• Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
  • CONTRACTORS
  • ENGINEERS
  • RADIANT & HYDRONICS
  • INSIGHTS
  • MEDIA
  • RESOURCES
  • EMAGAZINE
  • SIGN UP!
cart
facebook instagram twitter linkedin youtube
  • PRODUCTS
  • FEATURED PRODUCTS
  • CONTRACTORS
  • BATH & KITCHEN PRO
  • BUSINESS MANAGEMENT
  • HIGH EFFICIENCY HOMES
  • TECHNOLOGY
  • WATER TREATMENT
  • PMC COLUMNS
  • PMC COLUMNS
  • Dave Yates: Contractor’s Corner
  • John Siegenthaler: Hydronics Workshop
  • Kenny Chapman: The Blue Collar Coach
  • Matt Michel: Service Plumbing Pros
  • Scott Secor: Heating Perceptions
  • ENGINEERS
  • CONTINUING EDUCATION
  • DECARBONIZATION | ELECTRIFICATION
  • FIRE PROTECTION
  • GEOTHERMAL | SOLAR THERMAL
  • PIPING | PLUMBING | PVF
  • PME COLUMNS
  • PME COLUMNS
  • Christoph Lohr: Strategic Plumbing Insights
  • David Dexter: Plumbing Talking Points
  • James Dipping: Engineer Viewpoints
  • John Seigenthaler: Renewable Heating Design
  • Lowell Manalo: Plumbing Essentials
  • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
  • RADIANT COMFORT REPORT
  • THE GLITCH & THE FIX
  • INSIGHTS
  • CODES
  • GREEN PLUMBING & MECHANICAL
  • PROJECT PROFILES
  • COLUMNS
  • SPONSOR INSIGHTS
  • COLUMNS
  • Codes Corner
  • Natalie Forster: Editorial Opinion
  • Guest Editorial
  • MEDIA
  • EBOOKS
  • PODCASTS
  • VIDEOS
  • WEBINARS
  • RESOURCES
  • INDUSTRY CALENDAR
  • DIRECTORIES
  • PM BOOKSTORE
  • CE CENTER
  • MARKET RESEARCH
  • CLASSIFIEDS
  • EMAGAZINE
  • EMAGAZINE
  • ARCHIVE ISSUES
  • CONTACT
  • ADVERTISE
  • PME EMAGAZINE ARCHIVES
search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
    • FEATURED PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • EBOOKS
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!
Columns

Throttled Into Submission

By John Siegenthaler, P.E.
July 31, 2000
Underreving injection pumps needlessly waste control accuracy, warns John Siegenthaler.

What do you get when you install a 1,000-horsepower engine in a Volkswagen Beetle? Answer: A vehicle that probably could cruise along at 70 mph with the gas pedal only pressed down a quarter of an inch!

Imagine what it would be like driving such a car. Those of you with a steady right foot could probably keep it moving somewhere between 60 and 80 mph on the freeway. But even NASCAR's Jeff Gordon might look a bit erratic trying to drive this little hot rod around in city traffic.

The problem with this scenario is that the driver is forced to use a tiny fraction of the gas pedal's movement to regulate the car's speed over its entire useful range. (Unless, of course, he decides to challenge the world's land speed record.)

As far-fetched as it sounds, lots of variable speed pump injection-mixing systems have a similar "rangeability" problem. The injection pumps in these systems only reach a fraction of their full speed, even while supplying heat at design load conditions. While many of these systems produce a comfort level that's apparently above the callback threshold of their owners, the ability of their injection controls to fine-tune water temperature is largely wasted.

Figure 1

Underrevving Pump Syndrome

Two factors often prevent a variable speed injection pump from operating above the lower portion of its speed range, an affliction I call underrevving pump syndrome (UPS).

1. A relatively low injection flow rate, which is needed by many low temperature radiant systems supplied from conventional boilers.

2. An infinitesimally small head loss, which is associated with the short injection riser piping used in most systems.

UPS is a relatively new ailment within the hydronics industry, first appearing as variable speed mixing systems gained a foothold in the market a few years ago. As the popularity of variable speed injection mixing has grown quickly, so have the instances of UPS.

Fortunately, UPS is fully curable. First we'll examine the causes and then discuss the antidote.

