Wiring a garage heater for "fan only"

[Brad Sperr]

I have a 5000W 240V Dimplex garage heater that I will be installing and controlling with an external thermostat. I'll be bypassing the heater's internal thermostat and would also like to wire up a separate relay to control the heater's fan with the manual fan control on the thermostat (for air circulation on hot days). The way the heater operates normally is that one leg of the heater coil and fan are unswitched. The internal thermostat switches the other leg of the heater coil and a thermodisc switches the other leg of the fan once the heater coil reaches 135F (shown in the second picture). A capacitor (shown in the first picture) keeps the fan running after the thermostat turns off the heater coil to dissipate the heat. In the picture of the capacitor below, the blue wire (Line 1) is unswitched and the red wire (Line 2) is the one switched by the thermodisc. Would there be any issue with me wiring a separate relay for the thermostat's manual fan control into Line 2 after the thermodisc to achieve a "fan only" mode? Of course the fan would continue to run a bit even after I switched it off because of the capacitor, but would I have any other problems? Thanks.

[David L Morse]

IF this
CUH05B31TWiring.jpg
is the wiring diagram for your heater then it's not really simple. You will either need a thermostat rated for 240V at the rated current of the heater or a 24V transformer, 24V thermostat and relays. It appears that the blower is tied to the heater except for delay operation so you will need a spdt relay to make it run by itself from the fan only switch on a typical 24V thermostat.

[John Lanciani]

The capacitor is not what is keeping the fan motor running, the therodisc in your second picture is. There are two sets of contacts in there, one for the fan and one for the heater. If the wiring diagram that David posted is correct, you just need to jumper around the "fan delay" with a switch.

[Brad Sperr]

The capacitor is not what is keeping the fan motor running, the therodisc in your second picture is. There are two sets of contacts in there, one for the fan and one for the heater. If the wiring diagram that David posted is correct, you just need to jumper around the "fan delay" with a switch.

That's the right heater, and that wiring diagram clears some things up for me. You're definitely right about the thermodisc controlling the fan delay. What does the capacitor do in this arrangement? I'll be using a 24V heat only thermostat with a transformer and 2 relays as David mentioned. Relay 1 will be on the W terminal of the thermostat and Relay 2 will be on the G terminal. Does this wiring diagram (overly simplified of course) make sense?

[John Lanciani]

The capacitor is to get the motor turning in the right direction when it starts.

[Brad Sperr]

Looking at that diagram again, it looks like I may have to use some kind of DPDT relay to control the fan without powering the heater coil. Does this diagram make more sense?

[David L Morse]

Looking at that diagram again, it looks like I may have to use some kind of DPDT relay to control the fan without powering the heater coil. Does this diagram make more sense?

If what you mean is this
CUH05B31TWiring_2.jpg
then yes. The DPDT could be a SPDT although DPDT might be easier to find.

There's something about the way the fan delay thermodisc is diagrammed that's doesn't seem quite right but using a DT relay should make that a non-issue.

[Brad Sperr]

There's something about the way the fan delay thermodisc is diagrammed that's doesn't seem quite right

It doesn't make sense to me either. It seems like the common terminal is labeled wrong or else it wouldn't close the circuit for the fan when it heats up and keep it closed until it cools down.

[David L Morse]

It doesn't make sense to me either. It seems like the common terminal is labeled wrong or else it wouldn't close the circuit for the fan when it heats up and keep it closed until it cools down.

Exactly. It seems like the common terminal should be connected to the fan.

It looks like you've already spent some time tearing it apart so maybe a few more minutes with an ohmmeter would show us how it's really connected. A heat gun or hair dryer should be enough to cycle the thermodisc.

[Bruce Wrenn]

The capacitor is to get the motor turning in the right direction when it starts.

This is actually a run capacitor instead of a start capacitor.

[David L Morse]

After further thought I think you should wire the thermostat relay so that it replaces the original thermostat. The fan delay may be needed to keep the residual heat from overheating the fixture. This could be a safety issue so I strongly discourage disabling the fan delay feature.
CUH05B31TWiring_3.jpg

[Rollie Meyers]

There is a difference between can one do it & should one do it, if it is a UL or recognized testing lab listed item it is not a good idea to modify the factory wiring unless your willing to accept the liability, or pay the big bucks for a field examination by the testing lab. Just my opinion....

[Brad Sperr]

Just an update in case anyone else wanted to try this. I got the heater wired up with an external thermostat with independent fan control. Here is the basic wiring diagram and pictures of the heater, control box, and thermostat. The fan delay should still be functional in this arrangement as long as the thermodisc is within the right temperature differential. I would definitely recommend an external thermostat. This thermostat in particular has a very low set point, like around 35F, so it can be used for freeze protection in my garage. In hindsight, the independent fan control was not worth the trouble because the fan on this heater is not really powerful enough for air circulation, but it was a neat project to work through anyway.

[David L Morse]

Just an update in case anyone else wanted to try this. I got the heater wired up with an external thermostat with independent fan control. Here is the basic wiring diagram and pictures of the heater, control box, and thermostat. The fan delay should still be functional in this arrangement as long as the thermodisc is within the right temperature differential. I would definitely recommend an external thermostat. This thermostat in particular has a very low set point, like around 35F, so it can be used for freeze protection in my garage. In hindsight, the independent fan control was not worth the trouble because the fan on this heater is not really powerful enough for air circulation, but it was a neat project to work through anyway.

I see you changed the wiring diagram of the Fan Delay Thermodisc. That certainly seems to be the only way it could work. Did you actually verify that the manufacturer's schematic was wrong?

Oh, and nice looking installation!

[Brad Sperr]

I see you changed the wiring diagram of the Fan Delay Thermodisc. That certainly seems to be the only way it could work. Did you actually verify that the manufacturer's schematic was wrong?

Oh, and nice looking installation!

Thanks David. I meant to test the fan delay while I had the heater disassembled, but I got in a hurry to get this project finished. Everything on the heater seems to be working though. I'm going to email the manufacturer about the diagram and see if they'll acknowledge it's a mistake.