HVAC Overloads and Safety Switches Don't Just Fail
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- čas přidán 26. 02. 2022
- Bryan teaches the Kalos technicians Overloads and Safety Switches, including float switches and thermal overloads, and why they Don't Just Fail.
A float switch is wired into the thermostat wiring to kill power to the system if the drain system begins overflowing. The switch opens to create an incomplete circuit, similar to how an open drawbridge stops traffic. Float switches are normally closed, and they open when they are filled with enough water.
When a float switch opens, the first thing you should do is check to see if there is water inside; you want to see WHY the switch is open, not just replace the switch. If there is NO water in the open float switch, then you’ll want to check for a path with an ohmmeter. If there is no path, then it’s a failed switch. However, if a switch is failed open, it’s likely that it has been opened and closed many times in the past.
Thermal limits can sometimes be called overloads as well. Those safeties open when a component overheats, such as a surge protector. Modern surge protectors use normally closed thermal limits that open when the surge protector gets too hot. High-limit and rollout switches on furnaces as well as thermal overloads on compressors and heat strips are thermal limit switches. Fuses work similarly, but they break completely instead of merely opening like a switch. Again, make sure you figure out why thermal switches opened instead of just replacing them.
In furnace systems, poor airflow can cause the high-limit switch to open. Dirty coils, filters, and blowers are potential causes of thermal switches opening. A compressor could go out on thermal overload due to a failed fan motor. However, sometimes, thermal switches reset themselves quickly due to electric overheating on the windings. If you replace the capacitor and correct the locked rotor condition, then you won’t have the thermal overload anymore.
Compressors also get too hot when there’s insufficient or way too much refrigerant moving through the compressor. When the compression ratio is high, a compressor will run hotter than it should (especially due to low suction pressure and high heat pressure). Systems with long runtimes and excessive superheat can also cause compressors to overheat. All sorts of restrictions can also cause overheating. In those cases, the compressor should be cooled with water from a hose.
To tell if a thermal switch has been reset, put your meter on the ohm scale with the ringer on and put the leads on your contactor across the two legs. Generally, the condenser fan motor will have high enough resistance to prevent your meter from ringing, but the compressor has low enough resistance to make the meter ring.
Low and high-pressure switches are also safeties that look similar on schematics. The high-pressure switch opens when the pressure increases (the line is above the right circle on the schematic), and the low-pressure switch opens when the pressure decreases (the line is below the right circle on the schematic).
If a pressure switch is stuck open, check the conditions that may have made it open (such as undercharge). Jump out the low-pressure switch and see why it went out on low pressure. Do the same to the high-pressure switch if it has failed (possibly due to failed condenser fan motor in cool mode, failed blower in heat mode, poor airflow in heat mode, or overcharge).
In all cases where you encounter open switches, the most important thing you can do is find out WHY those switches opened.
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this video talks to my soul. unfortunately i have a colleague who is an old timer whos been in business for 50 years and 20 years in this trade and still doesnt know refrigeration basics and he just replaces the entire unit if he has to so a 5minutes job can take up to 5 days instead
Great explanation. No one tech us that very well in the field. Great job as always 👍👌
Up here in Canada an AC manual reset high side pressure safety switch can trip over winter. Not a failure of the system operation but due to temperature fluctuations over winter while the system is not in use. A quick reset and the system will run fine all summer.
Siempre con muy buena informacion para quienes estamos empezando en la Industria 💯👍
Great lessons. Thanks from Western Australia
Awesome class thanks for posting these!
Great video. Thanks Bryan
Great video Bryan!
Awesome class thank you
Air at the circulating pump can result in the flow switch opening because the circulator cannot produce water flow if it has air at the impeller.
Great video
If it is a chiller and you bypass the safety flow switch it can result in freezing & breaking the evaporator if other safeties don’t stop the compressor. The low pressure cut out safety switch should stop the compressor if it responds quickly enough to prevent the pipes from freezing. Big compressors may temporarily bypass the low pressure switch on start up and if that bypass stays closed, the low pressure control will not work to open the circuit. This would be a disaster.
this is fun. I should of been an A/C guy. what about heater boards dirty with dead lizards.
I have come across failed open float switch’s due to .. low voltage short .. float switch is rated at 1.25 amp max in a 3 or 5 amp circuit..
Yes agree
I want that shirt, when you gonna have that for sale?
👍
can we bypass any of these switch
For troubleshooting purposes, sure. Never leave a safety bypassed. It’s a safety for a reason.
If you know the switch and bad and don’t have a new one on you, sure. Then follow back with a switch when you get it.
I would also tell the customer what you are doing to the system. Some customer may want you to wait to get part.
@@d3adp94 tbh I would be that customer. Why risk system/house/life when the part is likely in stock just not on the truck? I probably already waited longer than that for one o' y'all to come out in the first place.
@@d3adp94 No... You never bypass a high pressure switch, float switch, any high limit on a furnace or a vaccum switch furnace. Period.
@@_n8787 if you know a low or high pressure switch is bad and not actually going out on either high or low. Why not. It’s 90 degrees outside and the house is 80. You might consider it. Lennox is known for this. I would never by pass a high limit or float or roll out. Condenser, I would.
YA THEY DO JUST FAIL. TODAYS SWITCHES ARE MADE TO A MINIMUM SPEC IN. CHINA. AND THEY DO JUST FAIL
Agree, have run into plenty switches that are defective. The new gas valves Rheem and Richmond use have pilot safety circuits that just fail “ bad valves”
Ya rarely will a manual reset go bad but the thermal limits can be somewhat common I change 5-10 every winter and the new style high and low pressure switches are a joke they go bad quite often
Fundamentals are fun!
Thanks again for the videos!
🍺🍺🍺🥃🥃⛳🎳
Stay safe.
Retired (werk'n)keyboard super tech. Wear your safety glasses!