the high, and can be as low as nighttime room temperature in non-heating season if no tankless coil is in use. The temperature/pressure gauge may help in checking for normal conditions before and during boiler operation. However the gauge can be wrong! This gauge shows a typical in-boiler pressure of under 20 psi, and a temperature of about 190 degF. (The boiler had just cut off on a heating cycle.)

What Are the Normal Hot and Cold Operating Pressures of Residential Hydronic (hot water) Heating Boilers?On a residential heating boiler the automatic water-feeder/pressure reducing valve that automatically provides makeup water to the heating boiler if pressure drops below 12 psi. So 12 psi is the typical "cold" pressure for residential boilers.
If your building is taller than two floors, the installer may have needed to boost the starting "cold" water pressure for your boiler to overcome the hot water distribution piping head pressure - otherwise your heating circulators may not be able to circulate hot water.
At PUMP, WATER PRESSURE BOOSTING we explain the relationship between building height and water pressure, and we illustrate the water pressure decrease in building water supply piping with building height. But a look at the basement water pressures in this illustration also explains the pressures that a basement located hot water heating circulator pump has to overcome.
12 psi is for typical U.S. / Canadian residential heating systems normal cold temperature starting pressure. U.K. and european heating systems should be pressurized to between 1 and 1.5 bar - cold.
20 psi is typical for U.S. / Canadian residential heating systems normal hot operating temperature, up to just under 30 psi, depending on the high-limit temperature setting on the boiler limit control. If we set the boiler high-limit much over 210, on many residential systems the system pressure will exceed 30 psi and we'll see water leaking from the pressure/temperature relief valve - ultimately an unsafe condition.

Where & How do I Set Boiler or Furnace Temperature?Remember that the building THERMOSTATS set the desired temperature in the occupied spaces in building, not the actual temperature in the heating boiler or furnace itself. In most heating systems, turning up the thermostat simply causes the boiler or furnace to turn on. The temperature at the boiler or furnace is controlled by local safety devices mounted right at that equipment.

• Boiler temperature, pressure setting controls: For details how, to what number, and on what controls the hydronic heating boiler temperatures are set, see AQUASTAT CONTROL Functions, and Limit Switches, Boilers.
• Steam boiler temperature, pressure setting controls: For details how, to what number, and on what controls the steam boiler Pressure settings and pressure readings are set see Steam Boiler Pressure Gauge and Normal Pressure Ranges.
• Furnace temperature setting controls: Warm air furnace temperatures are controlled at the [size=-1]FAN LIMIT SWITCH.)
  

For more diagnostic aid on finding the cause and executing the cure of abnormal heating boiler pressures see

• LEAKS at BASEBOARD, CONVECTOR, RADIATOR - how to find leaks at baseboards, radiators, etc.
• BOILER LEAKS CORROSION STAINS - how to diagnose & evaluate leaks on or in heating boilers
• BOILER LEAKS, HOW TO LOCATE - how to determine that a boiler or hot water system is leaking and how to find hidden leaks
• EXPANSION TANKS - if the expansion tank is waterlogged, the boiler and hot water heating system pressures will become excessive and spillage may be found at the pressure/temperature relief valve.
• Pressure/temperature relief valves - can also leak on their own - an unsafe condition. See /heat/ReliefValves.htm
  

Steam Boiler Pressure Gauge and Normal Pressure Ranges

Pressure and Temperature gauge on steam heating boilers: Residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi. You should see similar settings on the pressure gauge (at left in our photograph) and on the steam pressure control switch (the gray box at right in our photo) on your boiler. The controls in this photo are discussed in detail at [size=-1]STEAM HEATING SYSTEMS and in detail at [size=-1]Steam Pressure Gauge

What Are the Normal Hot and Cold Operating Pressures of Residential Steam Heating Boilers?Residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi.

If your residential steam boiler is operating at higher pressures (take a look at the dial setting on your Pressure Switch, Steam Boiler) , that may be an indication that a service technician or owner was having trouble getting heat distributed through the building. Rather than finding and fixing the problem, someone is trying to "force" the steam around the system.
An experienced steam heat service technician will look at the operating pressure of your steam heating boiler and if it is not set to a normal level, the technician will look for the reason. Examples of problems that can affect the flow of steam heat through the system, leading to attempts to over pressurized the system include

• Blocked condensate returns at individual steam radiators
• Steam radiators tipped the wrong way
• Improperly relocated steam piping that has the incorrect slope
• Radiator valves that are not operating,
• Radiator steam vents that are not operating properly - Steam Vents
  

For details about radiator problems see [size=-1]RADIATORS and see LEAKS at BASEBOARD, CONVECTOR, RADIATOR. Also see HEATING LOSS DIAGNOSIS-BOILERS. More about steam boiler gauges is at Steam Pressure Gauge.


