Viessmann WB2B 35.

Viess

Hey my Radial Fan NRG118 that feeds gas to the burner has gone bad. I'm waiting for a replacement. I know the blower runs a bit before the gas valve opens to make sure there isn't an excess of gas in the burner when it lights. But can someone tell me if the same thing is supposed to happen when the gas valve shuts. Is the blower supposed to run a bit to clear the chamber? The reason I ask is because when the gas valve shuts the gas off the blower immediately shuts off too. There's a gas smell and some back flow coming from the venturi tube. It smells like raw natural gas. Any help would be appreciated, Viess.

Paul Pollets

The pre-purge function is compromised by the radial fan's failure. You shouldn't run the unit until the fan is replaced.

Viess

Thanks Paul,
The pre-purge part is working. I was wondering how long after the gas valve closes and the flame goes out before the fan should shut off. It shuts off immediately shouldn't it run awhile to post purge?

Robert O'Brien

No fan post purge.

Viess

Okay thanks that's good to know. It's the bearings/sleeve that's going bad, making a funny noise. What would be causing the gas/combustion chamber air to be coming back through the venturi tube. Is there a back flow valve on the sealed combustion chamber keeping it from coming back inside?

Viess

I meant to ask is there supposed to be some sort of back flow device that keeps the gas/exhaust from coming back inside through the venturi tube/fan assembly?

GW

I may be wrong yet i don't thing that model has a flapper.

Viess

I think you are right. Other models have the flapper gasket right on top oh the fan blower housing. This one just has a gasket.

GW

I'm confusing myself. Some have a flapper just above the blower assy

CMadatMe

Doesn't have a flapper. Wasn't introduced until B2HA when they approved common venting. Need to check vent system for cross contamination. Vented coaxail or 2 pipe? Also stick the analyzer up at the top of the HX where the Vent adapter enters into it. There's a push gasket the holds that vent adapter in place similar to a no-chaulk shower drain. If it gets pinch from over aggressively sticking the vent pipe in you can get exhaust leaking into the cabinet and sucking some exhaust into the venturi and blower. There's usually a reason and a cause why the blower went and 90% of the time it's a vent issue that cause the problem.

Paul Pollets

I've seen pinched vent gaskets 2x when the installer didn't lube the gasket at top of cabinet and forced the vent connector through. In both cases, the inside of the cabinet had large amounts of rust and corrosion.

Viess

I had the HVAC guy replace the fan assembly he then checked the unit for a gas leak. The only gas he found was what I had found. There was some coming from the venturi tube after the gas and fan shut off. He thinks this is normal because that part of the cabinet is somewhat sealed. All of the piping to and from the HX, including the gas line are rubber grommeted. The cabinet cover has a thick rubber gasket like material that seals the front of the HX box. I can't smell it now with the cover on but I still don't know if there is supposed to be backflow gas/exhaust coming from the venturi tube when the boiler shuts down.

GW

I've had one or two Viessmanns leak gas by the valve; what a pain that was.

Viess

Your saying the gas valve didn't fully close? What seems to be happening in my situation is when the boiler shuts off and the gas valve shuts some gas/exhaust comes back through the venturi tube then it stops. There isn't a steady flow just a small amount right after it initially shuts.

GW

I don't recall the exact details, only that I had a WB1A and a WB2B that needed a new valve due to gas odors (customer complaining of smelling raw gas)

CMadatMe

Viess said:

I had the HVAC guy replace the fan assembly he then checked the unit for a gas leak. The only gas he found was what I had found. There was some coming from the venturi tube after the gas and fan shut off. He thinks this is normal because that part of the cabinet is somewhat sealed. All of the piping to and from the HX, including the gas line are rubber grommeted. The cabinet cover has a thick rubber gasket like material that seals the front of the HX box. I can't smell it now with the cover on but I still don't know if there is supposed to be backflow gas/exhaust coming from the venturi tube when the boiler shuts down.

I'm going to assume when you say "Gas" you mean exhaust. Once the burner shuts down the flame safeguard performs a gas valve proof of closure test. If flame safeguard see's no flame boiler then goes into a post purge speed test and gas valve should be fully closed.

Are you sure its not flue gas recirculation? How is the boiler vented?

GW

I should have been more clear, the odors were detected outdoors in the area of the vent termination

Viess

It uses a coaxial vent pipe. The boiler has a 90 attached at top of the boiler from there it goes through the wall to the great outdoors. Total run around 24 inches or so. Don't think it a venting problem. Now that the cover is back on I don't smell anything.

CMadatMe

i'm going to assume the protective screen was used. Did anyone stick their analyzer into the Fresh Air Test port at the flue adaptor and check for flue gas recirculation? That will takeaway the "I don't think."

The protective screen has that half dollar size solid circle dead center and I've seen many times where you will get flue gas recirculation as the flue gases bounce off it and are pushed down towards the fresh air intake on the termination dependent on wind and location of the vent termination.

They didn't think they had a vent issue in the pic.

Viess

No Chris they didn't put the bird cage screen over the end. I see what you mean though. No the guy who swapped out the inducer motor didn't use an analyzer to check the fresh air port for flue gas recirculation. Now that you bring it up seems like a simple and logical thing to do. I guess all serviceman aren't created equally. Where can a fellow get a basic flue gas analyzer at a reasonable price?

GW

If you didn't have the bird cage you're likely not recircing. Those things are a real head scratcher for sure.

So you're saying you Can smell it when the cover is off?

CMadatMe

GW said:

If you didn't have the bird cage you're likely not recircing. Those things are a real head scratcher for sure.



So you're saying you Can smell it when the cover is off?

I'd still check. He's venting Coaxial and could be coming from a connection. Also would check inside the cabinet above the HX where the flue adaptor seats into the HX.