Month: October 2018

Here is another excellent article from Michael Housh, owner of Housh Home Energy and a regular contributor to HVAC School. Thanks Michael!

I thought I would go through a simple example of sizing a hydronic circulator for an application.  This is a made up scenario, but I sketched out a 20’ x 20’ square home with baseboard radiators that encompass mostly all of the exterior walls.  It should also be mentioned that this article does not go into detail on choosing a radiator/panel, this is a hypothetical situation to show the steps involved in determining the total head loss (resistance) through a piping arrangement.

I equate the following to what Manual-D would be for duct-design (the air-side of HVAC).  Before I move on there are a few points that I need to make clear. Because hydronic systems are a closed loop it’s important to realize that the lift (or height) from the pump to the highest point of the system has no effect on sizing the circulator, the only thing that matters is the drag/resistance of the pipe, valves, and fittings in the system.  So whether a system is like the one in the drawing or you have a loop that travels 1000’ in the air, the process to size the circulator is the same.


Another thing I will just gloss over here is that for any of the following formulas it’s assumed that the piping system has turbulent flow (Reynold’s number between 2,300 – 200,000).  Turbulent flow means the water molecules are not traveling in a straight line, they gravitate from center to the wall of the pipe and back again, this adds increased head/resistance but it allows heat to be transferred through the wall of the pipe/radiator more easily.


While this is an imaginary project and I’m going with ¾” piping for the entire loop, it is important when designing and selecting the size of the loop that we keep our velocity inside of the pipe between 2-4 ft/sec.  The 2 ft/sec is the minimum velocity required to be able to get air to move downwards throughout the piping system, if we get above 4 ft/sec then we can have noise issues or damage the pipes.


The primary difference between this exercise and doing a Manual-D is that we must consider every pipe, valve, and fitting in the entire circuit, as opposed to only using the longest Total Equivalent Length (supply and return) for design.


For some it may be easier to think of this like an electrical circuit (Ohm’s Law), we add the resistance through the entire circuit to come up with the total resistance (ft. head) our system will have to pump against.


The next step is totaling up all the equivalent lengths of everything using charts available (most all of the charts that I use are in Modern Hydronic Heating by John Siegenthaler and because of copyright I can’t publish them, but is a good place to look for these types of resources).  I created a spreadsheet from the data in my drawing and came up with the following data.

You may notice that the air-separator has a Cv (Flow Coefficient), I’m not going to go into details in this article on how I determined it was equivalent to 11’ of ¾” copper, but I will give the definition of Cv. Cv is the flow rate of 60° water to make a 1 psi pressure drop across the device.  If you read my last article on setting up a pump, then you should know that a pressure difference can be converted to ft. of head, so there are some formulas that can be used to determine the equivalent length of a device for a certain pipe-size but is a bit complex for this article. Below I combined the important totals to give us our total equivalent length of 162.8’ of ¾” copper.

While this article is getting long, the above information is gold!  It allows us to create a fingerprint for our system and determine the performance over a range of flow rates.  With this information, we can create the System Head-loss Curve, which is defined by the following equation.

The reason I say this is gold is that we are able to create our system fingerprint over an array of flow rates, which will give us the head-loss we must deal with, as in the following graph.

In our hypothetical system we are going to design around a 180° output temperature from the boiler and a 20° Delta-T, giving us an average temperature of 170°.  Since density and viscosity are dynamic this is the number we will use to determine the Fluid Properties Factor for our application. We can solve for this using the following formula.

Next all we need is to determine our pipe size coefficient, which is just a multiplier based on the internal dimensions and roughness of our tubing.  This is something that is looked up in a chart as well, and for ¾” copper pipe is equal to 0.061957. I’m also going to pick a generic flow rate for the application of 4 GPM, so we can wrap things up.  So now we can substitute all of our values into our head-loss equation, solving for our design flow rate of 4 GPM.

So currently we would need to select a pump that would produce 4 GPM at 5.13 ft. of head.  We would do so by matching up manufacturer’s data/pump curve to match our desired output. This is a deep subject, some of which carries over into the air-side of HVAC, but some of the topics I have just skimmed the surface on.  While it may seem overwhelming, the main take away from this article is going to be to relate the piping system to an electrical diagram.

