• 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
  • 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
  • PODCASTS
  • VIDEOS
  • WEBINARS
  • RESOURCES
  • INDUSTRY CALENDAR
  • DIRECTORIES
  • EBOOKS
  • 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
  • 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
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!
ColumnsJohn Siegenthaler: Hydronics Workshop

They're not boilers

By John Siegenthaler, P.E.
February 1, 2012
Designing hydronic systems around outdoor wood-fired furnaces.

Figure 1.


Interest in wood-fired hydronic heating systems continues to rise along with the price of conventional fuel. One heat source used for such systems, especially in rural areas of North America, is the outdoor wood-fired furnace (Figure 1).

Even though heat is carried away from them by a stream of water, they are called “furnaces” rather than boilers because they have unpressurized water compartments that surround their large fireboxes.

Although intended to be fueled with firewood, especially by the U.S. Environmental Protection Agency, I’ve learned that for some people, just about anything that fits through the firebox door is fair game for fuel. I even heard how one of these furnaces allowed a certain person to make a deer, shot in violation of location regulations, disappear when he heard the game warden was on his way.

Outdoor wood-fired furnaces bring some “baggage” to system design. First, because they are unpressurized, water in piping connected to the furnace, and above the water level inside it, will be under subatmospheric pressure when the circulator is off. Second, because they are “open loop” heat sources, no ferrous metal components, such as cast-iron circulators, should be used in portions of the system piped directly to the furnace.

Figure 2.

Big bang theory

Figure 2 shows how sub-atmospheric pressure exists in piping above the water level in the furnace. This is the case when the circulator is off. When the circulator is on, the pressure at a given location in the piping may be positive or negative relative to the atmosphere. It depends on the head added by the circulator, the pressure drop caused by head loss along the piping, as well as the elevation of the piping.

In theory, water can remain in piping under subatmospheric conditions, provided that certain conditions are met. The water has to remain above its vapor pressure. When water reaches its vapor pressure, it boils. This vapor pressure depends upon the temperature of the water (Figure 3).

This graph shows the relationship between boiling point and absolute pressure. For example, at an absolute pressure of 14.7 psi, which corresponds to normal atmospheric pressure at sea level, water boils at 212º F. However, let’s say water finds itself at a pressure 5 psi below normal atmospheric pressure (e.g., 14.7 - 5 = 9.7 psia). According to Figure 3, it will boil if it reaches a temperature of about 190º.

This could occur the instant a circulator shuts off in a distribution system where the 190º water is about 12 ft. above the water level in the outdoor furnace. The sudden drop in pressure, relative to when the circulator is on, could lower the local pressure below the vapor pressure. The result will be strong “banging” sounds from the piping as the water flashes to steam. It’s not a sound anyone wants to hear coming from their heating system.

The higher the top of the distribution system is compared to the water level in the furnace, the more negative the water pressure, and the lower the temperature at which the water will boil. Formula 1 can be used to estimate the negative static pressure at the top of the system each time the circulator turns off.

Formula 1:

Where:
pstatic = static gauge pressure of the water at a given location (with circulator off) (psi)

H = height from top of system piping down to water level in outdoor furnace (ft.)

For example, determine the negative static pressure of water located 16 ft. above the water level in the outdoor furnace. If the temperature of this water is 190º, will it boil at the top of the system when the circulator turns off?  

First, determine the extent of the negative pressure using Formula 1:

The water will boil if its static gauge pressure is lower than (more negative) the vapor pressure corresponding to 190º. Figure 3 shows the vapor pressure of 190º water to be 9.5 psi absolute. This corresponds to a gauge pressure of 9.5 - 14.7 = -5.2 psi. Because the static gauge pressure at the top of the system (-6.9 psi) is lower than the vapor pressure of the water (-5.2), boiling will occur. This is a situation that must be avoided.

Formula 3.

Tiny sucking sounds

Another nuance of open hydronic systems is that air will enter through any possible leakage path located where the local pressure is subatmospheric. Such paths include float-type air vents, valve packings, circulator flange gaskets or less-than-perfectly sealed threaded connections.

As air enters the piping, water slides back to the outdoor furnace. Over time, the water level could drop several feet, depending on the elevation of the furnace. When the circulator turns on, it may not be able to push this air pocket back around the circuit and refill the piping. Even if it can, who wants to listen to air bubbles gurgling through piping as the circuit refills?

