Boiler Pressure ProblemBoiler Pressure Problem - Hello, where I live, it’s mostly gas furnaces with air conditioners or heat pumps. My boiler just started causing problems with high pressure. I work in HVAC as a technician but do not have a lot of experience with hot water or steam boilers. I’ve been doing this for 15 yrs but don’t have a lot of experience diagnosing anything water-related. Additionally, it’s a 6-year-old (approximately) Burnham boiler with old cast iron radiators.

This year it started having problems with the relief blowing. The boiler set temp is 170 at the aquastat. It’s a gas boiler if that makes any difference for a boiler system. Finally, can you help me out with the boiler pressure problem? Thanks!!

Boiler Pressure Problem - Expansion Tanks

You have a hot water gas boiler as opposed to a steam boiler. Each type of heating system works differently. Furthermore, the piping systems are also different along with operating pressures for hydronic heating systems.

There a few specific things you need to check first. Most of the time, when I find the pressure relief vent purging, it is a boiler expansion tank problem. Furthermore, of the two types of boiler expansion tanks there are 1) the bladder type expansion tank 2) the steel barrel expansion tank, both types can give you problems with causing high-pressure problems.

Boiler Expansion Tank Types

  • Hot Water Boiler Expansion TanksBladder type expansion - check the pressure and fill accordingly. The proper air pressure for a bladder-type expansion tank is “fill pressure” plus four psi. Make sure to isolate the expansion tank before setting the pressure. Most expansion tanks have isolation valves to isolate them from the circuit. Furthermore, if the expansion tank will not hold pressure, you could have a hole in the bladder or the tank itself. In that case, replace it.
  • Steel barrel-type expansion tanks - These tanks can, over time, become waterlogged. They will be overfilled and need draining to about 1/3rd of the tank. In that case, you can only hope you have an expansion tank water level valve. Lastly, this will help you get the proper amount of water (1/3rd) in the tank leaving 2/3rds air.

Make-up Water Valve - Boiler Pressure Problem

In all boiler systems, you have a makeup water supply. In residential systems, this is usually a pressure reducer valve. Some of these have fast-fill latches built in while others have a bypass coming from the makeup water supply. The bypass is for the fast-fill of the system. It has valves to turn the water supply on and off to bypass the pressure reducer valve. If you are having pressure problems with the boiler, always check the pressure-reducing valve and any bypass valves coming from the makeup water supply.

Since the “supply water” for the makeup is higher than the needed boiler pressure, it is essential to make sure this is not the problem. Furthermore, a problem with either the pressure reducer valve not working correctly or the makeup water bypassing. Additionally, remember that the boiler water level is super important. Always make sure you have a good makeup water source that can directly feed the boiler. Finally, if the boiler does go low on water, the low water cutoff will cut it off. Therefore, you will have no heat.

Domestic Hot Water Coils - Boiler Pressure Problem

Boiler Pressure Problem

Pulse Boilers

Lastly, some boilers have a domestic hot water coil for making hot water for showers, washing dishes, so on and so forth. Sometimes, after prolonged use, a domestic coil inside the boiler jacket can develop a pinhole in the tubing. If this happens, the boiler pressure is affected by pressure from the domestic coil. In this case, the domestic needs to shut off and replaced. Another option is to abandon the domestic hot water coil in the boiler and use a water heater.

Sometimes, domestic coils link into an indirect water heater meaning a storage tank that holds the hot water. Additionally, in addition to the boiler system providing space heating inside the home, it also makes hot water for the house with the domestic coil. The water temperature and water pressure are different for the dual application. Lastly, this happens through the boiler controls.

What Should the Boiler Pressure Be Under Normal Boiler Operation?

The pressure for hot water boilers will vary from the boiler system to the boiler system. Most boilers operate with a pressure of 12 PSI. Again, that pressure will differ from one boiler to boiler.
Additionally, within the loop, the pressures will vary. As noted above, the expansion tank has a lot to do with the proper pressure inside the boiler loop. The make-water delivery will also affect this pressure. Most city water pressures will deliver water to your home higher than 50 PSI. The water feed system to your boiler has a pressure reducer valve. That reduces the pressure delivered from the city (or even a well system) so that the loop is not over-pressurized.

The setting on most pressure reducer valves is 12 PSI. That is likely the pressure on your boiler system. The pressure reducer can be adjusted to increase or decrease the pressure depending on the boiler system. Additionally, if you have a bladder-type expansion tank, then the pressure for the tank should be checked for proper pressure. Follow the manufacturer’s instructions for proper adjustment settings for the expansion tank. Methods and settings vary from manufacturer to manufacturer. In any case, the pressure for your loop should never exceed 18 to 20 PSI.

What is Normal Boiler Pressure and Where to Check Boiler Pressure? | How to Set Boiler Pressure

Normal boiler pressure should be around 12 PSI.  There are some boiler systems that are slightly higher. To determine the proper pressure for the boiler system requires filling the system and determine the isolated static pressure. Then add 4 PSI. Here is a graphic showing the pressure reducer valve with a pressure gauge.

Boiler Pressure Reducer Valve and Gauge

Some pressure reducer valves do not have a fast fill feature. Instead a small loop with valves with being piped around the pressure reducer valve. If the pressure for your boiler system is excessively high, make sure the bypass valve is closed.


​For boiler pressure problems, you need to check these three specific areas.

  • Expansion tank
  • Pressure reducer valve and any bypasses in the system for makeup water - don’t confuse this with a pressure relief valve.
  • If so equipped, a domestic hot water coil

If you are uncomfortable with a self-fix boiler repair call for boiler service from a pro. They will have all the boiler parts for boiler servicing and basic boiler maintenance. Heating equipment, especially water heating systems such as water boilers, needs the attention of an expert from time to time. Therefore, efficient boilers are well-maintained boilers.

Whether it is a gas-fired boiler or an oil boiler, they all will require boiler service periodically. As a homeowner, you want to reduce your energy bills and save on home energy use. Finally, save energy and make your home heating system more energy efficient by having regular maintenance done.

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Boiler Pressure Problem

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