--- product_id: 281303858 title: "Honeywell Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay" brand: "honeywell" price: "7211946₫" currency: VND in_stock: true reviews_count: 5 category: "Honeywell" url: https://www.desertcart.vn/products/281303858-honeywell-aube-technologies-rc840t-120-electromechanical-relay-with-built-in store_origin: VN region: Vietnam --- # Honeywell Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay **Brand:** honeywell **Price:** 7211946₫ **Availability:** ✅ In Stock ## Quick Answers - **What is this?** Honeywell Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay by honeywell - **How much does it cost?** 7211946₫ with free shipping - **Is it available?** Yes, in stock and ready to ship - **Where can I buy it?** [www.desertcart.vn](https://www.desertcart.vn/products/281303858-honeywell-aube-technologies-rc840t-120-electromechanical-relay-with-built-in) ## Best For - honeywell enthusiasts ## Why This Product - Trusted honeywell brand quality - Free international shipping included - Worldwide delivery with tracking - 15-day hassle-free returns ## Description Honeywell Rc840t-120 Electromechanical Relay 22 A 2640 W Built-in Transformer 120/24 V Review: Reliable Heating Control Transformer - The Honeywell RC840T-120 Electromechanical Relay is a high-quality solution for controlling electric baseboard heaters, convectors, and fan-forced heaters. Featuring a built-in transformer, it seamlessly connects low-voltage thermostats to high-voltage heating systems, ensuring efficient and consistent operation. Designed for 120V applications, this relay provides quiet switching and reliable performance, reducing electrical noise and wear. Its compact and durable construction makes installation easy in residential and commercial settings. Users appreciate its efficiency, ease of wiring, and compatibility with various heating systems, making it a trusted choice for temperature control solutions. Review: EDIT: works well, no overheating! - EDIT: this RC840T-120 works well, no overheating. Have tested with 15A 120V heater with 1400W showing at the kill-a-watt. Using a multimeter, there is 5.6VAC at red LOAD wire, even when thermostat is OFF. Apparently this might be normal OFF voltage, as the other model RC850 manual indicates that the OFF voltage should be <5VAC. So if 5.6V is above the breakdown resistance of the resistor (I think yes for most heaters as it's just a very simple wire?), you will consume 3.6W all the time, even when heater off. Which is 31kWh per year about 50c. I can't tell with the kill-a-watt as the kill-a-watt does not turn on. I use a switch in line, so I can turn off the whole system during the summer. Use gray for that switch and the outlet so it's clear that it's not a normal outlet. It DOES work very easily. Just add black (C common) white (W) & red (R) small wires (I used 14Ga stranded) and join the white and red for the heater to be on. My opinion: DON'T buy a return. Get a NEW one. The "very good" return I was sent did NOT work. The NEW one works great. NO excess heat when running a 15A 120V heater. I have seen more excess heat from a GE extension cord when running 15A than from this. A+++ on the excess heat issue, which is an important issue for me. The other kind of in-line thermostats seem to suffer a lot from excess heat issues (unless people are just wiring the 240V versions wrong, which might be the case. I saw one video where a guy was referring to one of the wires as "neutral") I use another 4 square metal box for it. I don't just hang it off the side of a 4 square box. So I have a 2 gang box with a light switch on the left and a blank on the right and then I have another 2 gang box above that with a blank plate over it, just for this thing and a couple of connections. Although I partly did that because I was anticipating excess heat because you can't even buy an extension cord these days that is rated for 15A that doesn't have excess heat. But there was no excess heat so maybe the hanging it off the side of 4 square box method is OK. Power is proportional to V squared at constant R, so if I was running a pure resistive load, it would consume 0.24% of full power all the time. So 1.5kW resistive heater would consume 3.6W all the time. "if my calculations are correct" For scale, you lose 0.76W in a 2A resistive fuse. So this power loss is like the power loss in 5 fuses. 