24Vac from Honeywell 7224 for wifi thermostat's c wire

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Old 01-01-21, 11:39 AM
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24Vac from Honeywell 7224 for wifi thermostat's c wire

Hi all,

My home has a Honeywell L7224 aquastat controller (manual) which is connected to a couple of two-wire Hunter battery-operated thermostats, which I am looking to replacing with Honeywell RTH9585WF Smart Thermostat (wifi) which require a C wire. At my skill level I can run wires so I'm not concerned about that.

Unfortunately the L7224 doesn't have a obvious C wire connection but I found an old but promising thread here, Wiring a new Honeywell Thermostat to Honeywell Aquastat Controller in which @PJmax suggested that there may be 24Vac between terminals marked T and 2 in the schematic. I measured this yesterday and I measure ~27Vac across those terminals, and ~13Vac between T and 1.

My question: can I connect the C wire to terminal 2 of this aquastat and pull it up to the thermostat?

Secondly, I have another zone in the basement, and also a hot water heater connected to this aquastat. How would I handle the second thermostat?
 

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01-02-21, 11:10 AM
doughess
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Here is photo of DH zone control panel mentioned in Post #7 showing easy to see wiring. No zones were active so only LED for 24 volt power was lit.

White page on upper left List 4 step to activate zone and basic color coded wiring for all zones. Labels on right side on have blue tape for thermostats across top and yellow tape wire for zone valves on bottom. Terminal blocks with screw make wiring and changes easy.

Panel is protected by removeable Plexiglass plastic sheet.

A second text sheet, out of picture, at bottom explains color codes and physical location of zone vales.

Bring all zone wiring to central panel makes life easier for both DH and future repair person. Post this hoping it will make it easier for others.
 

Last edited by doughess; 01-02-21 at 11:38 AM.
  #2  
Old 01-01-21, 12:43 PM
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GoHelloGo: To both questions the answer is YES.

Modern thermostat use very little power for their electronics. The heating contacts, RH and W usually rated at 3 amps, far less than most heater T T controls draw.

How much spare power the L7442 has for other things, i.e. zone valve is another issue.

To simplify things am using one 24 volt 75 VA transformer for everything.... 8 zone vales, 5 thermostats, electrics stack vent, out-door-reset/aquastat, etc..

Caution, there can be issues using several 24 volt sources in one residence. That is why have separate RHeating and RCooling terminals for 24 volts to activate unit. If system has only one combo heating and cooling terminals are jumpered
.
 

Last edited by doughess; 01-01-21 at 02:36 PM.
  #3  
Old 01-01-21, 12:56 PM
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If you have two thermostats/zones then normally the thermostats do not directly connect to the Aquastat. Please verify where they actually connect to.

For that Aquastat the first T is W, the T/3 is R and the 2 is C.
 
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Old 01-01-21, 05:18 PM
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@doughness Thank you for your reply and confirmation. Based on the following, I may not even need to worry about 24Vac from the aquastat...

@Astuff, You're correct, the thermostats are not directly connected to the aquastat. Your reply made me look again to see how the current wiring is set up, and while doing that I found something I didn't notice the first time: up high there is already a Honeywell AT72D1683 transformer which is supplying 24Vac to the thermostat on the hot water heater and zone controllers (valves?).

So only the two thermostats (1st floor and basement) are on old, two-wire connections. Given that, my plan is to pull new 18/5 to/from the two thermostats to replace the existing 18/2, and then hook up the Common wire from the AT72D1683 transformer to the Blue wire in the 18/5 bundle to the new thermostat. Does that sound right?
 
  #5  
Old 01-01-21, 05:41 PM
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If GoHelloGo has typical system, thermostats contacts start zone valve motor. When valve opens, end switch closes to activate circulator or its controller. Wires from several end switches are connected in parallel to circulator for those zones. DH has 2 circulators activated by 8 send switches.

This another instance of Honeywell causing confusion by labeling L7224 an aquastat when is multi function item..

“The L7224U controls the circulator, oil burner and boiler temperature. “

https://s3.amazonaws.com/s3.supplyho..._PROD_FILE.pdf
 
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Old 01-01-21, 07:03 PM
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Welcome to the forums.

That transformer is switched thru thermostats to zone valves. The zone valves then activate the boiler.
It would be best to post a picture or two of the wiring at the zone valves to confirm their wiring and C connection.

How-to-insert-pictures.
 
  #7  
Old 01-01-21, 07:34 PM
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DH 1057 split level home started with one line feeding all elements. Over years, split to 8 zones, with thermostats and valves in various places and 2 circulators.

