HVACWebConnection.com Home Page


     Search     

 
 
 

 


  Connect to your HVAC Community  

 
  

 

 

 

 


   

 
  
          HVAC Articles

VENTING TODAY – A COMPLEX SUBJECT
Part Two


With this continuing discussion on venting of Modulating/Condensing equipment it is once again not the purpose of this article to define exact installation requirements. The attempt here is to address some typical types of requirements for venting this equipment.

When vertical venting is an option on this equipment it is important to follow manufacturers rules as to EQUIVALENT LENGTH as it relates to air intake of vent. An example would be:

90-degree concentric elbow 4.5 feet
45-degree concentric elbow 4 feet
A 3” 90 degree elbow 5.5 feet
A 3” 45 degree elbow 4 feet

Many times failure to observe these equivalent lengths in calculating the maximum length of the vent can result in inadequate air supply or problems with the pressure switch not making. It is often a good idea if the system will not work with the vent connected to remove the vent temporarily and see if the equipment will operate with it disconnected. If it will and it will not work with it connected you obviously have a problem with the vent. One of the things to address is length and also equivalent lengths of fittings. There may also be other problems such as blockage or damage to the vent.

There may also be other requirements addressed in the following examples:

Permitted Terminals for Vertical Venting - A straight termination is installed in the end of the vent pipe. The air inlet terminal consists of a 180-degree elbow (or two 90 degree elbows) with a rodent screen. Vent manufacturer part numbers for these screens vary.



Vertical Vent Terminal Locations - Observe the following limitations on the location of all vertical vent terminals. 
· The top of the vent pipe must be at least 2 feet above any object located within 10 feet. 

· The vertical distance between top of the vent and air inlet terminal openings must be at least 12". 

· The bottom of the air inlet terminal must be at least 12" above the normal snow accumulation that can be expected on the roof. 

· The air intake terminal must be located on the roof and must be no further than 24" horizontally from the exhaust pipe. 

Wall thimbles - Wall thimbles are often required where single wall vent pipe passes through combustible walls. 

Pitch of Horizontal Piping - Pitch all horizontal piping so that any condensate, which forms in the piping, will run towards the boiler: 

· Pitch horizontal concentric venting 5/8" per foot 

· Pitch Stainless steel venting 1/4" per foot. 

Supporting Pipe - Vertical and horizontal sections of pipe must be properly supported: 
· Support concentric venting near the female end of each straight section of pipe. 
· Support stainless steel venting as called for by the vent manufacturer's instructions. 


I next want to cover some of the code requirements, which must be addressed by manufacturers when setting down the rules for installation. The code rules can be added to but never made less severe in their application. It is however important to note that manufacturers installation requirements have precedence.

To use the chart determine from the rating plate on the equipment the venting category and then proceed down that column to determine requirements and perimeters for venting that particular category. For example, a listed Category I appliance can be vented using a Type B gas vent, chimney, single-wall metal pipe, chimney lining system that is listed for gas venting, or a special gas vent listed for the appliance. The chart refers to appliance Category I through Category IV. The categories are based on vent temperature and pressure. 

The term non-positive vent (negative pressure) means that even if fans or blowers are used in an appliance or vent system, venting is accomplished by natural draft. Natural draft is created by temperature difference (delta T) and the height of the vent. (The vent pressure is lower than atmospheric).

The term positive vent pressure means that fans, blowers or other means are used to propel vent gases through the vent at above atmospheric pressure.

Note that the definitions reference "a vent gas temperature that may cause excessive condensate production in the vent.") A specific temperature is not provided, because it is not the same for all appliances. The ANSI Z21 standards for appliance categorization can be referenced for this information. Annex L contains a complete list of the ANSI Z21 standards. Note that the installer should not need this information, since the appliance vent category is included in the appliance manufacturer's installation instructions and on the appliance nameplate. The criteria in the ANSI Z21 standards are based on a flue loss of 17 percent of total energy. The 17 percent flue loss is the same flue loss built into the vent sizing tables for fan-assisted appliances. In this way, the standards ensure that the appliance will work properly with the vent system. The term nonpositive vent pressure means that the pressure in the vent will be lower than the surrounding atmosphere if the vent system meets the requirements of the National Fuel Gas Code Chapter 12 and Chapter 13. The incorporation of a fan into the appliance does not always mean that the vent pressure is positive. If unsure, check the appliance nameplate or manufacturer's instructions for the venting category, or check the vent pressure with a manometer or other pressure gauge when the appliance is operating. 







