[Moto Roto]

I've set up a 240V heater in my garage but due to space restrictions it's mounted on a shelf about 7' above the floor. To keep the air circulating I've added a shroud to the intake of the heater with a 6" round duct to the floor for a cold air pick up.

The heater's element seems to be running too warm so I'd like to add an auxilary fan in the 6" duct to improve air flow while the heater is running.

Can I use a 4 wire stat to control the 240V heater with 2/4 wires and a 120v fan in the duct with the other 2/4 wires? I believe the stat is a double pole unit, something I will double check with my volt meter, meaning it basically has two circuits/switches that are opened & closed by a single relay. It's a Honeywell stat #CT410B

Please confirm or correct me :)

Thank you.

[Moto Roto]

I think I just answered my question. Sorry I should have check this out before posting.

As long as the stat is set within the temp range it controls one line and the second line stays hot. The second line is opened only when the stat is turned to OFF.

Does anyone know of a stat I could use for my situation? Could I use a 24v stat with relays to close two circuits?

[KISS]

That 4 wire stat breaks both hot sides of the 240V which is what you should do. Probably the easiest thing to do is to use a 240 V/120 V transformer to run the fan. L1 and L2 go to the t-stat. Switched L1 and Switched L2 go to (heater L1)/(transformer L1) and (Heater L2)/(Transformer L2). The fan connects to the 120V terminals of the transfomer.

You can use a 24 VAC or the stat you have but the "relays" will be called contactors.

You didn't mention the size in VA or HP or watts of the fan. This can influence the route taken.

It says it's a DPST stat which is a double pole single throw stat or two SPST switches that operate together.

I don't really like large current contacts switching a very low current load. Everything you come up with will have advantages and disadvantages.

You have to be comfy putting controls together.

[Moto Roto]

Thank you for your help, I'm now making some headway.

Since I posted last I did come across some transformers that might work. It's good to have this confirmed.

Basically add the thermostat and then after the t-stat I would split the line, one to the heater and another to the transformer. Correct?

The heater is rated at 20A and 4800W. The stat is rated for 22A. The fan I'd be adding is 0.60A and 35W. The breaker for the 240V line is 30A. Is there any concern here?

Just to clarify. The thermostat only breaks one hot side during temperature control. The second hot side remains closed unless the t-stat is switched to the "Off" position breaking both hot sides. You had mentioned that both hot sides should be broken, I don't think this is what will happen during normal temp. control, any concern regarding that?

Also can you explain your concern regarding this comment.
"I don't really like large current contacts switching a very low current load. Everything you come up with will have advantages and disadvantages. "

[KISS]

917-8021 and 868-7502 will both work from Allied Electronics - Electronic Components Distribution.. The latter has an integral secondary fuse holder. They are both 50 VA.

Basically add the thermostat and then after the t-stat I would split the line, one to the heater and another to the transformer. Correct?

Yep.

The heater is rated at 20A and 4800W. The stat is rated for 22A. The fan I'd be adding is 0.60A and 35W. The breaker for the 240V line is 30A. Is there any concern here?

Nope. You have 2 * 220 Watts to spare.

Just to clarify. The thermostat only breaks one hot side during temperature control. The second hot side remains closed unless the t-stat is switched to the "Off" position breaking both hot sides. You had mentioned that both hot sides should be broken, I don't think this is what will happen during normal temp. control, any concern regarding that?

No concern here. The god part is that when it's turned off, the power is removed from both legs.


If the secondary of the transformer is fused, it should be fused at (1.5 or 2) * 50 VA/120 VA. I'd use a 1 A slow blow fuse.

"I don't really like large current contacts switching a very low current load. Everything you come up with will have advantages and disadvantages. "

There is a parameter called contact wetting current, that really doesn't apply here. Contacts can oxidize when they don't switch a minimum amount of current. This current is usually less than 0.010 amps.

Advantages/disadvantages
Cost - time and materials
Case
Ease of construction
Complexity
Programability
Ease of operation

Ease of operation. Usually these stats use the counterclockwise position to be off and every time you turn it on, you play with the knob.

