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MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
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NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
OIL FILL PIPE LEAKS
OIL SPILL CLEANUP / PREVENTION
PLASTIC PLEXVENT ULTRAVENT RECALL
PUFFBACKS, OIL BURNER
RELIEF VALVE LEAKS
RESET SWITCH, HEATER PRIMARY CONTROL
RESET SWITCH, ELECTRIC MOTOR
RESET SWITCH, STACK RELAY
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOLAR HEATING SYSTEM DESIGNS
SOOT on OIL FIRED HEATING EQUIPMENT
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: HEATING SYSTEMS
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
ZONE VALVES, HEATING
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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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.
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).
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.
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.
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.[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
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
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:
Warm Air Furnace Temperature Settings
Furnace temperature setting controls: Warm air furnace temperatures are controlled at
See FURNACES, HEATING for a complete discussion of warm air heat.
Additional furnace controls include register and duct dampers -
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.
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
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 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".
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.
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
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 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.
Take a look at the dial setting on your PRESSURE CONTROL, STEAM BOILERS
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
For details about radiator problems see RADIATORS or select a topic from the More Reading links shown below.
Continue reading at WATER FEEDER VALVES, HYDRONIC BOILER or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: effect of furnace fan limit switch not working & other worries
(June 21, 2012) abbas said:
what the problem if furnace high switch not working
(Sept 20, 2011) John said:
(Apr 1, 2012) Muhd Nazlan from Malaysian said:
Abbas the problem with a Hi Limit switch that is not working is that the furnace is unsafe and can overheat, damaging or even ruining the heat exchanger.
Question: automatic water feeder not working
(Oct 13, 2012) Frank B said:
Use the email found at our CONTACT US link at page top or bottom to send photos and we'll be glad to comment further.
And see WATER FEEDER VALVE, STEAM
Question: steam leaks from steam heating system
(Nov 28, 2012) Tonda Ladson said:
(Nov 20, 2014) Anonymous said:
Look for a steam vent that is not operating properly and needs replacement. If you see steam escaping from piping or from the boiler itself that's a different matter. In that case shut off the system and ask for help from a trained steam heating service tech.
Question: boiler turns on but no hot water at faucets
(Feb 25, 2013) Ed said:
Question: Lochinvar model rbn 135 serial d029094 settings
(Feb 16, 2014) henry said:
Check the aquastat settings: someone may have set the LO limit too close to or even above the HI limit. If the settings are correct then I suspect a control or control sensor problem.
Question: test to see if the aquastat control has a bad sensor?
(Feb 16, 2014) Anonymous said:
(Feb 16, 2014) henry said:
Interesting question, Henry, I don't know but a wiring diagram for your control might answer it. If you are talking about an aquastat on a heating boiler, typically there is a boiler temperature sensor that is inserted into a well that itself projects into the boiler water; the manufacturer recommends that the copper tubing connecting the sensor to the aquastat be bent enough to assure good pressure or contact between the sensor and the well sides, and they also may recommend use of a heat conducting paste to assure good contact; If contact is poor (or if the sensor has failed) the control may fail to respond properly to the actual boiler temperature.
So my thinking starts by observing how the boiler actually responds compared to the gauge temperature.
Unfortunately to check the sensor mount and contact you'd have to remove the whole control - a lot of trouble so not what people would try first.
That's why I stressed the importance of looking at the HI LO and DIFF settings first
(Mar 15, 2014) momma said:
To answer your question I'd need to understand more accurately how your building provides hot water (at sinks etc): is it by
- a separate, stand-alone water heater, and if so how is that heater powered: gas, oil, electricity?
- a tankless coil on the heating boiler itself (probably your case) ?
- an indirect water heater (tank) heated by a loop of heating water circulated from the heating boiler?
Question: boiler pressure problems, gauge readings, leaks at the relief valve: debugging & diagnosis
(Dec 15, 2014) Roger said:
you want to see this diagnostic article
to get at the underlying cause: could be a bad expansion tank, pressure regulator, or other causes.
(Dec 16, 2014) Roger said:
Roger pressure over 32 psi is already too high in the boiler. When the boiler is COLD the pressure would normally be around 12 psi, heating up to under 30 PSI at maximum boiler operating temp. Higher starting pressure is needed for taller buildings.
