My house has many hardwired smoke detectors that are interconnected. I only have one hardwired heat alarm in the center foyer ceiling that I am lookin the replace.

The existing heat detector:
ESL model 429CT (photo below)
Wire bar has two - and two +
Junction box has a red and green wire just tucked away
Junction box black wire connected to one of the two positive posts
Junction box yellow wire connected to one of two negative posts
There is no white wire in the junction box (just black, yellow, red and green)
All wires are a very small gauge

Replacement heat alarm (photo belowp:
First Alert model HB6135FB
Came with a three wire lead (terminal end); black, red and white

Do I connect the yellow junction wire with the white lead on the new alarm cable? Since I only have one heat alarm I will assume it is not interconnected with any other alarm in the house, so the red wires remain unconnected. I'm at a lost understanding the yellow lead to negative post on the old alarm and the white neutral lead on the new alarm.

Any help appreciated!

 

It is likely that the "yellow" wire is just a white wire that has been discolored. You can confirm this by using a meter and check for voltage between the black and the "yellow" wire. You should get 120 volts between the two wires.

 

The ESL 429 CT is a 12vdc smoke head that only connects to an alarm panel.
It cannot interconnect with 120v smoke detectors.

 

In other words, the new First Alert heat detector is 120V and won't work on a low voltage alarm system.

 

Interesting! I totally skipped the 6/12v fire panel reference in the installation guide. Guess I will have to purchase the same exact unit (or a replacement model) as the one I purchased is incompatible. Much appreaciated.

 

Need to elaborate....

I didn't read your original post close enough. You have an alarm panel with multiple detectors on it and you want to replace the one in the picture. That is a photo smoke and heat detector. These are two wire low voltage smoke detectors that are designed to work with a particular system. I couldn't read the date code on yours but smoke detectors are required to be replaced every ten years.

The 429CT may be a discontinued unit.