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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.
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.
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 voltage 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.
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
Vent piping should not be run in the vestibule area unless it is the sealed joint type, to avoid flue gas leakage and re-circulation 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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
“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.
VAV Applications – Minimum Airflow
HM and HD Series duct furnaces may be applied to VAV (Variable Air Volume) applications with 2 speed or variable frequency drives. In these applications the minimum allowable CFM supplied to the duct furnace can be 67% of the minimum CFM listed in the ratings table.
Furnaces must be applied as follows:
• Unit is equipped with 2 stage (TS) or electronic modulating gas controls (MD or MR)
• System includes a discharge air temperature controller (provided by others) and set to prevent the unit from firing above its maximum rated temperature rise at or greater than the minimum CFM. The controller would reduce the furnace input (low fire) if the maximum rise condition is reached.
• The system is not controlled by a thermostat in the heated space. Since it is located remote from the unit, operation on the thermostat could cause the unit to over-fire at the reduced airflow and result in overheating and possible damage.
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.
Rule 1111 applies to residential and small commercial fan-type central furnaces having input ratings less than 175,000 Btuh and provides an emission limit of 14 ng/Jl. We are not aware of any commercially available products that comply with the current Rule 1111 limits.
• Entering Air Temperature (EAT)
• Supply Air Temperature (SAT)
• Temperature Rise Required = SAT – EAT
• Design Airflow Rate (ACFM)
• Site Elevation (0-2000 ft. = Sea level)
HEAT LOAD (Sensible)
Heat Load (Btuh) = lbs. air/hr x Sp. Ht. (Btu/lb-oF) x Δ T (oF)
or = (ρair x 60 x ACFM) x 0.24 x (ΔT) (1)
The familiar formula used for calculating required heat load is:
Heat Load (Btuh) = 1.08 x CFM x Δ T (2)
It is important to note that formula (2) is based on a 70 oF inlet air temperature and 29.92 “Hg barometric pressure (Standard air density) and is fairly accurate for return air applications. However, for make-up air applications this formula may result in an undersized heater.
Example: Design Conditions – Make-up air application EAT (10 oF), SAT (80 oF), ACFM = 3100, Sea level installation
Using standard formula (2) above the Heat Load = 234,360 Btuh
Using design formula (1) above the Heat Load = 264,278 Btuh
At design conditions the fan is moving more lbs of air and using the standard formula (2) the heater selected would be undersized by 30,000 Btuh for this make-up air application.
Sizing the Heating Unit
Ratings for most heating units and duct furnaces are based on input rating in Btuh.
The heat load determined above is the output of the heating unit or furnace.
In order to select the appropriate heating unit based on input divide the heat load (heater output) by the rated efficiency for the furnace:
Heater Input rating = Heat Load (Heater Output) Btuh
In the above example for a furnace with 80% Steady state efficiency:
Standard Calculation: Heater Input = 234,360 / .80 = 292,950 Btuh
Preferred Calculation: Heater Input = 264,278 / .80 = 335,347 Btuh
The correct heater for this application would have an input rating of 350,000 Btuh. Using the standard formula an undersized 300,000 Btuh heater would be selected.