When I was probably 12, I put a space heater with a 24 VAC thermostat and contacter under my workbench.

There is a real easy way of doing a professional job using "DIN terminals". Like here: http://www.alliedelec.com/Images/Pro...le_5020179.pdf

It would start with a rail, usually "T shaped". A End stop, 2 terminals + jumper bar, separator, 2 terminals+ jumper bar, an end section, 1 or 2 ground terminals, and another end stop.

This would easily do the splitting of wire to the heater and transformer.

It would take a few more terminals to make it neat and have places for everything to go in a clean and neat fashion. You basically make a box where you have t-stat in, Fan out, and heater out terminals and appropriate grounds in a well defined area in the box.

Enclosures with backing plates are usually used. The backing plate is an aluminum panel set off from the back of the box.

One alternative is wire nuts and pigtails.

[tkrussell]

This can be as simple as using the controlled leg of the stat to control the fan, only need a neutral for the 120 volt fan.

The double pole stat would control the heater, as normal. Since you now know that one switch of the stat closes as soon as it it turned On, and the other only closes when heat is needed, using the controlled pole of the stat to control the fan would cause the fan to come on when the stat needed heat, and the heat came on.

All you need to do is feed the stat with a 3 wire cable, two hots for the 240 volt, and a neutral for the fan motor.

You would only be adding the load of .6 amps to one leg of the 240 volt feeding the heater.

No relays, transformers, contactors, etc, etc, etc.

Can't get any simpler that this.

[KISS]

Tk:

I either disagree or don't understand. Let's hope it's the latter.

Let's describe the system with two switches:

Let K1 be the thermostatically controlled one with terminals K1c and K1a. K1c is connected to K1a when the t-stat calls for heat. K1c is the common terminal and is connected to line. K1a is the load terminal.

Let S1 be the switch that is off when the heater control is turned off. It has terminals S1c and S1a. S1c is connected to S1a when the knob is in any other position but off.
S1c is the common terminal and is connected to line. S1a is the load terminal.

The two terminals of the fan are Fan-T1 and Fan-T2. The Fan is 120V
The two terminals of the heater are Heater-T1 and Heater T2. The heater is 240V.

As I understand from the t-stat measurements.
Mode 1: OFF: K1 open, S1 open
Mode 2: ON: no heating call; K1 open, S1 closed
Mode 3: ON: heating call; K1 closed, S1 closed

Fan (Fan-T1) and one side of the heater (Heater-T1) is connected to K1a.
The other side of the heater (Heater-T2) is connected to S1a
The other side of the fan (Fan-T2) is connected to N
K1c is connected to L1
S1c is connected to L2

Mode 1: system is off
Mode 3: Fan and heater is on
I have no problems with this.

In mode 2:

S1a will be connected to L2 and (Heater-T1).
K1a will act as a junction between (Heater-T2) and (Fan-T1).
N will be connected to (Fan-T2)

Thus, in this mode, 120 VAC from L2 will be supplying the series combination of the heater and fan to Neutral.

Thus this is not Fan off, heater off when there is no heating call and the t-stat is set to a particular setpoint.

So, which is it. Am I wrong or did I misinterpret your answer?

[tkrussell]

Whew, you really don't follow your name.

Give me a break with all the K1 and s2 stuff, I am not going to read all of that, who can.

If I bring a 3 wire cable to feed a stat, I now have a neutral along with the two hot 240 volt feeds to LINE. The load side of the stat is 240 volts, each leg is 120 V to neutral.

The heat connects to the two loads for 240 volts. I also connect a 120 volt fan across the one controlled load leg and neutral, I will then turn on both heat at 240 volts and the fan at 120 volts with the same stat.

What is so hard about that?

[KISS]

Read carefully what Moto Roto stated:

"As long as the stat is set within the temp range it controls one line and the second line stays hot. The second line is opened only when the stat is turned to OFF."

