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LARGER VIEW of a heating boiler pressure temperature gauge Heating System Pressure & Temperature Controls

  • PRESSURE & TEMPERATURE SETTINGS, CONTROLS - CONTENTS: how to set heating system pressure & temperature for hydronic or hot water heating systems or for steam heat:What are normal hot water heat pressure & temperature levels & settings ? What are normal residential steam heat pressure & temperatures ? Where & how do I set the boiler pressure & temperature controls? Where & how do I set a steam boiler pressure & temperature? Where & how do I set the furnace temperature control? Sources of variation in pressure in hot water heating systems. Definition of PNPC point of no pressure change
  • POST a QUESTION or READ FAQs about heating system operating pressures, temperatures, and controls for hot water and hot air heating systems and for warm air furnace systems
  • REFERENCES
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Hot water heating boiler pressure & temperature settings:

Here we explain where and how and to what numbers the pressure & temperature are set or controlled on hydronic heating boilers. We also describe where and how the pressure & temperature is controlled on residential steam boilers.

We include notes and links to detailed articles about the operation and use of controls on boilers, furnaces, and water heaters.

This article series answers most questions about all types of central heating system controls in order to aid in troubleshooting, inspection, diagnosis, and repairs.



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What Are the Normal Hot and Cold Operating Pressures of Residential Hydronic (hot water) Heating Boilers?

Boiler gauge with typical pressure and temperature

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.

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.

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.

Causes of Variation in Pressure in Hydronic Heating Systems

Watch out: the actual pressure in a hydronic heating system is tricky to pin down. System pressure is only uniform throughout the system when the circulator pump(s) are off, there is no circulation by convection, and piping is entirely and only horizontal - not a realistic situation, right? ITT Industries (B&G et als) point out in technical publications that the pressure you read on the pressure gauge is only the system pressure at that location and at that particular operating condition (temperature and circulator on or off).[1]

The variables that determine the water pressure in the heating system include:

Definition of The Point of No Pressure Change - PNPC - where the Expansion Tank is Installed

The PNPC is the location in the heating system piping installation where the compression tank is connected to the system. Note that this is not really a point of "no pressure" since there is always some pressure in the system. It is a point of no pressure change [or pressure difference] on either side of the tank's inlet fitting.

Watch out: ITT notes that despite the instructions from manufacturers that an expansion tank can be installed anywhere, installing the tank on the discharge side of the circulator pump is a mistake.

The action of the pump [if the tank is installed on the discharge side of the circulator] now decreases the pressure below the non operating pressure of the system everywhere except in the small section between the pump and the tank.

In systems where the non operating pressure of the system is low compared to the pump head, the large reduction in pressure when the pump comes on could cause boiling in hot water systems, draw air in through automatic vents, or even result in pump cavitation. When the pump is located in this way, it also can cause a great deal of confusion.  [1]

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.

Where & How do I Set Hot Water Boiler, Steam Boiler, or Furnace Temperature?

Hot Water Boiler Temperature Settings

Photograph of a multi function combination control on a heating boiler

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.

[Click to enlarge any image]

The temperature at the boiler or furnace is controlled by local safety devices mounted right at that equipment, such as the heating boiler
AQUASTAT CONTROL
shown at left.

Hot water heating boiler normal pressure ranges as read on the pressure gauge, are described in this article, below.

The means by which a hot water heating boiler's pressure is set or controlled are described in this article below
at Where do I Set the Heating Boiler Operating Pressure?

For details how, to what number, and on what controls the hydronic heating boiler temperatures are set, also
see AQUASTAT CONTROL FUNCTIONS,

and LIMIT CONTROL, SINGLE.

Steam Boiler Temperature & Pressure Settings

Steam boiler pressure control switch

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:

Warm Air Furnace Temperature Settings

Furnace fan limit switch (C) Daniel Friedman

Furnace temperature setting controls: Warm air furnace temperatures are controlled at
the FAN LIMIT SWITCH.

See FURNACES, HEATING for a complete discussion of warm air heat.