22A rating is good ## Features - Immediate relay activation provides improved temperature control and comfort - Epoxy soundproofed relay ensures quiet operation - Built in 24 transformer for easy installation - Compatible with 24V thermostats with 2 wire connections R,W or 3 wire connections R,W,C - Direct replacement for any brand name relay ## Technical Specifications | Specification | Value | |---------------|-------| | ASIN | B00D5YLY2G | | Brand | Honeywell | | Brand Name | Honeywell | | Coil Voltage | 24 Volts | | Connector Type | Screw | | Connector type | Screw | | Contact Current Rating | 22 Amps | | Contact Material | Silver | | Contact Type | Normally Open | | Contact material | Silver | | Contact type | Normally Open | | Current Rating | 22 Amps | | Current rating | 22 Amps | | Customer Reviews | 4.6 out of 5 stars 272 Reviews | | Global Trade Identification Number | 00085267083508 | | Manufacturer | Honeywell | | Maximum Switching Current | 22 Amps | | Model | 9800767 | | Mounting Type | DIN Rail Mount | | Mounting type | DIN Rail Mount | | Operation Mode | Automatic | | Part Number | 9800767 | | UPC | 085267083508 775264840005 | | Wattage | 2640 watts | ## Product Details - **Connector Type:** Screw - **Contact Material:** Silver - **Contact Type:** Normally Open - **Current Rating:** 18 Amps - **Mounting Type:** Surface Mount ## Images ![Honeywell Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay - Image 1](https://m.media-amazon.com/images/I/81SpNsXNJlL.jpg) ## Questions & Answers **Q: How do i wire this to 2 wire 110v hot(Black wire) and neutral(white) wire, want to use "C" connection.** A: You don't need the common wire if you have 2 wire 120. The black is your hot. And the white is your neutral. **Q: I have two wires on a line 120 v oil burner thermostat can this be used to install a nest e thermostat? And if so how should I connect it thanks** A: you need to know '' the load '' on the circuit when 'oil burener thermostat' [I am not familiar with this, so let me say, this device may have a problem turning on and off an 'electric motor' [ there is a surge of current to energize the coils on an electric motor and this surge could be to much for the device] to me it does not sound like a thermostat [? but maybe we are not talking oranges and oranges but oranges and pears?] should draw much current, but if thermostat switched on a pump [like above] then need to know specs on the pump; I had a 120vac ''line thermostat'' controlling on/off water circ pump [very small hydronic circ pump] and I removed that thermostat and wired this device 'in between' [both the 'nest e thermostat and the 'rc840T-120' come with ''installation instructions'' and you might be able to youtube some videos [I do not know where or what ] but try '' nest e thermostat install in 120vac system you tube?'' goodluck .... I will explain the 'wiring diagram as shown on the RC840T-120 next: on the right are 3 contacts which are used to power the nest thermostat with 24 volts AC [verify your new device / thermostat operates with 24VAC input before wiring/buying] , when the 840T is wired to [on the left] the white [common/ neutral] and black [+110VAC hot wire/ 'line'] it will produce 24VAC current at contacts [on the right ] 'R' and 'C' [you see in details , it says this" Compatible with 24V thermostats with 2-wire connections (R,W) or 3-wire connections (R,W,C)" so if Nest wants 2 wires or 3 wires they are provided [check Nest wiring diagram/instructions to see what it requires ] OR ? spring for an electrician ? [ maybe invest in a volt meter? to check for 'where is the 110VAC and where is the 24VAC , as you install ] [Caution 110VAC will kill some people, deenergize before wiring, reenergize before 'calling for heat' [ eg set Nest thermostat at low temperature so that it will not close its contacts], and it is always good to have a manual switch at the furnace [in series with the 'load' ] [the Nest thermostat is 'a switch' = its contacts turn on and off the load [whatever you wire to the load wires on left side , I am just guessing it is a low flow small oil pump ? and maybe a flame igniter? not at all familiar but the 840 switching 'contacts' [red wire] are rated for 18 amps 120vac [so verify your load is less than that] also 18 amps is 'resistive load' [ read up resistive vs inductance loads] [ if you try and start a 1HP inductance motor with this, it will probably [pop/ smoke/ break?] so your question can not be answered without knowing more details about 'what is the load' ? [It is working for me, but I have a very small load] **Q: which wires to use on the transformer/relay when i have a black and white wire only to call for heat** A: There is information missing from your question. This device is meant to switch a 120 volt load when an independent (dry) contact is closed, say from a thermostat. The thermostat will close a contact when heat is being requested. These two contact wires should go to the "R" and "W" screw terminals. The black and white wires should go to the 120-volt supply hot (normally black) and neutral (normally white). The heater (load) is then connected to the red (hot) and white (neutral) wires from this device. This results in 3 white wires being connected together - the power feed, the relay and the load. When the thermostat asks for heat, its output contacts close, causing the relay to apply 120 volts to the heater. Hope this helps. **Q: Will this work to convert a 120v 2 speed evaporative (swamp) cooler thermostat for a smart thermostat?