The HM Series furnace modules are ETL Recognized Components. As such, certain additional airside testing is required by the Listing Agency to demonstrate proper operation in the AHU air tunnel design to complete listing in the manufacturer’s AHU.
The HD Series is an ETL Listed Duct Furnace and includes the air tunnel box enclosure per our listing. This series of heaters may be applied to an AHU without the need for additional testing, provided the delivered airflow is within the listing range for the heater model selected, subject to Listing Agency review.
The only duct heaters that are Class 1 Div 2 (Explosion Proof) are Electric. A couple of companies that manufacture them are
We can measure the draft pressure in the combustion blower during operation. Readings below a certain point would indicate possible vent system issues. See Attatched Sheet
Very few circulating air fans and duct work have uniform airflow or velocity profiles. Good enigneering practice suggests that the common duct on the dishcarge side extend 4 to 10 times the hydraulic duct diameter of the duct (d=2ab/(a+b) to establish uniform airflow before before branch lines are introduced. This allows time for mixing required for fairly uniform tmperature distribution. The back end of the heat exchnager typically exhibits the highest temperatures and the highest volume airflow should be directed over that area in zoned systems.
Many of our customers also provide a hotspot limit (Thermodisc 10H11) in zoned systems to prevent overheating of the heat exchanger.
Section 2.14.3 of ANSI Z83.8 requires that the average discharge air temperature shall not exceed of 160 oF above room temperature when adjusted for minimum airflow at its maximum input rating, so this is determined by specifiying the appropriate minimum air flow rates.
An additional test is conducted to determine at what point the heater will shut-off if airflow is reduced for any reason (blocked filters, low volatge etc.) The high limit location and setting is typically determined by this test.
The minimum and maximum rise is determined by input, airflow, efficiency and combustion performance, typically all functions of the unit design.
Yes you can. Each heater requires a 6” diameter air inlet pipe, with maximum equivalent length of 25 ft.. I supposed it could be PVC, but you could also use single wall or type B vent with taped joints which would probably be less expensive.
Condensation in the tubes may occur during air conditioning operation, as the tube surfaces drop below the dew point temperature of ambient air in the tube. The tube surfaces cool below the dew point and water vapor in the air inside the tubes condenses out. This condensate is typically no more corrosive than tap water, and the heat exchanger is designed to allow drainage of this condensate and prevent build up in the tubes. A drain tap is provided in the flue box to permit attachment to a condensate drain hose to remove condensate from unit.
During cooling operation condensation may also occur on vestibule surfaces or the burner assembly. In these applications, it may be necessary to insulate outside surfaces of heater to minimize condensation and accumulation of water in vestibule area, particularly if the installation is in an unconditioned space. A collection trough and drain should be provided if surfaces are not insulated
Type "B" vent is acceptable for HM Indoor installations that are vertically vented, have exhaust gas temperatures less than 550 oF and meet criteria of Category I vent.
Outdoor installations and horizontally vented indoor HM units are Category III venting systems. Category III applications should use the single wall AL29-4C sealed joint vent pipe. Outdoor units typically have a single vent connector which should be sealed at the point of attachment to the combustion blower. Vent piping should not be run in the vestibule area unless it is the sealed joint type, to avoid flue gas leakage and recirculation into the combustion air supply.
The HE's are Category III in all applications and should employ this pipe in all vent systems. I note that among the customer installation literature I've reviewed, mention of this type of vent pipe is made in such a way that it would leave the installer with the idea that it's use was optional in lieu of more conventional vent materials.
Both double wall classes of vent pipe are really intended for vent systems under negative pressure, since the joints in the assembled pipe are not pressure tight.
The combustion process generates water vapor equal to approximately 1 gal per hour for every 100,000 Btuh of input (100 cu ft. of gas). Typically very little of this vapor actually condenses in HM style heater. Most of the vapor is exhausted thru the venting system. If any condensation occurs, it is usually for a very short time at the beginning of the firing cycle.
Condensation is a bigger issue during AC operation in the summer, if the heater is downstream of the cooling section. The moisture in the humid air condenses as tubes are cooled by the conditioned air flowing over them.
Type 409SS is a ferritic grade of stainless steel and is prone to surface rusting when exposed to moisture laden air or surface water. When located downstream of cooling coils, condensation can occur on tube surfaces resulting in spot rusting. Outside air containing high moisture content or snow can deposit water on tubes, with similar results.
