Adding Low Voltage Thermostat Wiring to 240V Heater

I would like to bypass the grossly ineffecient thermostat in a portable 240V heater like this one ProFusion Heat Industrial Fan-Forced Heater — 5600 Watts, 19,000 BTU, 240 Volt, Model# PH936 | Electric Garage Industrial Heaters| Northern Tool + Equipment
by adding an external digital thermostat. I understand that this can be accomplished by adding a 24V relay with 240V transformer like this one RT850T-240 .
Is this correct, or can anyone suggest a better alternative? If this is correct, how would I go about wiring the relay and thermostat into the heater? Thanks for any details you can provide.

 

How can a thermostat be inefficient? All it is is a switch that turns on or off according to the temperature. Or do you mean that the "dead band" the differential between turning on and turning off is wider than you prefer?

Yes, you CAN use a low-voltage thermostat in conjunction with a transformer and relay (or contactor) to perhaps achieve better control but there are also line-voltage thermostats that have pretty close dead bands and that would be much easier to install.

Why don't you tell us how you are using this heater and why you need (or desire) better temperature control?

 

Thank you for the reply. Poor choice of words in the original post. The heater is not operating efficiently, because the inaccurate thermostat causes it to run longer than is necessary. The heater is used in a greenhouse and ideally maintains a temperature of 50°F at night. On sunny days, there is little to no heat required here until we get into the winter months, yet the heater is often running when the temperature is well above 50. Beyond that, the wife would like a digital thermostat to set, rather than the ambiguous knob on the heater. I am not an electrical engineer, just a diy guy trying to gain better control of the heater to save money and keep the wife happy. I have not seen any digital line voltage thermostats that are rated for 23 amps at 240V, but would certainly consider any cost effective solution. Please tell me more. I am open to any reasonable suggestions.

 

Welcome to the forums.

Honeywell makes a combination unit for just that unit. With this module and a low voltage thermostat you'll be all set. The module can be found at many HVAC parts sources.

HONEYWELL RT850T-240

 

I understand. I have worked on a few of those heaters and they have a separate thermostat that controls the fan motor. This is to allow the heating element to heat up for a short period of time before energizing the fan and also to allow the heating element to cool down a bit before stopping the fan. There may be a switch that allows for continuous fan operation and if so, turning that switch on will likely give you a bit better temperature control but probably not as close as you desire.

Since that is a 5500 watt heater (23 amperes at 240 volts) you would need a 30 ampere standard contactor or an electric heat relay rated at 25 amperes. Along with a 240 volt-to-24 volt control transformer you could then use any low-voltage thermostat remotely mounted to get a more accurate and repeatable control. Ideally the contactor would be wired in place of the internal thermostat so that the fan on-off delay still functioned but wiring it into the supply for the heater would probably be okay, it would just stop the fan and heating element at the same time when the thermostat was satisfied.

Let me know if you need detailed instructions and how to go about it. I'm really guessing right now but you might be able to get the relay and transformer for around $40 or a bit less.

 

I think we are all on the same page with this. It seems the relay/transformer/contactor I mentioned in the original post (RT850T 240) may be the best way to go and yes, I would love to see a diagram of the best way to wire that into the system (either internally, or externally). If you could also recommend a decent digital thermostat and add that to the diagram, it would be much appreciated. Thanks again for the help.

 

Diagrams are difficult for me to post but this is a fairly straightforward alteration. Understand that you WILL be voiding any guarantees and UL listings by making the alteration.

The relay/transformer that PJ linked to is probably the best best. It will come with instructions detailing the various connecting wires. You will have to open the heater casing and disassemble to the point of removing the internal thermostat which has two wires. Enlarge the hole where the original thermostat was located so that you can attach the relay unit with the wires threaded into the heater enclosure. The two wires that originally connected to the thermostat will connect to the "Common" and the "Normally Open" wires of the relay while the "line" voltage wires of the relay will connect to the two incoming power wires of the heater. Cut off any bare wire from the "Normally Closed" relay wire and screw on a wire nut. Push these wires away from the heating element and re-assemble the heater.

