What is going on with the flex watt?

Ok. I have a QT rack, it is two 20" strips of 4" flex watt wired in parallel. I added a second QT rack underneath. It is also wired with 20" strips of 4" flex watt. This one is NOT wired in parallel. Each strip is wired separately. All three plugs are plugged directly into a thermostat. My bottom shelf is running 92, and the top two are 95 and 96. The bottom one can be a bit off due to it being on the floor and being cooler. The 2nd shelf up concerns me. It is running at 88. What the heck is going on? All same length, size and wattage. IF anything I thought the two wired together would be cooler. Its not the plug in on the thermostat, because i have switched them all around and get the same results. I re crimped the connectors so I don't think its a bad connection. Could it be the power cord? what the heck is going on?

are they plugged into the same thermostat? I don't really follow what you are saying, exactly, about your wiring.

Just remember, wired in parallel means all paths have the same voltage. Wired in series means that all fw has the same current.

Think about it this way:

In your house you have two ceiling lights wired to the same switch. If they are in parallel they will always be the same brightness when you turn on the switch. Put them in series and the second blub is not as bright because the first bulb had a voltage drop across it.

Sounds kind of like what you are seeing.

Strip A and B are wired in parallel, and wired to a plug which goes to the thermostat.

Strip C is wired to a plug that goes to the SAME thermostat.

Strip D is wired to a plug that goes to the SAME thermostat.

The thermostat has 3 plug in places, and one temp probe.

All of the heat tape is the same length and width.

Strips A B and D are all fairly consistant. Strip C is running about 6 or 7 degrees cooler.

Why?

Could brand of power cord make a difference?

I don't know much about flex watt but I do hear of it failing sometimes, maybe just bad flex watt? I'm not sure though, if I read correctly there seems to be no reason for a 6-7 deg. difference. I would suggest maybe just changing out that strip, well thats what I would do anyway. --Josh

If the flexwatt was bought at different times, that could be your problem. Flexwatt is made with an ink based heating element, and the manufacturer will allow variances in the density of the applied ink. That could cause heat variances among the flexwatt. That's why it's best to order all your flex at one time if it's going to be running together rather than on individual thermostats.

What are you using to measure temps?

It all came off the same strip, and i am measuring temps with a digital thermo...actually many many digital thermos, and a temp gun, but the guns dont work so well on the tape.

Sounds like a bad section of flexwatt.
A few broken traces could do it and it would be hard to see. Also solder your connections. A bad connection could also cause this.