When issues arose, diagnostics, trouble shooting and repair was real challenge.

Some years ago brought all thermostat and zone valve wiring, laid out with labels in flow diagram pattern to 2 foot square plywood panel on wall at boiler. Also wires to activate circulator control on boiler and bring 24 VAC. LEDs show what's active.

For this low voltage & current wiring used round casing, interior line AT&T 4 wire #22 wire, and U shaped staples.

DH has long experience with complex systems, likes to simplify things.

When wife question temperature in some room show her boiler temp i.e. 165F then which thermostats and circulators active.
 

Last edited by doughess; 01-01-21 at 08:01 PM.
  #8  
Old 01-01-21, 08:23 PM
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The zone valves then activate the boiler
.
Zone valve end switches are used to activate something. Sometimes circulators or Honeywell L7440 aquastat controls, even boilers ... but not always
 
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Old 01-01-21, 08:43 PM
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Can we wait for pictures to comment further ???
 
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Old 01-01-21, 08:52 PM
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Thanks for the responses. Here are some pictures as @PJmax requested, which should clarify things: first an overall picture of the furnace; second a picture of the 24Vac transformer, and finally the junction box where I will make changes.

Refer to the junction box image: my plan is to replace the Thermostat #1 and #2 wiring which currently is two-wire, with a 18/5 cable, reconnect the two wires as they are today, and then use the Blue wire from the new cable and connect it to the "C" (white) wire from the 24Vac Transformer which is coming in from the top of the Junction box. With that done, my understanding is that I should be able to connect the new wifi thermostat which requires a C wire. Please review and let me know if I am about to do something crazy

First image is furnace, showing three zones (cellar, hot water and 1st floor), and the wiring which goes to the junction box


Furnace


Below is the transformer which supplies 24Vac and enters the junction box at the top


AT72D1683 24Vac 40VA Transformer

Junction box showing wires (labeled in image). Thermostat #1 and #2 are two-wire (old), rest are all 18/5, except for T-T and the wires from the transformer.

Junction box (yes, it's a mess!)
 
  #11  
Old 01-01-21, 09:35 PM
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It's a lot of connections in a small space.
Your transformer may be labeled C and R but it doesn't mean that it was wired that way.
When only two wires is used to connect the stat.... there is no polarity or direction of colors.

What is important is that the one of the transformer leads goes to the R of the thermostat.
The W of the thermostat goes to the TH terminal on the ZV.
The other transformer lead goes to the TR terminal.


On these zone valves.... TH-TR is just a splice point. It doesn't connect to anything in the ZV.
 
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  #12  
Old 01-01-21, 09:54 PM
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That 4” square electric box is not required for low voltage 24 VAC thermostat wiring. Maze of wire nuts and jumbled wires looks hard to work on.

Terminal 12 strip in link below can join several #18 to # 22 wires and be cut to number needed, Every easy to add or remove wires. Link also has many useful items for heating systems

https://www.allelectronics.com/item/...l-block/1.html

Rather than covering panel, use with clear plastic sheet area so LEDs are visible. Am not into cosmetics, prefer easy to see status and repair.
 
  #13  
Old 01-02-21, 11:10 AM
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Here is photo of DH zone control panel mentioned in Post #7 showing easy to see wiring. No zones were active so only LED for 24 volt power was lit.

White page on upper left List 4 step to activate zone and basic color coded wiring for all zones. Labels on right side on have blue tape for thermostats across top and yellow tape wire for zone valves on bottom. Terminal blocks with screw make wiring and changes easy.

Panel is protected by removeable Plexiglass plastic sheet.

A second text sheet, out of picture, at bottom explains color codes and physical location of zone vales.

Bring all zone wiring to central panel makes life easier for both DH and future repair person. Post this hoping it will make it easier for others.
 
GoHelloGo, zoesdad voted this post useful.

Last edited by doughess; 01-02-21 at 11:38 AM.
  #14  
Old 01-02-21, 06:44 PM
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PJmax The wiring is as you show in reply #11 except the C wire not there (yet).
Good news is the rewiring the thermostat #2 in the basement with 18/5 and C wire is complete. Unfortunately ran into a snag (literally) with pulling cable up to the first floor for thermostat #1, and going to stop for the day. More as I get along further.

doughess Your set up is well organized and giving me ideas to do something better than the junction box in the future.
 
  #15  
Old 01-03-21, 11:28 AM
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Happy to report that the 1st floor thermostat is also wired up with new 18/5, C wire connected to the smart thermostat, all running perfectly so far. Thanks to PJmax doughess and Astuff for taking time to reply to this thread. Much appreciated.
 
 

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