Heating, Air Conditioning, Fridge, HVAC/ESP


QUESTION: Hi Joe, I recently had a split system installed. All LENNOX 3 ton  down flow; furnace: G61MP-36B-071, Condenser: XC-036-230-1, coils CR33-30-36B-F, 18" ATCO 036 flex return air duct. I believe there may be a problem with the duct. On page 16 of the furnace installation manual under "DUCT SYSTEMS": NOTE - Operation of this furnace in heating mode (indoor blower operating at selected heating speed) with an external static pressure which exceeds 0.50 inches w. c. may result in erratic  limit operation. 1. My furnace is located in my garage; is this inferring that the furnace itself is indoors? Or just that the blower is encased in a blower compartment?   2. Is this referring to furnace being equipped with an ESP limit switch? 2a. Or is this referring to another limit switch? If so what switch? 3. Is the "selected heating speed" factory preset? Or does the installer set it in the field? 4. If installer selects speed; Is it possible to raise or lower the speed to achieve the furnace operating satisfactorily at a higher than 0.50 ESP w.c.?  Thank You   REG

ANSWER: Reginald,

I do not know what you mean "My furnace is located in my garage; is this inferring that the furnace itself is indoors  or just the blower is encased in a blower compartment."

I do not know what Lennox is referring to a ESP limit switch.

The selected heating speed is factory set, but can be changed in the field. Your looks to be a 4 speed blower. High, med. high, med. low and low. Typically you set the fan to run in high or medium high for cooling and low or med. low for heat.The contractor can change blower speed , typically you should have manual balancing dampers to adjust the amount of air per room. Trying not to have the furnace push to much air through the registers to where it creates a whistle noise. Your system being new ,you can ask your installing contractor what the static pressure is ,

The " may result in erratic limit operation " may refer to noisy air vents ( it does state in the previous remarks about having a uniform air flow through the duct work.) Your also suppose to have a certain amount of temperature rise across the heat exchanger.from the return side to the supply. I did not see in the instruction what lennox recommends.To little air flow you may trip a high limit. to much ,it may take longer than usual to heat the house, and noisy supply registers, you also may overamp the blower motor if duct work is not sized correctly .

I would discuss all your concerns with the installing contractor and if they do not answer your questions to your satisfaction ask them to have a technical rep. from Lennox contact you or come to your residence with the installing contractor. Lennox is a big name HVAC manufacture and they probably have a Tech. Rep. near you.

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QUESTION: Thank You Joe. My installer hasn't been forth coming with answers. I didn't know I could ask him these questions and they would have a representative come to my house. May I feel free to contact you again?    Reg


You can check your own static pressure ,you would need to buy a u tube or a manometer, not that expensive to purchase ,goggle how to check it. I'm guessing the installing Contractor did not check yours,reason they were not forthcoming with answers. Typically there is a start up sheet that comes with the system for the Contractor to check and adjust as needed.Such as the gas pressures ,refrigerant levels, amperage draw of blower motor, static pressure checks are in there also. Ask the installing contractor if they filled one out. Your static may be OK. I would have them come out and check it ( should be under the labor warranty ) and cost you nothing.Then if it's at a acceptable level your OK.You stated .50 " . If they are a reputable Contractor they should do that for you with no problem.Read up how to do it and the 2 location one in the return and one in the supply certain distance from the fan section, before they come to your residence

Static Pressure Testing, Step By Step
It typically should take less than five minutes to measure a system’s static pressure. Here are sample instructions for a furnace and a remote coil:

STEP 1: Locate the appropriate places to drill the test holes on the supply side (+) between the furnace and the coil, and on the return side (-) between the filter and the furnace. Center the holes for neat appearance. Stay away from any coils, cap tubes, condensate pans, or circuit boards to avoid damage. Always look before you drill.

STEP 2: Drill test holes using a 3/8-in. drill bit with a metal piercing tip. A bullet tip drill makes a clean round hole. Make sure to use your drill bit sheath (Described above) to prevent from drilling into the coil. If there’s duct liner inside, be sure to penetrate it to assure a good reading.

STEP 3: Push one end of the tubing onto the static pressure tip. Place the other end of the hose on the HIGH (+) pressure port of the gauge. If required, make sure the gauge is level and zero the gauge by adjusting the screw on the face with the small screwdriver. (Digital gauges each zero differently, so check owner’s manual.)

STEP 4: Read the supply or positive (+) static pressure by inserting the static pressure tip in the test hole into the air stream with the tip facing into the airflow. The magnet on the tip will hold it in place while the value is read and recorded. This measurement is the pressure the fan is "seeing" on the supply side of the system.

STEP 5: Read the return or negative (-) static pressure by moving the tube from the HIGH to the LOW-pressure port on the gauge. Insert the static pressure tip in the test hole on the return side with the tip facing the airflow. Read and record the negative static pressure. Don’t forget to insert hole plugs in the test holes when you’re done testing.

STEP 6: Calculate the system’s Total External Static Pressure by adding the two values. Since the negative and positive signs identify the type of pressure measured, you can ignore them when adding the two values together.

For example:
The supply static pressure reading is (+) .26 w.c.
The return static pressure reading is (-) .21” w.c.
The total system static pressure is .47” w.c.

.26” + .21” = .47” w.c.

Further testing may include pressure drop measurement to determine the contribution of each component to total external static pressure of the system.

When you read the static pressures in the systems your company designs and installs, you’ll begin to gain tremendous insight into the performance of your systems. Doesn’t it seem strange that the duct system is excluded from 98% of our industry’s service agreements? It’s clear that the pendulum has swung too far in the direction of the equipment and away from comfort.

If static pressure is too high, you have evidence of problems with the system causing low airflow. Check for blockage in ducts, closed dampers, improper transitions, offsets or kinked flex duct. The problems can also be from the equipment and system accessories like “High Efficiency” cooling coil or restrictive filters.

Low static pressure can also mean trouble. Low pressure may indicate leaking ductwork or plenums, missing filters, low fan speed or separated ductwork.  

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40+ years diversified experience.An HVAC/Refrigeration Self Employed Contractor since 1986. NATE Certifield. Answer questions pertaining to Residential and Commercial Air Conditioning , Warm Air Heating, Heat Pump systems.Mechanical and Electrical troubleshooting of these systems.Be as detailed as possible when describing problem.Packaged unit or split systems.No appliance questions.


Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration ( Walk In and Reach In Coolers and Freezers ) Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil , Natural and Propane Gas Heating systems.

Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

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