Spread the Love

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. Can you help me out with the boiler pressure problem? Thanks!!

Boiler Pressure Problem - Expansion Tanks

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.

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
  • If so equipped, a domestic hot water coil

Boiler Pressure Problem

To learn more about heating systems and HVAC use this resource.

See the About the Author page for information about the author of this article.

Share your HVAC Photos or ask a question about your HVAC System by uploading a photo of it.

Photo Uploads
Enter your name so we can credit you
If you have a website enter your website URL for additional credit
If you want to include a description then do so here.
Drop a file here or click to upload Choose File
Maximum upload size: 33.55MB
jpe, jpeg, jpg, gif, png, bmp, docx, doc, txt and PDF files allowed to be uploaded. Limit 15 files total and 100 mb/s max size.


Spread the Love