24 Vac Transformer Primary Open Problem

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  #1  
Old 09-09-07, 09:35 AM
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24 Vac Transformer Primary Open Problem

I installed a new hot water furnace. It has a standing pilot. The plummer filled it and turned on the power. There was a click and then nothing.
I traced the problem to the 120 Vac/ 24 Vac transformer. The primary shows that it is open circuit. The secondary appears OK. Of course, there is no output 24 Vac. I checked the zone valve and thermostat wiring and all appears OK. The transformer looks like a typical door bell type. I thought that they were rated for short circuit conditions. If there was a short, could this have caused the transformer primary to open with some sort of internal fuse link? One of the wires on the zone valve was loose and could have been shorted to ground. (It appeared to be connected but was actually broken when I tried to move it a bit.
 
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  #2  
Old 09-09-07, 10:06 AM
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some sort of internal fuse --

They should be able to handle a short. Sounds bad. Try a new one.

You may need a larger x-former.
 
  #3  
Old 09-09-07, 10:37 AM
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The transformer is part of the furnace. It has the transformer and the control relay mounted to a plate that is then mounted to a 4" box inside the furnace. I guess that I will need to get a new part from the furnace manufacturer. Have you ever heard of one of these transformers blowing at turn on?
 
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Old 09-09-07, 10:42 AM
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Rare, But yes.

Have you disconnected the LVolt side, and check JUST the x-former?
For LV out put, maybe let it cool down and try again.

You should be able to get one at the big box. Match the
VA.
 

Last edited by lectriclee; 09-09-07 at 10:55 AM. Reason: store
  #5  
Old 09-09-07, 11:06 AM
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I think it worked for less than a second and then NO output. I disconnected everything from the output and there was no voltage. Since the primary winding reads infinite ohms, my guess is that the primary is open. The problem is that the output has a built-in terminal board that is customized for the furnace OEM. It is a bakelite type board with several terminals and screws. They include a C, Y, R, W and G. A standard bell transformer from the bigbox place only has two screws on the output.
 
  #6  
Old 09-09-07, 11:25 AM
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To verify an open in the Primary winding , connect one Primary lead to the Neutral (may already be connected ), and test for 120 volts between the other Primary lead and the Black wire of the 120 volt "Feed-In" circuit from the panel. Zero volts = an open Primary winding.

IF the persom who installed the WH also purchased the WH , he should replace the defective control since he "sold" you the WH. He can "pass the buck " to the supply-house who sold HIM a WH with a defective component.

Who wired the WH?
 
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Old 09-09-07, 11:26 AM
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10-4. Good luck.

It should not be hard to find.
 
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Old 09-09-07, 11:39 AM
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PATTBAA:
I did the electrical and a plumber did the water/gas and valves. It was a one for one replacement, that is, the same zone valves and all other stuff as before. I was not downstairs when he turned on the service switch on the side of the boiler. He said he heard a "click". I am positive that the primary is open circuit.
 
  #9  
Old 09-09-07, 11:54 AM
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Does this control have "Th" ( thermostat ) terminals, and does it control the gas valve ?

If you wired the controls powered from the 24 v secondary output, and you made a mistake that "shorted" the secondary, you might "smoke" the transfromer, but that would not cause an immediate open . If the Primary had a defective internal "high-resistance "connection , the first application of power could cause the connection to open.

Who purchased the furnace , and from where ?
 
  #10  
Old 09-09-07, 01:40 PM
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It has R and G for the thermostat. Actually, the thermosats, 3 of them, control the zone valves and then the zone valves make the R and G circuit that closes a relay that then starts the furnace and operates the circulator pump. It was purchased from a wholesaler in NH. The guy will call them in the morning and get a new subassembly that has the transformer and relay. I checked all the wiring agin and it's correct. I even compared it to another fully working system, next door, and it is 100%. I really think the transformer is defective.
 
  #11  
Old 09-09-07, 01:55 PM
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Wink

That is not just a bell transformer you need a 24V and about a 50 VA one there. Also on the tstat dont sound right R is power W is heat G is fan and Y is compressor. Sounds like its one for a furnace with AC on it. I like to run the boiler temp on a just a aquastat And the tstat's control the valves and pump
 
  #12  
Old 09-09-07, 03:00 PM
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I think I made have mispoke. Maybe it was R and C. R and C is 24 Vac that goes up to the thermostats and then back to the zone valves. The zone valves, terminals 2 and 3 go back to the furnace R and G. When there is a contact closure at R and G, the furnace fires up. When the thermostat stops calling for heat, the zone valve opens and the R and G circuit opens thereby shutting down the furnace.
 
  #13  
Old 09-10-07, 09:16 AM
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I'd be inclinded to use a sperate transformer to power the zone-valves.

The 3 end-switches on the Z-V's could be wired in parallel , and connect to the "T" (thermosat ) terminals of a switching-relay (S-R) . One normally-open contact on the S-R could close a circuit to the gas-valve , and the other N-O contact could close a 120 v circuit to the circulator.

A dual-function aquastat could "maintain" the water at a hot temperature , and also---- Very Important !! --- provide "hi-limit" protection .

DO NOT wire furnace -safteys in a low-voltage circuit !!! #18 wire is too fragile for protection wiring.

What can you tell us about how the gas-valve is wired-- does the G-V operate on 24 volts , and was it possibly prewired to the "W" terminal of the S-R that has the defective transformer ?
 
  #14  
Old 09-12-07, 04:40 PM
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Here is what happened.
Two of the zone valved failed when the last transformer blew up. It had nothing to do with the water. Three new zone valves went in. I put in a new zone valve controller that has a built in 40 VA for three valves according to TACO. The valves wire to this new box that has indicators and more relays. It has an isolated set of contacts. This appears to be the way to do it. I talked to the TACO engineer and he explained that the furnace xfmr would work but the next trick is to use another xfmr. The trick is that the additional xfmr "floats" on top of the furnace xfmr. The 2 circuits are independent in that current only flows in each circuit. The DOWN side is that if there is a fault, like a short at a valve or at a thermostat, there is 48 Vac from the top of the valve isolation xfmr to ground. I look at the set up as a trick and do not like it. The zone valve controller is the way to go.
 
  #15  
Old 09-12-07, 08:59 PM
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Wink

I like to let the zone valves and pump work and call for heat on their own transformer say a 50 or better Va. Let the boiler run on its own transformer and control the water by its temp by a good immersion aquastat. That way they cant mix and have a down side.
 
  #16  
Old 09-13-07, 08:50 AM
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N-! can you please provide "product" info on the TACO multi -zone controller , such as the TACO catalog # ? -- Is this an immersion-type control ?

What protects the furnace from excessive water temperature, which is the "Hi-Limit" control ?
 
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