Guide to Heating System Pressure & Temperature Gauges

Heating System Gauges: Pressure and Temperature Gauges on Heating Boilers - Troubleshooting & Repair Guide
What are the normal hot and cold operating pressures of hydronic heating boilers?
What are the normal hot and cold pressures of residential steam boilers?
Where & how do I set the boiler pressure and temperature controls?
Where & how do I set a steam boiler pressure and temperature?
Where & how do I set the furnace temperature control?
Troubleshooting heating system boiler controls and switches
BOILER CONTROLS & SWITCHES - separate article, hot water heat
OIL TANK GAUGES - separate article
STEAM HEATING SYSTEMS & Controls - separate article, steam heat
Questions & Answers about heating boiler gauges; how to read heating system pressure and temperature, what are the normal system pressure and temperature? boiler gauge leaks, replacement, repair.
References

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Heating boiler pressure & temperature gauges: Here we provide an inspection and repair guide to Heating System Gauges: Pressure and Temperature Gauges on Heating Boilers. We also describe the normal operating pressures for hot water hydronic heating boilers and for residential steam boilers. You will also see that This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.

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 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 STEAM HEATING SYSTEMS and in detail at 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 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.

Frequently Asked Questions (FAQs) about heating boiler gauges

Question: How do I replace a leaky pressure/temperature gauge on my heating boiler

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

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. (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

Question: a few questions about commercial / power plant steam generators

1) How to size the safety valve for a boiler with no stamp?

2) What are the overall conditions responsible for the damage of superheater coils?

3) Why is deaerated water not used in boiler at the time of boiler hydraulic testing?

- Mohnish

Reply:

Mohnish:

Size or rating of the pressure/temperature relief safety valve on steam boilers & missing data tags

If the data tag identifying the safety valve has been lost, and considering that this is such an important safety device, and considering the importance of installing a valve of adequate capacity, I'd just replace a valve that was in question for any reason whatsoever. We're talking about a residential part price of $20. U.S. (or less).

In some jursidictions even on residential steam heating boilers and certainly on a commercial or power plant steam generator in particular, I wouldn't be surprised if regulations and maintenance guidelines would view a missing steam boiler pressure/relief valve data tag as a safety violation. Also see RELIEF VALVES - STEAM TP VALVES

Superheater coils on steam boilers, sources of damage

I presume you are referring to super heater coils that are used in a steam-heat system boiler to generate saturated steam. The super heater coils are specially used in Boilers of Power plant. These are not residential heating system components, and with respect, are beyond our expertise. In my OPINION an important consideration is to assure that the proper safety pressure/relief valves are mounted at the proper locations and are in good working condition on superheater coils as well. See references for superheater coils and super heater coil damage. [1]

Use deareated water in steam boilers during testing?

That question too is not pertinent to residential steam systems. Deareated water is water used in commercial or power generating boilers from which dissolved oxygen or other gases is removed to avoid corrosion in the system. (Oxygen and other gases are removed during deaeration to a level as low as feasible - it won't quite be to zero. typically oxygen is removed down to about 7ppb during deaeration of water for steam systems. [2]

OPINION: it may seem reasonable to test a steam generating boiler with deaerated water, arguing that you're trying to duplicate the in-service condition.

However I speculate that

The corrosive effects of a brief hydraulic pressure test of a steam generating system are perhaps very low or even below measurement thresholds, as compared with in-service use water, perhaps moreso if performed at low temperature
The focus in steam generating boiler testing is on pressure tolerance and leak detection over a short interval nothing like the in-service life over which hot water in a steam generating boiler, if not deaereated, would be expected to have very corrosive effects.

Take a look at Steam: Its Generation and Use for details. [3] and see our articles on residential and light commercial steam boilers beginning at STEAM HEATING SYSTEMS.

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Technical Reviewers & References

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

[1] Power Technology and Engineering (formerly Hydrotechnical Construction), Volume 43, Number 4, 247-250, DOI: 10.1007/s10749-010-0105-4 - Thermal Power Stations, Temperature regime for damaged steam superheater coils, V. A. Bogachev (this is a PDF file - if you can't find contact SpringerLeak)
[2] "Deaerator", Wikipedia, web search 12/18/11. Quoting:
The deaerators in the steam generating systems of most thermal power plants use low pressure steam obtained from an extraction point in their steam turbine system. However, the steam generators in many large industrial facilities such as petroleum refineries may use whatever low-pressure steam is available.
[3] Babcock & Wilcox Co. (2005). Steam: Its Generation and Use (41st ed.).
[4] Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering (2nd edition ed.). McGraw-Hill Professional. ISBN: 0-07-019435-1.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST, HVAC CONTAMINATION STUDY An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
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.
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,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
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).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"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
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
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
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
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.)
Links to our list of additional information on heating system inspection, repair, maintenance
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