Why do Cast Iron Boilers Leak/Crack? 

Cast iron boilers are very durable, dependable and offer very little resistance to flow. Through the years many cast iron boilers have failed due to no problem of their own. This means that it was not a product failure it was caused by an installation error. Don't take this the wrong way; I am not saying the manufacturers are never at fault. If you manufacture anything there will at times be issues. I am saying that the failures are due to manufacturing or installation problems, although it is easier to always blame the manufacturer then the installation. I do believe most leaks are caused by thermal stress, piping or water quality.  I have been in this industry for 34 years and have worked 20 of those years as a service/installation tech, service manager for a fairly large oil company. I than worked 14 years with a manufacturer in tech services and training which also included job site visits. This page will address the reasons I have found for cast iron hot water and steam boiler failures through my many years of experience.

Boiler manufacturers that make cast iron boilers will assemble cast iron sections using a metal push nipple between the sections or a gasket type of material on what is called a nipple port. The nipple port is where the water flows from one section to another. There may be 2 -4 nipple ports per section. There are upper and lower nipple ports. They may or may not be different sizes. Steam boilers will have larger upper nipple ports than lower nipple ports. The most common reasons for cast iron boiler failures are thermal stress, condensation, water quality and oxygen in the water. If there is a manufacturing problem it normally will affect many boilers with similar failures.  I will say the iron thickness and iron quality is better than it has ever been. The computer age, improved cast iron pouring process and advanced machining equipment has greatly improved over the last few decades

Cast Iron Hot water boilers

I will address the hot water boilers first and the next segment will address steam boilers. Hot water boilers will expand and contract as the boiler heats and cools. A slow expansion and contraction is not a problem for cast iron. When the expansion and contraction is rapid that is a problem which can cause push nipple and gasket leaks or a stress cracks. This happens when hot water returns to a cold boiler or cold water returns to a hot boiler. On residential applications the larger problem is normally cold water entering a hot boiler. The two major flow problems are extremely cold water entering the boiler or cool water entering the boiler at a high flow rate. The rapid and constant movement may also cause undo wear on the push nipples or gaskets. The ideal section movement is slow due to gradual temperature changes. Another reason for stress cracks or thermal stress would be an oversized boiler which causes quick iron movement as the water heats quickly, cools down during normal operation and fires to heat up quickly again. This happens with oversized boilers multiple times on a thermostat run cycle and thousands of times a winter. This causes more wear on push nipples and gaskets, also more stress on the cast iron sections.

Boiler protection against thermal stress must control temperature and /or flow. Here are piping strategies that help protect the cast iron boilers to avoid these problems.  A simple boiler bypass pipe would be the minimum protection that should be used, here is a simple drawing to show a boiler bypass.

Another reason for water boiler failures is oxygen causing rusting from the interior of the iron boiler section. The oxygen must be removed from the heating system and remain oxygen free from that point. I like to refer to what I call the boiler triangle. It takes three things to cause a boiler to rust out from the inside, iron, water and oxygen.

If we remove one of the three items we will eliminate oxygen corrosion. We cannot take away the iron or the water. We must remove the oxygen. How do we do that? Proper near boiler piping. This means placing the circulators on the hot supply side of the boiler pumping away from the expansion tank connection. Eliminate automatic air vents at the high points of the system, instead use manual vents. If your expansion tank does not have a rubber bladder eliminate all automatic air vents. Every time this style tank is drained fresh water is added and in the make-up water is oxygen, minerals and chemicals. None of which is good for the heating system. Avoid the addition of fresh water when ever possible by using strategic placement of service valves and repairing heating system leaks. The addition of too much make-up water can introduce minerals. The minerals will attach to the hottest iron surfaces which is where the fire is just under the iron. The mineral build-up will create hot spots and cause cracks.

Flue gas condensation will result when the hot flue gasses pass through the flue pass with cold water in the cast iron sections. Condensate occurs when the flue gasses are cooled enough to meet dew point temperatures. This is like dew on the summer grass except this moisture contains hydrochloric, sulfuric and carbonic acids. As the cast iron sections are wet the acids eat at the iron and when the cast iron sections get warm enough the flue passes will dry. It is very important that the flue passes are dry before the boiler shuts down. The temperature that the flue gasses stop condensing is dependent on different parameters. The industry standard is 140F. The actual number may be as low as 126f. The same boiler protection as stated above will also help here. The boiler bypass will slow the flow through the boiler thus allowing higher iron temperatures to avoid flue gas condensation.

Maybe this will help understand why cast iron boilers fail due to stress and condensation.

I did some research and found a boiler from the 50's with an input of 160,000 BTU's. Let's say the return has a flow of 7 gpm. As you see the boiler has almost 6 times more water than the return is bring back. With this ratio between return water and boiler water the internal temperature change is very slow. When the boiler is replaced with a new cast iron boiler today that boiler will hold about 6 gallons of water.  With the same return flow as the old boiler had there is 1 gallon more water returning to the boiler than the boiler holds. This will cause very quick and drastic temperature changes.

When a boiler bypass is added some of the flow from the return will flow through the bypass which means less flow through the boiler so the boiler sees less stress and the iron heats up quick enough to avoid flue gas condensation. The flow through the bypass may be three times the boiler flow. In this example the flow through the bypass is 4 gpm which means the flow through the boiler is 3 gpm. The flow through the boiler is 1/2 of what the boiler holds. Again it warms up faster than it would with the 7 gpm flow.

Steam Boilers

With steam boilers don't worry about flue gas condensation as there is no flow into the boiler until you start making steam. We still have the mineral and oxygen problems you experience with water boilers from adding too much make-up water due to steam or water leaks. The oxygen will cause pinholes at the water line as that is where the oxygen comes out of the water.

Another cause of leaks in a steam boiler is chlorides. When chlorides cause a leak it will create holes in the iron at the top of the boiler. Some of these holes can get pretty large. I have seen holes large enough to stick your fist in. The water level in a steam boiler should be maintained at the manufacturer’s specs. This level may be 1/2 to 3/4's of the sight glass. This means the boiler is not full of water. The iron exposed above the water level get much hotter than the iron below the water level as the water takes away some of the heat. The boiling in the boiler is very violent. The water will bounce off the internal top of the boiler sections. The water will flash off rapidly and leave the chemicals and minerals. The chlorides will eat away the iron over time and you cannot stop it.

Avoid adding too much make-up water from leaking air vent, valves or fittings. Do not add softened water, use water before the water softener. Do not drain the water from the boiler in the summer. If you have an electronic Low Water Cut-Off (LWCO) do not drain any water from the boiler. If you have a float type LWCO drain once a month or follow manufacturer's spec's and drain just enough water to get clear water.

Disclaimer: The information found on this website is for informational purposes only. Any and all preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.