Guide to Setting Heating System Pressure & Temperature Controls

PRESSURE & TEMPERATURE SETTINGS, CONTROLS on hot water heating systems:
- 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
Questions & Answers 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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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.

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

This website answers most questions about all types of central Heating System r Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

.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 upper limit of water pressure that should occur in a hydronic heating system is determined by the setting of the pressure relief valve (PRV). System pressures should not normally reach this upper limit.
The lower limit of water pressure that should occur in a hydronic heating system is a little more complex to state but is nominally the cold-fill pressure, measured at the pressure/reducer - water/feeder valve.

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

The initial cold water pressure established when the system was filled with water. Typical pressures (such as 12 psig) were discussed just above. Taller buildings will require higher pressures in order to actually circulate hot water (the circulator pump does not have sufficient lift capacity).

As a rule of thumb, 40 deg F. "cold" water (near standard density) gives us 0.43 psi per foot of altitude. (see above). The pressure setting of the pressure/reducer water/feeder valve is set to maintain the system minimum "cold" pressure for proper operation. But notice that the location of the pressure/reducer - water/feed valve will affect the system pressure that it senses. Located above or below the boiler, for example, causes the valve to see lower or higher pressures respectively on the boiler side of the valve.
The operating temperature of the heating system. In a closed system the expansion of water as it is heated will increase the system pressure.

Water does not expand at a uniform rate in response to temperture rise; based on the ITT article we cited above we figure on about a 5% increase in operating pressure at normal heating temperatures.[1] This is enough pressure to dump the PRV unless the system has an expansion tank (or "comnpression tank" in some literature) installed. Because the pipes containing heating water also expand slightly when hot ITT figures a net expansion of about 4.5% when the heating system is up to design temp.
The design, location, & condition of the expansion tank (compression tank). The expansion tank must be properly sized (based on system volume, operating temperature range, relief valve setting, initial pressure, and relative heights of the expansion tank, relief valve, circulator pump, fill point (pressure/reducer water feed valve), and the highest point or "top" of the system.

Note that expansion tank manufacturers say the tank can be installed "anywhere on the system" - that's because of the operating range capacity of a properly-selected tank, not because the tank sees the same pressure everywhere it might be installed.
The circulating pump(s) pressure differences caused when the pump is operating. Pump on or off changes pressures in the system.
Location of the boiler pressure gauge in relation to the circulator pump's discharge point (and other piping and system components)

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?

Photograph of a multi function combination control on a heating boiler

Hot Water Boiler Temperature Settings

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, such as the heating boiler Aquastat 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 Switches, Boilers.

Steam Boiler Temperature & Pressure Settings

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:>

Furnace fan limit switch (C) Daniel Friedman

Warm Air Furnace Temperature Settings

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 - 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 degF.

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

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 Functionsfor details.

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.)

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

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

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.
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 Switches, Boilers.
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 or Limit Switch. If these controls fail, the boiler's temperature and pressure are released by one or more RELIEF VALVES - STEAM TP VALVES

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 & 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.

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 degF.

The steam boiler pressure is controlled by the Pressure Switch, Steam Boiler - the gray box shown in our photo at left and in closeup at our photo, below.

The controls in this photo are discussed in more detail at STEAM HEATING SYSTEMS

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. Click to enlarge and you can see the actual pressure settings on the steam boiler control shown at left.

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

Frequently Asked Questions (FAQs)

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

[1] "[Heating] System Pressure in Typical Hydronic Systems", TechTalk, Vol. 20, Issue 1, January 2005, ITT Industries, Fluid Handling [copy on file]
[2] Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
[3] National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
[4] The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
[5] Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
[6] The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
[7] Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
[8] "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
[9] "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
[10] "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
[12] Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
[13] Installation Guide for Residential Hydronic Heating Systems
[14] Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
[15] The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)

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Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
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