Mechanical or electrical device used to control a heating, ventilation, and air conditioning (HVAC) system, or other temperature reliant system.

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78k views

Why won't my A/C turn on after I replaced the thermostat batteries?

Our thermostat died yesterday, but after replacing the dead batteries with fresh batteries, and confirming the LCD screen was operable, our AC no longer turns on. I have attempted a factory reset on ...
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0answers
52 views

AC Unit does not receive 24 volts on outside unit

My outside unit does not turn on itself, but it does work. I just did a quick repair just thinking it was the capacitor and it was only 15 dollars for a new one so I changed that out and still had the ...
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1answer
35 views

Vent now blows on thermostat

A long story how this got this way but there's a house with two furnaces and two thermostats. One of the furnaces feeds a long hallway with the return vent at one end and four feeder vents evenly ...
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1answer
15 views

HVAC compressor turns on when calling for fan only

I just moved into a new apartment with HVAC, and I'm looking to control the thermostat with a few Phidgets (I had already invested in that ecosystem, so I just needed a couple of relays). Anyway, ...
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1answer
326 views

Why won't my thermostat control the HVAC anymore after trying to switch thermostats?

I bought a new Nest thermostat which is supposed to be compatible but needs a professional installation (advice that should have been heeded by non-professional electrician me). This is to replace an ...
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1answer
129 views

How do I wire a thermostat with an external 24v transformer?

My current wiring has 5 wires, R, Rc, Y, G, and W wires. No C wire. I've added and external 24v transformer and connected one wire to the C connector but don't know what to do with the other. If I ...
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4answers
3k views

How to wire a relay for an evaporative cooler to a thermostat?

I have an evaporative cooler with 120VAC control that I want to switch on and off using a relay that is activated when the cool call comes from my thermostat. I plan on just using 1 relay to activate ...
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0answers
53 views

How do I connect a honeywell wifi thermostat with no C wire at the furnace?

I purchased a Trane XV90 furnace in 2001 & I'm upgrading the thermostat to a Wi-Fi model. This thermostat needs a wire labeled C from the furnace & of the 4 wires coming from the furnace none ...
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1answer
172 views

Wiring new 'c' wire to heat pump system. Do I need to connect at both air handler and condenser?

I am planning on installing a new WIFI thermostat (Honeywell RTH6580WF). I have a heat pump system (outdoor condenser unit and air handler in attic). Old thermostat did not have a 'c' wire, but it ...
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2answers
3k views

Can I piggyback a thermostat with a C Wire hookup on a furnace?

I'm trying to install a new thermostat that needs a C wire hookup. There are currently 4 wires running from the thermostat to the furnace, none of them are a c wire. When I look at my furnace itself ...
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1answer
146 views

Why does my gas furnace run once then wont come back on until reset when temps are below 30 outside?

THE PROBLEM: My gas furnace will run one cycle and then not come back on until the switch is reset. Once the switch is reset it will kick on immediately run once and again I will have to reset before ...
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1answer
59 views

Central Air Unit not Sending Enough Power to Thermostat

Right now I don't have specifics on my central air unit (make, model, etc) but hopefully the info I provide will help. I have a Second Generation (and Third Generation) Nest Thermostat hooked up to ...
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2answers
2k views

thermostat batteries replaced, AC unit won't start

Came home from work to find no AC. I checked the thermostat and the display was blank. After replacing the batteries and resetting the program, my AC unit never started up. I checked: - The AC unit ...
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1answer
226 views

Can I shorten my in floor heating cable?

Just wiring up an in floor heating thermostat. I have two separate two wires going into my flooring as well as another coming from the panel. I am wondering about the two wires going down to the ...
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1answer
2k views

Where do I connect the C wire in my furnace?

I am upgrading my thermostat to a Nest, and would like to add a common wire. My current furnace controller doesn't have a C terminal, but the current thermostat wire has 2 unused wires (black and ...
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1answer
744 views

How can I add a C-wire to a Venstar T5800 thermostat?

I am trying to replace my existing thermostat. My old thermostat has: Rh, W (coming from the heating unit). Rc, Y, G (coming from the AC unit). On the Venstar T5800, I can map as follows: Rh+Rc ...
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1answer
3k views

why does my heat pump wiring diagram show 7 wires going to the thermostat but the thermostat only says it needs 5?