Formula 4.

Multiple solutions

There are techniques that can help prevent “nuisance boiling” in open loop systems supplied by outdoor wood-fired furnaces. One is to lower the water temperature. Another is to lower the height of the distribution system relative to the water level in the furnace. Still another possibility is to locate the furnace at a higher outside elevation given the constraints of the property, building locations, etc. All these options have their limitations.

In my opinion, the best solution is to avoid both the nuisance boiling and air admittance problems by using a stainless-steel heat exchanger to isolate the unpressurized outdoor furnace from what will then be a true closed/pressurized indoor distribution system. The concept is shown in Figure 4.

The closed-loop portion of the system can contain the auxiliary boiler, cast-iron circulators and the standard “trim” that would be present in any modern system. Just think of the heat exchanger as the “boiler” in the system and design accordingly.

When this approach is used with an auxiliary boiler, make sure the circulator in the outdoor furnace is turned off whenever the auxiliary boiler is operating. It’s also a good idea to install a check valve so that heat produced by the auxiliary boiler doesn’t thermosyphon back outside.

Finally, please don’t scrimp on the piping or insulation system between the outdoor furnace and the interior of the building. Use a quality pre-insulated piping system specifically intended for buried installation. Size the piping for a reasonable small head loss, and be sure it’s compatible with the temperatures that outdoor wood-fired furnaces can create.

The emissions from outdoor wood-fired furnaces have been a contentious issue for years. So much so that some municipalities in my area have either banned any further installations or put significant restrictions on such installations, such as distance from property lines and required chimney heights. The EPA, working in conjunction with manufacturers of outdoor wood-fired furnaces, have developed a “white label” standard that holds emissions to higher standards (e.g., lower particulates).

Even with these changes, it’s hard for me to imagine outdoor wood-fired furnaces surviving the ever-tightening standards for higher efficiency and lower emissions. Other technologies, such as wood-gasification, are likely to prevail in the long term. Still, tens of thousands of outdoor wood-fired furnaces are installed each year in the United States and Canada. If you’re going to get involved with one, be sure to stay mindful of what we’ve just discussed.

Links

  • Contact Plumbing & Mechanical
  • Follow PM on Twitter!
KEYWORDS: hydronic heat

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
Subscribe For Free!
  • eNewsletters
  • Online Registration
  • Subscription Customer Service
  • eMagazine
  • Manage My Preferences

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

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

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

IPEX celebrates grand opening of new Florida distribution center

IPEX celebrates grand opening of new Florida distribution center

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

The interior of a government building.

President Trump signs executive order promoting skilled trades and apprenticeships

Figure 1 is a sketch of the flow problems of the current plumbing system.

Hydronic heating glitch solved: Why adding a circulator won't fix primary loop flow issue

Underfloor heating installation with drain sewer hole in bathroom close up on water floor heating.

Using hydronics to leverage time-of-use electrical rates

PMCE Home-X April 29 Free Webinar: From Legacy to Leadership: Preparing Your Home Services Business for the Next Generation

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

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

Download the FREE Water Conservation, Quality & Safety eBook: Plumbing Trends Increasing Safe Water Availability

Related Articles

  • John Siegenthaler Hydrolics Workshop

    Energy and Power

    See More
  • John Siegenthaler Hydrolics Workshop

    Our industry needs more focus on distribution systems

    See More
  • Siegenthaler PM

    John Siegenthaler: How to use a variable-speed circulator for boiler protection

    See More

Events

View AllSubmit An Event
  • August 26, 2014

    Free Webinar: Low Temperature Heat Emitter Options in Hydronic Systems

    With proper design, you can create systems that require supply water temperatures no higher than 120° F under design load conditions.
  • 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
  • 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
    • PODCASTS
    • VIDEOS
    • WEBINARS
  • RESOURCES
    • INDUSTRY CALENDAR
    • DIRECTORIES
    • EBOOKS
    • PM BOOKSTORE
    • CE CENTER
    • MARKET RESEARCH
    • CLASSIFIEDS
  • EMAGAZINE
    • EMAGAZINE
    • ARCHIVE ISSUES
    • CONTACT
    • ADVERTISE
    • PME EMAGAZINE ARCHIVES
  • SIGN UP!