gas valve diagnostics

rob

Those old CAC/BDP gas controls actually had a small resistance heater inside that acted as a time delay. The pick, hold and main style valves. Think about that, a reistance element in the same area as methane gas.......
No wonder UTEC got out of the gas control biz..
BTW, the gas valve is outside the furnace jacket the gas control is inside and has wires attached to it.

GW

gas valve diagnostics

heat guys,

I remember a service call I did a year or two ago that left me feeling a little jostled. I'm trying to get some advice so I can feed my Palm Treo with some good info (I have a good library of technical notes going). Unless I'm simply ill-informed, there is no data on ohm readings for gas valve coil windings. I would very much enjoy having some guidlines on this.

Tim, if your out there you helped me out on this when this happened, I even mailed you the bad valve so you could possibly use it in you classes.

Thanks for any help on this!

gary



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Unknown

Gary, I have

most of that data in my books at the training center. I will look it up for you on Monday and post it here in case others are also interested.

Darin(in Michigan)

Most of the gas valves I"ve seen were between 20 and 100 ohms. However, the old Carrier 3 wire gas valves(the blocky ones with 5 terminals) had resistances as high as 230 ohms and still worked.

Darin(in Michigan)

Thanks rob.

I'll keep that in mind

Unknown

Actually Rob the heater element was isolated from the gas

its job was to delay bringing the valve on . They were called time delay valves. The other valve manufactuerers Honwywell, White Rodgers and Robertshaw also had these valves.

The pick and hold valves where first devloped by White Rodgers and used on gas dryers. They later found there way into use on heating systems and Robertshaw had a system also that was used with a mercury pilot.

The old Bryant time delay valves Models 638, 639 and 640 I believe were made by Bryant I do not think UTEC ever produced a gas valve.

The time delay principle is still in use today.

rob

Its probably

just about the only control they don't make. I've wondered why HW or WR seems the best the control of oem choice.
Edit... and I'll bet the new TDR gas controls do not use a resistance heater or do they??

Unknown

What does TDR stand for

I may know them by a different title?

Unknown

Gary, here you go

Let me first say that using Ohms readings on gas valve coils is not the best way to troubleshoot them. One of the reasons is that some gas valves today use a diode built into the gas valve. This means that when 24 Volts AC is fed to the valve it is convertd to DC before it goes to the coil. So the reading you would get when going across the diode would be infinity on one side and resistance on the other. You are better off using proper 24 volt troubleshooting procedures which I teach in my classes that are foolproof. You can also look at the amperage draw of the gas vale and then using an Ampmeter see if it matches up.

Here because you have asked are some resistance readings for gas valves.

Honeywell VR8200 & VR8300 Series (Has a diode) 50 ohms one way 85 ohms the other.

Honeywell V800 series (Older valves) 30 ohms

Robertshaw 7000 series 50 ohms

Robertshaw 7200 series (New with red lever) has diode 55 ohms one way infinity the other way.

Robertshaw (700-406) and Honeywell(VS820 and VS821) self generating valves (Powerpile) the coils are 1.5 to 2.0 ohms and the power units are 11 ohms.

Honeywell VR8345 Universal repalcement PV to MV/PV 30 ohms one way infinity the other due to diode. MV to MV/PV 85 ohms one way infinity the other due to diode.

Robertshaw 720-079 Universal replacement gas valve P to C 50 ohms M to C 100 ohms

Most White Rodgers valves 36C, 36D and the new 36E are solenoid valves and will read around 90 ohms across the solenoid.

Hope this helps.

Empire_2

Tim, A Q for you

Just for my own curriousity, what or why would anybody even care what the ohms on the coils are, (unless commercial expensive valves) if the valve dont open it's bad. right? Just looking to see if I may be not doing my job correctly. I dont have time to ohm out a coil,...If I am applying power and the valve does not open you bet I will replace it.

Unknown

Yes that is correct

just be careful that you do not have a broken ground wire after the gas valve. The way to prove that is this: if you have 24 volts to the valve and the burner does not light (assuming the gas is okay) then check on the other side of the valve you should have "0" zero volts, if you read 24 volts on both sides of the gas valve you have a broken ground wire. Then to prove the broken ground wire run a new wire back to the "C" common terminal of the transformer the valve should work. Thats it your done problem solved.

Dale

gas valves

Tim's insightful as usual. True 99% of the time if you have power and no flame you have a bad valve. Sometimes though, and this will get you with spark pilot the gas is restricted and while the pilot lights fine the main burner doesn't so, watch the upstream gas pressure gauge when the valve is powered and then move to manifold pressure if upstream is OK. Also, some power stealing stats have you seeing 24 volts with a good digital meter when a test light won't light because of the ultra low amps, then there's the shorted valve that takes out your tfmr, if you ohm the coil and see it's too little ohms on both sides you have a tfmr or fuse killer. It's an interesting business.

GW

valves

Yes, 99% seems like a good number to me. It's that 100th one that will bite you firmly. The issue I had was continuous call backs, so on the 3rd visit I brought a new valve and compared the ohms and sure enough I saw a difference and replaced it.

Gary

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Unknown

Gary I assume the

valve was not working every time. The higher resistance would tell you that but so also would an increased amp draw from the rated amp draw on the gas valve.

You also have to be careful with ohms readings on gas valves as they are doing so many things internally to valves to make them last longer and todays ohm reading may not be the same tomorrow.

I took five valves out of the box at my center today and tested all of them and they all were slightly different.

Darin(in Michigan)

Hey Tim

I've enjoyed your input on everything you write on but I would like your input on the statement you made about an open ground circuit. You stated(as I read it) that if you read 24 volts on both sides of the valve, the ground circuit is open. For troubleshooting purposes because there is no path to the other side of the source(i.e. the transformer) the voltage may read anything between 0 and 24 volts(what I like to call funky voltage?)
Whoops read it again and it made sense.