There are many types of boilers that do a lot of different things, but most all of them have some of the same basic components. Some because they are required by regulatory agencies, some because they are necessary for proper operation and safety of the boilers. But no matter how large or small the boiler is, you can probably find most, if not all of these components. Unless noted otherwise, these are typical for hot water boilers.
Every boiler needs some source of heat, obviously to heat the water. The type and sizes of burners vary so much that a few complete articles would be required to really get into detail as to how they operate and the specific operations of each burner. All burners serve the same function, which to safely and efficiently burn fuel and create heat. Typically, the flame safeguards are integrated in the burner control sequence, in that certain conditions need to be met in order for the flame to light, similar to gas furnace pre-ignition sequence of operations. A blower with air dampers for adjustment is often part of the burner, and air/fuel ratios are adjusted at the burner during combustion analysis. Ignition transformers/control boards, ignitors, draft pressure switches, flame sensing devices (flame scanner/ flame rod), and a primary controller to sequence all of it together are all apart of most burners. Honeywell, Fireye, Siemens, and a few others are common burner controls that are all different, but essentially do the same thing. Some of the most common burner manufacturers ( at least in my world) are Power flame, Webster, and Beckett, but there are a lot of burners out there. It is common to see a dual fuel set up on larger burners, meaning they are capable of burning 2 types of fuel, typically gas and oil. Steam and hot water boilers both use burners.
Gas Fired Boiler
Most water boiler systems are sealed, meaning that they are filled with water, the air is bled, and the same water is circulated throughout the system. In a perfect world, no additional water would be required, but most systems lose water and pressure through a variety of ways. A automatic water feeding valve is used to keep the system at a set pressure. There are many types with many different pressure ranges. The proper term would be pressure reducing valve, but i’ve always heard them called water feeders, so that is what I call them here. Back flow preventers are often used with water feeders. Once the water enters the boiler system, it should not be allowed to go back and re-enter the domestic cold water system. Boiler water can be pretty gross, and often contains chemicals for water treatment so be mindful and safe when opening the water side of any boiler system.
Relief valves are used to protect the boiler pressure from rising above the safe maximum that the boiler is rated for. The pressure rating on the relief should never be above the pressure that the boiler is rated for. Also, relief valves come with a BTU rating and are sized to match the fire ratings of the burner. This is crucial to keep in mind when replacing a relief valve. A valve that is too small may open prematurely, and a valve that is too large may not open at the pressure it is supposed to. There are calculations and recommendations that are used to size relief valves that I’m not gonna give here, but if you are replacing one or having issues with one and you are unsure, ask a senior tech or contact the manufacturer for recommendations. But keep in mind that the relief valve may be the last line of defense in preventing a boiler explosion, so treat it as such. NEVER plug a leaking relief valve, it is kind of illegal. Found on both steam and water boilers.
IF YOU DO THIS, YOU ARE FIRED!
These are controls to maintain a set range for water temperature, the burner modulating from high to low fire (for modulating burners), and as a safety to prevent temperature rise above set point. These are used in both water and steam boilers, and i will go over them in more detail in the next article.
Are used to move water through the boiler and the system. Some pumps are controlled on and off by the boiler, some are controlled by building automation, some are just on and run constantly. Flow sensors are often used to insure proper water flow is present in the boiler, and will disable burner operations if the flow is decreased below what is recommended.
Pumps come in all shapes, sizes, and voltages.
When a boiler gets low on water, it can be a very dangerous situation. Low water safeties are used to disable the burner when low water conditions are present. Steam and water boilers both require protection, but low water controls for steam are generally much more crucial than a typical water boiler, as the risk with steam and boiler low on water can be severe.
This is by no means a comprehensive list, just a general overview, and I’m sure i missed something that you all will let me know about. With the huge variety of boilers out there, it would be tough to list every single thing that you might run into. These are all very common and the things that i seem to replace or have issues with the most. I will expand on steam specific controls and components in the next article.
Boiler piping is a much-debated topic in the HVAC trade. In fact, many books have been written on the subject. Should the circulator pump be on the supply or return? Where should the expansion tank be located? The best way to bleed radiators? If you talk to 10 different technicians, it is very possible to get ten different answers. And the short answer is, they are all correct. Because there is no “one size fits all” approach to boiler piping and layout. What works on one boiler system may not on another, and when a new boiler is installed on an existing system, there are plenty of potential issues which could be unique to that specific set up. Water is weird, sometimes. Like air, water doesn’t always do what you engineer it to do. I couldn’t count the times that I’ve been involved with projects with issues that left engineers scratching their heads because how they designed the water to flow through a system vs. what the water is doing is completely different. A technician need to be able to identify and correct what causes flow and heat exchange issues when we find them, and to do that, we need to know how it’s supposed to work.