Hot Water Heating System Controls Inspection, Peripherals, Key Components

How to determine what type of heating system is installed:
Warm Air Heating Systems - Furnaces: If the heat in your building is provided by warm air that flows out of ceiling, wall, or floor air supply registers into the occupied space, or if your heating system uses a water-to-air heating system then the air which warms the living space is probably being delivered through large or small diameter ducts, registers, air filters, and a furnace blower, and the air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system. See FURNACE CONTROLS & SWITCHES and for details see FURNACE OPERATION DETAILS.

Hot Water or Steam Heating Systems - Boilers: If the heat in your building is provided by warm or hot metal radiators, heating baseboards containing finned copper tubing, or wall convectors that look like a radiator but contain finned copper tubing, or if heat is provided by flexible rubber, plastic, or metal tubing run in building floors or ceilings, then the warm or hot water circulating in those devices is probably being delivered by piping circulating water heated by a heating boiler, or possibly by a steam boiler or a heat pump or geo-thermal system. See BOILERS, HEATING and RADIANT HEAT Floor Mistakes to Avoid.
If your heating radiators have valves which hiss and let air escape as heat is coming on your heat is probably being delivered in pipes which circulate steam from the steam boiler up through radiators in the occupied space. See STEAM HEATING SYSTEMS .
This website provides description of all of the major components of hot water or steam heating systems, how to recognize or find each component, what it looks like, what goes wrong, and how to maintain, repair or adjust the component.


From: InspectAPedia®

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Question:
My pressure temperature gauge is leaking on my Mclain p-wtgo-3 oil fired boiler. It seems like an easy fix. How to I safely drain and refill the water in the boiler to effect this repair? By the way, the boiler is only about 3 years old. It doesn't speak well for Mclain products. - J. Wojciechowski


Reply: Basic outline of how to replace a heating boiler pressure/temperature gauge

J. Wojciechowski, You're right that just replacing the pressure/temperature gauge itself is usually mechanically simple - as long as you buy an OEM part you'll have exactly the same item as before.

Watch out: the actual gauge replacement itself is not usually technicall difficult as long as the gauge mounting port on the boiler has not become rust damaged. But there are some serious pitfalls to watch out for - enough that for most people I'd recommend having the job done by a trained heating service tech, particularly because you might have difficulty purging all air out of the system after you get the gauge installed.

Basically to replace a heating pressure/temperature gauge one would:

• Purchase an OEM replacement part so that we won't waste time trying to jury-rig something that does not fit on your boiler. You should be able to find a replacement gauge set from your local heating supplier or from the boiler manufacturer. With the new gauge checked for a match, prepared with teflon tape or pipe dope as needed, the general procedure that an HVAC tech would follow would be as follows:
• turn off electrical power to the boiler - just using the service switch at the boiler should be fine
• let the heating system completely cool down to room temperature
• if there are valves that isolate the various zones, close them (that may save leaking air into the zones and leaving them airbound).
• close the water supply valve to the boiler - usually the boiler water shutoff is just ahead of the automatic water feeder (Water Feeder Valves, Hydronic Boiler)
• attach a drain hose to the boiler drain and run it outside or to a floor drain
• open the boiler drain to drain water pressure off of the system.
  

Wet process vs draindown process for boiler gauge replacement

At this point there are 2 choices:

The "wet" process for changing a boiler gauge (this process can avoid having to purge air from the heating system)
1. when pressure is off, and with the new part ready to install, IF it is a direct screw-in part, some installers just allow things to get a bit wet (protecting electricals, burner etc from splashing), and with PRESSURE drained off of the boiler but the boiler not completely drained, they remove the old and install the new working calmly but quickly to minimize the spillage

OR The "dry" drain-down process for changing out a boiler gauge (this procedure will require purging air from the heating system)

2. with pressure off, drain enough water from the boiler to get the water level just below the height of the gauge attachment point (you don't need to drain the whole system) - and install the new part.

You may need to open a service valve, remove (and then install a new) Air Bleeder Valves or even open the pressure/temperature relief valve to let air INTO the boiler (once pressure has been relieved therein) to speed water draining out of the boiler.
Watch out: messing with a relief valve can leave it leaky and lead to need for replacement. See RELIEF VALVES - TP VALVES for details
Watch out: You may later have to purge air from the boiler, and even from the heating zones, depending on how much air enters the heating system during this procedure. ([size=-1]AIRBOUND HEAT SYSTEM REPAIRS)

Returning the boiler to service after gauge replacement

With new boiler pressure/temperature gauge in place, you will need to:

• Check that the P/T relief valve or any other air inlet source you opened has been closed and is not leaky
• Make sure that the Boiler drain is now closed and drain hose removed
• Turn on Water supply to the boiler
• Check and confirm no leaks at the gauge nor elsewhere
• Purge air from the heating system boiler and if needed from the zones. If necessary see our air purging methods beginning at AIRBOUND HEAT SYSTEM REPAIRS
• If you closed valves that isolated heating zones and circulators from the boiler remember to open them
• Turn power back on to the boiler and if necessary set the thermostat up to call for heat so that the boiler runs
• Let boiler run up to full operating temperature and check for leaks and for airbound conditions
• If any heating zones are not heating properly they may be airbound and may need additional venting