Converting a line level boiler thermostat to low voltage

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  #1  
Old 01-11-16, 05:17 AM
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Converting a line level boiler thermostat to low voltage

I have a strange setup in my home.

There are three zones of a/c and baseboard heating from an oil boiler.

In two zones, the thermostats are combined (a/c and heat). In the third, an extension built much later than the rest of the house, there are separate a/c and heat thermostats (all three zones had heat first; a/c was added later).

The separate heat thermostat is line level; I confirmed this by opening it up, and it has "high voltage" printed on it; a multimeter showed standard household AC voltage. All of the other thermostats are low voltage (I switched one to a Nest), as is the a/c control in that room.

What is involved in converting that single heat controller to low voltage? Is it something best done by a HVAC specialist or an electrician? Could they reuse the existing high voltage wiring?
 
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  #2  
Old 01-11-16, 10:49 AM
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You have a strange setup in that the two zones are 24vac and one 120vac. That doesn't make a lot of sense to convert two and not the third.

Do you have one circulator and two zone valves or two circulators ?

You can reuse the existing cable, probably a two wire cable, for just heat control.

We'd need to know where that stat connected to offer further help. You could take and post a few pictures of your wiring and we might be able to go from there.
http://www.doityourself.com/forum/el...-pictures.html
 
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Old 01-11-16, 11:34 AM
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Such a conversion typically involves a 120/24 VAC transformer, a 24 VAC thermostat and a 24 VAC relay used to switch on the 120VAC when the thermostat calls for heat.


Honey well used to have a box they manufactured that included a transformer and relay in a neat package. I don't know if they still manufacture that --- you might find it as someone'stock on ebay.
 
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Old 01-15-16, 03:37 PM
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Thanks for the replies.

There are four circulators: there were four zones, but after Hurricane Sandy destroyed our basement we didn't repair the basement baseboard, electing to let the basement be heated by the boiler itself (all partition walls are now gone, so that's feasible). I think that fourth zone was low voltage, too (it was just one of those round Honeywell thermostats). So, there are three working circulators for the three zones, and 3 a/c blowers.

The 120vac zone is the newest heating zone, part of an addition. The other zones weren't converted to 24v in my memory, meaning that whenever that conversion was done, it was done a LONG time before the addition was built. The contractor did line level even though the rest of the zones were 24v. I can't fathom why.

I don't know if this is what you need, but this is the controller:

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(Sorry for the delayed reply; I'm subscribed to the thread, but I didn't see any notifications of replies.)
 
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Old 01-15-16, 04:20 PM
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Ok.... that is a 3 zone controller. It will accept up to three stats and can run zone valves or pumps.

If the stat wire that you are working with connects here..... it's 24vac.... not high voltage.

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You said you checked the wiring for 120v and you did measure 120VAC.
Was that across the wires at the thermostat ?
 
  #6  
Old 01-16-16, 03:51 PM
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Yes, measured at the thermostat, which itself is also marked as being for high voltage. Does that mean that this low voltage connects somewhere that converts it to line voltage?

Isn't that... odd? Why would someone do that?
 
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Old 01-16-16, 06:34 PM
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I suspect one of the zones wired to the relay panel was originally for the basement zone. You should be able to use that set of terminals for the addition & move the wiring for the basement zone circulator to the addition's circulator.
 
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Old 01-16-16, 06:53 PM
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It is not uncommon to use an old line powered thermostat for 24vac. It's basically a switch that connects the two wires together.

BUT if you measure 120vac across those thermostat wires.... then you do have a 120v system. How it's connected could be anyone's guess. Your going to need to find out where it's connected. Possibly you can look in the basement and see if that cable.... probably BX.... comes out thru the floor.

If not..... post more pictures of the wiring and any other control boxes that are there.

That controller is a Taco SR503 and it is not wired to control the boiler so there are more controllers.

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  #9  
Old 01-17-16, 11:17 AM
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The boiler is likely controlled by a triple aquastat.
 
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Old 01-17-16, 11:37 AM
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I agree..... it's just strange that the zone control panel is not telling the aquastat it needs more heat. I'm guessing when one of those three zones called for heat.... the boiler temp would sag and then the aquastat would increase the heat.
 
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Old 01-17-16, 12:04 PM
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I know what you mean, Pete but I've seen hundreds of boiler set up with RA89A switching relays for each zone & nothing except the aquastat controlling the boiler.
 
  #12  
Old 01-18-16, 07:57 AM
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Unless I'm tracing it wrong, it looks like the wiring set at the far-left bottom (i.e., the leftmost "???" that PJmax labeled in post #8) is the one that goes to the 110vac zone.
 
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Old 01-18-16, 01:23 PM
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This wiring is making no sense at all to me. This control is not wired to match either of Taco's diagrams.

Vanderdecker: Can you tell us to what the wires on terminals 'ZC' & 'H' connect?
 
  #14  
Old 01-18-16, 04:03 PM
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ZC leads, as best as I can trace, to the high voltage thermostat--i.e. the jacketed cabling that the ZC is a part of goes to an enclosure, and the only other wire that comes from the enclosure goes up into the wall around where the thermostat ought to be.

H goes out to the external boiler power switch.

FWIW, here's the unit. H goes to the switch mounted midway up on the closest side of the unit. You can sort of see the jacketed cable with the ZC wire looping down from the left side of the controller, up to and alongside the red switch box and then up (it doesn't enter the box at all). That bundle of wires going up from the switch box also includes all of the thermostat wires, though of course most of them bypass the switch... they just tied those wires to the incoming conduit coming down from the ceiling.

There's a second boiler switch just out of sight on the ceiling; the electrician put it there after the basement was rewired, following the post-Hurricane Sandy flooding. I don't know why the boiler guy added a second switch.

But all this wiring weirdness predates that. As I said, there were four zones at that time, so I don't know if they used a different sort of controller that accommodated four zones directly or if there was some other workaround.


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  #15  
Old 01-18-16, 04:28 PM
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I can see the wiring except to the pumps. If you could take another picture just like the second one here with and with the ties cut from the cables that supply the pumps.

Here's the updated information. That's a real kluge job. The left two cables are the 120v stat and the circulator pump it controls. They just took power off that control box. Basically used it as a splice box. The white romex is the stat.... correct ?

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