Here is a basic drawing of a hot water boiler system. This is an optimal setup, in my opinion. I like to feed water into the supply side before the expansion tank. Typically this area is the lower pressure which allows feeding easier. A lot of the air should go to the expansion tank, and the rest will go out into the system, which is ok because if it is in the system, it can be bled as long as there are bleed valves. Also, it allows the cold feed water to heat up before it enters the boiler, avoiding shock. I prefer to put the circulator pump on the return side, as well. I’ve had better luck with flow and pump life when the pump is pulling from the system, rather than pushing into the system. Gravity and convection play a part as well. Hot water naturally wants to rise, and this natural circulation helps the return side pump move water much easier than if the pump were on the supply side of the same system. If a pump must be installed on the supply side, I prefer to install it after the expansion tank. Also, as indicated by all of the X’s, install shut off valves wherever you can. It will save you a ton of time and hassle later on when a repair is required.
This is all my opinion, and is based on my personal experiences. However, if someone calls and asks me a boiler piping question, my first suggestion is to do whatever the manufacturer recommends. Most boiler installation literature shows diagrams on piping set up, and that is the baseline for installing a new boiler, and possibly diagnosing a flow issue. Some manufacturers show the circulator on the supply, some on the return, and some don’t care either way. If you follow the manufactures specs, to the tee, 99% of the time you won’t have a ton of issues with flow and boiler operation.
Bleeding air from systems is necessary from time to time. Some boiler systems are much easier to bleed if they are piped to allow air to be removed by automatic vents or go to radiators to be bled. On system drain down and refill, I will typically bleed air after filling the system, while it is still cold and no pumps are on. After that initial bleed, I turn the boiler and the pumps on and allow the boiler to heat up to operating temperature. Once it is hot, I shut everything off, boiler and pumps. This allows the air that may be traveling with the water to go up, either to higher radiators or bleed points. I bleed air, turn everything on again, turn it off, and repeat until all the air is out.
A lot of older boilers guys I have worked with only bleed air with the pumps running. I could never get a satisfactory answer as to why, and I have had much better luck bleeding air from a system with the pumps off. If you do things differently, and it works for you, that’s great. Again, most of this is my opinion based on my experiences, and there is more than one way to skin a cat. Also, increasing boiler system pressure while bleeding helps speed things up. Most automatic water feed valves are factory set to keep the pressure at 12 psi, which is a pretty standard pressure. If your system is 12- 15 psi, bumping it up to 20-25 psi will help speed up the bleeding process. Always make sure you aren’t exceeding the pressure rating of the relief valve if you increase system pressure to bleed. And don’t forget to bleed excess pressure off after you have completed.
A basic steam system is much simpler than a hot water system. The natural rising of the steam allows it to move through the system, so there is no need for a circulator pump to move steam. Steam is a vapor, so there is also no need to bleed air, and no need for an expansion tank on a basic steam system. However, piping pitch is much more crucial to this system. The piping must have pitch or fall to help the steam rise, and more importantly, to allow the condensate to flow back to the boiler. Level piping holds water, so it must have fallen. Also, a Hartford Loop is required to connect the supply and return. This is basically an equalizer to balance the pressure between the two sides of the system. Also, as part of the loop, the condensate return line connects 2’’ below the water level of the boiler. The loop is used to prevent water from leaving the boiler through the return if the pressure is lower than the supply, or if there were to be a leak on the return. This piping configuration was mandated by code for a long time as a prevention for low water conditions causing the boiler to dry fire. With the invention of more advanced low water protection devices, it isn’t required by code everywhere but still is a good idea. It allows added protection if the low water safeties were to fail.
Steam traps are integral to steam systems as well. A steam trap is a check/float valve that allows condensate to pass through and return to boiler while preventing steam from passing. Steam traps are locating on the return (outlet) side of steam heat exchangers, radiators, etc. The goal is to keep the steam in the radiator as long as needed for it condenses to liquid water, thereby releasing as much heat to the radiator as possible. Without steam traps, the steam would blow right through the radiator, and would not stay there long enough to properly heat it up.
There are an infinite amount of piping configurations that you will run into of the course of a career, some much better than others. And certain situations dictate changes and configurations that may allow one system to work well, and the same configurations could cause a different system to function poorly. In short, every system is different, and a lot of times I am required to think outside of the box to make a poorly designed system work. But as mentioned at the beginning, if you have a basic understanding of how things are supposed to work, it makes diagnosing why it isn’t working a lot easier.