- Tech Tips
We had a really great conversation on the HVAC School Facebook Group about some belt tension best practices and it turns out that even a lot of really smart and experienced techs are not aware of all the factors related to belt tensioning.
Myth #1 is that amperage is used to set belt tension. Now don’t get me wrong, checking amperage before and after changing belt tension is an excellent practice to ensure you are not binding the bearings from over tension, it does not tell you whether or not the belt is at optimum tension.
I think Browning summarizes it best in this statement from their Browning tool box technician app
Ideal tension is the lowest tension at which the belt will not slip under peak load conditions
Getting a belt too tight shortens the life of the belt and bearings and can cause high amperage. Leaving a belt too loose will shorten the belt life and result in loss of airflow and noise.
Many techs confuse the sheave adjustment, designed to alter the pulley ratio and the airflow with the belt tension adjustment. These are not the same thing and serve separate purposes.
The adjustable sheave allows the pulley faces to adjust closer or further from one another, resulting in a belt that rides closer to the hub when looser (halves further apart) or closer to the edge when tighter (halves further separated) THIS ADJUSTMENT IS FOR FAN SPEED ONLY NOT TENSIONING
A properly tensioned belt should not slip significantly when starting, it should not be noisy and it should not bounce around. If you tighten the belt check the amps before and after and the motor should not overamp.
The correct tension method is to get the belt close to the correct tension by feel with a deflection of 1/64 of an inch for every 1″ of distance between the two pulley centers. You can then use an app or a chart like THIS ONE to find the proper force to generate this deflection.
You would then use a belt deflection tool like the one shown above to test the deflection force required and adjust accordingly. The video below demonstrates this.
I like what Jeremy Smith stated in the group “Belt tension has less impact on motor amperage than pitch diameter of the sheave and how that affects total airflow. Use the Emerson tool and the app (or paper chart if you’re all stone age) Record tension and other data (sheave diameter, center to center length, rpm and proper tension) on the blower housing.”
Check those belts during commissioning, maintenance and service and change them as needed.
Depending on what segment of the business you are in and what area of the country you work, you either work on pump down solenoid systems all the time or YOU HAVE NO CLUE what they are.
A liquid line solenoid is just a valve that opens and closes, it has a magnetic coil and depending on whether the valve is normally open or normally closed it opens or closes when the coil is energized.
If you work on refrigeration or straight cool units up north, you are likely very well acquainted with “pump down” solenoids. If you do residential HVAC in the south you may have never seen one.
You know that you pump down a system by closing the liquid line? That’s all a pump down solenoid does. It closes when the system is running causing the system to pump all of the refrigerant into the condenser and receiver (if there is one).
The trick is that in order for it pump down the compressor needs to be running and then it needs to SHUT OFF once it is done pumping down. This means to need a good, quality, properly set low-pressure switch near the compressor to shut it off when the suction pressure gets low enough but not TOO low. The goal is to get all the liquid pumped into the condenser not to pump down to zero.
There are a few benefits of a pump down solenoid. First, it helps prevent liquid refrigerant migration down the suction line into the compressor. When liquid refrigerant migrates into the compressor it dilutes the oil and can cause a “flooded” start.
The other cool thing is you don’t need any Low voltage controls between the indoor and outdoor unit (in some cases). The solenoid is in the liquid line near the air handler inside, so by opening the valve the suction pressure increases and the compressor turns on and when it closes the compressor pumps down and shuts off.
Obviously, this would not work on a heat pump system because in heat mode it would attempt to pump down into the indoor coil which would not work. They also won’t work in most cases when you have complex or proprietary controls.
In some cases the liquid line solenoid is not used to “pump down”, it simply closes during the off cycle preventing refrigerant flow and migration in that way.
So… there are places where a liquid line solenoids make sense and applications where they don’t but they are fairly simple and easy to understand.
When we teach electricity to new techs we use a lot of “water” metaphors. We talk about volts like PSI, amps like flow and Watts like GPM. Even the word “flow” gives us a vision of water moving.
Then we talk switches and circuits and we say “open” to mean no path / no flow and “closed” to mean a path or flow.
That’s the opposite of water…
With water, we “open” the tap when we want flow and close it to stop the flow.
With a switch, we “close” when we make a circuit and we “open” when we break a circuit.
Someone pointed out to me that describing an “open” switch or circuit like a drawbridge may be better. Cars (electrons) can move when the bridge is closed and cannot move when the bridge is open.
It struck me that this water metaphor may be one reason newbies struggle to grasp relays.
Or maybe I’m just overthinking it.