In future articles this will allow us to break down complex piping systems into more manageable pieces.  As with most things in our industry, it’s not always about the information you have in your head, but your ability to find the correct information/formulas/resources when they’re needed. Keep on learning!

— Michael


This article is written by one of the smartest guys I know online, Neil Comparetto. Thanks Neil!

Recently I posted a question in the HVAC School Group on Facebook, “when designing a residential duct system what friction rate do you use?”. As of writing this, only one answer was correct according to ACCA’s Manual D.

I feel there is some confusion on what friction rate is and what friction rate to use with a duct calculator. Hopefully, after reading this tech tip you will have a better understanding.

So, what is friction rate?

Friction rate (FR) is the pressure drop between two points in a duct system that are separated by a specific distance. Duct calculators use 100′ as a reference distance. So, if you were to set the friction rate at .1″ on your duct calculator for a specific CFM the duct calculator will give you choices on what size of duct to use. Expect a pressure drop of .1″ w.c. over 100′ of straight duct at that CFM and duct size / type.

Determining the Friction Rate

First, you need to know what the external static pressure (ESP) rating for the selected air handling equipment is. ( external static pressure means external to that piece of equipment. For an air handler, everything that came in the box is accounted for, including the coil and typically the throwaway filter. For a furnace the indoor coil is external and counts against the available static pressure)

Next you have to subtract the pressure losses (CPL) of the air-side components (coil, filter, supply and return registers/grilles, balancing dampers, etc.). Now you will have the remaining available static pressure (ASP). ASP = (ESP – CPL)

Now it’s time to calculate the total effective length (TEL) of the duct system. In the Manual D each type of duct fitting has been assigned an equivalent length value in feet. This is done with an equation converting pressure drop across the fitting to length in feet (there is a reference velocity and a reference friction rate in the equation). Add up both the supply and return duct system in feet. It is important to note that this is not a sum of the whole distribution system. The most restrictive run, from the air handling apparatus to the boot is used. Supply TEL + Return TEL = TEL

The formula for calculating the friction rate is FR= (ASP x 100) / TEL
This formula will give you the friction rate to size the ducts for this specific duct system. If you test static pressure undersized duct systems are very common, almost expected. This is because a “rule of thumb” was used when designing the ducts.

This is just an introduction to the duct design process. I encourage you to familiarize yourself with ACCA’s Manual D and go build a great system!

— Neil Comparetto


When you work on a heat pump system and you want to test defrost there are so many different test procedures to follow to test the board and sensors. Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.

Lots of pins and jumping involved.

But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.

A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use an ice bath to test it. If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the cond fan off and see when it closes by using an Ohmmeter.

On a defrost thermostat you can also easily jump it out to test the board since it is just open an closed.

A defrost sensor is a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.


You CANNOT jump out a thermistor.

— Bryan

P.S. – A podcast about Heat pumps is available HERE

This article is written by Michael Housh, owner of Housh Home Energy and a regular contributor to HVAC School. Thanks Michael!

This is another article in trying to relate the air-side of HVAC with the water-side.  In this article I’m going to talk about how to determine pump flow (GPM) based on the pressure difference across the pump.


I relate checking pump flow to checking air flow with static pressure on the air-side.  Without knowing how many gallons per minute we’re moving the less we can truly tell about what our system is doing.  Most of the equations that revolve around hydronic systems (or air-conditioning systems) revolve around how many pounds of a substance we’re moving through the system.


The trouble with many of the systems out there is that most installers/designers don’t add anyway for us to check this in the field.  I’ve primarily only seen any sort of pressure port or gauge on larger commercial/industrial systems, or sometimes older pump flanges that have a pressure port built-in.  Most people today are installing isolation flanges on their pumps, so I strongly encourage everyone to spend the few extra dollars on isolation flanges with a drain port built-in, this can allow a technician or installer access to checking the pressure difference across the pump.


