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THERMOSTATS, HEATING / COOLING
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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.
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?
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 at STEAM HEATING SYSTEMS
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 CONTROL, STEAM BOILERS) , 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
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.
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