This rust occurs only on the surface of the tube and typically does not penetrate the tube material. Usually this rust can be removed from the tube surface by wiping with a rough cloth or fine steel wool, if the appearance is objectionable.
In the rack assemblies, we pipe all the units in each rack assembly to a 1” PVC header, so there is one condensate discharge per rack.
The reason we have all the orientations listed and have always requested the direction of airflow be specified at time of order is to provide heat exchangers with dimples oriented to provide for drainage of condensate in applications where heater is mounted downstream of the evaporator coil (approximately 90% of all commercial HVAC applications). Dimple form alignment depends on airflow direction and burner tray alignment. Vertical burner trays with horizontal airflow tubes have dimple forms aligned differently than units with horizontal burner trays and vertical airflow tubes. See the attached General Product Specification which we have published since the introduction of the HM Series product line. I’ve highlighted the paragraph relating to the dimple forms.
As to the water in the draft fan, our fan vendor does provide a hole in the bottom of the fan housing for RH discharge combustion blower and the housing would drain naturally for LH discharge. For vertical discharge units we attempted to add a hole during unit assembly, but it is too easy to damage the fan blade drilling the hole on our production line. We requested that our vendor add the hole, however they have a much larger customer using the same housing who did not want the additional hole and so they refused our request. We felt that the condensate would have to fill the flue box at least up to the fan orifice plate in order to fill the fan housing up to the fan blade, and thought this was not likely given the drainage of the tubes in their proper orientation.
We will be looking at a new fan assembly later this year and we’ll see if we can have another hole added to eliminate any possibility of condensate accumulation.
To the best of my knowledge all our literature contains statements and warnings concerning the possibility of condensation for applications downstream of refrigeration systems. We will review the information we have and see if we need to bring more specific
Most commercial and industrial regulators service a large range of gas flows for any given body size and therefore are typically equipped with a flow orifice for a particular capacity range. It is likely that the orifice in the regulator installed is to small for your required gas flow rate. Your combined input rating should be less than the maximum flow rating of the proper sized orifice for the desired pressure drop. If the orifice is sized correctly, be sure that the spring provided with the regulator is for the desired pressure range.
The maximum gas supply pressure on either Natural or Propane Gas is 13.5 inches w.c. This is the maximum pressure that the combination gas valve is listed at. Also, certain combination valves incorporate a regulator design that will reduce or prevent gas flow above this pressure, while others do not. Excessive inlet pressure could lead to nuisance shut downs or ovefiring unit and damaging heat exchanger.
The minimum inlet pressure for Natural Gas is 5.0 inches w.c. and for Propane gas is 11.0 inches w.c. If inlet pressure is less than the noted levels you will not be able to achieve maximum rated input to heater. In some locales Natural gas supply pressures are less than 5.0 inches. The burners should function properly down to 3.5 inches inlet pressure, but input rate will be reduced and temperature rise and maximum airflow rates need to be reviewed for the application.
Our furnace modules are listed for a minimum of 20 oF rise, as is all other equipment due to the standard requirements. Also, below 20 oF rise the risk of tube surface temperatures dropping below the dew point of the flue gases and continuous condensation increases. This specification would require the use of an air bypass with an adjustable damper. During high heat requirements, the damper would be closed, but as the input modulates down the damper would have to open to allow some air to bypass the heat exchanger. This reduces the airflow over the heat exchanger maintaining a higher temperature rise and higher tube temperatures to avoid condensation of flue gases. In the example you provide, 50% of the air volume would be bypassed at minimum input , reducing the air flow over the heat exchanger to 50% and providing a temperature rise of 20 oF.
HM furnace modules can be installed in series or in parallel to achieve higher rises if necessary. Modules can be stacked up to five high or four deep to obtain the necessary temperature rise. Refer to the Applications section “Multiple Unit Installations” for recommendations on sizing and arrangements. Regardless of the rise requirements, the Duct furnace standard limits the maximum discharge temperature to 160 oF for any combination of listed heaters.
Insufficient oxygen supply to the burner causes similar problems to flue gas recirculation (see above). The most common problem is sizing the opening based on the physical size of the opening alone. The recommended opening size would provide 1 sq. in. of free area for every 4,000 Btuh of input. So a 10in x 10 in opening (2would be right if no screens, louvers or expanded metal is applied to the opening. Screen materials typically vary from 60 to 85% open area, expanded metal, 45 to 70% and louvers 40 to 60%. If the opening is louverd with average louvers the opening dimension would need to be 2X larger to provide the required free area, or 200 sq. in.