With boiler cold drop pressure to 12-18 psi. Watch to see if pressure rises while boiler remains off. I understand in cold weather when you want heat this can be difficult.
If the boiler contains a tankless coil shut water off coming into the coil to see if that is the leak source
(Dec 20, 2014) Roger said:
IF you set the boiler pressure to 12 PSI when the system is cold and leave the boiler off the pressure should not increase.
IF it does there is a leak into the boiler - sometimes from a leaky water feeder / pressure reducer valve or possibly from a tankless coil (or leaks into the boiler circuit heating an indirect-fired water heater) when the boiler has one.
The TP valve releases heat and pressure trying to avoid a dangerous BLEVE explosion.
(Dec 20, 2014) Roger said:
When I leave return open and boiler off the the pressure rises sounds like it might be the indirect fired water
Did you tap the pressure gauge to see if it's sticking?
(Dec 21, 2014) Roger said:
(Dec 21, 2014) DanJoeFriedman (mod) said:
Roger if the gauge pressure does not fall when water is removed from the boiler, it's probably stuck or debris clogged and needs replacement. Try measuring boiler water pressure at a boiler drain using an independent gauge.
WATCH OUT for scalding burning hazards.
(Dec 22, 2014) Roger said:
gauge goes to zero when I drain boiler doesn't go up to 20 until i cut water back in
boiler just shut off at 175 degrees with 30 psi boiler cools down to 120 and now pressure is 27 boiler has been off for an hour and pressure is staying at about 27 i have replaced the expansion tank, feed valve, and air valve when i put new expansion tank in i didn't check air pressure as it was supposed to be preset amtrol filltrol but when i cut in water gauge went to 20 psi it seems that should have been 12 can't figure why psi only drops 2 or 3 psi after boiler cools down
Roger you'll only see 12 PSI when the boiler is *cold* and your system, if your home is 3 or more floors tall, may actually need a higher starting pressure. So when the boiler is off for an hour its temperature is still pretty hot. Still I'd think pressure would drop more than 2-3 psi.
If the relief valve is not spilling (which would be a second indicator of high pressure or temperature) I'd move on to make an independent measure of the actual boiler pressure, taking care not to get scalded.
At inspectapedia.com/water/Water_Pressure_Measure.htm we show how you can buy or make an inexpensive independent water pressure test gauge that would work on the boiler drain - provided your boiler drain can be opened and shut without leaking.
(Dec 22, 2014) Roger said:
the relief valve is spilling the house is a ranch with boiler in basement it really seems that the gauge is accurate the spillage doesn't occur until gauge reaches 30 which is valve setting
Can i close the return from indirect water heater and close zone valve to water heater to check for leakage from water heater into boiler circuit and turn boiler on it would be isolating the water heater from boiler
You should indeed be able to isolate the indirect water heater from the boiler - as the water heater too has its own pressure relief valve.
Question: monitor boiler pressures when water supply is shut off - to check for a boiler leak
(Dec 23, 2014) dean said:
But after 4 to 5 days, the cold pressure goes to 0. I this normal? Can it be caused by something other than a leak?
Cold pressure in a hot water heating boiler (hydronic) won't normally fall to zero unless there is a leak. Typically it'd be around 12 psi. A steam boiler pressure will drop to zero when cold.
I'd be looking for a leak.
Question: gurgling sounds in hot water heating baseboards
(2 days ago) roger said:
Reply: check for and purge air in the hot water heating boiler & piping system
Usually gurgling means there's air in the system that has not been purged.
See inspectapedia.com/heat/Air_Bound_Heating_System.php for help with getting air out of the piping, baseboards, radiators. The bleeding you've done may not have been effective.
If the relief valve is leaking the system may be operating at too high a temperature or pressure or might be (more rarely) suffering from a water hammer problem as a circulator pump starts and stops. Or of course the TP valve itself may need replacement.
Start by purging air, then
Continue by checking for a water-logged expansion tank on your boiler.
Mike Burke said:
Thanks for the nice note, Mike. We work hard on this material to make it accurate, useful, unbiased, so I'm naturally elated when a reader finds it actually helpful. We also welcome content suggestions, questions, criticism. Working together makes us smarter.
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