Thus the stat operates as a thermostically-controlled SPST switch when set to a particular setpoint and a DPST disconnect in the "TURNED TO OFF" position. One side of the 240 V heater always has power unless the stat is "TURNED TO OFF". This is safe, but unexpected behavior. It's because of this PECULIAR behavior, your method won't work.

"TURNED TO OFF" is not the same as heater off.

If it was a thermostatically-controlled DPST switch, your method is fine.

[tkrussell]

He states:
Just to clarify. The thermostat only breaks one hot side during temperature control. The second hot side remains closed unless the t-stat is switched to the "Off" position breaking both hot sides.

This is the typical operation of a double pole stat, one switch closes as soon as turned on, the other opens on temp rise.

Connect the fan to the controlled leg.

Should I provide a wiring diagram?

[KISS]

Tk:

This is what I envision as the schematic. Follow the red lines. Is your schematic any different?

[tkrussell]

That will do it, and it will work just fine.

Cannot get any simpler that that.

Definitely subscribes to the "Kiss" method.

[KISS]

Isn't this a 120 V source powering the series combination of a 220 V heater and a 120 V fan in the state shown?

Converting to resistances, which I can't really do
Heater = 240 V/20 A = 12 ohms
Fan = 120 V /0.6 A = 200 ohms (Not an exact transformation. Really need winding resistance)

I = 120 / (200+12) = 0.56 A

This would leave (0.56)(200) or 112 V across the fan and
(0.56)(12) or 6.72 V Across the heater

Because I don't know the DC resistance of the motor and it's proabably a lot less than 200 ohms, the motor will burn up to protect the fuse (OCPD). I don't know what would happen if the motor was Impedance protected (able to operate stalled with no damage). This is typical of small fans used in equipment.

In any event, this analysis would show that the fan would run at a reduced voltage when it was supposed to be off if the loads were purely resistive.

[tkrussell]

You really make things harder that it is.

Ever see a dryer, with 5000 watt 240 volt heater and a 1/3 HP 120 volt motor?

Same thing.

To do resistance calcs, you do not add those that are in parallel.

Total resistance is calculated:

Rt= 1/ R1+R2+R3...

Total resistance or impedance will be lower than the smallest load.

So your voltage drop calcs are way off.

[KISS]

I give up! The loads are in series not parallel as drawn in RED in this particular switch state. HEATER OFF, CONTROL ON is the state described by the red lines.

TURNED OFF works properly. HEATER ON works properly.

I'll assume that there is already a fan in the heater and the 120 V fan is the AUX fan.

If the contactor is DPST, then the schematic works. In this case it is not.

You CANNOT interrupt just L1 because your backfeeding through the 220 V heating element and L2 (120 V with respect to Neutral)

BTW the formula for resistances in parallel is Rt = 1/ (1/R1+1/R2+... 1/Rn) which is usually written as 1/Rt = 1/R1 + 1/R2 +... 1/Rn


The transformer I suggested can be removed from consideration in one of two ways:
a) Use a DPST 240 V coil contactor activated by the t-stat.
b) Wire the 2 contacts of the t-stat in series to a 120 V coil DPST contactor.

Wire the motor and heater very similarly to the schematic, except K1 and S1 are the contactor's contacts. They both will open and close together and not independently. Both of these options involve pulling a neutral.

[tkrussell]

Ok I see what you mean now, sorry to be so dense, you are absolutely correct.

Good thing when you gave up you gave a better, shorter, explanation.

I wonder what the OP has done?

[parttime]

OP may have ducked for cover.lol

[tkrussell]

I hope not, only having a civil discussion here.

I do hope he either did as Kiss mentioned, or waited to see the outcome of my getting head out of butt on this one.

[parttime]

I was just kidding, everyone suffers from cranial rectal inversion at sometime. I think you give awesome assistance.

[KISS]

Tk, labman:
No harm done. I won't even give you a reddie. Even experts can be wrong sometimes. Teachers, in particular don't like to be corrected in front of the class. I remember the question "What wire is hot? White or black? Teach got it wrong.
When you know too much and take an intro course, the answer expected by the teach may not be the correct answer.