The injection flow rate needed to supply a given heating load can be determined using Formula 1:

Formula 1:

fi = the required injection flow rate (in gpm)

Q = the load being supplied (in Btu/hr.)

b = a constant (use 490 for water, 470 for 30 percent glycol, 450 for 50 percent glycol)

Thot = temperature of ingoing injection water (in degrees F)

Treturn = temperature of cool fluid returning from distribution system (in degrees F)

It's often amazing just how small the hot water injection flow rate needs to be. For example, say we want to supply a design heating load 80,000 Btu/hr. to a slab-type floor heating system that has a return water temperature of 100 degrees F. Assume boiler water is available at 180 degrees F. The required injection flow rate is attained using Formula 1:

Formula 1

The filter pump in your kid's aquarium could probably keep up with this flow requirement.

The injection piping usually consists of 4-8 feet of 3/4-inch copper tubing coupled the boiler loop and distribution loop with pairs of closely-spaced tees as shown in Figure 1. The closely spaced tees uncouple the head loss of the injection riser piping from those of the boiler loop and distribution loop. The injection pump only operates against the head loss of the injection risers.

So what's the head loss of, say, 6 feet of 3/4-inch copper tube at a flow rate of 2 gpm? According to my calculations, it's about 0.06 feet of head. A requirement so low that even a 1/40-horsepower circulator is as much overkill as a 1,000-horsepower engine in a Volkswagen. You can see this in Figure 2, where the desired operating point of 2 gpm and 0.06 feet is plotted along with the pump curve of a 1/40-horsepower wet rotor circulator.

The system resistance curve for the assumed injection piping (6 feet of 3/4-inch copper) is shown in blue. This curve intersects the pump curve at a flow rate of about 9.3 gpm. If you install this pump into the assumed injection piping and allow it to run at 100 percent speed, 9.3 gpm is the injection flow rate you'll get - more than four times higher than necessary under design load conditions.

Figure 2

So What?

Maybe you're thinking, "What's the big deal if the injection pump never needs to run more than 10 percent or 15 percent speed? Just think of all the reserve capacity that pump has and all the electricity I'm saving. Besides, since I usually size my boilers two or three times larger than necessary, why not do the same for the injection pump?" (If you identify with these statements, I'll bet you also think a 1,000-horsepower Volkswagen would look pretty cool on car night down at the drive-in.)

Unfortunately, injection controls can't "drive" heating systems with improperly setup injection pumps any better than you could drive that souped-up VW. Here are some problems that will occur:

Problem No.1: The ability of the injection control to "fine-tune" heat input using an underrevving injection pump is severely handicapped.

Imagine driving your car if the gas pedal could only be set at three or four fixed positions. You could probably get by, but the ride's certainly going to be a bit jerky.

Similarly, at the lower end of their speed range, most injection controls adjust pump speed in steps rather than as a continuous process. The more restricted the pump speed range, the fewer speed steps the control has to work with. Although higher mass radiant systems can partially mask this deficiency, operating a system in this manner wastes much of the resolution the injection control is otherwise capable of.

Problem No. 2: An injection control matched with an underrevving pump repeatedly crosses the speed thresholds where the boiler is turned on and off.

Although most injection controls are programmed to keep the boiler on for some minimum time once fired, that time may be shorter than what could otherwise be attained if the boiler remained enabled, and was set up with a wider operating differential. The latter allows the thermal mass of the boiler to be "exercised" to generate longer, more efficient burner cycles. (See last month's column.) An injection control that repeatedly turns the boiler on and off is usually suffering from UPS.

Figure 3

Spin Those Impellers!

One way to cure UPS is to use an injection pump that's only capable of a few gpm with a head gain of, say, 0.5 feet.

Go pull all your pump catalogs off the shelf and see if you can find one with these specs that's also capable of lasting a couple of decades in a typical hydronic system. If you do, it probably costs quite a bit more than a garden-variety, cast-iron zone circulator. Some pump manufacturers are beginning to address this need with low flow/low head/low cost models specifically designed for injection-mixing applications. Thanks, pump guys. This is definitely a step in the right direction.

When a typical zone circulator is used as the injection pump, the cure for UPS is to add flow resistance to the injection riser piping. A throttling valve in the return injection riser does the trick. Think of this valve as a sort of rheostat for water. The more it's closed, the greater its resistance to flow.

The throttling valve should be closed until the injection pump is running at full speed while delivering the design injection flow rate calculated from Formula 1.

Envision the blue system resistance curve in Figure 2 as getting progressively steeper as the valve handle is turned toward its closed position. When the system curve is steep enough to pass through the pump curve at the design injection flow rate, the valve is set properly.

If we make a few assumptions about the injection riser piping, it's possible to calculate the Cv of the throttling valve necessary to force the injection pump to the design load's full speed.

