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[MichMark]

Hi all.  Been lurking for about 5 years now, since I did my install.  Now I am helping a friend, and his T-stat is different than mine and we need some help figuring it out.  Here is what his set up looks like:  There is only 2 wires between existing T-stat and gas furnace (NO CENTRAL AC), a red wire and a white wire.  The existing T-stat (digital w/ batteries) has the following terminals:  W  RH  B  O  RC  Y  G  C.  The furnace has these terminals:  R  C  W  Y  G.  The red wire goes from RH on T-stat to R on furnace.  The white wire goes from W on T-stat to W on furnace.  Also on the T-stat, there is a jumper wire going from RH to RC.  Clear as mud, right?

He wants to be able to use his gas furnace like others do, as a back up if the boiler goes out or cools down too much, in which case the gas furnace would come on at a lower set temp to keep thinks from freezing and such.

So, now the questions.  Can this be done with one T-stat (even if have to buy new one), or will it take the addition of a second one jumped into the first?  I have a second on mine, but with central air and such, I have more wires.  Hoping it not more complicated than that (no relays, aquastats on pipes and such).

Although RSI seems to be the local "go to" guy on T-stats, thanks to any that help in this matter, as neither of us knows much about T-stat wiring.
 
I have learned so much from all of you over the years, and wanted to take this time to thank you all as well.

Mark
His furnace says it was installed in 2005.

[smitty33]

So his thermostat is a Heat pump Stat? If it is it should be capable of running two different sources. If you don't have anymore wires, you may need to run a new wire to the stat. maybe wiring gurus will chime in. what I was thinking was have another wire from the Y terminal at the thermostat go to the G terminal at the furnace to control the fan for the boiler. Keep all the rest of the wiring the same. If he wanted to just use the furnace could put the thermostat in emergency heat. Use it like a heat pump stat? What do you guys think? Mine is a heat pump stat, I wired mine like that, but I put a switch at my indoor unit to go back to heat pump if I need to.

[MichMark]

Thanks for the reply. I don't know if the T-stat is for a heat pump or not, but right now it is wired to a conventional natural gas furnace. He would like the boiler to heat his house via the heat exchanger we put in the plenum of the furnace, but have the gas automatically kick on when the boiler goes out. Pretty standard set up, just don't know how to make that happen through the T-stat. 

Any other advice out there?  Scott, Yoder, SlimJim, and others?  Probably easy for those of you that deal with this regularly, but we don't know how to do it. 

Looking for the simplest way. Wish I had the book "T-stat Wiring for Dummies !"

[Jwood]

Is it possible to run a 4 or 5 wire

[RSI]

There is no way to do it with 2 wires. Does the furnace have a G terminal? If not, you will probably need a relay to get fan only. (only likely if it is a fairly old furnace) If you can't run another wire to the thermostat you may be able to just put a 2nd thermostat somewhere else where you can get wires to. The advantage of having them together is that the temperature is the same there so less chance of backup heat coming on when it isn't needed.

You would need a 2 stage thermostat if you only want one. I don't care for this method because most are more complicated to get setup. Also, most have a fixed differential between where the gas would come on. Since it is fairly low, if you decide you turn the temperature up in the house it starts burning gas because you went past the 2nd stage setting.

[RSI]

Actually there is a way to do it without running more wires. They make wireless thermostats. Here is a 2 stage that should work to run both the OWB and backup heat. http://www.alpinehomeair.com/viewproduct.cfm?productID=453059927
I didn't read anything about it so I don't know how well it will work.

[MichMark]

Thanks so much for getting back to me.  We can pull a new wire.... shouldn't be a problem.  The furnace does have a G post.  If you look back at the original message, it shows what the furnace has for connecting posts.

I would personally prefer the 2 t-stat option (like mine at home).  It is a no-brainer to adjust.... wife even has it figured out!!  I don't he would like the idea of him or his wife having to program a single 2 stage t-stat.

So.... could you explain to a dummies such as us what type of wire to pull and how to hook up a 2 t-stat system. Thanks. 

The install is complete other than this, and would like to get the t-stats wired in so he can start burning wood.  He will be moving in this weekend as well, so the time couldn't be better.

A big THANKS to you and all who have chimed in with help so far.

Mark

[RSI]

Just another cable same as what is there will be fine. Connect the R in the furnace to RH in the 2nd tstat and the G in the furnace to the W in the 2nd tstat.

If the thermostat you get has a C connection then you may need to run 3 wires instead of 2. Just connect to C at both ends. If thermostat is digital and doesn't use batteries the C may be required. If it has a C connection and can take batteries it will make the batteries last longer if you hook it up.

[MichMark]

Thanks for speedy reply!!!  I think it will be much easier in this case to run a whole new 5-wire(?) verses pulling an additional 3-wire.

So what about this set up if you have time to ponder right now:

1)  pull an new 5-wire and while removing the old two wire.

2)  wire the furnace to this basic Heat/Cool t-stat to run the furnace gas heat (in case he adds AC sometime down the road):
  http://www.lowes.com/ProductDisplay?partNumber=230858-74493-CT87N1001&langId=-1&storeId=10151&productId=3142157&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1] [url]http://www.lowes.com/ProductDisplay?partNumber=230858-74493-CT87N1001&langId=-1&storeId=10151&productId=3142157&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1[/url]

3)  jump this heat only t-stat to the first t-stat to control the blower for the boiler heat:  http://www.lowes.com/pd_230878-74493-CT87K1004_4294821952%2B4294962546__?productId=3142159&Ns=p_product_qty_sales_dollar] [url]http://www.lowes.com/pd_230878-74493-CT87K1004_4294821952%2B4294962546__?productId=3142159&Ns=p_product_qty_sales_dollar|1&pl=1&currentURL=%3FNs%3Dp_product_qty_sales_dollar%7C1&facetInfo=Honeywell[/url]

If that will work could you run through all the connections from the furnace to the first t-stat, then from the first t-stat to the second t-stat so we don't screw it up.  Sorry for the hassle, but "we ain't smart".  Then let me know how much money to send you for your advice!

Mark

[MichMark]

RSI, if you don't want to respond to that last post I think I got it figured out after putting it on paper.  So much easier to understand in picture form verses words!!

Thanks again.  I will let you know how it works out after the weekend, if you want.

[RSI]

It will work fine replacing the existing cable with more conductors. Since it will be one cable, you can use the same wire from the furnace to connect to both RH connections in the two thermostats if you want.
You really only need 3 wires total but never hurts to run extra in case they might be used in the future.

Those thermostats should work fine. I usually use these because they are about as simple really simple. http://www.menards.com/main/heating-cooling/thermostats-freeze-alarms/honeywell-basic-digital-thermostat/p-1911741-c-12864.htm

[MichMark]

Yes, I agree.  I think we might as well just pull something like a 5-wire and then will have extra for use down the road if needed.

Those t-stats are along the line of what he wants.  Might go with those rather than the old dial style.

Thanks again!

[MichMark]

Couldn't wait to the weekend, so went to buddies house and we pulled a new wire then hocked up the two new t-stats.  It worked like a charm.  Thanks to all!!