VENTING CATEGORIES:

GAS-FIRED EQUIPMENT

Operating

 

 

 

 

 

characteristics

Category I

Category II

Category III

Category IV

Pressure in the vent

Negative

Negative

Positive

Positive

Temperature of

Above

Below

Above

 

Below

vent gas (4)

275°F

275°F

275°F

 

275°F

Annual efficiency

Below 84%

Above 84%

Below 84%

Above 84%

Condensation

Not acceptable

Possible

Possible

In heat

 

(1)

(in vent)

(3)

 

exchanger

Design requirements

 

 

 

 

Gas (air) tight vent

No

No

Yes

 

Yes

Corrosion-resistant

No (1)

Yes

Possible

(3)

Yes

vent (water tight)

 

 

 

 

 

Vent into

Permitted

No

No

 

No

masonry chimney

(1) and (2)

 

 

 

 

Combined venting

Permitted

No

No

 

No

Condensate drain

No

Ask

Possible

(3)

Yes

 

 

manufacturer

 

 

(At

 

 

 

 

 

equipment)

Source of

N.F.G.C.

Manufact.

Manufact.

Manufact.

information

Fuel gas code,

literature

literature

literature

heating equipment

 

 

 

 

 

and vent system

 

 

 

 

 

manufacturers

 

 

 

 

NOTE 1 Usually, there is no problem when high vent gas temperature equipment is vented into double-wall vent or into a lined masonry chimney; but condensation could occur if mid-efficiency (80% to 84%) mechanical draft equipment is vented into a vent that has highly conductive walls, cold walls, or massive walls. In this case, design a vent system that minimizes the wall losses (use double-wall pipe for the whole run and avoid long runs through cold spaces). 

NOTE 2 Install either a rigid or flexible metal liner inside of the masonry chimney and use a double-wall connector when venting mid-efficiency (80% to 84%), mechanical draft equipment.

NOTE 3 Condensation in the vent is posing on the ambient temperature and the conductivity of the vent walls. In this case, design a vent system that minimizes the wall losses (use insulated pipe and avoid long runs through cold spaces). A corrosion-resistant flue and a drain may be required if condensation cannot be prevented - refer to the manufacturer's recommendations. 

NOTE 4. The dewpoint of the vent gas depends on the fuel (natural or LP gas), the amount of excess air and the amount of dilution air. The limiting case occurs when the dewpoint of the vent gas is at a maximum, which is about 135°F. This maximum is produced when natural gas is burned with no excess air or dilution air. Therefore 275°F = 135°F dewpoint + 140°F 

It is a good rule of thumb when looking at vent temperature that any temperature from 300° (F) down is a concern for condensing and needs to be addressed in the case of Category I and III venting.



We are offering a great deal on four of our manuals:

Troubleshooting Gas Electric Ignition Systems
Integrated Boiler Controls
Integrated Furnace Controls
SmartValve


All four would normally go for $170.00 plus $10.00 shipping and handling. We are offering them for $150.00 until the end of the year and we will pay shipping and handling.

We are also offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling. 
We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling. 

We also conduct seminars; we have recently revised all of our seminars and have changed the titles of some of them on the following topics and many others:

· Gas Combustion Fundamentals 
· Circuitry and Troubleshooting
· Hydronic Controls Circuitry
· Gas Heating Electric Ignition Systems
· Gas Heating Advanced Electric Ignition Systems
· Powerpile Systems/Pool heaters
· NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT


Some new seminars we are now offering are:

Multi-Zone Panels
Integrated Boiler Controls
Special Venting for Category III and Category IV High Efficiency Equipment

Electrical Troubleshooting for Condensing/Modulating Boilers and Condensing Furnaces

If you are interested in information call 401-437-0557 or write to: 

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915

E-mail gastc@cox.net


                                                  Venting Today Part Two

                                                                            ###

HVAC Web Connection ©