FAN PRODUCING NEGATIVE ON DISCHARGE

Jackmartin

I have just finished a job that has Geothermal heating and backup electric duct mounted heater. The guy did not have heat out of his duct heater since he moved in, it is a new house. We tried to have the air proving switch complete on pressure from the fan discharge side it was gimpy at best. We went back with our manometer and the discharge pressure off the fan was reading -3 inches of water. The house is getting good air flow even if the reading is strange. This is a Trane package Geothermal unit. We then took a reading on the return and we had a reading of -15. So we used the air proving switch on the negative for proving and the electric heat contractor is now working correctly. He phoned five contractors before us and no one would look at his problem ,including the jerk ,who installed his system because he has a Nest thermostat.They told him the unit could not operate off a Nest Stat. I am a Nest dealer for their stats and they are just a little more advanced than the regular electronic stats we install day after day. My question is ,after a very long time in this business ,I have never seen a negative pressure of the fan discharge,;please someone tell me what is going on? Thanks and all the best Jack

EBEBRATT-Ed

I don't have a clue on the negative pressure unless the whole building is in a big negative. Did you use a water manometer or a digital thing of some sort?

To my way of thinking because the air pressure is wacky I would use an air switch as a differential switch...sense both fan discharge and suction pressure connections to the switch. Also maybe a current sensor on the fan motor wires for added safety

ratio

It's exceedingly difficult for the discharge of a fan to be negative w/r/t the space it's blowing in to. Remember, high pressure goes to low pressure. Always, and if it's not it isn't. Check the zero of your instrument first. After that, it's quite possible to find low pressure, even negative pressure, in a duct, especially if you're close to the blower or an obstruction or change of direction. What are you using to probe? How & where are you doing it at?

I've got a selection of Magnehelic gauges that I use for the hard questions, mainly because they aren't digital, short-term trends are easily noticed, the mean pressure is apparent, and they are truly differential. Also they're proof against up to IIRC 15 PSIG, even the ¼ IN WC model! Anyway, one of those and a pair of static probes punched in to the duct at the right locations should give you a reliable number. Check the Trane service manual.

If it's just proof of airflow you're after, a sail switch proves mass flow, but a differential pressure switch might be cheaper; & physics says it'll prove flow, as "High pressure goes to low pressure. Always."

GW

You must be sticking your tip into a vortex, you can’t be nagative on the positive side