Formula 2

Cv = the required Cv setting of the throttling valve

fi = the calculated injection flow rate from Formula 1 (in gpm)

Hp = the head of the injection pump at the injection flow rate fi (in ft.)

r = 0.058 when the injection riser are 3/4-inch copper

r = 0.25 when the injection risers are 1/2-inch copper

This formula assumes that the total equivalent length of the injection risers, excluding the throttling valve, is about eight feet. Be sure to use the correct (r) value, depending on the pipe size of the injection risers.

For example: To achieve a full speed flow rate of 2 gpm through 3/4-inch injection risers using the 1/40-horsepower circulator of Figure 2, the throttling valve should have a Cv of:

The Cv rating of a valve is the flow rate (in gpm) of water at 60 degrees F, which produces a pressure drop of 1 psi across the valve in its fully open position. At any partially open position the valve has a unique Cv value. The more the valve is closed, the smaller its Cv value becomes.

Some balancing valves have a Cv indicator built into their handles. If that's what you're using, just make the calculation using Formula 2, set the handle to the indicated Cv, and you're done.

Another option is to install a flow meter, or a precision-balancing valve with flow metering ports, into the return injection riser piping. Close the valve until the flow rate calculated with Formula 1 is attained, and you're done.

So what should you do if you don't have a valve with a Cv scale or a flow meter? In a future column I'll show you a way to use temperature readings to set the throttling valve on an injection-mixing system. This method is especially helpful during system start-up, when the water temperature returning from the floor slab can be quite a bit lower than under normal operating conditions.

In the meantime, don't be afraid to choke down the throttling valves on your variable speed injection systems. Your injection control will return the favor by driving the system with the same deft precision as a NASCAR champion.

Share This Story

Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!

Siegenthaler

John Siegenthaler, P.E., is a consulting engineer and principal of Appropriate Designs in Holland Patent, New York. In partnership with HeatSpring, he has developed several online courses that provide in-depth, design-level training in modern hydronics systems, air-to-water heat pumps and biomass boiler systems. Additional information and resources for hydronic system design are available on Siegenthaler’s website,  www.hydronicpros.com.

Recommended Content

JOIN TODAY
to unlock your recommendations.

Already have an account? Sign In

  • Worker using the Milwaukee Tool SWITCH PACK drain cleaner

    Pipeline profits: Drain cleaning, pipe inspection create opportunities

    Drain cleaning and inspection services offer lucrative...
    Green Plumbing and Mechanical
    By: Nicole Krawcke
  • Uponor employee, Arturo Moreno

    The reinvestment in American manufacturing and training

    Plumbing & Mechanical Chief Editor Nicole Krawcke and...
    Plumbing News
    By: Nicole Krawcke and Natalie Forster
  • March 2024 Women in Plumbing hero image of woman engineer overlayed by circle of hexagon shapes with numbers from 1 to 10

    Celebrating 10 Influential Women in the Plumbing Industry

    Celebrating Women's History Month and Women in...
    Plumbing News
    By: Nicole Krawcke
Manage My Account
  • eNewsletters
  • Online Registration
  • Subscription Customer Service
  • eMagazine
  • Manage My Preferences

IPEX celebrates grand opening of new Florida distribution center

IPEX celebrates grand opening of new Florida distribution center

NIBCO Press Solutions

NIBCO Press Solutions

AI can boost efficiency and profitability for plumbing, HVAC contractors

AI can boost efficiency and profitability for plumbing, HVAC contractors

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

Bell & Gossett Illustrates Path to Net-zero at AHR Expo

More Videos

Sponsored Content

Sponsored Content is a special paid section where industry companies provide high quality, objective, non-commercial content around topics of interest to the Plumbing & Mechanical audience. All Sponsored Content is supplied by the advertising company and any opinions expressed in this article are those of the author and not necessarily reflect the views of Plumbing & Mechanical or its parent company, BNP Media. Interested in participating in our Sponsored Content section? Contact your local rep!

close
  • J.J. Keller CMV vehicles on road
    Sponsored byJ. J. Keller & Associates, Inc.

    The dash cam game-changer for small business safety

Popular Stories

Bath & Kitchen Pro 2025 Cover 900x550 hero image

eBook | Fifth annual Bath & Kitchen Pro

Integrate a heat pump with a boiler mounted on a brick wall.

How to integrate a heat pump with a boiler for dual temperature hydronic heating and cooling

Plumber using the DEWALT 20V MAX* Plastic Tubing Cutter to cut a white tube.

