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QUESTION: I have a gravity flow furnace currently connected by 2 wires to a battery powered t-stat (just R and W). Due to the home / location of the t-stat, I want to upgrade to the Honeywell YTH5320R1017, which requires a "c" wire.

Question: can I wire a 24v transformer so the common wire goes to C on the t-stat, the other wire from the transformer goes to the R slot and then the 2 wires from the furnace would wire to W and R (so there would be 2 wires into R)? Would I need a 24v relay switch?

The YTH5320R1017 is a bit expensive and I don't want to fry it by connecting the 24vac incorrectly. Thanks!

ANSWER: Neil,

You should have an existing transformer for your controls, and guessing your current stat is 24volt AC ,if so just run a wire to your new stat hook to C other end goes to your C common of your existing low voltage side 24 volt ac transformer. the other low voltage side of the transformer goes to R of the stat. Installing another transformer will not work, they need to be kept separate ( electrically ) does not the Honeywell stat show you how to wire your furnace, they usually show many configuration in the install guide.Make sure your current transformer is 40 VA minimum rated.Also your t-stat wire size are suppose to be 18 Gauge copper,does not matter if stranded or solid , but 18 gauge copper.

---------- FOLLOW-UP ----------

QUESTION: Hi Joseph -

Unfortunately there is no existing transformer for the gravity flow furnace.  I believe it uses a generator to open the gas valve.  

The current t-stat is run via batteries (2 AAs) and the only wires in the t-stat are red / white.  When looking at the furnace, only 2 of the 4 wires in the bundle are connected.

Since there is no 24volt current but the wireless system will need one, if I install a 24v transformer I'm assuming I can connect the common wire directly to the wireless transponder but for the other wire coming from the transformer, where does that get connected in this system (to complete the circuit)?

Thanks!!
Neil

Answer
Neil,

After rereading your first question more closely , you can try jumping the R and w wire at the t-stat, if your furnace operates OK then all you have to do is buy a standard 24 volt COIL rated relay with couple normally open dry contacts. and hook up one of the R wires from the stat to the N/O contact, the other side of the N/O contact hook up the W wire. then one side of the relay 24 volt COIL hook to the new 40 VA rated transformer C.the other wire of the 24 volt side of the transformer goes to R of your new stat. then the W terminal wire at the stat goes to the other side of the 24 volt relay COIL.The C terminal wire  from your new stat goes to the C of the transformer,( which has one wire from the relay coil also ) so when you power the new stat the R to W contacts closes sending 24 volt power to the relay ,closing the N/O set of contacts and your furnace should start.

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40+ years diversified experience.An HVAC/Refrigeration Self Employed Contractor since 1986. NATE Certifield. Answer questions pertaining to Residential and Commercial Air Conditioning , Warm Air Heating, Heat Pump systems.Mechanical and Electrical troubleshooting of these systems.Be as detailed as possible when describing problem.Packaged unit or split systems.No appliance questions.

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Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration ( Walk In and Reach In Coolers and Freezers ) Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil , Natural and Propane Gas Heating systems.

Education/Credentials
Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

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