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Why Does My Combi Boiler Keep Losing Pressure?

This is a question we get a lot from our customers. But most of the time, it’s an easy fix.

Your combi boiler can be losing pressure for a number of reasons. Pressure loss may be caused from a leak in the pressure relief valve, an issue in the expansion vessel, air in your system, or a leak in the heating pipework itself.

In this article, we give you a few options that you can do on your own, before you call a licensed HVAC technician. You don’t need to be an expert in order to do a little troubleshooting on your own! Just follow the simple steps below and there’s a good chance you’ll figure out your problem in no time.

Check for a Leak in Your Combi Boiler Pressure Relief Valve.

If you have had a continuous flow of water running through your pressure relief valve, then there’s a very good chance that some debris may have got lodged inside. This is most likely your problem and you’ll need to call an HVAC heating specialist. The valve may need to be replaced of cleaned, which is a fairly inexpensive repair 9 times out of 10.

Is the Expansion Vessel Faulty?

The combi boiler expansion vessel is there to accommodate any expansion created when water is heated up to heat your home or shower, wash dishes, etc. If the expansion vessel is faulty, chances are it will not be able to handle the expanding hot water. The pressure on the vessel will then rise rapidly and water will be ejected through the pressure relief valve.

If that doesn’t seem to be the issue, next, find the Schrader valve on the expansion vessel (it looks like an air valve found on a car or bike tire). Unscrew the plastic cover on top of the Schrader valve and briefly press the pin in on the valve. If air doesn’t come out and water comes out of the valve instead, the expansion vessel is bad. The diaphragm inside the vessel is ruptured and the vessel needs to be replaced.

Still not the problem? Quite possibly, your expansion vessel may need to be pressurized. If the pressure appears to rise rapidly in the vessel, then suddenly drops when the pressure relief valve opens, it’s time for your expansion vessel to be recharged. But you’ll need an expert to do a pressure check to be certain that’s really the issue. The vessel will have to be drained first and then the pressure checked.

Pressurizing your expansion vessel should typically be done as part of an annual service, which will prevent issues from happening in the first place, but it’s an easy repair none the less. But be warned, we recommend that this work be done by a licensed professional heating technician only. This can be too dangerous for a home novice to attempt. We’re talking scalding hot water and gas here.

Is There Air in the System?

Checking for air in your combi boiler system is often times an easy thing to check yourself. You just might be able to diagnose your problem with just a few checks, before you call a tech.

Before you do anything, try bleeding your radiators (if your system has these). Bleeding your radiators is rather simple. To bleed your radiators, either use a radiator key or a flat screwdriver (depending on your valve type). Find the radiator bleed valve and slowly turn the valve counter clockwise until water starts dripping out. This will release trapped air and let hot water into the cold fins. Make sure you bleed the radiators in sequence. And make sure you’re using a towel or something else to catch any drips from the valve, unless you want a mess!

If bleeding the radiators doesn’t work, contact an expert. You may have a bigger issue such as air lock in the system, or may need to the system itself flushed out. This is something we don’t necessarily recommend you doing on your own, unless you feel completely comfortable.

Is There a Leak in Your Central Heating System?

A leak on your central heating system can cause your combi boiler to lose pressure too. Look for signs of leakage such as a stain your ceiling, a damp floor, leaky pipes, or a small hole in a radiator somewhere. Corroded pipes and radiators can also be a good clue to where a leak may be located.

Finding a leak may seem like finding a needle in a haystack, but it’s worth a shot, before calling a heating tech. Most all HVAC companies charge an hourly rate, which can become quite expensive, if they spend hours troubleshooting your system.

Did We Help Solve Your Problem?

If we didn’t please feel free to ask questions in the comments section below. Or, if you’re in the Connecticut area, feel free to call us. @ (860) 818-3891. We would love to help.

About the Author

Ken is the president and co-founder of County Wide Mechanical Services, LLC. He has over 15 years of experience in the HVAC and plumbing field. Ken has been instrumental in the growth of County Wide Mechanical Services in Connecticut. He believes that customer satisfaction is the backbone of the company.