Flue gases contain Carbon Dioxide (CO2), Nitrogen (N2) and water vapor and may have traces of carbon monoxide (CO). Proper combustion of gaseous fuels depends on a supply of Oxygen (O2) present in the air drawn into the burner. The re-circulated flue gases displace a portion of the fresh air and reduce the amount of oxygen drawn into the burner resulting in higher CO2 and CO levels in the flue gases. Eventually the oxygen supply is insufficient to sustain a stable or clean burning flames, and causing a floating flame with loss of flame signal and lockout, or trip of the rollout switch. The poor combustion can also resulting in sooting of the heat exchanger tubes that restricts the tube causing poor combustion and reduced operating efficiency. If blockage is significant, floating flames or flame rollout can occur.
The coefficient of heat transfer is affected by the direction of airflow and velocity over the tube surfaces. In order to maintain the required minimum 80% thermal efficiency, the maximum rise is limited to 60 oF in configurations where air flow changes direction in passing through the heat exchanger or is directed through the open side of the heat exchnager tubes.
IRI Information IM.4.3.1, covering Space Heaters identifies IRI position as follows. “ Use heaters that are listed by a nationally recognized testing laboratory (NRTL), that provide periodic inspections of the listed equipment or materials. Install heaters with minimum clearances to combustible materials as shown on plate attached to the heater.”
No. All vent terminations should be an approved type rain and windproof vent cap. In addition to preventing water from entering the vent system, these terminations compensate for wind and atmospheric conditions, and prevent downdrafts to insure proper venting of flue gases.
Category I appliances operates with a non-positive vent pressure and at vent gas temperatures at least 140 oF (78 oC) above its dew point, sufficiently high enough to avoid condensation. Because the vent pipe pressure is not positive, single wall or Type B vent pipe may be used, depending on the application, and requirements of Z223.1 the National Fuel Gas Code. HM series modules for indoor applications and vertically vented are Category I.
Categories were established to define the conditions in the vent piping under which the product operates. Category III operates with a positive pressure in the vent and at vent gas temperatures at least 140 oF (78 oC) above its dew point. Vent gas temperatures are sufficiently high enough to avoid condensation. Because the vent pipe is under positive pressure vent pipe listed for Category III applications, having sealed joints, must be used to avoid products of combustion from entering the heated space.
It should not. The HM Series module are listed as non-condensing heaters, either Category III or Category I. Flue gas condensate is highly corrosive and even the best grades of stainless steel used in duct furnace construction (Types 304L, 316 and 321) are subject to accelerated corrosion and premature heat exchanger failure if condensation of flue gases occurs in heat exchanger tubes. This also voids the heat exchanger warranty. Typically, condensation can be avoided by maintaining airflow over the heat exchanger to maintain a minimum 20 oF rise across the heat exchanger under any operating condition. Refer to the Airflow Considerations in the application section for more details.
Yes you should. Some condensation may occur in the flue box itself during operation of the air conditioning system. In applications with a horizontal burner tray, condensate will drain from the open end of the tubes. A collection pan with a drain hose should be provided at the base of the vestibule adjacent to the vest panel to allow for removal of this condensate.
For applications requiring a 20 oF to 90 oF temperature rise, aluminized steel can be used provided that the minimum return air temperature is above 40 oF and the minimuim input rate is greater than 50% of maximum input.
Stainless steel should be used if 40% or more outside air is used, return air temperature are less than 40 oF or modulation below 50% of maximum input is provided. See Material Selection Guidelines in “Applications” section for more detailed guidelines for best material choice to provide maximum heat exchanger life.
At 800,000 Btuh input the rise would be 45 oF. The minimum rise for our heaters is 20 oF and therefore the minimum input would be 175,000 Btuh for the modulating heater (assumes that ½ the airflow passes over this heater) without any bypass.
For lower rises, a % of air would have to be bypassed and mixed with the heated air to provide the desired 5.6 oF minimum rise. As an example if 50% is bypassed, airflow over the heater bank would be (6600 cfm) and airflow over the modulated heater would be approximately half of that or 3300 cfm. At that airflow with a minimum modulated input of 100,000 Btuh, the rise would be 22 oF. Under this operating condition 75% of the air would be flowing over unheated surfaces and 25% over the heated surface, which should approximate a 5.5 oF overall rise condition, provided the air can be properly mixed.