One dumb question: T/F Electricity flows from positive to negative. (nonsense) That should be worded "Conventional current flows from positive to negative". (True) This question: T/F "Electrons flow from positive to negative" is FALSE.

My psychiatric evaluation suggests that I don't think like everyone else. This is a prime example. I was convinced that the schematic with the voltage path would have done it. Also, no one would help me with our argument. I guess the word that did it was "backfeed".

I learned something to from Moto and you: How line stats operate. It's probably more reliable and possibly easier to build that way. It might even be able to have a lower profile.

Moto Roto:
I hope you don't get the impression that this site is like this all the time. It's not.

Again re-iterating options:

1) 240/120 Transformer operating the fan
No neutral needs to be pulled. The transformer primary and/or secondary should be fused. The transformer may be able to be mounted in the heater. Might not. It might be the simplest to wire if the stat is already in place. Advantage is that the sat can act as a disconnect.

2) 240/120 Transformer operating a contactor controlling the fan.
This requires a neutral too. The advantage is that the thermostat will act as a disconnect.
It too may be able to be mounted in the heater. This would be the method if the HP rating of the fan was larger. Again wiring is mostly done at the furnace.

3) 240 V control.
In general, this is not normally done. Wiring becomes a mess because the high current wiring goes directly to the contactor first. This is likely at the furnace. Neutral is required.
A transformer could be used to eliminate the neutral.

4) 120 V control
This is a typical control voltage in an industrial environment. Same problems as 3. Neutral may or may not be required depending on the size of the transformer. Various wiring schemes are . Same issues as 3. Transformer can be used to eliminate the neutral.

5) 24 V Control
The standard for HVAC equipment and therefore another good choice. This allows the possible upgrade to a programmable thermostat. If the 240 V wiring is already in place for the line stat, this is somewhat of a problem as the wires would have to be wirenutted together and a plate placed on the junction box and the 24 V stat would need to be located elsewhere.

It would require a transformer. Could be a 120/24 VAC or a 240/24 VAC. You can even get a 240/120 to 24 VAC transformer. The current requirement of the fan is low and therfore the control transformer and the fan transformer could be the same. Neutral may or may not be used.

You could use the 4 wire stat temporarily (contacts in series) until the upgrade to a programmable stat happens. You would need to run t-stat wire. You would need at least 3 wires. Common, Heat call and the return. You need the extra wire for electronic stats to avoid the batteries.

In any event, you do need to consider the need/necessity for a disconnect near the unit for servicing.

There is one thing I don't like about line stats is that you have to keep twiddleing with the dial to get the temperature right and they are not exact. That's why when I installed baseboard 120V heat on a porch, I combined a standard switch with the stat. You know when the heat is turned on and you don't have to twiddle the dial every time.

Remember to pigtail rather than put two wires under the same terminal.

As you can hopefully see when assessing the options there are many variables besides cost: convieneince, safety, servicability, reliability and difficulty are some.

I had a chemical engineer ask: Why does it have to be neat? Basically so it's easy to troubleshoot.

Why do you need a relay to light a light in a panel 50' away. I said you only communicate via contact closures. When I kill power to the box with the light, I expect it to be virtually dead. I don't want to affect something in the main box.

The telephone company taught me something. Bring in the inputs and outputs to an enclosure separately to convieniently accesable DIN terminals. Then wire. This is exactly how telco wiring is done in large buildings. The main trunk comes in. It's jumpered to individual locations or other risers.

I did mention that there are advantages and disadvantages for any method chosen. 1 and 5 are my favorites.
(1) because it's simple and uses the existing stat
(5) because it's upgradable to programmable

There is a product called a fan center which combines an energy limited transformer and a DPDT relay and has the t-stat codes in it. They usually mount in 4 x 4 junction boxes. I have my hunch that the current rating isn't high enough.

Don't forget to report back. If your proud of your work and want to show it off, take some pictures. Post them using the "Go advanced" button.

We don't bite - honest.

Happy New Year to all!