Munchkin, Blown Fuse on Circuit Board

Alan (California Radiant) Forbes

Plenty of Munchkins in my area, but I've never had to service one with a blown fuse on the circuit board. This boiler is a 140M, made in 2002 and had a new circuit board installed 2-3 years ago. There's probably plenty of of things that could short out; gas valve, blower motor.....Can I ohm these components out to see if they are good or bad?

The owner had called me a few days ago to say that her zone valve controller had died. I haven't gone out yet. The two seem unrelated unless there was a power surge.

DZoro

Low voltage connected from boiler to controller? Was it the actual zone valve? Could have followed the 24v wiring? Don't know the internal ohms of the gas valve, but certainly check to ground. Probably a good chance they maybe related.
D

ch4man

whats the fuse rated at?, that says lot.
the only times ive found a blown 5 amp fuse on a mod/con board the blower was seized because the blower had a grenaded impeller
ohming a seized blower wont show you that

HomerJSmith

I think it is a 6.3 amp fuse. A blown fuse indicates an excessive current draw. Look for a short to ground. Shut the boiler power off. Remove the molex plugs on the control except for the power to the board. Remove the gas and blower plugs and the input, output, flue temp sensor plugs. There is a chance that a sensor shorted out. Put in a good fuse. Turn the power on with a no call for heat. Did the fuse survive? If not, it is probably the board. The fuse protects everything that comes after it. If it survived, plug one molex plug at a time and turn the power on. Repeat this with all the plugs. You're trying to isolate the bad circuit.

mattmia2

Ideally try to plug things in in the sequence of operations so it doesn't stop at something you haven't plugged in yet before it powers the thing you just plugged in.

ch4man

where I was going with fuse size is a 6 amp fuse will be there to protect the large amp draw components (blower) and say a 2 or 3 amp fuse is there for low voltage, low power components. i left out circulators in this as they are line voltage but draw very little amps
these blowers are ECM design and are engineered to try to reach target RPM even it it has to draw more power than normal.
see when a fuse actually blows. most munchies upon power up the blower ramps up then down. if it does my guess is wrong

Zman

I have seen a locked up circ blow the fuse on the board.

JUGHNE

Is there only one circ motor?
If a second one usually need a relay.

HomerJSmith

Like I said, the fuse protects everything that comes after it against current overload. That probably includes the board and I suspect the fuse is on the primary of the transformer, protecting the transformer, too. The only way to be sure is to look at the foil on the back of the board and tracing it out,, but that's hard as the board foil is double sided. Companies keep the schematic information on control boards close to their vests.

In reference to Zman's comment, the 140M had a plastic impeller in the blower and gas reversion can destroy it, which was why I never used concentric venting. I would want to know whether the impeller was rotating, whether it had a piece of the impeller that broke off and lodged in the fan, keeping it from rotating. The replacement blower has an aluminum impeller.

You want to know how many 140M blowers that have failed sitting on my shelf, at least 5 or 6. Some were the electronics in them and some were the impellers.

HTP does give you a way to test the blower with a meter as I vaguely recall.