VS8420 Millivolt Gas Valve - is it dead?

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edleno

New Member
Mar 18, 2022
3
Snohomish, WA
Saw various posts on forums on the Honeywell natural gas fireplace valves (VS8420) - got one that the pilot will light and stay lit, but as soon as you attempt to switch the main valve (off/pilot/on control) to the ON position it will "thunk" and turn off the pilot. Some responders say this is a known issue with the Honeywell's, and suggest turning it on VERY SLOWLY (safe cracker hands suggested in one post). Tried that and it still kills the pilot when switching to ON. Tried a new thermopile, and from what I see thermopile is generating 600mv when pilot is held on and not connected, but down to 350mv when connected - data sheet says minimum 460mv when connected.

(1) did these valves have an active "pull in/down" that relied on the thermopile voltage in order to hold the valve in the ON position (belts AND suspender kind of thing, even though it can't turn on without sufficient voltage anyhow). I tried a couple of other fireplaces and I can disconnect the thermopile and turn on the valve - and of course the main burner doesn't light, but the pilot stays on.

(2) next issue - may also indicate a failed valve - I wanted to see if I could increase the pilot's fuel to increase the coverage of the thermopile, but turning the pilot adjustment (after removing the cover screw) has basically little to no effect. Same with the inlet and outlet pressure adjust valves - no effect on the fuel to the pilot assembly.

Time for a new control valve? I see that Honeywell no longer manufactures them, and from dates I would think this valve is probably 20 years old.
 

Fingerlakes Fireplace

Burning Hunk
Jan 9, 2017
188
Upstate NY
The thermocouple supplies power for the pilot selenoid, not the thermopile. Replace the thermocouple, but it probably is a bad valve.
 

edleno

New Member
Mar 18, 2022
3
Snohomish, WA
The thermocouple supplies power for the pilot selenoid, not the thermopile. Replace the thermocouple, but it probably is a bad valve.
I should have mentioned I tried that also - did the whole pilot assembly when I suspected the thermopile since the effort to get at the thermopile is enough that it makes sense to do the whole thing. It sure looks to be a failed valve
 

Jackfine1

New Member
Jun 22, 2022
7
Chattanooga, TN
Saw various posts on forums on the Honeywell natural gas fireplace valves (VS8420) - got one that the pilot will light and stay lit, but as soon as you attempt to switch the main valve (off/pilot/on control) to the ON position it will "thunk" and turn off the pilot. Some responders say this is a known issue with the Honeywell's, and suggest turning it on VERY SLOWLY (safe cracker hands suggested in one post). Tried that and it still kills the pilot when switching to ON. Tried a new thermopile, and from what I see thermopile is generating 600mv when pilot is held on and not connected, but down to 350mv when connected - data sheet says minimum 460mv when connected.

(1) did these valves have an active "pull in/down" that relied on the thermopile voltage in order to hold the valve in the ON position (belts AND suspender kind of thing, even though it can't turn on without sufficient voltage anyhow). I tried a couple of other fireplaces and I can disconnect the thermopile and turn on the valve - and of course the main burner doesn't light, but the pilot stays on.

(2) next issue - may also indicate a failed valve - I wanted to see if I could increase the pilot's fuel to increase the coverage of the thermopile, but turning the pilot adjustment (after removing the cover screw) has basically little to no effect. Same with the inlet and outlet pressure adjust valves - no effect on the fuel to the pilot assembly.

Time for a new control valve? I see that Honeywell no longer manufactures them, and from dates I would think this valve is probably 20 years old.
When a millivolt valve will hold a pilot and then drop out once turned the to on position it is an almost sure indicator the valve is bad or is going bad. As stupid as this sounds, give the valve a few stern hits with a rubber or plastic mallet/hammer. This will sometimes give the valve a little more time before needing replaced. The reason this method works is because there is a magnet in the valve that is activated using the Millivolts supplied by the pilot, over time the cavity where this magnet is will accumulate buildup from contaminates in the gas and wont allow that magnet to move. The force from the hit with a hammer will sometimes loosen up the buildup enough for it to work.