# Control Module to Gas Valve Questions



## poly (Oct 29, 2015)

Hey all,

I'm currently in school for hvac. Fairly new. We've been looking at schematics and wired a furnace with a spark igniter today. 

I did this up quickly to ask on another place http://i.imgur.com/D2QMVjr.jpg

Soon after I found the schematic that I saw that on much earlier http://i.imgur.com/l8L5Olq.jpg?1

So to sum things up:
Control module MV to Gas Valve TH
Control module MV/PV to Gas Valve TR
Control module PV to Gas Valve TH/TR
Control module 24V GND to Transformer Return
Control module GND 120 to Ground

Now we did this wiring today and the spark igniter and everything went fine. But the other place I asked this on a bit ago I got a response saying that's wrong. The other guy said (which is much different than how we did it):



> The valve coil itself is TH and TR. When you want the valve to open you need 24v+ on one and 24v- on the other. On your module MV is 24+ to the valve and MV/PV is your 24- common. There is no pilot (PV)
> 
> So, you only need two wires Leave PV on the module and TH/TR on the valve empty


Bare in mind I'm completely new to control modules and gas valve wiring. We have been on wiring for a couple weeks and always were told to skip them and to focus on everything else to troubleshoot the basics.

So maybe (probably) I'm not understanding the function of these connections.

MV wired to TH makes sense to open the valve for gas flow on a call for heat, but I don't really get the flow from there about what happens in what order.

Sorry for the long speech here. Just hoping somebody can dumb this down. Taking in so much information at once that even simple things seem confusing at times.


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## user_12345a (Nov 23, 2014)

No tech here, just from experience troubleshooting:

The reply in quotes makes zero sense.

You're dealing with intermittent pilot ignition; there's pv, mv, and common. (label will vary for common)

You do need three wires to the valve.

The common is shared; there are two solenoids, pv opens first, spark lights the gas, module detects flame then energizes mv as well.

Most modern furnaces sold now have hot surface ignition which doesn't use a pilot; ignitor heats up, then power is applied to the main valve. Uses a totally different valve.

-------
On the schematic, if you want to visualize it, draw a solenoid coil (the electromagnet that opens each valve) between mv to mv/pv and mv/pv to pv.

When the module applies power to pv, it travels through the solenoid coil that opens the pilot valve, to mv/pv back through the module.

When both are energized, the wire connected to mv/pv serves as a return path to complete the circuit for both valves.


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## supers05 (May 23, 2015)

All gas valves that you'll see in residential are called redundant valves. So they have 2 valves inside. One called pilot valve and one called main valve. They both need to open for it to work. It doesn't matter that we don't use standing pilots anymore, the name stuck. Sometimes also called Th and Tr respectively. Th/Tr and MV/PV are the same thing, which is a common between the 2 valves. 

(larger burners still use intermittent pilots though with the same terminology and wiring) 

Some valves are only 2 wire systems now (Th-Th) with internal wiring to the redundant valve. Others are "smart valves" which have the ignition module built in. 

Cheers!


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## user_12345a (Nov 23, 2014)

Intermittent pilot systems will always have three terminals on the valve with the module verifying a lit pilot before opening the main valve. This is different from smart valve, electronic ignition without pilot (2 terminals/wires, what happens internally is a different story), or standing pilot.

Intermittent pilot is still found on new boilers and commercial garage heaters, but has been extinct in new furnaces for at least 15 years. The most common is now hot surface ignition where the igniter burns out after so many cycles. Any pilot-less system will use a valve without a pilot gas connection, just main manifold.

The intermittent pilot to me is a very good system, but it probably costs too much to implement these days.


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## beenthere (Oct 11, 2008)

You probably got an answer from someone that is only familiar with standing pilot valves, or direct ignition valves. Your wiring explanation is correct as I read it.


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## poly (Oct 29, 2015)

Thanks so much guys. I was racking my brain trying to come up with a reason why I was wrong


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