Connect two-conductor thermostat wire to the low-voltage connections of the relay/transformer unit and run them to wherever you plan to install the remote thermostat. Mount the thermostat in an area that is representative of the whole and you want to control the temperature. Connect the wires to the R and W terminals on the thermostat.

I can't recommend a particular thermostat but you will likely need a battery-operated one. The less fancy (read non-programmable) the easier it is to use. You might want to look at something like the Robertshaw model RS2110

 

Thanks for the help. Sounds pretty straight forward. No worries with the warranty, well worth the modification. Now to order some parts and wait. Thanks again.

 

Finally got the relay and thermostat. Opened up the heater and found that there are three wires coming in, one to ground, a white wire directly to the existing thermostat and a black wire that goes directly to what looks like a temperature sensor. The complication is that there is also an on off switch. The white wire comes out of the thermostat and goes to the switch. There is also a red wire from the thermostat to the switch. The red wire from the switch goes to the other side of the temperature sensor as well as to the heating element. So now I'm not sure where to connect the relay. The relay has red and blue wires that connect to L2 and a black wire that connects to L1. How do I wire this to keep the on off switch, or should I just by-pass it completely? All details greatly appreciated. Photos attached, but can send more if requested. By the way, I'm trying this on a backup heater. I'm not touching the in use heater until I get this one working.

 

I'm not ignoring you but I have been trying to remember exactly how the internal wiring of those heaters is arranged. I know that there is a non-adjustable thermal switch connected to the fan motor that keeps the fan off until the heating element has generated some heat, and keeps the fan running after the adjustable thermostat disconnects the heating element for a cool-down period. I am NOT sure if the manual switch is wired to allow for continuous fan operation or is a master on-off switch for both heater and fan.

Several more pictures with the wires separated as much as possible will help. I'll see if I still have the instruction manual for one of these heaters. Also, what is the full part number of the relay assembly you bought?

 

Are you using the RT850T ?

That looks like some type of mini contactor. There is an actual adjustable thermostat.... correct ?

 

First of all, thanks for taking the time to attempt to help me with this. Much appreciated. The relay is the Aube RT850T. The heater in the photos is this model: DAYTON ELECTRIC MANUFACTURING CO./DAYTON Electric Space Htr,Fan Forced,208/240V - Portable Electric Heaters - 3VU36|3VU36 - Grainger Industrial Supply, also sold as ProFusion by Northern Tools: http://www.northerntool.com/images/d...als/700095.pdf . I've attached several photos, but may need to remove the fan assembly to give better views of the switch wiring. Just let me know what you need and I'll provide. If need be, I could attempt to sketch out the wiring and post it that way.

 

I just uploaded more photos and info. Waiting on MODs to approve the post.

 

I posted early this morning and was shown a message that said the post would show up after review by moderators. Any idea how long this usually takes? I included 5 photos in the post. Is that what triggered the review? When I posted 2 photos earlier the post went up immediately. I could repost individually if 5 pics in a single post is a problem. Just wondering.

 

Seems a glitch in the system. PM me if you still can't see them.

 

The "missing" post with the photos is visible now. Thanks for the help.

 

This should be pretty easy but I have two more questions, when you turn the thermostat up to turn on the heat does the fan start right away? Or is there a delay time while the electric element heats before the fan starts? How about when the thermostat is satisfied (or is turned down) does that fan continue to run for a cool down cycle or does it stop immediately?

I've been busy today and my brain is fried. I'll post more later.

 

I can verify after work today, but as I recall from turning up the thermostat on the heater, the fan does activate as soon as I hear the click from the thermostat. Likewise when turning the thermostat down; I hear a click and the fan stops. I haven't looked closely to see if the element is heating up before the fan kicks in, but can check it out later today and post back. Hope you have a quieter day and get "unfried".

 

Between Furd and I we'll get you going there.

 

It appears to me that the fan and heat come on and go off at the same time when the switch is set to "Auto". The switch is labeled on,off and auto. We normally leave it on auto and allow the built in thermostat to control it.

 

Don't meant to push, but I may just have to cut a few wires and see what happens. I appreciate the help so far, but need to move on with this.

 

I'm currently looking for the RT850T wiring diagram. We'll have you going this afternoon.

 

Here it is: http://www.aubetech.com/manuel/2/RT850T.pdf

 

Sorry to be so late in getting back, health problems.