Measuring pump flow (just like air flow) requires looking at manufacturer’s charts on the pump you are working on.  But instead of checking static pressure across an appliance we use the pressure difference across the pump. In the U.S. the amount of energy produced by a pump is measured in ft. of head, however all of the gauges that are used measure in PSI.  Luckily there is a formula to convert PSI -> Ft. of Head.


H = PSI / (0.433 * SG)




H = feet of head

PSI = pounds per square inch

0.433 = constant

SG = specific gravity (for rule of thumb we can use 1.02 for water)


The constant (divisor) can be simplified by multiplying 0.433 * 1.02 = 0.44:


H = PSI / 0.44


At this point I must give a WARNING about some of the pictures I’m going to show, as they will surely be controversial, but until my mind is changed on this subject Testo will have to remove H2O from their measurable media type.

Before I move on, let me lay some groundwork.  My latest system had a design load of @ 80K BTU.  This system utilized two zones each having a pump.  By using the “rule of thumb” sensible heat rate equation for hydronics, I can layout what my total system GPM should be to deliver the load.  Most hydronic systems are designed around a 20° deltaT on the distribution side, which is what I’ll use in the below equation (for more on the sensible heat rate equations you can read this tech-tip I wrote).


80,000 / (500 * 20) = 8 GPM


So this means my total system needs to move 8 gallons per minute in order to meet demand.  In my scenario I actually have 2 zone pumps (one does a bonus room which needs about 2-3 GPM, and the other the main house needing about 5 GPM).  I utilized the same 3-speed pump on both zones, and I will cover how I set the speed based on the above GPM that I need to deliver. Below is the pump curve (similar to a fan chart) that we need to compare to, each line representing one of the speeds of the pump.

I take the pressure at the inlet and the outlet of the pump to get my deltaP, using the drain port on the isolation flanges.    

DeltaP = 3.5 PSI (as you can see this is very hard to take from an analog style gauge).  I can then convert my deltaP to ft. of head using the formula above.


3.5 / 0.44 = 7.95 ft. of Head


We can then compare to the above pump curve (for low speed, lowest curve), which shows a flow rate of about 2.8-3 GPM, which is where I want to be for that zone.  I can then do the same for the other zone pump (set on medium speed).


This gives me a deltaP of 4.8 PSI and converted ft. of head of 10.9.  When comparing my deltaP to the medium speed pump curve my flow rate is about 5 GPM, giving me a total system flow rate of my desired 8 GPM.


To take it a step further, I then compare my system loop deltaT, which in this case actually worked out perfectly to 20°, so I won’t go through the formula again, but this proves that we’re delivering the desired 80K BTU to the space.

— Michael


When I started in the trade I would see these devices (above) in the field and I would hear guys call them a PTC or a “Soft Start” and I just accepted it and moved on. In fact, I’ve been calling PTCR (Postive Temperature Coefficient Resistors) a “soft start” for most of my career.

Turns out I was wrong

A PTCR is just a resistor (thermistor) that increases in resistance as it heats up. It connects from run to start in parallel with the run capacitor and allows a surge of current the start winding of a motor when it starts. When the PTCR heats up, the resistance inside increases and it essentially takes itself out of the circuit.

A PTCR by itself does allow a spike of current to the start winding but it does not create a phase shift. This is why some devices add a start capacitor and just use the PTCR as the “relay” to take it out of the circuit like the products shown above. It is a start device… but there is nothing “soft” about it.

Then there are the more traditional hard start kits that use the tried and true start capacitor and potential relay instead of a PTCR. This serves the same basic purpose, increase current and phase shift to the start winding for a fraction of a second and then remove it from the circuit. It just does the “removal” part of the equation in a more precise manner.

All of these technologies serve to increase current to the start winding quickly then drop out

The idea is to get the motor to 75% to 100% of running speed as QUICKLY as possible by increasing start winding current and phase shift.

There is a good reason for this. When a motor is stationary or running at low speed, it’s windings act as low resistance resistive loads, essentially really high amperage heaters. The longer the motor spends trying to start at full voltage the higher the current it will draw and the hotter the windings get.

