standing wall heater, milivolt/microamp volt readings?

[acguytx]

ok so i dont work on a lot of these but on a service call for no heat,

The pilot lights and stays lit, it puts out voltage to gas valve at around 105Ua, checked the t-stat and it works and i jumped it out just in case and no gas will come out of valve and I also cleaned the the thermocouple and checked all limits.. all seem to be working yet no gas coming from valve.

So two questions how many micro amps/volts does it take to open the valve and what is the reading supposed to be coming from a flame generated sensor?

Im pretty sure its the gas valve but just double checking..thanks

[martyinlincoln]

Good thermocouple should put out around 25 millivolts,no lower than 20. Meter can't be set on microamps to check thermocouples.

[DOGBOY]

are you talking about a thermocouple or a thermopile? above post is good for thermocouple but thermopile is around 750 millivolts. You can tell it is a thermopile as it will be larger than a thermocouple.

dogboy

[acguytx]

are you talking about a thermocouple or a thermopile? above post is good for thermocouple but thermopile is around 750 millivolts. You can tell it is a thermopile as it will be larger than a thermocouple.

dogboy

i guess its a thermopile because IT GENERATES THE MILLIVOLTS needed to power the tstat and gas valve on a wall heater.. how do you measure that to make sure its enough.. i had 105 microamps so it was putting out volts so where do i take the reading and whats the normal range?

[DOGBOY]

i guess its a thermopile because IT GENERATES THE MILLIVOLTS needed to power the tstat and gas valve on a wall heater.. how do you measure that to make sure its enough.. i had 105 microamps so it was putting out volts so where do i take the reading and whats the normal range?

AS stated above generally they put out 750 millivolts. there may be different outputs but the ones I used were 750mv. You got 105 millivolts. So I guess you check it the way you did, with the meter across the thermopile. Microamps has nothing to do with it.

look at the second page.

http://www.robertshawtstats.com/spaw...oltSystems.pdf

dogboy

[acguytx]

AS stated above generally they put out 750 millivolts. there may be different outputs but the ones I used were 750mv. You got 105 millivolts. So I guess you check it the way you did, with the meter across the thermopile. Microamps has nothing to do with it.

look at the second page.

http://www.robertshawtstats.com/spaw...oltSystems.pdf

dogboy

good link, thanks man..

BTW im not sure i was using my meter correctly but i was checking with my FP SC 56 meter with the dial select on the uA setting and that where i measure the 105 at across the first two terminals on the block while flame pilot was lit.. what setting on my meter do i use to measure milivolts ?

[suBcools]

good link, thanks man..

BTW im not sure i was using my meter correctly but i was checking with my FP SC 56 meter with the dial select on the uA setting and that where i measure the 105 at across the first two terminals on the block while flame pilot was lit.. what setting on my meter do i use to measure milivolts ?

On the sc56, set your meter to volts. Then, press the button to the far left (square with a circle around it) until mV is displayed on the screen. That will read millivolts at that point.

Sent from my Evo 3D CDMA using Tapatalk 2

[DOGBOY]

I would say vdc as your meter should auto range down to millivolts. Are you a technician? as this is pretty basic.

dogboy

[acguytx]

I would say vdc as your meter should auto range down to millivolts. Are you a technician? as this is pretty basic.

dogboy

yes and a damn good one but sometimes i run across crap like this that i dont see very often and want to make sure i am doing things correctly.. we are 80% cooling and 20% heating here in DFW,TX and i dont see many Millivolt systems(maybe 6 in 15 yrs) and we typically replace them not repair them so i wanted to brush up on this info..

[beb]

I would say vdc as your meter should auto range down to millivolts. Are you a technician? as this is pretty basic.

dogboy

Ok its millivolts DC. is the pilot strong? oh well just clean the pilot. in millivolt systems the pilot is the problem.
then we can go from there

[kdean1]

If it is a White Rogers thermopile throw it away and get a Honeywell. I've had so many bad White Rogers t'stats and other parts that I refuse to buy them any more.

[DOGBOY]

yes and a damn good one but sometimes i run across crap like this that i dont see very often and want to make sure i am doing things correctly.. we are 80% cooling and 20% heating here in DFW,TX and i dont see many Millivolt systems(maybe 6 in 15 yrs) and we typically replace them not repair them so i wanted to brush up on this info..

I have not seen one in at least 15 years myself. I think it was in the early 90's. My first hvac job was in dfw for td services.

dogboy

[DOGBOY]

another double post.

[Joehvac25]

I used to have a pic of me checking millivolts on my phone, I had to delete it because I was in my speedo but anyways the two Leads off the ppile you hook up to them and its vdc, my hs36 just ranges itself to millivolts. If you jump the stat wires at the gas valve and it works you can tell if the valve is bad or good.

[robatnorfolk]

What brand of gas valve is it? I will post a couple of troubleshooting tips tomorrow.
Does it have a thermocouple and thermopile or just one of the two?
If you pm me the brand i may be able to give you better suggestions.
I install and service wall heaters quite often.

[small change]

One thing to check if the millivolts check out would be a spiderweb in the burner orifice

The love to get in there and spin a web

[catmanacman]

clean the pilot and replace the thermopile

[ECtofix]

Make sure your gas supply gas pressure is good (7 to 14"WC - NO MORE than 14" {1/2 psi} though).

Common issues on millivolt systems:
1. Weak pilot flame due to improper gas pressure or obstructed pilot burner.
2. Faulty electrical connections. ANY poor/corroded/oxydized connection can be a major electrical resistance factor at such low voltages.
3. Faulty operating or limit thermostat.
4. Weak thermopile. Your pilot stays lit though, so I it may be okay.
5. Faulty gas valve.


Typical thermopile output is rated up to 750mVDC, But will drop under load as the main burner is engaged. So, with pilot only, you should read well above 600mVDC. When you set the thermostat to call for heat and main burner coil is energized, thermopile out will drop. You should see a SUSTAINED voltage of around 225 to 250 mVDC (if higher, that's okay) between the "TH" terminals on the combination valve.

In my experience, the main valve will have problems opening if it's only seeing 175mVDC and less. FYI - pilot lockout usually occurs around 75 mVDC.

[mike3]

PG-9 open read factory fresh-750mv
G-250 -------------------------250mv
PG-1---------------------------320mv
With the thermostat calling for heat, take reads at the gas valve as follows. 1- Across the PG terminals 2-across the thermostat terminals 3-Subtract 2 from 1 and that is the voltage left to operate the valve. Most systems will operate with 120mv at the valve, however that should be addressed. If you take an open read across the PG and then take a closed read, you will notice a drop in the reads. This is normal and may be as much as fifty percent. If you get a high read across the thermostat terminals, meaning probably 75 or more it is time to check the type of thermostat or poor splices. One of the problems I have encountered with PG's is the clips on the ends. I always remove, brighten wire and connect directly to gas valve terminals. The most COMMON problem: Dirty pilot