I have a coleman eb15b unit in my home. I first used the unit last night and noticed the heat eminating from the vent was only slightly warm. Today I tested the elements to see if this could be the problem. The unit has three elements at 4800 watts each. I wanted to check the resistance of each coil but it did not state the resistance. Had to envoke the laws of Ohm... lol. Using the voltage at 240 and the wattage of 4800 I calculated that each should be 12 ohms. Two of them are at 11.6 ohm which I am assuming is within tolerance. The other is at 8 ohms. I know that this is effecting the efficiency and heat output of the unit. But is it a large heating loss or only minimal and I should look else where? As the elements are $100 and up I just don't want to start putting money into something that is not the main culprit.

Without knowing the amperage each element is drawing, you cannot determime the resistance.

Ohms law Wattage/Voltage = Amperage
W/E = I

Each of the elements is rated for 4800 watts at 240 v

W =4800
E = 240

4800/240 = 20

Current = 20

Giving me the current at 20 amps

Voltage/Current = Resistance

E/I = R

240/20 = 12

Resistance = 12

Actually I could skip the two steps and get it directly if I used Ohms law of E(squared)/W = R

I understand Ohm's law but you are making the assumption that all the elements are being energized. On most electric heat packages, each of the coils has a limit switch which, if open, will not allow that strip to pull current. There are also sequencers which may not be closing. The easiest way to check is with a volt meter & clamp on ammeter. This will tell you real world instead of theoretical.

BTW, it is not uncommon for a heat strip package to have two 5 KW & one 2 or 3 KW coils. By using two 5KW coils & a 2.5KW it leaves enough available capacity through a 60 amp circuit to run the controls & blower. That combination is quite common since it saves having to pull another circuit just for the blower section.

Quote:

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Originally Posted by Grady White

I understand Ohm's law but you are making the assumption that all the elements are being energized. On most electric heat packages, each of the coils has a limit switch which, if open, will not allow that strip to pull current. There are also sequencers which may not be closing. The easiest way to check is with a volt meter & clamp on ammeter. This will tell you real world instead of theoretical.

BTW, it is not uncommon for a heat strip package to have two 5 KW & one 2 or 3 KW coils. By using two 5KW coils & a 2.5KW it leaves enough available capacity thru a 60 amp circuit to run the controls & blower. That combination is quite common since it saves having to pull another circuit just for the blower section.

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Okay got what your saying now. I have checked the thermal limiters and they are all conducting, while cool at least. The sequencers I have not checked. I don't have a clamp on so that limits me on my reading of the amperage. My meter will read, inline, up to 20A, but that should be around the amount of current the circuit is using. Not taking into account that it could be drawing more. So I don't think I should use that meter since its max is at the normal amperage of the circuit. Probably would blow the fuse in it on the inrush current at start-up. I will have to find if a friend of mine has a clamp on.

As far as the element watt ratings I get what you are saying that they could be different and I concur. I should have mentioned that I looked the circuit diagram up for the heater and the coil packs. The top coil pack is a dual coil and both are rated at 4.8kw totaling 9.6kw for the whole pack. The bottom pack is a single coil and is rated at 4.8kw. That is where I derived the "assumption", lol, that all three coils should be at the same resistance. The top coil in the top pack is only at 8 and the second at 11.8, bottom coil 11.8 readings in ohm. I was again assuming that since all are rated at the same kw and voltage that they should be equal in ohms. Probably didn't explain what info I had researched well enough. Sorry about that.

I didn't even check the sequencers. Totally slipped my mind, ahhh 3 year old twins, lol. Thanks for reminding me of those. I will do a power "on" check for the circuit, as well as I can, without the clamp on. Could be a sequencer failure where a coil isn't even being energized. Just saw that 1/3 loss on that coil and my mind locked on it. Thanks for all your help though it is greatly appreciated.

It is certainly easy enough to get tunnel vision. I've done it myself more than once. It does puzzle me as to why, if that single coil pack is marked 4800, why it reads a different resistance than does the either coil of the double.

I just checked the resistance of the sequencers. There are two on is a double layer the other is a single. The double goes to the top two coil pack and the single goes to the bottom single coil pack. The service manual says that the coils should be 70 to 90 ohms. On the double sequencer I'm reading 70 ohms, however on the single I am getting 130 ohms.

I started the unit and the double sequencer turns on within 20 seconds, checking both I see that both coils are supplied with voltage. The single within 120 seconds, supplying heater coil voltage. The manual says that both are set to activate within 110 seconds. I have the thermostat set at 90 with a current temperature of 74 in the house. I'm assuming with the higher ohms of the single it is causing the 120 second delay. I can't get the current across the coils yet as I don't have the clamp on meter. So I will have to wait until I get the clamp on meter to check the coils a little better.

Quote:

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Originally Posted by Grady White

It is certainly easy enough to get tunnel vision. I've done it myself more than once. It does puzzle me as to why, if that single coil pack is marked 4800, why it reads a different resistance than does the either coil of the double.

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This is what I see




|-------------------------------------|
| * Top coil 8 ohms * |
| | <------- Top coil pack with two coils 9.6kw
| *Bottom coil 11.6 ohms* |
|_______________________|

|------------------------------------|
| |
| *Single coil 11.6 ohms * | <------- Bottom coil pack of one coil 4.8kw
| |
|------------------------------------|

So its actually the top coil in the top pack. The double stack sequencer controls this one and activates both at the same time in around 20 seconds The single is activated by the single sequencer within about 120 seconds. The squares I drew are representing the plates that are holding the coils. Best I could anways. They also have the thermal trips for each coil mounted to these plates. All of the thermal trips check OK when coil and are testing OK "while heating" after 5 minutes still conducting and not tripped.

Hope this helps a little better with what I'm seeing. I can take a picture if that would help?

I'm really suspicious of that upper most coil. Is there any difference in appearance such as turns being fewer or further apart? Either would indicate it is shorter thus giving it less resistance.

The top and the bottom coil on the plate appear to be duplicate in size. They are following the same pattern back and forth and the coil count and size appear to be the same as well. Physically they appear identical. Besides the top appears marginally a little darker than the bottom. Otherwise closely identical.

Im going to jump in with my (most likely) worthless two cents.

Take an accurate thermometer and take a reading at the return grill of the furnace. Then take a reading of the supply air. Looking for temp rise here. The furnace may be working fine, but blower speeds, duct sizing, number of supply and returns, and proper sizing of unit play a big factor into correct operation. Also, many times the air coming out will feel only luke warm, but may be well above a hundred degrees, just feels cooler do to the actual air flow. There should be some sort of temperature rise information on the mfg plate, or in the furnace literature. See if it is within range. If it is, I say you may be looking to deeply into the electronics. If its not, you are on the exactly right track by checking all aspects of the electrical components.

This one has me up against a wall. I'd like to be able to get my hands on it. Once you get that clamp on & can measure amperages maybe we can figure out why that one coil has low resistance. Please keep me up to speed on anything you learn.

Thanks for all the help. Wanted to just do a check up on the system to make sure it is running at it's most efficient for the up coming winter. Defiantly don't need a huge electric bill for heating since we just moved in a couple of months ago I have no idea what to expect from it. Just seemed a little slow just moving up 2 degrees and the vents felt only luke warm as stated. I will get the clamp on when I can and post the current draws. Think I'm kind of at a stopping point with it otherwise. I got to order the temp probe to get the temperature difference between the two. My meter does have a temp setting but I don't have the probe yet.