Hard start devices can do nothing to actually reduce the current the motor draws when it is at locked rotor (stalled), the hard start device simply gets it out of that stalled / low RPM as quickly as possible.

Many of you may note that when you measure inrush current with a hard start in a place that it will show lower than when it is not in place. This is simply because a hard start shortens the time the motor remains in the locked rotor, not because it actually reduces the starting amps.

There are also some concerns about the added torque that a hard start provides over such a short time period and the side effects of that “torque shock” to the internal components of the compressor as well as the connecting copper lines.

Soft Start

Different methods of “soft starting” have been in use in large 3 phase motors for a long time. The purpose is the start a motor more slowly and therefore reducing the current inrush.

The devices shown above are single phase soft start devices. They reduce the voltage during starting to reduce the current associated with the start. These devices carefully control the voltage and therefore the current applied to the start and run windings to provide a lower initial current during start and slowly increase up toward full speed. These devices require advanced algorithms to do this which makes them significantly more expensive than the traditional hard start technologies and they are not capable of producing a large shifted / current boost to a start winding like a hard start.

This means that while a soft start is a great device to help reduce light flicker, decrease start amps and increase compressor life, it is unlikely that it will start that old, stuck compressor.

Soft starts and hard starts both serve a purpose but what they do and how they do it couldn’t be more different.

Have an old, locked compressor? Hard start is likely the best bet. If you have a complaint that lights are dimming on compressor start? A soft start will give a better result.

— Bryan

Here is a pretty dramatic demonstration of hard start and soft start HERE

And a great application guide on soft start HERE

In Brazing and soldering tubing, we have a few things we need to accomplish to make a proper connection

  1. We can’t overheat the joint to the point that it overheats the base metal or the flux where applicable
  2. We must bring the entire joint above the melting temperature of the brazing alloy
  3. Draw the alloy deep into the joint

New techs often underheat, overheat or take too long to complete a joint because they don’t use visual cues to apply the alloy at the proper time.

Every metal and alloy responds a little differently but we always use indicators of some sort to know when to start and stop.

Some things you need to know before making a connection are

  • What is the melt temperature of the base metal?
  • What is the working temperature of the alloy?
  • Is flux required and is it external or integral to the alloy you are using?
  • What is the thermal conductivity of the metals?

Most metals we work with will respond to heat in the same way with a color change shown in the chart below. Notably, aluminum will show no change in color before it hits its melt point.

Copper to Copper

Copper to copper connections require no flux when rods that contain phosphorus are used. This is why rods using a small amount of silver with the remainder of the rod being made of copper and phosphorus are common.

Most of these phos/copper alloys have a working temperature around 1200°F and copper has a melting temperature of around 1950°F. A quick look at the chart above will show you that a good brazing indicator would be copper in the “cherry” range of 1175 – 1275°F when applying the rod. We use the Solderweld 15% round alloy at Kalos.

Copper to Brass 

Brass is a metal that is made up of a mixture of copper and zinc. Brass has a lower melting point than copper but is great for casting so many valves and other refrigeration components will be made of brass. It is preferable to use a high silver content alloy with either an external flux or a flux coated rod like the Solderweld 56% rod.

You will then heat up both sides of the joint until you see the proper color on the copper and to a lesser extent in the brass as well. The flux will also act as an indicator because it will go completely clear and flat giving both base metals a “wetted” look at about 1100°F (for most appropriate fluxes). Both the color change and the clear flux can act as indicators that the temperature is correct on copper to brass.

Copper or Brass to Steel 

Working with copper or brass to steel will definitely require a brazing alloy with no phosphorus and flux. Steel changes color in much the same way as copper but it has less thermal conductivity which means that the heat you apply tends to concentrate in one spot rather that travel the way it does with copper. Steel doesn’t melt until 2500°F but the working range for the flux is generally 1100°F – 1600°F (depending on brand/type) so you can easily overheat the flux when working with steel as well as bump into the copper melting temperature of 1950°F if you aren’t careful.

