This weekend's project was an energy efficiency project - upgrading the furnace fan motor in my 20 year old Lennox fan coil furnace (thermopump / electric heat). When we moved into this house two years ago, we embarked on a series of energy efficiency upgrades - increasing attic insulation. Increasing insulation around attic duct work to reduce duct loss. Improving the building envelope - by sealing gaps and adding spray polyurethane insulation at critical places. With the improved sealing, we also installed a heat recovery ventilator (HRV) - to constantly exchange fresh air from outside with stale air from inside the house. Since the house has a ducted ventilation system, the HRV was connected to the return ductwork and required the furnace fan to operate constantly when the HRV is exchanging air.
The furnace fan ran with a typical permanent split capacitor (PSC) motor, with a 3/4 hp rating. This motor draws 5.5 Amps at 230 Volts when running - for 1250 Watts of enengy consumption, 7 days a week, 24 hours a day. At $0.095 / kW-hr for electricity, this motor costs $1071 / year to run. The motor was installed to run at a single, high speed.
Some research lead me to the Evergreen IM electronically commutated (EC) motor. This motor is a brushless DC motor with permanent magnets, one of the most energy efficient motor types currently available (similar in design to the motors found in electric cars). All the electronics are contained in the motor housing itself, and the electronics and electrical connections have been designed to mimic standard HVAC fan motor leads, with "taps" for low, medium, medium-high and high speed. These taps don't connect to line voltage, rather, they are control lines that are used to instruct the power electronics how to control the motor speed electronically.
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| The Evergreen IM EC motor, with belly band mount installed. |
Prior to purchasing the motor, I read the manual available online to understand its specifications, applicability to my furnace, and installation requirements. Investigating my current system I found my existing 3/4 hp motor to be installed with a shaft bearing mount - incompatible with the Evergreen IM motor. So I knew I needed to order a motor mount at the same time. Photos of the existing motor's data plate indicated that it was a "Frame 48" sized motor - the same size as the Evergreen IM.
Genteq sells a series of different sized motor mounts for its Evergreen line of motors - 10", 11" and 13". I got access to the current blower, measured the blower wheel diameter (11 1/2" in diameter), and realized I would require the largest mount - the 13" mount.
I shopped online, got pricing from Amazon and the usual suspects, but also called my local HVAC contractor - Ventilation PCP in Sainte-Julie. They provided a reasonable price for the motor and the mounts, and also some assistance in case I ran into any difficulty installing the motor. I felt that the little extra that I paid purchasing from my HVAC contractor might come in handy in case I ran into trouble, so I ordered the motor and it arrived from the distributor in a few days.
The first steps in replacing the existing motor is to baseline the performance of the existing fan, so that you can set the speeds correctly with the replacement motor. To do this, there are a few methods described in the Evergreen IM motor manual. I decided to do a Total External Static Pressure (TESP) test of my fan coil unit, and also do a Temperature Rise test of the 2nd stage electric heating circuit. These tests took about 2 hours total, taking my time to get it right. There are quite a few good youtube tutorials on how to do these tests, they are not complicated, if you are methodical it's pretty simple and straighforward. I'll list a few of my lessons learned below.
To perform the TESP test - you need an electronic differential pressure gauge. I borrowed a Testo 510 - very nice and simple to use. They cost about $200 on ebay to purchase, but since I don't expect to be doing this type of work very frequently, I didn't purchase one. The Testo reads differntial pressure in inches of water, and since the fan coil unit is pretty large, I used some vinyl tubing to run to my pressure test points - at the inlet (return) side of the fan coil - just before the thermopump evaporator coil - and at the outlet (supply) side of the fan coil unit - after the ventilation fan and electric heating stage.
My existing system ran at about 0.44 in H2O, which I understand to be a pretty good figure indicating relatively efficent (properly sized) ductwork. There are three principal returns, one from each level of the house that come to the return side, and three separate independant supply branches after the fan coil running to each level of the house. To do the test, I made sure all the filters were cleaned, all registers and dampers were open to minimize restrictions in the ductwork.
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| Testo 510 differential pressure gauge. |
To do the temperature rise test - the procedure is relatively straighforward. Basically, by measuring the air temperature entering and leaving the furnace heating unit, and measuring the energy consumption rate of the heating system in Btu/hr - you can calculate the airflow of the furnace using a simple thermodynamic calculation. The Evergreen IM manual only suggests that you measure the temperature rise before and after the motor upgrade, so you can set the speeds on the Evergreen IM to match the temperature rise numbers (and therefore match the airflow). In my case, by also measuring the electrical consumption of the electric heaters, you can use the sensible heat calculation to do a reasonable estimate of airflow. I used my Klein CL1000 clamp meter to measure the power consumption of the heating stage - which ran at 85.3 Amps at 230 Volts - from which you have to subtract the fan power consumption (5.5 Amps), which left me with approximately 80 Amps of electric heat consumption. By the way - when you're doing this, you'll have the connection panel (or your electrical panel) open to be able to access a cable to measure current. If you're doing this - you'll need to be very careful about live circuits. If you're not comfortable with this - just avoid this altogether - you really only need to measure temperature rise.
