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Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 22029
Experience:  40+ yrs. experience as a licensed oil & gas technician.
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I have a LAARS Mascot HT 1.330. The fan operates but nothing

Customer Question

I have a LAARS Mascot HT 1.330. The fan operates but nothing else. I have checked connections, fuses and read through the manual. The system is set up for central heating only. When the selector is set to on, the fan comes on but there are no display lights. I have tried reset on the selector with no change.
Submitted: 2 years ago.
Category: HVAC
Expert:  Rick replied 2 years ago.

Ok, if the fan runs but the burner won't fire up then the flue/exhaust could be blocked. It's also possible that you have a faulty draft pressure switch. The draft switch has to prove that there is adequate draft before the burner will fire up. Also check the small hose that connects the switch (top left) to the exhaust. If the hose is disconnected or falling apart that could also be the cause of the burner not firing.

Customer: replied 2 years ago.
I verified that the flue is not blocked. The hose is in good shape and plugged in to the top inlet. How do I test the switch? Can I jumper it? Also, shouldn't the info advisory come on regardless?
Expert:  Rick replied 2 years ago.

No you can't jump out the switch because the board tests for that and it won't fire up if it's jumped. Yes you should get an error but since you aren't this is the most likely culprit if you aren't getting an arc from the ignitor electrodes. If you are getting an arc you should hear a ticking noise. You can check for power to the electrodes as well.

Expert:  Rick replied 2 years ago.

You can test the switch by checking continuity across the NO terminal when the fan is running. Remove the leads so you don't overload your meter

Customer: replied 2 years ago.
I isolated the switch and got continuity between C and NC but no continuity with NO in any combination. I then tried it with the switch on and the blower running. Still no continuity. There is no clicking at the electrodes. As far as the info advisory on the boiler display, it sounds like you are not too concerned that the display is blank.
Expert:  Rick replied 2 years ago.

Sounds like a bad switch if you have no continuity across C & NO with the fan running and the vent clear. So if you want to confirm a bad switch then you need to put a manometer on the exhaust with the fan running to confirm adequate draft. Or you can just replace the switch which is a lot cheaper than buying a manometer if you don't have one. It's not that I'm not concerned about the lack of an error showing. It's just that you want to eliminate the cheap parts as potential culprits before you point the finger at an expensive control board and a switch failure is a whole lot likelier than a bad board.

Expert:  Rick replied 2 years ago.

You should also check to be sure the ports the vac hose connects to aren't blocked. You can poke a paper clip into the one on the fan housing not the one on the switch

Customer: replied 2 years ago.
I pulled the switch out and tested it by blowing in to the port. I get continuity between C and NO. I then made sure the port was clear on the flue side. I then plugged the switch leads back in and started the boiler. I could get continuity between C and NO by blowing through the hose but nothing else changed in the system.
Expert:  Rick replied 2 years ago.

The switch is calibrated to a very specific pressure. Blowing into it doesn't tell you anything. I've already said what you need to do to confirm a failed switch.

Customer: replied 2 years ago.
I will order a new switch and let you know how it goes.
Expert:  Rick replied 2 years ago.


Customer: replied 2 years ago.
I replaced the switch and no change. I also double checked that there was no blockage in the exhaust. The flow is strong and I can see all of the way back to the elbow. Any other thoughts?
Expert:  Rick replied 2 years ago.

Did you check continuity across C & NO on the new switch with the fan running?

Customer: replied 2 years ago.
I checked for continuity between C and NO with the fan running. There is no continuity. I poked down into the flue nipple where the tube is connected, again just to double check a clear passage. It is clear and I can feel air coming through the tube with the fan running. Also, just in case this matters, we are at 9600 ft. The technicians that got the system up and running last year adjusted for that.
Expert:  Rick replied 2 years ago.

Setting high altitude parameters is a one time operation. So that's not a likely cause. So since the new switch isn't proving adequate draft your options are: first and foremost take the flue and fan apart to be sure there are no obstructions. Once you have proof there's nothing inhibiting the draft you either put a manometer on the exhaust and check the draft or you replace the fan. I had a recent case where the customer insisted the vent was clear. After considerable prodding he finally took the fan apart and found moths in the fan blades.