Month: October 2017

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.

Oil Fired Boiler

Gas Fired Boiler


Water feeder

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.

Typical Water Feeders (PRV’S)


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.

If you do this, you are fired!


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.


Circulator Pumps 

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.

— Justin


This series of articles is written by senior boiler tech (and all around swell guy) Justin Skinner. Thanks Justin.


There are quite a few different types of boilers out in the world. They come in all shapes, sizes, pressures, and types of fuel burned. I'm going to go over some of the more common ones, their common components, and why it all matters.


First, lets establish what a boiler is. A Boiler is defined as a fuel-burning apparatus or container for heating water, in particular. The in particular part is thrown in there because a lot of boiler systems heat a fluid other than water. Glycol, oil, and process chemicals to name a few. But to keep things fairly simple, we will stick with water. So your basic boiler burns a fuel source to heat water. A water heater, basically. But water heaters are used to produce domestic hot water for showers,sinks, and other household hot water uses. Boilers are used to produce hot water for space heating purposes, dehumidification, and other processes (or even potable water indirectly through and exchanger).


For residential and commercial/industrial purposes, there are 2 types of boilers. The most common is the fire tube design. This would include the common sectional type boiler seen in most residential applications. The fuel is burned, and the hot gases pass through a series of flue passages or tubes that are generally steel. The steel composing the flue passages or tubes is heated by the gases passing through. This area does not contain any water, only heat and combustion gases. Water surrounds the flue gas area, but does not actually enter the area. Heat transfer to the water occurs by conduction, primarily. The steel comprising the flue passages becomes heated, and transfers the heat to water.  Hopefully these illustrations help.​


 Commercial Style Fire tube


 ​Residential style sectional, note flue passes

As you can see, the tubes or sections that contain the hot flue gases are surrounded by water, which is how this boilers transfer the heat to water. The passages that contain the hot gases also act as the heat exchanger in the boiler. The FIRE (flue gases) is contained in the TUBE (tubes or passages).


A water tube boiler is typically used in commercial and industrial applications, and not seen often in residential. As the name implies, the tubes contain the water being heated, and are surrounded by the hot combustion gases. This type often looks like a rectangular box with a burner mounted to it. Its virtually a large fire box with water tubes. ​


 Water Tube Boiler Design


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.

— Justin


Water Tube Boilers


Fire Tube Boiler

This article was written by Christopher Molnar, a licensed Florida mechanical contractor. While I'm not personally a practitioner of the “check static every time” doctrine, I certainly appreciate Chris and his passion for this topic. Thanks Chris!

Why Check Static Pressures on EVERY call 

You wouldn't go to the doctor’s office and walk away without having your pulse and blood pressure checked. To do so would be malpractice. Your pulse and blood pressures are a sign of how your blood is flowing through your body and how well your heart is pushing that blood around your body. Measurements outside of the standards are cause for further investigation. 

 Why is it acceptable to walk away from any HVAC service call without checking the air pressures in the ductwork? I would suggest that not taking these pressures on each and every service call is malpractice and you are cheating your employer and customer by not taking these readings.


What is static pressure? The definition is the outward pressure of the air (without movement) in the ductwork. The number is either a positive or negative based on which side of the blower/fan you take the pressure on. The air on the supply side of the system is always positive pressure, in other words the pressure is greater than that of the air outside of the duct. Duct pressures are generally measured in Inches of Water Column in the US.

 The air on the return side of the system is always negative, or a pressure that is below the air pressure outside of the ductwork. Think about it, it makes sense. In order to blow air into a space the air in the supply duct must be above the pressure in the room we are moving it into. In order to draw air into the return duct, the air in the return duct must be under that of the room we are pulling from. Air will always move from an area of high pressure to an area of low pressure.

 What causes this static pressure? Ductwork is just a pipe, correct? So why does pressure build up in this pipe, shouldn’t it just “Flow”?

 The blower is designed to work against a certain amount of pressure in order to provide comfort to the customer and achieve peak heating/cooling efficiency. 

The less air that moves through the ducts and over the coil the greater the temperature of the air will drop and more humidity it will give up. However, we also must maintain a proper level of air flow so that we do not overheat a furnace, or freeze a coil. For air conditioning this is about 400CFM (cubic feet per minute) per ton in most regions. 

On the data plate or in the specs of every air handling device or funace you will find the “total external static pressure” that the device can handle. For example, on the nameplate above you will see that the total maximum static pressure is .5 inches of water column. So, what does this mean?

A technician will take two pressure readings using a static pressure tip. One will be in the supply duct above the coil in the case of a fan coil / airhandler or between the blower and coil on a furnace. The other will be in the return duct right before the blower compartment, or sometimes in the return plenum. 

The number in the supply duct will be positive. The number in the return duct will be negative. To find the total external pressures you will drop the sign (+ or -) and use the absolute value. So, a +.25 will become .25 and a -.25 will become .25 as well. Add the two numbers together as .25 + .25 equals .50 inches water column (.50 “w.c.).


To properly get these readings you will need a mimumum of one static pressure tip and a manometer, such as the Testo Smartprobe 510i that interfaces with a smart phone.

You want to see a measurement as close to, but not over the “maximum external static pressure” on the data plate. If the number is too low it most likely means that the blower is not operating at the correct speed, or that there is a leak in the ductwork that is preventing static pressure from building up.


Low supply static pressure: Look for duct leaks. Look for missing register covers. Look for proper blower speed. Check for proper cooling (if system has a cooling coil that should be wet). Finally, double check supply duct sizing.


Low return static pressure: Look for duct leaks. Look for missing filters. Look for proper blower speeds.
High return duct static pressure: Dirty filter? Blocked return grills? Insulation or something else stuck in the return duct. Collapsed return duct (if fiberboard or rusted steel ductwork).

 High supply duct pressure: undersized supply ducts. Closed registers and grills. Closed doors in buildings with central returns. Collapsed ductwork. Blower set at too high of a speed.

 Static pressure that is too low will affect latent capacity (moisture removal) on PSC motors in some cases. 

Static pressure that is too high will cause noise, excess wear on the blower motor, high amperages, cooling and heating inefficiency, and in many cases can pull the condensation off of the air conditioning coil causing leaks and damage. 

 We all learn in trade school to measure voltage, amperage, and the resistance of components. Most technicians understand what causes low superheat, high subcooling, and pressures. We understand what high amperages can do to a system. We know that motors will burn out if the amperages are too high. We know that a clogged condensate line can cause a flood. Taking the additional 5 minutes to take two static pressure measurements when the system is operational will allow you to do a quick air flow check and make sure that there is not something else happening that will impact system health, performance, and customer comfort. Not taking static pressure on each and every Heating or Air Conditioning service call should be malpractice.

— Chris

Scroll to top
Translate »

Daily Tech Tip

Get the (near) daily Tech Tip email right in your inbox!
Email address