Making tough jobs easier: Most-used tools by plumbers in 2025

Nominations for Plumbing & Mechanical's NextGen ALL-Stars are now open. Submit your nomination TODAY!

Events

November 13, 2024

Future Proofing MEP: Navigating the 2026 High Efficiency Water Heating Standards

Join our deep dive into DOE’s new standards so you can future-proof your MEP practice.

EARN: 0.1 ASPE CEU; 1 AIA LU/HSW; 0.1 IACET CEU*; 1 PDH

August 14, 2025

Precision Under Pressure: Optimizing System Performance Through Balancing and Air Removal

We’ll cover how proper air separation and hydronic balancing help maximize operational efficiency, reduce maintenance and minimize downtime.

View All Submit An Event

Poll

Will business be up or down in 2025?

Do you anticipate business in 2025 to be up or down in comparison to 2024?
View Results Poll Archive

Products

The Water Came To A Stop

The Water Came To A Stop

See More Products

2025 Bath & Kitchen Pro eBook

Related Articles

  • John Siegenthaler Hydrolics Workshop

    Smoke Gets in your Eyes

    See More
  • heat pumps and buffer tanks

    John Siegenthaler: Combining heat pumps with buffer tanks

    See More
  • Figure 1 glitch drawing for February 2025

    A heat pump heat migration problem

    See More

Related Products

See More Products
  • greening steam.jpg

    Greening Steam: How to Bring 19th-Century Heating Systems into the 21st Century (and save lots of green!)

  • M:\General Shared\__AEC Store Katie Z\AEC Store\Images\Plumbing\new sites\classic_hydronics.gif

    Classic Hydronics - How To Get The Most From Those Older Hot-Water Heating Systems

See More Products

Events

View AllSubmit An Event
  • June 13, 2016

    Advanced Radiant Design with Mark Eatherton

    This course drills deep into the engineering aspects of tube length, tube center density, tube size and other factors affecting hydronic radiant heating systems.
View AllSubmit An Event
×

Keep your content unclogged with our newsletters!

Stay in the know on the latest plumbing & piping industry trends.

JOIN TODAY!
  • RESOURCES
    • Advertise
    • Contact Us
    • Directories
    • Store
    • Want More
    • Supply House Times
  • SIGN UP TODAY
    • Create Account
    • eMagazine
    • eNewsletter
    • Customer Service
    • Manage Preferences
  • SERVICES
    • Marketing Services
    • Reprints
    • Market Research
    • List Rental
    • Survey/Respondent Access
  • STAY CONNECTED
    • LinkedIn
    • Facebook
    • Instagram
    • YouTube
    • X (Twitter)
  • PRIVACY
    • PRIVACY POLICY
    • TERMS & CONDITIONS
    • DO NOT SELL MY PERSONAL INFORMATION
    • PRIVACY REQUEST
    • ACCESSIBILITY

Copyright ©2025. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing

search
cart
facebook instagram twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
  • NEWS
  • PRODUCTS
    • FEATURED PRODUCTS
  • CONTRACTORS
    • BATH & KITCHEN PRO
    • BUSINESS MANAGEMENT
    • HIGH EFFICIENCY HOMES
    • TECHNOLOGY
    • WATER TREATMENT
    • PMC COLUMNS
      • Dave Yates: Contractor’s Corner
      • John Siegenthaler: Hydronics Workshop
      • Kenny Chapman: The Blue Collar Coach
      • Matt Michel: Service Plumbing Pros
      • Scott Secor: Heating Perceptions
  • ENGINEERS
    • CONTINUING EDUCATION
    • DECARBONIZATION | ELECTRIFICATION
    • FIRE PROTECTION
    • GEOTHERMAL | SOLAR THERMAL
    • PIPING | PLUMBING | PVF
    • PME COLUMNS
      • Christoph Lohr: Strategic Plumbing Insights
      • David Dexter: Plumbing Talking Points
      • James Dipping: Engineer Viewpoints
      • John Seigenthaler: Renewable Heating Design
      • Lowell Manalo: Plumbing Essentials
      • Misty Guard: Guard on Compliance
  • RADIANT & HYDRONICS
    • RADIANT COMFORT REPORT
    • THE GLITCH & THE FIX
  • INSIGHTS
    • CODES
    • GREEN PLUMBING & MECHANICAL
    • PROJECT PROFILES
    • COLUMNS
      • Codes Corner
      • Natalie Forster: Editorial Opinion
      • Guest Editorial
    • SPONSOR INSIGHTS
  • MEDIA
    • EBOOKS
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!