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| Klein CL1000 Clamp Meter |
For measuring temperature rise, I used a digital meat thermometer (found in the kitchen). I found that with a 3/8" hole drilled in a duct, I could get the probe into the airflow pretty close to the supply and return sides of the furnace, and get a fairly accurate temperature measurement. It takes about 15 minutes for the furnace to come to a good equilibrium to allow you to take measurements. Then, each duct temperature measurement took about 1 minute for the reading to stabilize, indicating that the probe tempeature had matched the airflow temperature. If doing this again - I would purchase a second digital meat thermometer. They are not expensive, and by doing both the supply and return side at the same time, it would have saved some time in the process.
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| Digital meat thermometer for measuing temperature rise. |
The following photo is the meat thermometer inserted in the supply duct measuring the furnace outlet temperature (43.9 Celcius). Note that you have to place the thermometer out of direct line to the heating elements - otherwise heat radiation from the elements will throw off the measurement. You want to measure the temperature of the outlet air only.
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| Supply side temperature measurement using meat thermometer. |
While I was at it, I also took a few duct outlet temperature measurements, and compared them to the mesurement at the outlet of the furnace. The difference in temperature between the furnace outlet and the supply duct outlets give you your duct temperature loss.
Once the measurements were taken and noted, it was time to let the furnace cool down a bit, and then get started with the motor swap. The first step is to cut power to the furnace (from all potential power sources, in some systems from multiple breakers, especially if you have a thermopump), open the covers, and note all the existing wire connections for the PSC motor. You'll be using some of these connection points for your new connections, so take a few photos and some good notes before you start disconnections. I intend to keep the old motor as a spare - so I'll need to remember how it hooks up to be able to reuse it in the future.
Once the old motor is disconnected, you need to remove the entire blower assembly from the furnace. In my case, this was pretty quick and easy (very accessible).
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| Blower with old PSC motor - note the shaft bearing mount - not compatible with the Evergreen IM motor. |
Once the blower was removed, the next step was to separate the blower wheel from the motor shaft. I started with a bit of penetrating fluid, and then checked to see if any of my automotive pullers would work on the blower wheel. None would fit, and I didn't have a blower wheel puller. So - once the motor was unbolted from the blower cage, I set the blower cage up on some 2x4 blocks, facing up, centered the blower wheel on the cage for good support, and then tapped the motor shaft with a block of wood and a hammer. 3 or 4 stiff blows, and the motor dropped out of the blower wheel.
With the blower wheel removed from the cage, I noticeds quite a lot of oily crud buildup - probably a mix of dust and lubrication oil from the motor bearings. I took the wheel outside, and scrubbed each vane of the blower wheel with a toothbrush to get rid of the crud. This is an important step in the process, and I'll explain why later on in this post.
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| Blower wheel in serious need of a scrub. |
With everything apart, it's time to get the new motor installed. I carefully centered the belly mount cage on the blower cage, and marked my placement for new holes. NOTE THAT THE MOTOR NEEDS TO BE INSTALLED IN THE MOUNT TO MARK THE HOLES. If you don't have the motor in the mount - the motor mount will flex open too much, and your holes will be in the wrong place.
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| Marking holes for drilling for the new motor mount |
I drilled the holes without the blower wheel installed, and then carefully cleaned the cuttings out of the blower cage. It was a bit fussy to get the new mount installed - the blower wheel had to be in the cage first, because it would not slip past the mount installed. It was a bit tricky reaching in behind the blower wheel to place the bolts - but everything went well working farthest from the blower outlet first. The last two bolts go in easy from the blower opening.
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| Installing the belly band mounting bolts. |
Once the mount is taken care of, install the motor in the mount and the blower wheel on the motor shaft. I put a small amount of anti-seize on the shaft before installing it in the blower wheel - just to help avoid it rusting together over time, and complicating future replacement.
Prior to installing the blower, I had to replace the grommet leading from the fan / coil compartment to the wiring compartment, to fit the wiring harness for the motor. I found running the harness before installing the blower cage and motor gave me more space and made things easier. One nice thing about this motor is that the cable harnesses plug into the side of the motor and are removable. So - if you ever had to replace this motor, it's a simple swap out without the requirement to even access the wiring compartment of the furnace.
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| New rubber grommet for the wiring harness. |
The harness can be installed in the furnace before reinstalling the blower and motor.
