Original message
| KrapfHomes
 |  "Honeywell RTH7500 wiring questions"
, posted Sat 25 Oct 17:38  
I've scoured the net and am at a complete loss. I am replacing my old Honeywell T874R1822/Q674L1710 thermostat with a Honeywell RTH7500. The system that it controls is a Friedrich Climatemaster model 805036GUSSLTN0B. This system is a geothermal heat pump with electric backup heat. The system is old, probably close to 30 years if it's the origianl for the house, which I think it is, but it works just fine and doesn't beat me up ont he power bill too bad. I could find nothing in the way of manuals for it on the net. I was able to find the old t-stat manual though, and with that info I determined what the various control wires do. I have a total of 8 wires running to the t-stat. The old terminals and their controls are as follows:
G - fan relay
B - heat changeover valve
E - emergency heat relay
Y - compressor relay
R - thermostat power
X - ?
O - cooling changeover valve
W2 - aux. heat relay
I cannot 100% determine what the X terminal does. Per the schematic it is connected to all three LEDs in the old t-stat. I read on one site that the X wire is from the common side of the transformer, and is required with some electronic t-stats that have indicator lamps (which seems to make sense given the preceeding).
Based on the installation instructions from the RTH7500 I have wired the terminals as follows (old to new):
G - G
B - C (per pg.23 of the instructions)
E - E
Y - Y
R - R
X - (not connected yet, as correct terminal could not be verified)
O - O/B (per pg. 23 of the instructions)
W2 - Aux. (per pg. 23 of the instructions)
I could not find a site on the web where anyone was doing exactly what I am doing, and since I know incorrect wiring could damage my equipment I am not willing to guess. I think if I knew what the terminals on the RTH7500 were supposed to control that I could wire this properly, but I had no luck finding this information either, and they do not include it in the manual (my guess as to why is that depending on programming the terminals do different things). Any help would be greatly appreciated.
It always takes less time to do something right once than to do it wrong and try to fix it later.
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Replies:
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| Houston204
| "Re(1):Honeywell RTH7500 wiring questions" , posted Sat 25 Oct 19:56
If your heatpump requires both the O and B terminal, you will need a different stat. The Honeywell Focuspro and Visionpro series require that you configure the O/B terminal to be either O (energize reversing valve for cooling) or B (energize the reversing valve for heating).
It is uncommon for a heatpump to use both.
X is common.
"Tip of the Day" Remember to remove power first.
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| | KrapfHomes
| "Re(2):Honeywell RTH7500 wiring questions" , posted Sat 25 Oct 21:54
I looked at the instructions for the FocusPro 6000 series online. It looks like the TH6220D is the right one based on my system, which I now see as a heat pump with both emergency and aux. heat. I suppose that the system uses the aux. heat if it is having trouble keeping up with the heating needs. The emergency heat is what would kick on if the primary system failed, which is very costly to run, but at least you won't freeze to death.
However, I still don't see a correct correlation between the wires I have and the FocusPro. According to the info in the installation instructions the terminals are as follows:
G - fan relay
C - 24VAC common
E - emergency heat relay
Y - compressor contactor
R - power
O/B - changeover valve
Aux. - aux. heat relay
L - sends output when set to emergency heat
I still wouldn't have a seperate O & B terminals on the FocusPRO, so my guess is that I'd still wire the O to the O/B terminal and the B to the C terminal. Then in the setup I can set the changeover function to 0. I can also do this on the RTH7500, so I'm not seeing the difference between the FocusPRO and the RTH7500.
The Visionpro 8000 instructions show the terminal assignments as identical to the FocusPRO. There are simply a few more terminals in the Visionpro for systems with a second cooling stage or the need for remote contact with outdoor units.
From what I can see the RTH7500, the FocusPRO, and the Visionpro are all equally capable of running my system or equally not capable of running my system. None have seperate O & B terminals and the X wire is still in limbo.
The real question I should be asking is what the X terminal does on my old T874R1822/Q674L1710 t-stat. I am starting to suspect that if I connect the X wire to the L terminal on my RTH7500 that all will work properly. Any ideas?
It always takes less time to do something right once than to do it wrong and try to fix it later.
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| | KrapfHomes
| "Re(3):Honeywell RTH7500 wiring questions" , posted Sun 26 Oct 11:36
Upon closer examination I do not think that connecting the X wire to the L terminal is correct. The L terminal sends output when the system is in emergency heat. The X wire comes from the transformer in the unit. I don't think I want to send power back to the transformer under any circumstance.
After careful review of the schematic for the T874R1822/Q674L1710 it is aparent that the X terminal connects to the three indicator lamps in the t-stat and also is connected into the anticipator circuits for the primary and aux. heat. I do not think that the indicator light function is required with the new RTH7500 t-stat, which is digital. I assume that the digital t-stat will know when the various portions of the system are operating and display it on the LCD.
I also do not think that the anticipator function is required anymore, since the new t-stat can easily do this function digitally with the inputs it already has.
The X wire seems to have been required with the older t-stats because of the technology of the day. Analog circuitry could only do one function with each component. Since the introduction and widespread use of ICs(integrated circuits)and digital technology it has become possible to do a lot more with a lot less. This is one of the main reasonsy why electronic devices have been able to get so small.
I'm going to simply wrap the X wire up with electrical tape and hopefully everything will work properly. I will post back with results.
It always takes less time to do something right once than to do it wrong and try to fix it later.
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| | KrapfHomes
| "Re(4):Honeywell RTH7500 wiring questions" , posted Mon 27 Oct 06:58
The heat functions properly. I'll post back in the summer to verify AC function.
It always takes less time to do something right once than to do it wrong and try to fix it later.
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