AC thermostats acting up

I"ve got two AC units for my house. The outside part of this split system houses both AC compressors (It's a Freus water cooled unit) in one enclosure but they have separate Fused feeds. Inside the house there are two completely separate air handlers. Each system has it's own thermostat. In the past week both thermostats have started acting funny. The backlights for them come on and then fade out or "stutter" out and the LCD digits fade away and can't be seen whether the backlight is on or off. This happens about 2 seconds after you press the buttons to raise or lower the temperature or the button to turn on the backlight. I thought each thermostat was powered by a transformer in each of the air handlers. That's how the original system with two completely separate Carrier AC units was set up. The thermostats are Braebun brand. Has anyone seen these thermostats go bad this way or should I be looking at some electric issue in the AC unit? Seems quite a coincidence both thermostats started doing this at the same time if it's a problem with the thermostats.
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Are these thermostats only powered externally or do they also have batteries as a backup? Normally, one would expect that if they are powered at least mainly by an external source, that the source would be a seperate thermostat in each air handler. Try turning off the AC power to one handler at a time and see what happens.
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On Sun, 30 Jan 2011 23:57:34 -0800 (PST), snipped-for-privacy@optonline.net wrote:

yes they have batteries. I think they are powered by the air handlers. Further info from today. I pulled them off their bases and just let them run on battery power. They operate just fine on battery power, no fading backlighting or fading LCD numbers. Stick them back on the base and they act up. Measured the AC voltage on one of them and it's 28 volts. When that one acts up it's still 28 volts.
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Very strange indeed. Since the batteries are there to offer backup, it's odd that they work with batteries off the wall, but fade out when connected to the HVAC power. Even with power absent, they should still be OK and you also are measuring voltage present.
Do they have a calander? Maybe they reached pre-programmed obsolescence....
LOL
I'm assuming this worked OK for a long time and it's not a new install? Next steps I'd try would be to go look at the other end, determine how it's wired, if it's powered by only one Xfrmr or two, etc. If there is any wiring going from one air handler to the other, etc. And after recording wire positions, take one of them off and see what happens. Seeing if they are wired per install instructions would be a good idea too.
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On Feb 1, 9:22 am, snipped-for-privacy@optonline.net wrote:

Has the OP tried brand new batteries? Even if the thermostat works ok off the wall, but not on the wall, it is quite likely that the batteries are used for some function that increases the drain when the thermostat is on the wall. The only way to tell is to measure the battery voltage when the thermostat is off the wall, and then continue to monitor the voltage while the thermostat is mounted on its base and see if the voltage changes when it is mounted on the wall.
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On Sun, 30 Jan 2011 23:30:55 -0700, Ashton Crusher wrote:

" I thought each thermostat was powered by a transformer in each of the air handlers"
Better make sure of that. Unlikely both would fail in the same manner at the same time without something like a power source in common.
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I would assume they have batteries in them. Check that out. WW
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On 1/31/2011 12:30 AM, Ashton Crusher wrote:

Did the malfunction just start and has the system operated correctly in the past or is it a new installation?
TDD
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wrote:

Problem Solved. To answer a few questions, this was a 5 year old installation and nothing had changed but suddenly BOTH thermostats started acting up. One suggestion was that a voltage spike had taken out both thermostats. I checked voltages and whatnot in both air handlers and could find nothing amiss. So I bought two "new" thermostats on eBay and after making sure they were configured for my system (basic reset had all the right settings) I slapped them in place and both systems now work properly again. So I guess it must have been a voltage spike that took them both out at the same time (or the MTBF on this has been VERY accurately calculated). Very strange, no other electronics in the house went bad.
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Ashton Crusher wrote:

How about telling us the brand name and model of those 'stats?
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wrote:

Braeburn Model 5200 . They have enough programming features that an AC guy can stock just them (or whatever the current model would be) and install them for almost any installation including both single and dual stage gas and electric and heat pump systems.
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