Home ยป Tech Talk with John Wilson from Ravenheat testing boiler fans
Off the cuff Tech Talk with John Wilson from Ravenheat testing boiler fans Afternoon guys. Just another little follow in. We touched on the air pressure switch the other day, and the functionality.
On a later video, we’re going to talk about combustion and the movement of gas and air around an atmospheric appliance. Basically when the chamber gets hot with resistances on the air pressure switches, and when the chamber’s cold. We’ll address that on a different video.
I’m staying with older technology for the minute, and we’ll move on to pre-mix at a later date.
One thing we ought to just look at now guys, which is important, is the fan coil, okay? We spoke about the ohms resistances across the air pressure switch the other day, and we’ve done ohms resistances on earlier videos. But let’s just look at the fans. I’ve got a couple down here on the desk. The thing is with the fan, if the ohms resistance is breaking down, either increasing or decreasing, then yes, at some point in the not too distant future, that fan is going to burn out. And basically then you’ll get an OL situation on your multimeter.
And to be honest, as soon as you walk in Mrs. Smith’s kitchen, you can smell the fan. Because you get an acrid smell from the motor. You can see localised burning on either sides of the fan coil. But what is very important guys, there is a period of time where when that fan coil, with it being variable speed, is breaking down, it can create all sorts of crazy things.
Basically you can have a fan that’s running too slow, or you can have a fan that’s running too fast. Now when I was out on the road like you guys, what I had a black book, and every boiler that I went to, I took the ohms resistance of the fan and wrote it down in the book. Because you usually find, when a manufacturer picks an ohms resistance on a particular range of boilers, they will stick to that on that fan. And they won’t deviate from that. And a Ravenheat fan of this type is 45 ohms. So if I just … I’ve got my multimeter on the correct range there, and if I just take a reading here on this fan, we’ll see that we’ve around 45. So we’re 45.5. We’ll not split hairs; there’s 45 ohms.
Now if I take a reading on this fan, then again, you’ll see that the fan, 45 ohms. Exactly the same. But there is a massive difference, guys, between these two fans. If I spin this fan impeller, which is the impeller at the bottom, if I spin this cooling fan impeller here, you’ll see that that is as free as a bird. It’s still spinning. It’s as free as a top. Now, if I can move onto this fan, and I spin this fan impeller. Can you see there, guys, it grinds to a slow halt. Now, as you’ve witnessed, it’s not the coil that’s creating that fan impeller to be stiff. Basically it’s the ohms resistance is the same at 45. It’s here, at this point here in the casement of the fan at the top and underneath in the casement, we have a bearing. Now basically what’s happened there, there’s been some debris got underneath that bearing, and that bearing has swollen.
Now if we’ve got a brass boiler unit and we warm it up with this blow lamp, obviously the brass expands. Everything that’s plus and a minus, we can then open the brass boiler unit, two inch brass boiler unit with us Rothenbergers, with us pump pliers. But the problem is, the disadvantage with brass is it’s very soft. The bearing here is very very hard. Now the plus and minus is we can run a shaft through that bearing for a lifetime, and there’s no real calculable wear which is a fantastic material. The only problem is with the material, when we get debris at the back of it, it swells. And when it swells, it doesn’t go back to its original size. So you’ve probably done throughout your time out on the road, you’ve greased fans with your graphite paste, your duck oil, your goose grease, and you’ve greased that fan bearing, and within a few days, once that lubricant has dissipated, you’re back to still having a swollen fan. So it’s worth being a little bit tentative when you do that. Make sure that it’s not a swollen bearing, okay?
So check for 45 ohms. As we say, manufacturer’s fans do differ. As I spin this fan impeller and it grinds to a slow halt, with 230 volts to that fan, and if that chamber is cold, that air pressure switch will energise. That air pressure switch will drop out and it will go to flame failure when it gets hot in that chamber, because the pressure drops. Okay? So that could create nuisance flame failure on this particular older type atmospheric appliance.
So the moral of the story of this video guys, is always make sure that you check the ohms resistance of the fan, because if it’s running too slow, you could find that that fan impeller’s not stiff, but the fan is running too slow, it’s breaking down. Like we said, it’s variable speed, it will break down eventually. But if it’s breaking down and it’s running too slow, it can create a flame failure situation.

Allen Hart

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