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Electric Furnace Testing??


KY Ted
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Someone told me you can check the status of a electric furnace with a multimeter by grounding it to the floor and placing the red lead on different area on the cabinet. He also said it would tell (I guess) that it either is/or is not getting a complete circuit. Have you guys ever heard of this?

Thanks!

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KY Ted,

No, Never heard that one.

I admit that I have only worked on a hand full of electric furnaces. But, wouldn't that mean the first barefooted kid that comes by and touches the cabinet could be electrocuted? Seems if you find that situation, you better write it up right after you have killed the power to it.

Are you also saying the elements of the furnace will heat with an "incomplete" circuit?

George

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KY Ted stated:

“Someone told me you can check the status of a electric furnace with a multimeter by grounding it to the floor and placing the red lead on different area on the cabinet. He also said it would tell (I guess) that it either is/or is not getting a complete circuit. Have you guys ever heard of this?

Thanks!â€

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The other "only way" to test these is to get into the electric panel, which you would do anyhow, and put an ammeter on a furnace lead. Simply report the amp draw - 0= not working at all, 20 amp = one element working, 40 amp= 2 elements working, etc. Looking at the breaker will give you an indication of how many elements there are, given that it will be oversized by 10-20 amps over the nearest multiple of 20 amps per element. A 60amp breaker is for 2 elements, e.g.) This method is for us guys who don't want to even look like we're "dismantling" equipment (per ASHI SOP), and wouldn't want to set a precedent by getting into furnace cabinets.

-David Lee

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Originally posted by dlee

SNIP. This method is for us guys who don't want to even look like we're "dismantling" equipment (per ASHI SOP), and wouldn't want to set a precedent by getting into furnace cabinets.

-David Lee

David,

Good point, perhaps this is what KY Ted is trying to do. Do you check the temperature rise? If so, would that not give you sufficient information for a home inspection?. You know, tizz working, or taint working.

George

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Originally posted by a46geo

Originally posted by dlee

SNIP. This method is for us guys who don't want to even look like we're "dismantling" equipment (per ASHI SOP), and wouldn't want to set a precedent by getting into furnace cabinets.

-David Lee

David,

Good point, perhaps this is what KY Ted is trying to do. Do you check the temperature rise? If so, would that not give you sufficient information for a home inspection?. You know, tizz working, or taint working.

George

Sure. And that's the basics. But I find it easy enough to go this bit further and describe the situation where I only get 20 amps draw, and the breaker is a 60 amp. I don't know what's wrong, but that's enough to say that either one's there and not coming on, or the breaker is oversized. The real limitation of this method comes with newer heat pump auxiliaries - which sometimes have a limit switch that only allows one element to come on if the ambient temp is moderate. If I suspect that, then I just waffle: heater came one, or didn't come on.

-David Lee

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OK,

WHAT IF:

The thermostat was set high enough to bring on all the heating stages? Could you then compare the temperature rise with a rating on the mfg. plate?

I guess my question is; on an electric furnace, why can't inspectors determine all that we need to know with just a temperature rise?

Say for example the manufacturer says a full load TR is 70 degrees and we measure 50. Then we could write it up just like that along with a call for a professional evaluation?

George

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Originally posted by a46geo

OK,

WHAT IF:

The thermostat was set high enough to bring on all the heating stages? Could you then compare the temperature rise with a rating on the mfg. plate?

I guess my question is; on an electric furnace, why can't inspectors determine all that we need to know with just a temperature rise?

Say for example the manufacturer says a full load TR is 70 degrees and we measure 50. Then we could write it up just like that along with a call for a professional evaluation?

George

It’s a personal choice but I always like to pull access doors off and have a look see. I also like to check amps on electric resistance heating coils to make sure they’re firing. It doesn’t really take that much extra time once you have the inspection covers off.

Your mileage may vary.

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KY Ted, the method you describe is more useful for troubleshooting an electric furnace that has a problem than it is for determining if it’s working properly in the first place. It won’t tell you if current is flowing through a coil, for instance. All you need for a thorough inspection of an electric furnace is a clamp-on ammeter and a multimeter (or a combination meter). It’s easy to test them but it’s also easy to electrocute yourself or cause damage to the equipment. No offense, but based on your limited understanding of these beasts, I’d suggest that you limit yourself to a visual inspection of the furnace and a reading of the TD. If you want to go further, find a friendly HVAC tech who’s willing to have you tag along with him for a few days. Buy him some nice lunches, carry the heavy stuff and you’ll learn a lot.

David, taking the amp draw at the distribution panel can work, but it has limited value. You don’t get that hands-on gratification that you get from working directly with the equipment. (Kind of like phone sex.) Personally, I don’t see a lot of difference in risk between opening a distribution panel and opening an electrical compartment in a furnace cabinet. The real limitation of this method is that, when you find fewer coils firing than you’d expect, you don’t know if it’s by design or if something’s broken. If you’re in the furnace cabinet, 30 seconds of probing with a multimeter will tell the answer and you can save your client an unnecessary $80 service call.

George, measuring the rise is certainly an option. It’s probably the most sensible protocol for those who’re uncomfortable with working around live equipment or who don’t want to risk exceeding the standards by too much. However, setting the stat high will not always bring on all the coils, so you won’t necessarily get the max rise as stated on the data plate. In most modern heat pump systems, for example, the electric elements are controlled not only by the stat but by an outdoor thermostat which limits the number of coils that will fire depending on the outdoor temperature. On a day with moderate outdoor temperatures, you’ll find that at least one coil doesn’t work. Without access to the electrical compartment, you have no way of checking to see if the coil is receiving power, or of checking the coil’s continuity. The Rheem/Ruud line uses a watt restrictor instead of an outdoor thermostat. This reads the outgoing air temp and throttles back the heat strips accordingly. From the cabinet, you can see all of this.

Personally, I like to open the cabinet, see the toasty wires, determine exactly what is and isn’t working and give my client a full & meaningful description of the furnace’s condition. However, that certainly isn’t required by any of the home inspection standards I’ve seen. A careful visual inspection along with a notation of the temperature rise easily satisfies the ASHI & NAHI standards for the more demure among us.

- Jim Katen, Oregon

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Jim,

Thanks for the info. I have not seen an electric furnace since the late 70's. I don't believe I ever saw one used with a heat pump. The few heat pumps that I have worked on were installed with gas furnace back up. They too used outdoor sensors, but the sensors were easily jumped out. Its funny, just one state away from me electric furnaces are at least semi-common. Must be a public service commission/political/thing.

George

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