** A: Just make sure amp of the swamp cooler does not exceed the amp rating for the relay with a little safety factor. ## Customer Reviews ### ⭐⭐⭐⭐⭐ Reliable Heating Control Transformer *by T***Y on 19 February 2025* The Honeywell RC840T-120 Electromechanical Relay is a high-quality solution for controlling electric baseboard heaters, convectors, and fan-forced heaters. Featuring a built-in transformer, it seamlessly connects low-voltage thermostats to high-voltage heating systems, ensuring efficient and consistent operation. Designed for 120V applications, this relay provides quiet switching and reliable performance, reducing electrical noise and wear. Its compact and durable construction makes installation easy in residential and commercial settings. Users appreciate its efficiency, ease of wiring, and compatibility with various heating systems, making it a trusted choice for temperature control solutions. ### ⭐⭐⭐⭐⭐ EDIT: works well, no overheating! *by D***R on 18 June 2023* EDIT: this RC840T-120 works well, no overheating. Have tested with 15A 120V heater with 1400W showing at the kill-a-watt. Using a multimeter, there is 5.6VAC at red LOAD wire, even when thermostat is OFF. Apparently this might be normal OFF voltage, as the other model RC850 manual indicates that the OFF voltage should be <5VAC. So if 5.6V is above the breakdown resistance of the resistor (I think yes for most heaters as it's just a very simple wire?), you will consume 3.6W all the time, even when heater off. Which is 31kWh per year about 50c. I can't tell with the kill-a-watt as the kill-a-watt does not turn on. I use a switch in line, so I can turn off the whole system during the summer. Use gray for that switch and the outlet so it's clear that it's not a normal outlet. It DOES work very easily. Just add black (C common) white (W) & red (R) small wires (I used 14Ga stranded) and join the white and red for the heater to be on. My opinion: DON'T buy a return. Get a NEW one. The "very good" return I was sent did NOT work. The NEW one works great. NO excess heat when running a 15A 120V heater. I have seen more excess heat from a GE extension cord when running 15A than from this. A+++ on the excess heat issue, which is an important issue for me. The other kind of in-line thermostats seem to suffer a lot from excess heat issues (unless people are just wiring the 240V versions wrong, which might be the case. I saw one video where a guy was referring to one of the wires as "neutral") I use another 4 square metal box for it. I don't just hang it off the side of a 4 square box. So I have a 2 gang box with a light switch on the left and a blank on the right and then I have another 2 gang box above that with a blank plate over it, just for this thing and a couple of connections. Although I partly did that because I was anticipating excess heat because you can't even buy an extension cord these days that is rated for 15A that doesn't have excess heat. But there was no excess heat so maybe the hanging it off the side of 4 square box method is OK. Power is proportional to V squared at constant R, so if I was running a pure resistive load, it would consume 0.24% of full power all the time. So 1.5kW resistive heater would consume 3.6W all the time. "if my calculations are correct" For scale, you lose 0.76W in a 2A resistive fuse. So this power loss is like the power loss in 5 fuses. 22A rating is good ### ⭐⭐⭐⭐⭐ This relay works with the Nest thermostat !!!! *by W***N on 20 September 2019* This has been working very well for me and was easy to install with the instructions. You should have electrical building experience to install it though. It is installed to a 1900 junction box in my attic and the thermostat wire is ran from the relay down to the Nest thermostat in the bathroom. The Nest thermostat gets the right low voltage to operate and will request the 120 volt circuit to complete through the relay when it calls for heat. The heater is 700 watts. You can schedule the Nest when to call for heat, along with all the Nest options. I'm very happy to have found this relay it was the perfect match and cost for my project. P.S. This is a real review from a real home owner... I will check this review for any questions.. --- ## Why Shop on Desertcart? - 🛒 **Trusted by 1.3+ Million Shoppers** — Serving international shoppers since 2016 - 🌍 **Shop Globally** — Access 737+ million products across 21 categories - 💰 **No Hidden Fees** — All customs, duties, and taxes included in the price - 🔄 **15-Day Free Returns** — Hassle-free returns (30 days for PRO members) - 🔒 **Secure Payments** — Trusted payment options with buyer protection - ⭐ **TrustPilot Rated 4.5/5** — Based on 8,000+ happy customer reviews **Shop now:** [https://www.desertcart.vn/products/281303858-honeywell-aube-technologies-rc840t-120-electromechanical-relay-with-built-in](https://www.desertcart.vn/products/281303858-honeywell-aube-technologies-rc840t-120-electromechanical-relay-with-built-in) --- *Product available on Desertcart Vietnam* *Store origin: VN* *Last updated: 2026-05-22*