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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 15 yrs but don’t have a lot of experience diagnosing anything water related. It’s a 6-year-old (approximately) Burnham boiler with old cast iron radiators. This year it started having problems with the relief blowing. Boiler set temp is 170 at the aquastat. It’s a gas boiler if that makes any difference for a boiler system. 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. 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. 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.

  • Bladder type expansion - check the pressure and fill accordingly. The proper air pressure for a bladder type expansion tank is fill pressure plus 4 psi. Make sure to isolate the expansion tank before setting the pressure. Most expansion tanks have isolation valves to isolate it from the circuit. If the expansion tank will not hold pressure you could have a hole in the bladder or the tanks itself. In that case, replace it.
  • Steel barrel type expansion tanks - These tanks can, over time, become waterlogged. They will be overfilled and need to be drained to about 1/3rd of the tank. In that case, you can only hope you have an expansion tank water level valve. 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 make-up 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 fast fill of the system and 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 important to make sure this is not the problem. A problem with either the pressure reducer valve not working properly or the makeup water bypassing.

Domestic Hot Water Coils - Boiler Pressure Problem

Lastly, there are some boilers that have a domestic hot water coil for making hot water for showers, washing dishes, so on and so forth. Sometimes, after long 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, a domestic coil is tied into an indirect water heater meaning a storage tank that holds the hot water. In addition to the boiler system providing space heating inside the home, it also makes hot water for the home with the domestic coil. The water temperature and water pressure are different for the dual application. This is done through the boiler controls.

Conclusion - Boiler Pressure Problem

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, need the attention of an expert from time to time. 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. Save energy and make your home heating system more energy efficient by having regular maintenance done.

Boiler Pressure Problem

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