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| Installation of the motor wiring harness. |
Install the motor voltage jumper before reinstalling the blower and motor. If you get this step wrong, you can damage the motor, so take care with the instructions at this point. Here is the yellow jumper for 230V installed.
With the harness installed, replace the blower cage and motor.
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| Blower cage installed. Note the wiring harness run through the grommet to the wiring cabinet. |
With the blower reinstalled in the furnace, it's time to start making connections. Take your time, read the manual, and everything will go fine. I connected the two harness connections.
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| Harness connections and jumper installed. |
I won't go into the details of the wiring connections, the manual does a fine job of describing the connections. All in, removal of the old motor, and reconnection of the new motor and startup tests took me about 2 hours. In my case, I connected the high voltage signal cables to give me 1/2 to 3/4 hp operation, and then connected the high speed signal wire to the thermopump "Y" call for first stage heating or cooling. I connected the medium high speed signal wire to the "W" emergency auxiliary heat connection (electric heat) in my case. There was no need to connect a low speed signal wire, the motor defaults to low speed when there is a voltage on the high speed signal wire, and no voltage on any of the other control speed signal wires.
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| This looks a lot worse than it really is - most of these wires are running back and forth between the electric heat elements and contactors. The thermostat connections are at the top right of the cabinet. |
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| The fan relay - I took the time to attach it to the side of the cabinet, because for some reason it was just sitting in the bottom of my cabinet. In my case - all of my 230V connections were made directly onto the terminals of this fan relay, and the motor harness is pre-terminated with spade plugs that fit perfectly on my relay. Made for very quick and easy connections. |
With everything installed and running, I turned the furnace back on and ran a few tests. My first reaction was amazement at how quiet it was running in low speed mode - which I set up to run when my heat recovery ventilator is running. The furnace literally used to shake and rattle with the fan running at high speed all the time - and now, with a blower wheel scrubbed clean (and probably closer to factory balance) and the Evergreen IM motor - the furnace was now making less noise than the Venmar heat recovery ventilator - Amazing! Even at high speed the system was running much quieter than before.
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| Zoom into this spreadsheet to see pre and post Evergreen Installation Calculations |
The final step in the motor installation is to verify that all modes are working correctly, and very importantly - at the right speeds. It is absolutely critical that the ventilator speeds are well matched to the furnace operating modes - if you don't have enough airflow - especially with 80 Amps of electric heat in my case, or gas or fuel fired furnaces, you can quickly overheat the combustion or heating chambers, and hopefully your safety devices kick in and avoid damage. So - finish the installation with some Temperature Rise and TESP tests to compare to your previous blower speeds.
When I completed my checks - high speed on the new Evergreen Motor in 1/2 - 3/4 HP mode gave me an identical TESP static pressure drop across my fan coil - indicating identical airflow. My temperature rise / sensible heat calculation gave me an airflow of approximately 1520 cfm on the old system - which is a good match for my 3.75 ton thermopump, using 400 cfm per ton of heating / cooling - indicating that I require 1500 cfm minimum.
For the identical airflow in my system - 1500 cfm - my fan current dropped from 5.5 Amps to 3.75 Amps - a 31.5% efficiency gain.
I used the fan law to calculate my airflow when in electric heating mode - medium high speed on the Evergreen IM. The fan law uses your TESP measurement differences to calculate your airflow differences in two cases. In my case, my temperature rise when from 38.7 F with the old motor to 42.1 F with the new motor in medium high speed - an acceptable increase in my case, considering that my fan power drops even further to 3.06 Amps - a reduction of 44%.
Conclusions
The final stunning measurement is the new power consumption in low speed mode. The fan law gives me a low speed air circulation of 834 cfm - a little more than half the flow of high speed - for only 0.63 Amps - a reduction of 88.5% energy consumption for more than half the high speed airflow. Since my furnace runs most of the time in this mode - my savings should be very significant.
By my estimates of the duty cycles - 50% of the time the thermopump or emergency electric heat is running, and 50% of the time only my air exchanger is running - I calculate that I'll save just over $600/year in reduced electricity bills. That gives me a payback of less than a year for the motor and mounting bracket.
A few final observations - a good cleaning of the blower wheel has resulted in the elimination of vibration in the furnace - even at high speed. No more rattling and chattering noise, and no rattles coming through the ductwork - what a huge improvement. At low speed circulation mode - the flow of air in the house is now imperceptable - even at night when the house is silent - what an improvement. And being in the furnace / utility room is now much quieter as well. I have a small workshop there, the reduction in noise is very welcomed. So far - well worth the research time, and all in - about 3/4 of a Saturday to perform all the pre and post tests, and the installation itself.
Sources and Links
I hope you found this post useful. Feel free to ask questions in the comments section below. I answer all questions.
