HVAC Electrical Question ?

Question

I just started taking a course on HVAC. I am doing pretty good so far, but one thing is confusing me. When checking for resistance there is measurable, infinite and zero. What kind of resistance do you want on a properly running system and what kind is bad, that you do not want on a system.

Answer 1

"What kind of resistance do you want" depends on what and where you are measuring.

A plain old wire with nothing connected to it should have zero ohms from one end to the other, and infinite resistance to any other wires. If you measure something else, that means something is broken.

A relay without power applied to it, the NC contacts should read zero, the NO contacts should read infinite. Again, anything else is an error.

The coil resistance should read a discrete value, like 100 ohms. If it reads zero or infinite, it is shorted or open.

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Answer 2

I'm a mechanical engineer and I have no experience with HVAC. However, I do know that when looking at power relay terminals on compressors, an infinite resistance means the contacts are open and need to be replaced. Ideally, between terminals the resistance should be zero. This MAY be applicable to your question.

If not, please check out

www.hvac-talk.com

they should definitely be able to help you out.

Answer 3

Hey I am student of electrical engineering, and I think maybe you are a little confuse whit this, when it talk about this measurement, that doesn´t mean that those are different kind of resisitance wheter that is the condition in which this resistance are measure.

Answer 4

they use resistance measurement for troubleshooting. zero means there is a short somewhere, infinity means open circuit or good insulation. i suggest to purchase a cheap digital meter from radio shack that able to measure 3-8 ohms so you can differentiate between a short circuit and the resistance reading of heating elements or motors.