HVAC power vs. external transformer

[Chase32]

I bought the Filtrete 3M50 and had spoken to a gas company technician about it a few months prior to buying it. He recommended against hooking it up to the furnace and using an external transformer instead. I really wish I could remember why but I can't, only that what he was stating seemed like a very good, logical reason. Does anyone know the advantage of using the external transformer?

I was told by tech support that I only needed one wire on the external transformer so I fished one additional wire, with a lot of difficulty due to the location. I questioned it at the time, saying that I thought I'd need another to complete the circuit, but the tech assured me I didn't.

Rather than fish another one and deal with a 24VAC/40 VA external transformer that I can't get to register an output on my multimeter, I'd rather just hook it to the C wire unless there's a really good reason not to.

Thanks!

[blaine]

Use the C if it is 28VAC or under (C and R at the furnace).

Or use an adapter at the furnace connecting one of its wires to R at the furnace and the other wire to the spare wire to the Thermostats C terminal. Range is 24VAC +15% -42% or 14VAC to 28VAC

http://www.condorelectronics.net 1-800-346-6873 Part # 802STA4116 16VAC 250ma
Radio Shack 18VAC adapter

[Chase32]

Thanks for the reply. I will run the C wire from the HVAC unit then.

The transformer I bought was a Honeywell 24VAC, 40VA from Home Depot that was in the HVAC section, so I think it should power it fine if for some reason I can't use teh C wire.

Thanks!

[Chase32]

Ok, getting really frustrated because nothing is working and I've spent a lot of time on this.

I was going to wire it to the furnace, but there is no C terminal there. I have a W R Y T and G. I tried measuring across the terminals to get 24V but was getting 65-72 volts. So I hooked up my extrenal transformer, fished another wire with a lot of difficulty, hooked the C on the external transformer to the C on the thermostat, hooked the R on transformer to the RC (jumpered to RH) on thermostat, and still no power.

I measured the voltage of the 24VAC transformer and my multimeter says it's putting out 65V. I'm thinking my multimeter (1 month old, only used one other time, but a Radio Shack piece of garbage) might be off to get two 65V readings when they're supposed to be 24V, but I still get no power to the thermostat.

Now the thing won't even turn on with batteries. Maybe it was 65V and blew something?

[blaine]

Does not sound good, when you get the 3M50 replaced consider using a lower voltage transformer.

http://www.mouserelectronics.net Part # 802STA4116, specs 16VAC @ 250ma, about $11.00, phone 1-800 346-6873

http://www.digikey.com Part # T1129-P5-ND specs 16VAC @ 250ma, about $11.00, phone 1-800 344-4539

http://www.mpja.com/18VAC-20VA-Plug-Tra ... /17560+PA/ 18VAC W/Terminals and wall screw, about $4.00, phone 1-800-652-6733

http://www.sciplus.com/singleItem.cfm/terms/5103 Part # 27671 ADAPTER, specs 18VAC/100mA, about $2.00, phone 1-888-724-7587

[Chase32]

Thanks for the links. I had went and picked up another transformer from an electrical supply shop that is used in HVAC units. It's a 24VAV/40A, actual output is 27.1V (I bought another multimeter. I think my other one was screwed up). I exchanged the thermostat, set it up and everything is working great now!

Thanks!