IF the heater that you want to control is the same as the one that you are experimenting with, that is, the fan comes on immediately with the thermostat and goes off the same way then you do not need to change any wiring internally but can use the relay to control the receptacle to which you plug in the heater. To do it this way you will likely need to add an extension ring to the existing receptacle box to allow for the connection of the relay unit.

The wiring would follow either figure 1 or figure 2 with the wires on either side of the word LOAD going to the receptacle as it does presently. You will need to use #10 type THHN single wires in any color other than white, grey or green. All green or bare wires need to be connected together and to the steel box.

No internal modification to the heater will be necessary, just turn the internal thermostat to maximum and you can leave the fan control switch in either the automatic or manual position and it will work automatically only.

If the heater does NOT have the fan start and stop immediately when the heater's thermostat is manually changed from maximum to minimum then internal wiring changes will be necessary for best life of the heater.

 

Sounds great. Will get some wire today and give it a try. Thanks again for the help. Hope you're feeling better soon. Will report back once I get things in place.

 

Picked up a PVC junction box for the greenhouse and tried wiring the relay into the receptacle supply line. I must be missing something. Here's what I did (with no luck). There are 4 wires leading to the (3 prong) receptacle for the heater; black, red, green and an unused white. According to the RT850T instructions, I connected the relay red and blue to the red wire leading to the receptacle and then connected the relay black to the black wire leading to the receptacle. There is nothing else on the line (direct path from breaker to the receptacle). The other part of the job seems fine (connecting the thermostat to the relay). The thermostat functions as intended according to the chosen setpoint. The problem is that even with the heater unplugged, when I throw the breaker for the receptacle, it faults out immediately. No pop, no smoke, no heat, just dead fault. What am I doing wrong? Should I be attaching the black wire from the relay to ground instead of to the black wire leading to the receptacle?

 

Those wiring diagrams they supplied are confusing. I'll try to simplify it for you.

 

The black from the relay goes to black power wire.
The blue wire from the relay goes to the red wire.

The red wire from the relay goes to the receptacle where the red wire was previously.

 

Black from the circuit breaker connects to the black of the relay.

Red from the circuit breaker connects to the blue of the relay AND to the Y terminal of the receptacle.

Red from the relay connects to the X terminal of the receptacle.

White from the circuit breaker is not used and should be capped/insulated.

All bare copper and green insulated wires connect together.

(Looks like PJ beat me to it.)

 

Thanks for the diagram and description. According to the diagram PJmax posted, the red wire from the breaker connects to the blue wire from the relay and then the red wire coming out of the relay goes to the receptacle. I had both the red and blue wires from the relay connected directly to the red wire from the breaker, so that appears to be where I went wrong. The diagram that came with the relay shows a straight line with both the red and blue wires connected directly to the red wire from the breaker (no gap between where the blue relay wire attaches to the red breaker wire and where the red wire from the relay attaches to the red breaker wire). If that's all I need to do to make this work, I'm all set. Will have to try it tomorrow evening and post back. Thank you both (again) for the help.

 

Tried the latest wiring. Thermostat works fine, but the heater/fan never comes on. I'm stumped.

 

I'm assuming you've turned the thermostat on the heater to the maximum setting.

Do you have a second module to try there ? That short may have blasted it open.

 

If you mean "do I have another relay like the RT850T", then no. I purchased the one I have along with thermostat, wire, etc for around $80. I'd have to think about spending another $40 for a new relay, when the heater itself is only around $100. Might have to just buy another heater and live with the built in thermostat. Any other ideas? If I were to attempt wiring inside the heater, bypassing the switch and internal thermostat,would that do me any good? I know it's hard to say without knowing the condition of the relay. Is there an easy way to check the relay before making any other decisions?

 

Sorry for the delay, life happens. I had the RT850T checked and was told it is fine. While I appreciate all the help on the t-stat issue, it appears that the heater I've been using is just not going to work the way I'd like it to. So my next question is for recommendations on a new heater. Would have to be 220V (have 30A service) and be compatible with the RT850T and would like to have the same (or more) heat ouput as the old heater (5500W). Any suggestions for a cost effective heater that would fit the bill?