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240 volt Electric Baseboard Heater with Aube RC840T-240 relay runs full blast.

240 volt Electric Baseboard Heater with Aube RC840T-240 relay runs full blast.


Old 09-24-15, 12:39 PM
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Join Date: Sep 2015
Location: Canada
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240 volt Electric Baseboard Heater with Aube RC840T-240 relay runs full blast.

I have a family member with a 40 year old house that has two 240 volt electric baseboard heaters in the living room. Each baseboard heater in on a separate circuit breaker and separate 240 volt relay with internal transformers. The two relays are mounted on the breaker box in the basement. When the house was recently purchased, the two relay were mismatched set from different electrical manufactures. One of the relays was original (say 40 years old) and the other might have been changed 15 years ago. The 24 volt side of the two relays are wired in parallel to a single mercury bulb thermostat in the living room. The 24 volt wire from the breaker panel relays to the thermostat is only a two wire. No external transformer in the circuit.

Original problem was one relay (lets call it relay "A") was locked closed when ever breaker A was on. This was the newer 15 year old relay. Baseboard "A" heat was full on whenever breaker "A" was turned on regardless of what you did with the thermostat. Only way to turn off baseboard "A" was to turn off breaker "A".

Breaker "A" had the newer of the two relays (15 years old).
Breaker "B" had the older of the two relays (40 years old).

A first I thought relay "A" was the problem, so I bought one Aube RC840T-240 relay (T= internal transformer).

To date
- I have installed a modern circular Honeywell thermostat in the living room that has connections for 2 wire or 3 wire. This replaces the old mercury bulb thermostat which was recycled correctly.
- I have installed the one Aube RC840T-240 relay on the breaker box to replace the suspected faulty relay "A" which was 15 years old.

So now I have
Relay A as the Aube RC840T-240 relay (internal transformer)
Relay B is the 40 year old relay with an internal transformer.
Their 24 volt circuits (the internal transformers) are wired in parallel to the new thermostat (same connection as mentioned before)

If both breakers "A and B" are turned on together everything works OK, except for a faint red light glows on the face of the new Aube RC840T-240 relay. Not sure why the faint red glow? Otherwise. both baseboard heaters can be commanded on and off automatically by the new Honeywell thermostat in the living room. Nice! Great! All seems good until you read below.

New Problem:
So new problem is when I shut off (only) breaker B, which depowers the 40 year old relay, and leave on breaker A (new Aube RC840T-240) , the new Aube RC840T-240 stays in full on (closed ) mode, even when the thermostat is set to off (low). Basebaord heater "A" goes on full blast. Kind of sounds like the original problem I had with the 15 year old relay. I'm thinking the new Aube RC840T-240 relay "A" is seeing the resistance of 40 year old "B" relay's 24 volt coil, then turns "on" and stays "on".

Helps - Solutions Needed Please.
I only visit here twice a year and the electrical supply store is 1.5 hours away in the city. Not an easy combination. I'm figuring the 40 year old relay may have been the original culprit and needs to be changed. No problem. What do I do?

Aube doesn't show a diagram for connecting two Aube RC840T-240 (with internal transformers) to one thermostat. Several posters have suggested that you should NOT wire two Aube RC840T-240 to one thermostat, other posters have said this is OK. After doing all this research and testing, it doesn't seem right to me to wire two transformers together.

Aube's website solution for multiple relays with one thermostat is to use two Aube RC840-240 (no internal transformer) with an external transformer. This seems best to me. Wish I had done more research before buying the one Aube RC840T-240 for $60.

1) Since I already have a good Aube RC840T-240 (with internal transformer) installed on the breaker panel, is their any way to buy one Aube RC840-240 (no internal transformer) and make it all work? This would avoid 2 transformers connected in parallel.
2) if above isn't possible, can I buy another Aube RC840T-240 (with internal transformer) and connect the 24 volt circuit in parallel with the existing Aube RC840T-240 (with internal transformer), then connect to single thermostat. I don't like this solution but don't have past experience here.
3) or should I follow Aube's website suggestion (plus suggestions from some of the DIY posters) and use two Aube RC840-240 (no internal transformer) and one external transformer??

Sorry for the long post but I read so many posts where members are asking for more details from the poster.

Thanks in advance for replies.

FYI - Rest of house has 240 volt thermostats on the wall of each room. Not sure why the living room was wired with relays, except to use one thermostat in the living room.
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Old 09-24-15, 12:57 PM
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Join Date: Feb 2005
Location: Near Lansing, Michigan
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I don't really like the idea of two transformers, on two different primary circuits with the secondaries interconnected. That would create a shock hazard when one of the breakers is off and the other is on. Power will backfeed through the "off" transformer and create a potentially hazardous situation for a serviceman who expects the circuit is dead when the breaker is off. Transformers are bi-directional. If you put 240V in one side and 24V comes out the other side, also means that if you put 24V in, 240V comes out.

The best solution is to use only one transformer, whether internal or external doesn't matter, and power both relays from it. An alternative solution on the primary side is to put both heaters together on a larger circuit and either use a larger single relay or parallel the two relays as they are now.
Old 09-24-15, 02:06 PM
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Location: Northern NJ - USA
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You can use two of those 840T's together but you need a small 24vac isolation relay.
The stat gets connected to the one 840 and then the small relay triggers the second 840 in place of a stat.
Old 09-26-15, 08:57 PM
Thread Starter
Join Date: Sep 2015
Location: Canada
Posts: 2
Cool Thanks

Very understandable explanation on what's wrong with current relay/ wiring setup.
It's been that way since the house was built 40 plus years ago.
No wonder the living room baseboard heaters never worked right for the current owners.

Thanks to both for your help & multiple ideas on corrections.

I'm liking the idea of the two Aube RC840-240 relays (no internal transformer) and one external transformer on the thermostat circuit. Aube's website has a wiring diagram for this setup. For other readers it's located at Figure 3 http://www.aubetech.com/manuel/2/RC840.pdf.

Next chance I have to work on the project will be Feb or March 2016. I will update this post then. In the meantime , both breakers A and B are switched "off" & the fireplace will be used to heat the living room if needed; there are no water pipes nearby.

Thanks again .
Old 09-27-15, 01:03 AM
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Location: Northern NJ - USA
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You can use an RC840T to run an RC840.

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