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New thermostat periodically losing power - Honeywell T8775A1009 round digital

New thermostat periodically losing power - Honeywell T8775A1009 round digital


Old 12-18-13, 10:32 PM
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Join Date: Dec 2013
Posts: 31
Question New thermostat periodically losing power - Honeywell T8775A1009 round digital

I just replaced an old round mercury with a Honeywell round digital T8775A1009 thermostat a couple weeks ago and noticed that it is acting dead occasionally. The digital screen goes dead and the zone becomes cool. It is on a low-traffic area(upstairs bedroom hallway) and over two weeks I noticed it dead 4 times. After playing with the switches and pressing the backlight it comes on after 10-20 seconds for some reason.

The installation is for heat only/furnace so it is a simple red wire/white wire hook up as described in figure 2 of the installation manual, linked below. The dip switches are all correctly set in the off position as well as the fuel switch to 'F'. I picked up a replacement of the same model today and I noticed that the issue is still occurring, so that rules out the possibility of a faulty unit.

The power for the digital screen comes from the wiring itself, hence it does not have a battery like some of the other digital therms' in our house. I am trying to figure out why it may not be getting power periodically, and what to check before having to get a house call. It seems to be functioning perfectly at least 90% of the time but today I woke up to a cold bedroom, brr.


Installation manual:
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Old 12-18-13, 11:07 PM
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In the instruction it says come with a 4074FAB resistor. In your application it appears it may be needed to supply the thermostat with more current for charging.

In the link below is a tech bulletin on the resistor but since I'm not sure what make and model your furnace is I can't ascertain if it would work for you.

Honeywell TechLitDocuments_4074fab.pdf
Old 12-18-13, 11:43 PM
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Join Date: Dec 2013
Posts: 31
Thank you very much for your informed response.

That resistor sounds like it may be applicable for my 'power-stealing' thermostat installation. I just don't know exactly where it goes. If it is too difficult perhaps I should just return these 'stats. I kind of like the classic round look though for where they are being placed.

Our trusty ol' furnace is a 'vintage' Burnham corp V-14A-T and the zone switch relay control reads Taco SR-501. I will provide anymore requested info

I'll check back later
thanks, again
Old 12-19-13, 02:36 PM
Join Date: Nov 2010
Location: USA
Posts: 123
If you have the resistor and have access to the Taco zone value screws, it is an easy install. I have the same thermostat, and I like the retro look as well.
Just be sure to shut off the power and check to make sure it is off.
Where the wires attach to the zone valve, loosen one screw and attach one lead of the resistor. Tighten and make sure both the resistor lead and the original wire is secure. Then repeat on the other terminal. Turn power back on.

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