# [SOLVED] Replacing old T87 thermostat with a WiFi one



## brucerpeterson (Nov 18, 2013)

I have a hydronic system with T87 thermostats and V8043F zone valves. I just bought an RTH6580WF thermostat and I want to replace one of the T87s with it. I tried to follow the directions but my setup is different from the "standard" Honeywell setup so that didn't work. I then contacted Honeywell tech support India and got nowhere with that. All the guys there could do is work with a standard system and couldn't suggest how I might get things to work in my system.

So I wired it up like I thought might work and made progress - at least the thermostat powered up and I set it up on WiFi and was able to access it remotely. The only problem was that it doesn't work as a thermostat. That is, you change the temperature setting and it doesn't turn the zone valve on (or off). That's the history. Now on to what my setup looks like. I drew up a crude diagram of my system.

One initial problem was that since the old system only required two wires, one for R and one for W, I would need to get another wire for the C wire that Honeywell said was needed. Luckily, the wire that goes to the thermostat had 6 wires and 3 went to R and the other 3 went to W. So at both ends of the wire, the thermostat and the zone valve/transformer, I broke things up into 3 sets of 2 wires instead of 2 sets of 3 wires. Then I hooked it up as the Honeywell documentation said I should and the thermostat did not power up. That's when I contacted Honeywell tech support and got nowhere. So I hooked it up like I thought might be the correct way and as I said, it powered up but didn't act as a thermostat. The second attachment shows how Honeywell said to hook it up and then how I hooked it up. (The upload is not allowing me to upload more than 1 file so unless it starts working again, I'll just have to explain the wiring). I tried again and still no luck with the upload.

Honeywell said to just leave the wiring the same - transformer C to thermostat W, transformer R to zone valve Tr, and zone valve Th to thermostat R and then connect transformer C to thermostat C also. With that setup the thermostat did not power on. My wiring was transformer C to thermostat C, transformer R to thermostat R and zone valve Tr, and zone valve Th to transformer W. As I said, that allowed the thermostat to power up. 

I don't know enough about the innards of zone valves - or the new WiFi thermostat too for that matter - to know what needs to be connected to what to get this to work. I have a suspicion that I may need to use the Th/Tr terminal on the zone valve but I wouldn't know how. The Honeywell tech support seemed to indicate that I should connect R & C from the transformer to R & Rc on the thermostat but then they couldn't tell me how to hook up the zone valve to the thermostat. Any help would be greatly appreciated. Thanks.


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## Vegassparky (Nov 24, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*

I just installed the same Tstat in my home with a regular fan forced furnace. It's a great convenience.

Are your valves 24VAC? If so, then hooking them up should be simple. Keep XFMR R(24VAC) and C(common) to stat R and C(like your drawing). Stat W is a 24VAC output. The valve will have at least an input and common connection(unless it's a modulating valve). _Your_ drawing shows 24VAC going to valve from xfmr, and stat, without a Common. That's why it doesn't work now. If you can identify the valve terminations, hook stat W to the valve input, and valve C to xfmr C. Post a valve part# for me to look up and I can tell you for sure.


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## Vegassparky (Nov 24, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*

Just to add, out of the box the stat had a small wire jumper between R/Rc. If you are only controlling the single valve, that jumper needs to stay intact.

Also look through the included manual for specific set up procedures. Some are appropriate for your situation, and different than default settings.


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## brucerpeterson (Nov 18, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*

The valves are V8043Fs from Honeywell. After more thought I think I understand why the way I wired it would not work. Tr on the valve is directly connected to R on the transformer. When the thermostat is closed, it connects R to W on the thermostat which means that R is now also on Th on the valve (if you follow the path from R on the transformer to R on the thermostat, through the thermostat to W and then on the the valve at Th). So there is no voltage difference there. My latest thought is that if I connect Tr on the valve to C on the transformer instead of R, then when the thermostat is on (R shunted to W), there will be 24VAC across Tr and Th on the valve and it will open. I think that that may be what you said in your post but am not completely certain.


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## Vegassparky (Nov 24, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*



brucerpeterson said:


> The valves are V8043Fs from Honeywell. After more thought I think I understand why the way I wired it would not work. Tr on the valve is directly connected to R on the transformer. When the thermostat is closed, it connects R to W on the thermostat which means that R is now also on Th on the valve (if you follow the path from R on the transformer to R on the thermostat, through the thermostat to W and then on the the valve at Th). So there is no voltage difference there. My latest thought is that if I connect Tr on the valve to C on the transformer instead of R, then when the thermostat is on (R shunted to W), there will be 24VAC across Tr and Th on the valve and it will open. I think that that may be what you said in your post but am not completely certain.


You got it. :thumb: Basically in the "your way drawing", you have either an open circuit, or 24VAC to both valve leads when calling for heating. With no potential difference, the valve doesn't move. Move Tr to C, instead of being on R, and enjoy the heat. That valve isn't polarity sensitive, but Th to W, and Tr to C is proper.


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## brucerpeterson (Nov 18, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*

Thanks for your help. I'll give it a try that way.


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## brucerpeterson (Nov 18, 2013)

*Re: Replacing old T87 thermostat with a WiFi one*

Just got it hooked up the way we thought it would work and it does. Thanks for your help.


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## Vegassparky (Nov 24, 2013)

No problem. Glad you got it working. :thumb:


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## kendallt (Feb 21, 2007)

So, exactly why is being able to change temp when you are not at the house a good thing?
Not to be a pain, but I am on a 'why is tech so good' kick.

Frankly, have a shed' that my grandson calls 'Grandpa's house', because that is where I spend most of my time when not at work.
I have tried every advertised method for 'efficient' temperature control available, and the most efficient is to leave the thermostat (electric space)at 50, and crank it up to 70 when I go in, then back down when I leave for the night.
I've tried various 'smart' thermostats, and that is the best way (going solely by electric bill) I can come up with. (efficient for me means money pad for comfort obtained) I still have 108 months left for the cheapest 'efficient' thermostat I bought....


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