Wits End with Minimax NT

[internet]

I've got an old Pentair Minimax NT that I've dumped a modest amount into reviving, and it's worked for a while until now and I'm totally stumped. Heater worked fine the first two years we were in the house. It had some issue come up that led me to open it up and discover just a ton of rust and buildup. Cleaned it all up, replaced the heat exchanger baffles, resealed the box (as best I could with aluminum tape), buttoned it back up and it ran fine. Then the motor crapped out. Took it off and realized the blower fan was completely rusted. Replaced that and everything ran fine again. Then blower motor crapped out. That was a big expense, but I replaced that and, once again, we were back. Until 2-3 weeks ago.

The heater fires up, runs about 10-15 minutes, and then cuts off with E04 error code. I did an ohm test on the air flow switch and it appears to work (switch closes both on "suck test" and when attached at the motor vacuum port). The blower motor is new. The only difference is that the new blower motor runs at 3000 instead of 3400 RPM, but I can't see how that would make a huge difference. I don't think it's overheating because even with the side cover taken off it still shuts down after 10-15 minutes with E04.

Can anyone give me any advice that might help me get another year out of this darn thing? I'm relatively handy with mechanical and electrical stuff. It's on its last leg and once its dead I'll go with a gas and solar combo, but I don't want to just toss this out just yet. Thanks in advance.

 

[danpik]

Surprisingly 400 RPM can make a big difference or how one of these operates. Working on house furnaces I see this a lot. When a motor gets old or warm it tends to slow down a bit and does not give a good enough vacuum signal to the switch. The best way to test this is to put a "T" in the vacuum line and connect a manometer to monitor the vacuum strength and see if it drops off as the unit is running. This is the recommended way to test. we also sometimes put a vacuum pump on the switch to give a constant vacuum signal and see if the unit stays running. My guess is that as the unit is running the motor get warm which causes it to slow enough to open the switch. These switches have a vacuum rating on them. You should be able to close the switch with a hand vacuum pump at the rating on the switch.

Of course this is assuming there is no blockage in the exhaust which is the primary reason for these flow switches.

Dan

 

[internet]

The motor itself is brand new, but of course it does run at a lower speed. I'm fairly confident there isn't a blockage - i checked the pipe. Tell me if this is a thing - the run capacitor on the motor is slightly smaller than the one on the 3400 RPM motor, but the motors themselves seem to be the exact same. I know an underrated run capacitor will cause a motor to run too slow. If I hooked up the larger run capacitor would that simply increase the speed of the motor and possibly address the issue? I realize saying that it sounds sort of crazy, but I don't really know a lot about how motors respond to that kind of thing.