Wifi Thermostat c-wire going to switching relay blown fuse

trizzo3399

Hey All,
Hope you can help. My current setup is a basic 2 wire (R & W) Honeywell thermostat wired to a Tekmar switching relay. This thermostat is on zone 2. I am trying upgrade to a Honeywell wifi thermostat, by using the green wire as the C wire at the switching relay. I did this and everything seemed OK for awhile (@ 2 hours). Then the fuse blew on the switching relay.

Can I used the green 3rd wire in this manner as the common wire?
Can I use the common wire off the switching relay?
What other option should I use?

Thanks in advance

EdTheHeaterMan

Electric is colorblind. What is the green wire connected to in the Tekmar Relay box? Pictures of the wiring connections would help. also the model number of the Tekmar control

trizzo3399

Tekmar 304P Switching Relay
The setup is on T-Stat2. Right now the green wire hanging from the left is unplugged.
I had it plugged into the "C" slot on T-Stat2 and it was working OK for a few hours, but then started to blow the fuse.

EdTheHeaterMan

See EDIT below
OK, i need to do some reading on this control. At first glance your wiring looks correct, but Tekmar is not always the same as other brands of the same type part. This is so you use tekmar thermostats. Tekmer makes a WiFi thermostat that they want you to buy. The 24V design of the Honeywell, Nest and Ecobee thermostats may not be compatible with your Tekmer relay

BRB

Mr.Ed

In reading the info on this Tekmar 304, it appear that it is compatible with 24v thermostats. So you now need to determine if there is a problem with the wiring to the thermostat.
First... is the thermostat used for heating only or for heating and cooling?

Heating only. Check the see if there is a short or bare wire in the three wires that go from the thermostat to the Tekmar. This may be an intermittent short that makes and/or breaks the short circuit based on a temperature change in a pipe the wire may be touching the wire. This is the only explanation I can think of off the top of my head.

if it is a Heat.cool system.
You need to use the Rc and Rh terminals of the thermostat to separate the two different systems. You need to use the C from the cooling system transformer to power the Honeywell wiFi thermostat.

There may be other causes but those are the ones that come to mind.

trizzo3399

Hey Ed - yes this is a heat only system. I just noticed that when they installed, the run of the cable was short, so they spice another 2 foot section of 2 wire (R,W,G) - I will inspect those first to ensure a good connection.

Here is my full setup:

T-Stat1 - goes to the upstairs theromostat. That thermostat controls heat & cooling ( central air). The heat comes from this Switching Relay in T-Stat 1. The C wire is coming from the 2nd wire feed that goes to the AC Handler direct, NOT through the switching relay.

T-Stat2 - goes to the downstairs thermostat. That thermostat is heating only.

See drawing below - its crude but hopefully shows the layout. The issue I had last night was that the relay switch fuse burned out and I am not sure why. My best guess is some type of feed shorted it. If it is as simple as making sure that splice is tight great. Otherwise would think of possibly using external power source for the downstairs thermostat

EDIT: I guess my question is can I have the C wire powering the upstairs thermostat from the ac handler and the switch relay powering c wire to the downstairs thermostat. Is there an issue that both are plugged into the switch relay for heat?

trizzo3399

Solution: Ended up just running external 24v power supply.

EdTheHeaterMan

I am sure that you have some wiring issue in the design of the heat cool thermostat and the tekmar relay where using the heat only thermostat worked just fine but trying to use the common wire on a smart thermostat caused the two transformers to talk to each other and make 48 volts 24 from air handler + 24 from tekmar = 48. Can't tell you exactly how this happens without actually looking at the wiring design, I just know that this happens more often than you might think. Good thing there was a fuse to resolve the problem, otherwise you would be buying a new transformer or control.

trizzo3399 said:

Solution: Ended up just running external 24v power supply.

This is the easiest solution!

Mike_Breault

Mike Breault from tekmar here, is the upstairs thermostat powered from the tekmar? or the air handler? is RH1 isolated from the other transformer? WHICH fuse is popping? (#)

Jersey2

I have a heat only battery powered thermostat and the instructions say optionally it can be powered with the C terminal. My boiler has a constant on 27 volt relay that powers the 24v zone valves when the thermostat calls. My question is, can this transformer also power my heat only thermostat by jumping the W to the C inside my thermostat?

Jamie Hall

No. You need to connect C on the thermostat to the same terminal on the transformer that the zone valve is connected to.

And the colour code on the wires is backwards -- not that that makes a difference.

EdTheHeaterMan

Here is the correct diagram. You need to add another conductor from the 24V transformer to the thermostat. (as illustrated by green wire)

Your original diagram might let the smoke out of the 24v. transformer or the thermostat