When working with copper or brass to steel use the metal color in that “cherry” zone as well as the quiet, clear flux as an indicator of proper brazing temperature.


Aluminum gives you no indication of when it’s going to melt which makes it more tricky to work with. It also melts at 1220°F which means that if you are working with aluminum to other metals you are in the danger zone as soon as there is any redness in the other metals. In brazing aluminum to aluminum, patching aluminum or working with aluminum to other metals you need to rely heavily on the aluminum flux to tell you when it is time to apply the alloy.

For aluminum to aluminum work we use Alloy-sol from Solderweld because you can apply as much flux as you need and it gives you a great indication of when to start applying the rod when the flux goes clear at around 600°F.

For aluminum to copper we use Al-Cop, it has a flux built into a channel on the rod that can be helf to the joint and will melt and run out when the proper temperature is reached.

In all cases we are looking for visual cues rather that overheating and damaging the base material or burning flux or underheating and globbing up rod onto the joint.

— Bryan

I’m not sure who first started calling evaporator coil odors “dirty sock syndrome” but I really wish they hadn’t. Nowadays every tech out there can’t help but categorize every odor from the system as dirty sock syndrome” and customers just LOVE to hear that they have a “syndrome” named after a dirty locker room smell.

What is Dirty Sock Syndrome? 

Loosely defined, dirty sock syndrome is a smell that comes from an evaporator coil, especially when it first comes on or shifts from heat to cool. The smell comes from biological material that dries out on the coil when the system is off or in heat mode.

Customers will notice this smell when it first comes on more than once it has been running a while and often describe it as smelling musty, moldy or like….. well…. dirty socks.

True dirty sock syndrome is almost exclusively found in humid climates and is most common with heat pump systems. Gas furnace air temperatures are generally high enough to kill the biological material (at least that’s the working theory).

Many have noticed that the occurrences appear to be increasing which some have attributed to porous recycled aluminum used in coils though this hasn’t been proven.

What Can be Done to Stop It?

There are many different approaches used by manufacturers and contractors but the first thing to do is to give all of the obvious parts a good inspection and cleaning.

Inspect and Clean-

  • Both sides of the evaporator
  • The return box
  • Drain Pan
  • Blower Wheel

Also check for

  • Standing water around the unit
  • Dirty filters (duh)
  • Gaps in return ducts
  • Signs of rodent or other pest intrusions

Many manufacturers have turned to using special coatings to help prevent the biological material (or biofilm) from attaching to the metal. These coatings can be factory or aftermarket applied and can provide pretty reliable results.

In the field, it is often more practical to use a combination of filtration, cleaning and UV-C bulb installations to help keep stuff from growing on and bonding to the coil. Many techs fail to realize that a HUGE part of the effectiveness of UV-C is how it is installed and keeping the bulbs regularly changed.

The effectiveness of UV-C depends completely on it’s proximity to the coil. It will do a great job of irradiating portions of the coil it is close to but the effectiveness will fall off VERY quickly the further the bulb is from the surface.

When installing UV make sure to –

  • Protect or shield any wires or plastic parts that may be damaged
  • NEVER look at the light when it’s on
  • Install according to manufacturers specs
  • Inform the customer of the ongoing maintenance and costs of bulb replacement

Specialty Cleaning 

As long as “dirty sock” syndrome has been around, folks have been spraying stuff on coils trying to stop it. I read one place that bleach works well… I bet it does until your coil corrodes out and your customers canary dies!

There are many products that techs swear by but largely the industry is going away from spraying harsh chemicals or chemicals known to cause human reactions because whatever you spray on that coil is going to make it into the customers home or business.

This is why we use the following approach to prevent and remedy dirty sock coil odors –

  • Keep good filters in the system (MERV 8 or better, 4″ whenever possible)
  • Keep drain pans and drain lines very clean
  • Use Evap+ from Refrigeration Technologies on the Evaporator Coils
  • Install UV-C at the coil if the issue persists

In 13 years in business this strategy has worked well for us although I’m sure there are more extreme cases where coatings or coil replace necessary.

— Bryan




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