Additional furnace controls include register and duct dampers -
See ZONE DAMPER CONTROLS

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

Typical location of a boiler gauge

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 °F.

Typical operating temperatures on a hydronic boiler call for a high temperature (boiler cuts off) of 180-200 °F.

If we set the boiler upper temperature too high over 200 degrees F. we're at risk of spilling at the pressure temperature relief valve.

If we set the boiler upper limit too low, there may be no relief valve problem but under some conditions we may reduce the operating efficiency of the boiler and heating system, thus increasing heating costs.
See AQUASTAT CONTROL FUNCTIONS for details.

Boiler gauge with typical pressure and temperature

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 °F. (The boiler had just cut off on a heating cycle.)

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

Where do I Set the Heating Boiler Operating Pressure?

The operating pressure of a heating boiler (hot water or hydronic heat) is read at the pressure and temperature gauge (see above) and controlled by

  1. The starting or "cold water" pressure at the boiler - typically at 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. Most heating boilers are provided cold makeup water through a pressure-reducing, back-flow preventing, automatic water feeder valve -
    see WATER FEEDER VALVES, HYDRONIC BOILER. The valve is adjustable but should not be changed except by a trained heating service technician.
  2. The ending or "hot water" temperature at the boiler - typically 180 deg F to 210 deg F, controlled by the heating boiler's combination control, aquastat, or limit control -
    see AQUASTAT CONTROL FUNCTIONS
    and
    see LIMIT CONTROL, SINGLE.
  3. Heating equipment has multiple safety controls designed to prevent damage to the equipment or unsafe conditions. Indirectly, or in emergency, the heating boiler pressure is limited by first, the high limit set on
    the AQUASTAT CONTROLS or LIMIT CONTROL, SINGLE
  4. Heating equipment relief valves: If the temperature limit controls should fail, the boiler's temperature and pressure are released by one or more pressure/temperature relief valves:
    see RELIEF VALVES - TP VALVES on hot water systems

    or RELIEF VALVE, TP VALVE, STEAM BOILER on steam heating systems. The operating temperature or pressure and relief capacity of these safety devices is matched to the BTUH input of the heating appliance. Pressure relief valves on residential and most commercial heating equipment are not adjustable.

The building THERMOSTATS do not normally directly control the temperature or pressure in the heating boiler. The thermostat sets the desired temperature in the building, but to the heating boiler it is working as a simple "on" - "off" switch, turning the boiler "on" until the thermostat is satisfied (the building is warm enough), then turning the boiler "off".

Residential Steam Boiler Normal Temperature Range

Steam pressure gauge on a steam boiler 

Temperature gauge on steam heating boilers: 

Because a steam boiler makes heat by producing steam - by boiling water, at sea level, the temperature at the boiler will be boiling or 212 °F or close to that figure. Details are
at STEAM BOILER TEMPERATURES

Residential Steam Boilers Normal Operating Pressure

Steam boiler pressure control switch

Residential steam heating systems are almost always designed to operate at very low pressures, perhaps around 0.2 to a maximum of 0.5 psi - that' s 1/2 of one psi. Click to enlarge and you can see the actual pressure settings on the steam boiler control shown at left. High rise building and some commercial steam heat systems operate at higher pressures.

Details are at STEAM BOILER PRESSURE - home.

If your residential steam boiler is operating at higher pressures 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.

Also take a look at the dial setting on your STEAM BOILER PRESSURE CONTROL SWITCH

...


Continue reading at PRESSURE & TEMPERATURE SETTING, CONTROL FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see WATER FEEDER VALVES, HYDRONIC BOILER

AQUASTAT CONTROL FUNCTIONS

EXPANSION TANKS, BOILERS

GAUGES on HEATING EQUIPMENT

PRESSURE GAUGE, BOILER

PRESSURE CONTROL, STEAM BOILERS

RELIEF VALVES - TP VALVES

RELIEF VALVE, TP VALVE, STEAM BOILER

STEAM PRESSURE GAUGE

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INDEX to RELATED ARTICLES: ARTICLE INDEX to ARTICLE INDEX to HEATING BOILERS

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