Here is a link to the wiring diagram for my heat pump. The thermostat that I am planning on buying has the following terminals C, G, Y, O/B, RC, R, Aux, E, and L. The instructions say to jump the RC ...
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1answer
513 views

Using a second thermostat to balance temperatures thru fan-only control

Like many others, temperatures between the two levels of my house are uneven. This is particularly noticeable as spring arrives and the upper floor (above ground) enjoys solar heating (so the heater ...
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1answer
53 views

Wiring for New Thermostat

I just bought new programmable z-wave thermostats (Honeywell TH8320ZW). I thought I had the wired correctly, but I woke up to a pretty chilly house this morning. The fan is working, but it seems to ...
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1answer
363 views

Correct Wiring for Furnace Blower Motor

After getting my furnace blower motor replaced, it now runs only in the following scenarios: 1. Mode "Cool" Fan "Auto" 2. Mode "Cool" Fan "On" 3. Mode "Heat" Fan "On" In other words it will not run ...
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2answers
122 views

Diode in a link wire for Thermostat?

What I have is a back boiler that runs my central heating via a pump. The domestic hot water is gravity fed. At present I have to turn the domestic hot water on via a timer switch this then gives me ...
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1answer
90 views

New Thermostat installed but compressor comes on immediately

I have a new Insteon Thermostat that I'm using to replace my prior Honeywell t-stat. This is installed to a relatively new HVAC system (both AC and Heat) that DOES have a "c" wire. I've attached the ...
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1answer
50 views

Can't get fan working on thermostat

I recently purchased a new thermostat. My old thermostat worked for heat (no AC either) but the fan wouldn't go on when I turned to "On" (in hopes to move some cool air from the basement up during the ...
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1answer
101 views

Updating thermostats to single system

I have a three floors in my house, each controlled by it's own thermostat (separate air handlers and mechanical valves on furnace). Baseboard heat. The three thermostats are controlled separately, ...
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1answer
70 views

How to power Honeywell wifi thermostat with external transformer

I want to install A Honeywell RTH 6580 WF thermostat in my garage for heat only . My heater is a simple LP gas, hanging unit heater; heat only and my thermostat has two wires labeled W and R . I ...
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1answer
26 views

Why might a thermostat refuse to switch from Heat mode to Cool mode?

I have this Honeywell RTHL2410C programmable thermostat and typical Florida electric AC system. Starting this Spring, it has started behaving strangely when switching from Heat to Cool mode. The ...
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2answers
46 views

Boiler heating extra zones

Background: This is a new house, with a boiler system I'm not familiar with. Instead of zone valves, each zone has its own pump (4 zones). There is a fifth pump on the other side of the boiler. Not ...
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1answer
36 views

RUUD Furnace blowing air but not heat

I have a RUUD furnace that stopped blowing hot air last night. I was thinking that it was the high limit switch gone bad BUT the coil ignites and lights but then goes out again after a minute, so ...
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1answer
1k views

Can I replace this Airwell thermostat?

I am in market to replace my simple A/C control unit with one that integrates with home my z-wave based home automation system. However, before I make the splash and buy a compatible thermostat, I ...
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1answer
33 views

Temperature of water in the shower fluctuates - will a cheap thermostatic tap help?

I recently moved to an apartment where temperature of water in the shower fluctuates and we called for a plumber. He either cleaned up or replaced one part (I think it was the mixer - nothing visually ...
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1answer
20 views

New thermostat for electric radiant ceiling heat

I live in the Pacific NW where radiant ceiling heat is common in older homes. I a have home built in 1969 and want to upgrade the thermostat to something newer that we can program and save money. This ...
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1answer
24 views

One thermostat with a switch to control 2 heating systems

I have a natural gas boiler and a pellet stove. I have a Honeywell programmable thermostat (2 AAA Batteries) that I have used independently with both of these systems. Switching from one system to the ...
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2answers
22 views

My furnace has a vibration for about a second at the end of the heat cycle.