The above example assumes that return air temperature can be within 5 oF of the desired discharge conditions, since you are concerned about the +/- 5oF discharge air control.
Side Wall venting is permissible. Refer to the OEM Instruction Manual for HE's, click the link button. Page 4 has diagrams indicating maximum and minimum equivalent vent lengths for horizontal and vertical venting. HE's are category III in all applications and therefore you must use approved vent pipe for this category (has sealed joints because vent is under positive pressure). See Attatched Sheet
The NYC code states “ Pressure regulators that require a vent……..” which clearly recognizes that not all regulators require an external vent. Combination gas valves used on HM series units are listed under ANSI Z21.78 / CSA 6.20 which permits the use of integral vent limiters on the regulator as provided on the Honeywell VR8305 Series controls.
Regulators that are not integrally vent limited are required to have a port marked “Vent” for connection to an external vent by the ANSI standards for regulators and combination controls, hence the requirement above in the NYC code. Click here for more information from Honeywell
For constant volume airflow systems the minimum airflow is that marked on the heater listed or in the specification table.
For multi-speed or variable air volume systems (VFD control) the duct furnace can be operated at 66% of the marked minimum airflow provided:
1. The heater is equipped with a modulating gas control system
2. The unit manufacturer provides a factory installed discharge air temperature controller
3. The system does not include a room thermostat.
The discharge air temperature controller must be set to prevent the unit from firing above the maximum rise specified for the duct furnace by monitoring the discharge air temperature and reducing the heater input.
The current Gas Duct Furnace Standard under which we are evaluated is a harmonized standard (ANSI Z83.8 and CSA 2.6). Our Listing Mark designates listing in the US and Canada, so the listing on your product will cover Canada as well. Your rating plate would need to bear the mark shown below indicating listing in both Canada and the US. Click for ETL Information
The minimum velocity is 500 fpm / the maximum velocity is 1,500 fpm. The typical velocity we see our customers design to is in the 900 fpm range.
The highest static we've tested with cetifying agencies is 3.0", which is typical for most duct heater manufacturer's. However, some customers do leakage tests up to 10" static pressure without any issues.
HTA heaters- Temperature Rise up to 125 F and Max Discharge Temperate of 160F. ANSI Z83.8 Gas-Fired Duct Furnaces. Only approved for the positive side of the air circulating blower.
HPA heaters- Temperature Rise up to 125 oF for individual heaters and 250 oF for tandem installation. Maximum discharge air temperatures to 325 oF. ETL / ITS Recognized Air Heater Module – IAS 9-90 Gas-Fired Desiccant Dehumidifiers, ANSI Z21.47 Central Furnace and ANSI Z83.8 Duct Furnace. Application flexibility - Heaters listed for operation on negative pressure (suction) side of circulating air fan. Maximum discharge air temperatures to 325 oF. Designed to provide regenerative heating for Desiccant Dehumidifier Applications.
Natural Gas consumption = 1cfh of gas per 1000btu input Propane Gas consumption = 1cfh of gas per 2500btu input.
Fresh air requirements = 10 cfh air/ 1 chf gass
60% excess air =16 cfh air/ 1 chf gass
500,000 btuh/1000 btu =500
Gas X Excess Air X Air= cfh/min=CFH
500x16+500= 8500 cfh
8500cfh/60min= 142 cfm
Separated combustion means that air required for combustion is brought to the heater by means of a duct connected to the outdoors. Both the vent and air intake can be provided by the customer.
Sealed combustion means that air required for combustion is brought to the heater by means of a duct connected to the outdoors. When installed that way the heater is stand alone , not affected by barometric or pressure changes within the room. The manufacture must provide the vent system, the fresh air duct work, and should not have more the a +8% leakage.
By convention the industry favors mounting heating units on the positive pressure side of the circulating air fan. The rational is that this location minimizes the likelihood of flue gases entering the circulating air stream in the event of a breach in the heat exchanger. Heating units listed under ANSI Z83.8 (Gas-fired Duct Furnaces) must be installed on the positive pressure side as a requirement of their listing. The Central Furnace standard (ANSI Z21.47) has construction provisions that would permit installations on the negative pressure side of the circulating air fan. Heat exchangers located on the negative pressure side of the fan must be welded to provide an air tight joint as determined by the certifying agency and comply with a heating element cycling test. Our HM, HD and Rack series units are listed under the Z21.47 standard when employing a welded, stainless steel heat exchanger. At the time of quotation, you would need to advise us that the application is for negative pressure side application.
