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Old 06-21-2010, 07:52 PM   #1
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Thermostat replacing


I have a cheap digital thermostat and I was looking to upgrade to a programmable model. Will I just be able to disconnect the wires at the thermostat now or will I have to change the wires going to the Furnace?

BTW my furnace is 18 years old.

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Old 06-21-2010, 07:58 PM   #2
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You can reuse the existing wires. Make sure you:
1) Shut off power to the furnace
2) Label the wires and terminals on the old thermostat (draw a pic of what you have) B4 removing them
3) Buy a good quality thermostat. You get what you pay for. The Honeywell Vision or Focus Pros are good from HDepot.

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Old 06-21-2010, 08:31 PM   #3
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Originally Posted by yuri View Post
You can reuse the existing wires. Make sure you:
1) Shut off power to the furnace
2) Label the wires and terminals on the old thermostat (draw a pic of what you have) B4 removing them
3) Buy a good quality thermostat. You get what you pay for. The Honeywell Vision or Focus Pros are good from HDepot.

Thanks. I was planning on going with a Honeywell anyway. I have heard good things. The one I have now is a POS.

Appreciate the advice.
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Old 06-22-2010, 01:40 PM   #4
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Thermostat replacing


take note .........on the newer digital stats they might call for a true 24Vs at the subbase.you will need to connect C down at the TR and have it come up to the subbase on the new stat.the old one worked on a back feed without a C wire the digital it will lite up but not call anything in
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Old 06-22-2010, 04:42 PM   #5
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take note .........on the newer digital stats they might call for a true 24Vs at the subbase.you will need to connect C down at the TR and have it come up to the subbase on the new stat.the old one worked on a back feed without a C wire the digital it will lite up but not call anything in

I have no idea what this says.
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Old 06-22-2010, 06:47 PM   #6
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Thermostat replacing


That means you might need 5 wires instead of 4. The extra will hook up to C on the furnace or air handler.
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Old 06-22-2010, 06:52 PM   #7
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no problem...just google your stat choice to see if you need a C connection on the subbase
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Old 06-22-2010, 07:13 PM   #8
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The C connection is an option to run with no batteries and the majority run off batteries so 4 wires is all you need. A few exotic ultraslim ones don't use batteries but they are rare. If it says uses batteries on the box you are okay. The 2 Honeywells both run off batteries without a C.
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Old 06-22-2010, 08:48 PM   #9
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Originally Posted by biggles View Post
no problem...just google your stat choice to see if you need a C connection on the subbase
All that will do, is give him multiple sites wishing to sell him the thermostat, and multiple that want to infect him. Best bet, is always go to the manufacturer website for the info needed, not do a blind web search.
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Old 06-22-2010, 08:55 PM   #10
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Thank you guys for the info.

Something like this would be cool
http://yourhome.honeywell.com/home/P...e/RTH2520B.htm

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Old 06-22-2010, 10:19 PM   #11
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Nice but it probably costs mucho dollaros $$. Very few problems with the Honeywells lately and you can buy them at HDepot. Contractors can get the White Rodgers but they are not sold at big box stores. RTH is the DIY consumer variant but still good. Contractors can get the models with 5 yr warranties.
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Old 06-23-2010, 02:24 PM   #12
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Quote:
Originally Posted by yuri View Post
The C connection is an option to run with no batteries and the majority run off batteries so 4 wires is all you need. A few exotic ultraslim ones don't use batteries but they are rare. If it says uses batteries on the box you are okay. The 2 Honeywells both run off batteries without a C.
Aren't the batteries only used when power fails, as a memory keep-alive?
A one or two volt drop across the 'closed' t'stat contacts (as opposed to supplying a true 24v) would be enough to power the guts of the thing.
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Old 06-23-2010, 05:07 PM   #13
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Originally Posted by Yoyizit View Post
Aren't the batteries only used when power fails, as a memory keep-alive?
A one or two volt drop across the 'closed' t'stat contacts (as opposed to supplying a true 24v) would be enough to power the guts of the thing.
Those power stealing thermostats have cost the general customer on a whole probably over $100,000 in service calls over the years. For all the trouble they can and do cause.
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Old 06-23-2010, 07:29 PM   #14
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The batteries completely operate the tstat unless it is wired into the the C and a wire run from C to the board. Good quality tstats will sometimes go up to 3 yrs w/o replacing the batteries. With the C wired up the batteries work in a power failure. Good quality tstats have a temporary memory so they won't need reprogramming if the batteries are removed for a minute or so. You get what you pay for.

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