- Tech Tips
Good friend and contributor to HVAC School Neil Comparetto made this video showing the way in which he creates access ports for static pressure and gas combustion analysis. As techs we find ourselves in the tough position of needing to drill access holes to take measurements but the drilling and sealing of the holes can sometimes create real and perceived issues with the equipment. Many techs use high temp RTV Silicone, Rectorseal Duct Seal compound or even tape. In some cases these sealants may be appropriate but Neil shows how he uses plugs to make a good permanent access point. Always make sure to leave any work is a well sealed and workmanlike condition.
You can find many of these items at Trutech tools HERE
This article is the second in a series on boiler basics by senior boiler tech Justin Skinner. Thanks Justin.
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 Powerflame, 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.
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.
Pressure/Temperature Relief Valves
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.
Operating/Modulation/High Limit Controls
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.
Low Water Protection
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.
This series of articles is written by senior boiler tech (and all around swell guy) Justin Skinner. Thanks Justin.
Whats the difference in application and why use one type over the other?
Water tubes are generally considered safer. They contain much less water than fire tubes, so if a disturbance occurs (tube breaks, boiler melt down) there isn’t as much water/steam to have the potential to escape the boiler.
The main determining factor of water or fire tube is application. Water tubes are able to handle much higher pressure ( 1000s of psi), and fire tubes generally aren’t designed to be used over 350 psi. Water tubes are available in much larger capacities than fire tubes, and are able to recover a lot faster from a large increase in load demand from a pressure stand point. Meaning if the pressure drops on a fire tube boiler, it takes longer to come back up than on a similarly rated water tube boiler. Fire tube boilers typically have lower operating and maintenance costs, have easier access to the fire and water sides for inspections, and its much easier to replace tubes on a fire tube than a water tube. Generally speaking, if you have fluctuating demand and large swings in steam requirements, a water tube is probably a better fit. If you have a pretty constant load requirement without a lot of swing in steam demands, a fire tube would work fine. My next article will cover boiler components and safeties.
I’ve heard a lot made of clocking gas meters over the years and honestly, in Florida there isn’t too much call for for heat and even fewer furnaces.
I was pleasantly surprised when I found out how easy it actually is. Here is how you do it, step by step.
#1 – Make sure all gas appliances are off other that the one you are clocking. Even shut off pilot lights or it can mess with your reading.
#2 – Make sure the appliance you are checking is running at high fire (max output)
#3 – Get a stopwatch (your phone has one)
#4 – Watch the smallest unit dial on the gas meter, it will often be 1/2 cubic ft
#5 – Time how long that dial takes to make one full revolution with the stopwatch
#6 – Multiply the dial size by 3600 (3600 is the # of seconds in an hr) so if it’s a 1/2 cu/ft dial it would be 1,800
#7 – divide that # by the # of seconds it took. So lets say it took 22 seconds that would be 1,800 / 22 = 81.82
#8 – Multiply that # by the BTU heat content of 1 Cu/Ft of gas provided by the utility. If it is 1,000 (which is common for NG) the total BTU per hr would be 81,820
The complete formula is Cubic Feet per Hour (CFH) = (3600 x Dial Size) / Time (seconds)
Then to get the ACTUAL device output in BTU’s you would multiply for the AFUE efficiency. In this case if it was an 80% furnace the input is 81,820 btu/hr and the output would be 65,456
Pretty cool huh?
One of the most common parts to fail on a single phase HVAC system is a run capacitor, so much so that we sometimes refer to junior techs as “capacitor changers”. While capacitors may be easy to diagnose and replace, here are some things many techs may not know.
Capacitors Don’t “Boost” the Voltage
A capacitor is a device that stores a differential charge on opposing metal plates. While capacitors can be used in circuits that boost voltage they don’t actually increase voltage themselves. We often see higher voltage across a capacitor than the line voltage, but this is due to the Back EMF (Counter electromotive force) generated by the motor, not the capacitor.
Current Doesn’t Flow Through The Capacitor, Just in and Out of It
Techs notice that the one side of power is connected to the C terminal or the side opposite the run winding. Many techs imagine that this power “feeds” into the terminal, get’s boosted or shifted and then enters the compressor or motor through the other side. While that may make sense it isn’t actually how a capacitor works at all.
A typical HVAC run capacitor is just two long sheets of really thin metal, insulated with an insulation barrier of very thin plastic and immersed in an oil to help dissipate heat. Just like the primary and secondary of a transformer the two sheets of metal never actually touch, but electrons do gather and discharge with every cycle of the alternating current. For example, the electrons that gather on the “C” side of the capacitor never go “through” the plastic insulation barrier over to the “HERM” or “FAN” side. The two forces simply attract and release in and out of the capacitor on the same side they entered.
The Higher the Capacitance, the Higher the Current on the Start Winding
On a properly wired PSC (Permanent Split Capacitor) motor, the only way the start winding can have any current move through is if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. If the capacitor is completely failed with 0 capacitance it is the same as having an open start winding. Next time you find a failed run capacitor (with no start capacitor) read the amperage on the start winding with a clamp to see what I mean.
This is why oversizing a capacitor can quickly cause damage to a compressor. By increasing the current on the start winding the compressor start winding will be much more prone to early failure.
The Voltage Rating is What it Can Handle, Not What it Will Produce
Many techs think they must replace a 370v capacitor with a 370v capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370v with a 440v but you cannot replace a 440v with a 370v. This misconception is so common that many capacitor manufactures began stamping 440v capacitors with 370/440 just to eliminate confusion.
You Can Test a Capacitor While the Unit is Running
You simply measure the current (amps) of the motor start winding coming off of the capacitor and multiply it times 2652 (on 60hz power 3183 on 50hz power) and then divide that number by the voltage you measure across the capacitor. For a full write up on the process, you can look here