need to wire a simple thermostat
We service a furnace and the customer is in her nineties and is really afraid of digital thermostats and defiantly thermostats with batteries. She and her care giver asked us to install a HW "Round" that they had picked up. I installed it and it worked for a bit. But after a few minuets, it quit which I thought might happen. We had previously installed the simplest battery thermostat we could find Honeywell TH1110E1000/U but she just wants a simple non battery thermostat.
I picked a HW CT31A which is a mechanical thermostat but it does not have the RC connection which is needed for this furnace/air handler.
Almost every elderly person I run into wants a simple Honeywell Round thermostat but HW shifted the rounds to power robing design and it really pisses me of that it is often incompatible and they won't step up to the plate and provide what people want. But I digress.
The Furnace/AC is a Boyerton LBD85 with a coil. this diagram shows a T87F thermostat which I believe is a mechanical, mercury, model.
I read that a resister can be install between W and C on the control board but when I called HW tech support they said that was a custom modification and they could not advise it.
Any advice on how I can better support this customer would be appreciated
Robert
Comments
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Get a T87F from eBay. They aren't common, but they are available.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
The RC wire is for a separate transformer in the condenser, when that was an add-on to a furnace. It's been a while since that was a thing, most everything is all fed from the xfrmr in the furnace nowadays. If there's no separate transformer to energize the contactor in the condenser, no RC wire is needed. If there is, an interposing relay (I suggest a RIB U1C) can be used, energized from the Y call from the thermostat, to isolate the two transformers.
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and at $17 that's a relatively inexpensive solution
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Actualy I ran across those on EBAY last night and am considering getting a few for these circumstances.
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Easy Peasy
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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If there's a spare wire, just power the TH1100E with 24 volts. No batteries needed.
If no exra wire, do they REALLY need G for fan only? Jump Y an G at the furnace fan center relay, NOT at the thermostat, and use the extra wire previously on the G terminal to connect to Common. The heat side doesn't use G for the fan as it goes through the fan/limit control for a different speed.
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This is obviously an oil burner situation. The RIBU1C is a great fix for thermostat problems. And @HVACNUT has a great fix for getting an additional wire for the C terminal if needed.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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looks like exactly what you want:
round mechanical, can support separate heat and cool transformers if you remove a jumper
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er, reading the fine print the honewell has a battery that might require attention every couple decades
this wr/emerson doesn't
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when I use that Honeywell T87N thermostat I sometimes take the fan switch off of the control subbase and break the lever so little old ladies don't touch the wrong switch and call you with a problem. A pair of side cutters does the job nicely.
Just an old trick from an old man in a wheelchair.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I don't know what's going on at Supplyhouse but, Honeywell no longer makes a mechanical round (T87F) thermostat. The manual for the T87K & N clearly states:
This thermostat provides electronic control of:
• 24 VAC heating systems (T87K)
• 24 VAC heating and cooling systems (T87N)0 -
Actualy I ran across those on EBAY last night and am considering getting a few for these circumstances.
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Honeywell does not make any mercury thermostats anymore. All thermostats are either mechanical contact types or electronic. That has been the case since 2010. Mercury is poisonous and cannot be left for us common man to handle. Our government is protecting us from ourselves. Thank you Uncle Sam.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Hey Ed, thank you for responding as you always do in a well thought out help full manner. I think you are responding to the part of my question about using a power robbing T87K or N thermostat. I'm having a hard time wrapping me head around part of it. Normally R and W or RH and W are nothing but a contact that closes when the room temp falls below the setpoint (heating mode) . In this case connected to TT on the Burner Primary Control. When I'm working on a configuration like this I can jumper TT if I need the burner to run. I your example using the RIBU1C, its clear that power between W on the thermostat and W on the fan control close contacts that short TT on the primary firing the burner. but what happens with W on the thermostat, R(RH) is not in play so what is W connected to in the thermostat that completes the circuit?
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the jumper is in place in the thermostat that connects rh and rc together so w connects to rc and powers the relay off of the cooling xfmr which then is an isolated contact closure for the primary control.
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The Honeywell round electronic stat is not a reliable option in my opinion. I had one in my own house and it worked for a while and quit.
And I have tangled with them on other jobs as well.
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I don't know if it's allowable to say this here, but I have at least 20 of the old mechanical HW T87 tstats that I haven't been able to get recycled if anyone really needs one. PM me if you are interested.
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there is a buy sell trade section. not so sure about mailing mercury.
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When the thermostat comes from the factory, I believe there is a jumper between R and Rc. That should stay in place (or you may need to reinstall it). Some power-stealing thermostats have a preference for which transformer they draw power from. I am not familiar enough with the T87N internal electronics to tell you which transformer it prefers when connected to the oil primary with R and W, and also Rc and Y for the AC unit, with the jumper removed.
As a result, you may have issues if the primary control transformer is insufficient for power stealing. By making only one transformer available—the 40 VA air-conditioning transformer—you eliminate any guesswork about which transformer is being used for power stealing and whether that transformer is powerful enough to handle the job. 40 VA has plenty of power for power stealing. Using the RIBU1C makes that easy as you said
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
1
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