It runs fine for the approximately 15 minute heat cycle. It appears as though it happens as the thermostat is satisfied and the gas valve shuts off. The vibration is heard upstairs and throughout the ...
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1answer
118 views

How to Change Chinese HVAC Thermostat

I am looking to put Nest thermostats in the house, I have now found out my entire house (one in each room, 6 of them) has been wired with HVAC Chinese U12A UNIVERSAL A/C CONTROL SYSTEM urrgh – they ...
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1answer
34 views

What could be causing my Thermostat “R” wire to only be 8 VAC?

I am trying to install a Nest thermostat. It says Rh not powered. I checked with a meter and I get approx 8 VAC reading. What could be causing this? The furnace itself ain't pretty. Update: ...
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2answers
48 views

How do RH/RC/R and Y/W relate for HVAC?

I have the following notes for HVAC (that I've compiled from reading through here): Thermostat wiring: C low voltage power wire that brings power from heating/cooling equipment to thermostat ...
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1answer
96 views

How can I connect a WIFI thermostat to a Honeywell R845A switching relay?

I'm trying to install a wifi thermostat and it won't power up. The old thermo was battery operated 2 wire but there were spare wires. At the R845A relay, red and white whites connect to terminals 5 ...
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1answer
26 views

Stove Temperature Control broken

I have a (Simpson Mercury) electric stove - and 2 of the 4 elements do not work properly. They heat up, but it is not possible to set the appropriate temperature for them - they are either on or off. ...
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1answer
52 views

Why might a central heating programmer no longer override a room thermostat?

My central heating and hot water is governed by a Danfoss FP715 Si. Using this I can set the time each day when the central heating and the hot water come on and off. When the central heating is on it ...
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1answer
685 views

Why did I have to change the wiring to use a smart WiFi thermostat?

I just installed a Emerson Sensi wifi thermostat.It works, and I ran a separate C wire. I had a 24 volt 5 wire Robertshaw programable thermostat that ran my boiler, air handler and AC. It is all ...
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0answers
11 views

AC electronic thermostat went black

I have a new (2013) Lennox AC unit. There is an electronic thermostat that just went black a few days ago. This has never happened before as, even when the AC is off for the winter, the thermostat ...
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1answer
16 views

12 gauge on 30 amp circuit - after thermostat

I have 2 2500 watt baseboard heaters that I'm planning on running off of a 30a 240v breaker. My question is, because the thermostat is close to the panel, can I run the 10/2 JUST to the thermostat, ...
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2answers
47 views

Why does my heating work poorly in moderately cold temperatures?

My house has a gas-fired boiler and hot-water radiators. I've noticed a strange problem since moving in a few months ago. If the weather is moderately cold (in the 30F - 40F range), my house feels ...
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3answers
4k views

Should I replace my mercury switch thermostat?

We have an old mercury switch thermostat in our house, You know the type, gold ring, very common. We are in new England on a gas/steam system. There is no central AC. We have been told by more than ...
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3answers
29k views

Adding a C wire to a new Honeywell WIfi Thermostat

I am trying to install a new Honeywell touchscreen WIFI thermostat and need to add a C wire to my terminal inside the furnace (using the help found at How can I add a "C" wire to my ...
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1answer
35 views

Does thermostat need to be grounded?

If a thermostat is connected to an air handler, which is connected to the main panel and has a ground wire, should one of the thermostat wires be connected to the ground?
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0answers
24 views

Thermostat replacement in a double wide

We have a new double wide and I want to replace the thermostat with a new programmable one. We took off the old one and it has 3 red wires, 2 yellow, 1 white and 1 green. I get the 2 red indicate a ...
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3answers
2k views

Which furnace thermostats support variable swing/deadband?

My home furnace is currently controlled by a programmable Honeywell thermostat, but it regularly thrashes the furnace by turning the furnace on for just ten or twenty seconds. It seems that the ...
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1answer
119 views

I changed my thermostat, not my heat does not work… help!

I know I made a mistake, but here's the storey. When I was hooking up the new thermostat I did it with live wires, and I used metal pliers (metal touching metal). The wires may have also touched ...
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0answers
380 views

Honeywell WiFi thermostat newly installed--not working

I just replaced a 2 wire thermostat with a wifi one that required a c wire. There was an unused blue wire which I attached to the C connector on the transformer at the furnace. At the thermostat, the ...