On HMB and HDB models, a high temperature ceramic fiber gasket is used on the flue cover panel. By design, the flue collector box on these models is located in the air stream and the gasket insulates the
cover panel to reduce heat flux into the vestibule area. Even though the gasket is compressed when
installed, the gasket does not provide a water tight seal and will “wick” water (condensate), especially
during periods when the AHU is operating in the cooling mode. This may cause rust on exposed surfaces
and /or water accumulation in the vestibule base. This may be more evident in applications with vertical
burner trays, since all the condensate formed inside tubes will drain to the collector box.
In order to minimize wicking, be sure drain tubing from the collector box is pitched downward to maximize flow of condensate though the drain tube. Inspect and clean drain tube(s) regularly to
maintain maximum flow through the drain tube. Additionally, provide a drain hole in the vestibule base
and provide a drain connection.
In applications with horizontal burner trays, the condensate primarily drains from the open end of the
tubes, and minimal condensation occurs in the collector box. An optional drain pan is available to collect
condensate which drains from the tubes.
Typically, the “wicking” is minimized in heating mode, as condensate formation is minimal and the
adjacent temperatures are sufficient to evaporate moisture and dry out the gasket material.
All other models employ an external collector box with a different method gasketing and insulating the collector box. However, on heaters where “silicone free” construction is specified, a woven ceramic fiber
gasket is used and will similarly wick condensate as outlined above.
Additionally, some surface condensation is possible on uninsulated vest panel and induced draft fan
surfaces during cooling operation.
The American Gas Association (AGA) sponsored laboratories referred to has been out of existence over twenty years.
Product design certifications to ANSI standards are now conducted by Nationally Recognized Test Laboratories (NRTL’s) including Underwriters Laboratories (UL), Intertek/ETL, Canadian Standards Association (CSA) and a number of other approved agencies. All of our products are listed by Intertek/ETL and certain models are listed by UL.
We would suggest you contact the specifying party regarding their acceptance of a recognized NRTL.
All units need to be sized at sea level and Heatco will de-rate them if your elevation will be higher than 2,000 ft. See below for a detailed explanation:
Gas furnace capacity ratings are based on sea level operation. Furnaces are designed and listed based on heat exchanger size, combustion volume and vent capacity. For any given gas furnace design, the heating capacity depends on the density and oxygen content of the air, along with the pressure drop across the heat exchanger. Altitude affects each of these factors, since density and oxygen content of air decrease with an increase in elevation. “Thin” air carries less heat, resulting in a loss of furnace capacity compared to sea level ratings. Therefore, the gas input of the furnace selected, based on sea level ratings, needs to be reduced to maintain comparable airside (temperature rise) and safe combustion performance at altitude.
Current guidelines for gas furnaces sized at sea level ratings, provide for a 4% de-rate per 1000 feet of elevation for elevations over 2000 feet.
All of our HM/HD and HE/HF units are approved to Mass Code. Please click the following link for confirmation: Click Here
Typically residential furnaces employ fan controls for both on and off operating points. Circulating fan operation is delayed until the temperature rises to a certain point to avoid circulating cool air at the beginning of a heating cycle and continues to run for a short period of time until the circulating air temperature drops to a predetermined point at the end of the heating cycle, to utilize the residual heat from the heat exchanger.
Commercially, the choice of a Fan Switch to delay operation or maintain fan operation after burner shut down would depend on the application. Delaying the fan operation might be beneficial in applications where a significant % of outside air is utilized. Operating the circulating fan at the end of the heating cycle cools the heat exchanger and prevents heat buildup in the cabinet and provides the most benefit with On / Off heater operation. For modulating systems, operating the fan after the heating cycle completion would probably have less benefit since it is very likely that the system would be operating at a reduced input rating just prior to shut down.
In multiple heater installations, the modulating heater would be the last to turn-off, so again operating the fan at the completion of the heating cycle would provide minimal benefit.
Below is a link to a page from FM Loss Prevention Data Sheet 6-20 for Space Heaters, outlining FM’s positions on acceptance of equipment. Our HM/HD/Racks heaters are rated at less than 2,500,000 Btuh per heater and are listed by nationally recognized testing laboratory agency, either ETL or UL, and therefore are acceptable to FM Global. Controls provided meet or exceed the recommendations outlined in Table 1 of the data sheet. Click Here