Checking on Voltages in a furnace

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Old 12-05-15, 02:55 PM
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Checking on Voltages in a furnace

PJ Max provides consistently excellent advice on this forum.

However, I have a different method of checking on 24 VAC switches and connections in furnaces that I find simpler and less confusing.

PJ Max recommends:

<<The way to check a pressure switch is to connect your leads to the two terminals. When the furnace is not calling for heat (inducer off) you should measure 24vac. Once the inducer starts and is running you should measure 0 volts.>>


With this method, you measure 24 VAC when the switch is open, and zero volts when the switch is closed.


The method I prefer involves connecting one test lead of the AC voltmeter to the furnace sheet metal (ground). The voltmeter is set to AC and to a voltage range above 24 VAC ---- such as 50 volts or whatever.

Then, you can easily "walk the circuit" from point to point within the furnace and is there is 24 VAC to that point, your meter will read 24 VAC (or whatever)

So you can check the AC voltage at the R terminal, then the W terminal, then the voltage supply side of the pressure switch and the other side of the pressure switch, which would show 24 VAC when the pressure switch is closed.

I find this simpler and faster to do than connecting two wires to the pressure switch to determine if the pressure switch is closed.

Similarly, it's easy to check the status of the high temperature limit switch by measuring the input and output voltages rather than connecting test leads to both sides of that switch.

PJ Max's method works. I simply suggest the method above as being simpler and less confusing.
 
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Old 12-05-15, 10:25 PM
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Thanks SP.

The reason I don't usually suggest measuring to ground is because not every furnace uses a grounded C connection.
 
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Old 12-07-15, 03:51 PM
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<<The reason I don't usually suggest measuring to ground is because not every furnace uses a grounded C connection.>>


Heh, heh! They don't?

Milliviolt powered systems, I suppose.

It's up to you, of course. Your method works.

Strictly speaking, I'd say you ought to disconnect at least one of the wires to a part being tested, since it otherwise you might get a bad read.

Thought I'd mention it.

People are lucky to have your consistently excellent advice!
 
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Old 12-07-15, 04:03 PM
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In particular, your collection of pictures and illustrations are valuable in posts. 1 being = to 1000 words, as they say.

I haven't had the patience to do that. Posting links to manuals from time to time is as far as I go by and large.
 
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