Trane negative pressure gas valve and Magnehelic

Empire_2

Sorry about that,.. The Mag has to be connected to the lo port or neg of the differential and , No do not hold you finger on the other port. What you are reading is atmospheric minus diff. = neg. manifold pressure. in "HG.. A standard manometer cannot measure this gas valve because it has no differential capability.


Mike T.

Help_2

Reading negative pressure with Magnehelic

Hoping somebody could explain the proper way to read neg pressures with a Magnehelic. Use bottom (low pressure port)? Do you have to hold finger over other port for accurate reading? Thanks to anyone that could lend a little knowledge.

Brad White

The negative (low) pressure port

will measure the negative pressure (as a positive value). You do not block the positive port, rather you want to leave it open to atmosphere as a reference.

If you are measuring a differential pressure, the high or positive pressure port will be connected (obviously) to the higher pressure source.

I am not familar with setting up gas valves so this is just general Magnahelic application. I use them for balancing and instrumentation.

Help_2

Thanks guys. Ben

Empire_2

Hello Brad.

Trane RTU's at least some models use the induced draft to no only introduce combustion air on the burner side, but also use the Neg. pressure to draw on the GV. This is Tranes design and for the most par it is pretty darn reliable. Problem is that u must have the Mag to get your readings. It's only a diff reading so that you can obtain the neg draw on the valve in relationship to what the exhaust blower and what it is doing. Example: cross winds, slowing rpm's,. anything that slows the draft motor will affect the manifold pressure. Usually couses sputtering, choking etc...:-)

Mike T.

Brad White

Thanks, Mike

I wonder how one tests such a valve on an RTU with wild weather conditions and such. I mean the cabinet would be open I suppose.

Always learning and I thank you for this!

Brad

EBEBRATT-Ed

I use a Dwyer inclined manometer on those gas valves and they work great for that. I think the Tranes run a negative .02 if I recall. On the newer ones you can't adjust the gas pressure anyhow. I think it is more accurate than a magnehelic in that application.

ED

Unknown

The Dwyer Slant Gauge is

the only way to get a true read on negative pressure gas valves and yes the valves are ususally set to -.02 as a standard and it is not adjustable.

Inlet pressure problems can sometimes cause problems especially if the inlet pressure drops below 4" W.C. this causes the internal negative pressure regulator to try and compensate for the pressure drop and this will usually result in a shut down.

I have an Troubleshooting Guide I wrote on Negative Pressure Valves and how they work and how to troubleshoot.