New Venstar T-Stat. Need battery backup for clock. Help!

[David D]

I recently bought and installed a new Venstar T1900 thermostat. All was well until we lost power last night and it didn't get restored until 13 hours later. The thermostat is powered by the 24VAC from the air handler and apparently has a non-volatile memory for the programming, but not the clock. :angry: In my ignorance, I didn't even think to check into that prior to buying the unit as I had always used battery-powered programmable thermostats prior to this one and it wasn't an issue. It seems kinda silly to have such a sophisticated thermostat that can't retain the time so that the programming is on schedule once power is restored...

I contacted Venstar and they responded quickly, but without resolution. I have replied back to their email, but I don't know how much information they are willing to share with regards to adding some sort of battery backup to the thermostat circuitry.

Basically, I see two options. One idea is to add a battery backup to the feed from the air handler. Of course, the air handler supplies 24 volts AC, so adding a battery to the circuit requires the thermostat be able to accept a DC power source. It would also involve isolating the battery backup from the air handler AC power somehow.

The other option is to inject a battery backup directly into the clock circuitry of the thermostat. I'm sure this would void any warranty, so this option, while possibly simpler, is a little less attractive.

Anybody have any thoughts on how to make this work? <_<

 

[dykwok]

Are you sure that they use 24 Volts AC? I thought they are using
24V DC directly from the circuit. I have two T1800 and they should be
similar. I see no reason that a thermostat would require AC power.

I recently bought and installed a new Venstar T1900 thermostat. All was well until we lost power last night and it didn't get restored until 13 hours later. The thermostat is powered by the 24VAC from the air handler and apparently has a non-volatile memory for the programming, but not the clock. :angry: In my ignorance, I didn't even think to check into that prior to buying the unit as I had always used battery-powered programmable thermostats prior to this one and it wasn't an issue. It seems kinda silly to have such a sophisticated thermostat that can't retain the time so that the programming is on schedule once power is restored...

I contacted Venstar and they responded quickly, but without resolution. I have replied back to their email, but I don't know how much information they are willing to share with regards to adding some sort of battery backup to the thermostat circuitry.

Basically, I see two options. One idea is to add a battery backup to the feed from the air handler. Of course, the air handler supplies 24 volts AC, so adding a battery to the circuit requires the thermostat be able to accept a DC power source. It would also involve isolating the battery backup from the air handler AC power somehow.

The other option is to inject a battery backup directly into the clock circuitry of the thermostat. I'm sure this would void any warranty, so this option, while possibly simpler, is a little less attractive.

Anybody have any thoughts on how to make this work? <_<

 

[DavidL]

Since 24AC is standard for HVAC...I would think it does operate off of AC and not DC.

You could plug the 24AC transformer into a UPS....just keep in mind that that transformer is also powering up other portions of the HVAC system. So....will require some thinking / rewiring.

Is that thermostat PC compatible? Possibly the PC could re tell it the time?

 

[dykwok]

Ops.. I thought they were DC. Obviously, I was wrong... great that I learned
something. <_<

Since 24AC is standard for HVAC...I would think it does operate off of AC and not DC.

You could plug the 24AC transformer into a UPS....just keep in mind that that transformer is also powering up other portions of the HVAC system. So....will require some thinking / rewiring.

Is that thermostat PC compatible? Possibly the PC could re tell it the time?

 

[David D]

The UPS idea would work, although that would be a rather expensive option.

It's my hope that the t-stat will accept a DC power input. That would make things much easier. The Venstar rep that emailed me back has been giving me "cookie cutter" responses and I'm not sure how forthcoming they will be with the kind of information that I need. It'd be nice to find someone that has already done this so I don't have to re-invent the wheel or power supply as it were.

The t-stat doesn't have a com connection (RS485, RS232, etc.) that I know of. It's certainly not one of the connections. It DOES have an accessory port on the bottom that's intended for an Insteon interface, phone interface, remote control, etc. I'm sure that there's some form of communication there as well as some sort of power, but without a schematic, I don't know.

I'm hoping the right person will see this thread and have the solution!

 

[David D]

UPDATE: Received another email from Venstar. Along with the requisite disclaimer about modifying and potentially damaging the t-stat, they gave me the component location where I can install a supercap or battery. Apparently, their commercial t-stats have a supercap installed in this location, whereas the residential units have a conventional cap installed. This is great news for me as this is the only issue I have with the unit that I bought.