Static management

[jambsi]

I have a retrofitted 4 ton Climatemaster Tranquility 27 which makes an irregular 'booming' noise @ 1400CFM. The noise seems to emanate from the supply plenum. Upon investigation by a serviceman he said it was caused by insufficient static. He installed a plate just inside the main supply duct which restricted the airflow. This eliminated the noise, but caused the ECM fan to ramp up thereby increasing energy consumption.

Is there another way of solving the problem without causing the ECM fan to work harder?

[aaronforprez]

i'm going to guess that the 1400 CFM is too much for the ductwork. he basically brought the pressure down so the duct didn't blow up like a balloon.

oh - and if this is the case you either have to 1) add duct to support the 1400 CFM 2) decrease the air flow and the capacity of the unit 3) live with the noise and the curtains blowing off of the windows.

a 4 ton TT27 unit will run at roughly 1700 CFM in high stage... so considering your "1400 CFM" comment, perhaps i'm not understanding your problem.

[jambsi]

Thanks for the reply.

As I understand it the ECM ramps up to keep the CFM at the 'set' level, so no reduction in airflow through the ducts is caused by the restricting "plate". The serviceman's statement was that the duct sizes are fine but the static in the plenum was too low.

Any other opinions of how to solve the 'booming' (not air moving) noise without causing the ECM to ramp up would be welcome. Plenum shape/design?

BTW -
TT27 049 CFM Stg2 heat @ ECM TAP setting:
1 = 1200 CFM
2 (factory default) = 1430 CFM
3 = 1650 CFM
4 = 1850 CFM

[jambsi]

I don't know why it worked, but making the plenum 4" shorter made the problem go away. With artificial duct restrictions removed the ducts are quiet even at max speed (1850).

[RoBoTeq]

Do you have return ducting on this unit or is the return not ducted? ECM motors require a certain amount of static pressure to operate properly.

[dash]

Can you post a picture??

The plenum is best sized to the "Take-off" size (metal around the unit supply discharge),if it's larger,like the entire unit size,this could have causd it,shortening the plenum may have helped.

[jambsi]

I think the plenum set up is quite conventional. Yes there's a (huge) return air side intake.

The way this problem developed / solved was a side-effect really. Originally I had a 4 ton unit. With this unit I had the very annoying booming noise as mentioned in the original post. Because of issues not related to airflow this unit was swapped out for a 6.5 ton unit (a very long story not yet concluded). The 6.5t is 4" taller than the 4t making the plenum 4" shorter. The noise went away.

I don't like "unknown" anymore than the next guy, but I'll take "unknown solution" over "unknown cause" any day.

[tuckerin]

I just gotta ask: Any chance the plenum was NOT cross broken and was just flexing?

[ACFIXR]

I just gotta ask: Any chance the plenum was NOT cross broken and was just flexing?
oh yeah;)

[jambsi]

I just gotta ask: Any chance the plenum was NOT cross broken and was just flexing?

I don't know what cross broken or not would look like sorry

[skippedover]

When flat sheet metal will be exposed to strong pressure changes, those changes could cause the large metal areas to flex or "oil can", a term used when the sheet metal acts like and old oil can that was tipped upside down and the bottom pressed inward to push the oil out. In order to prevent 'oil canning', the sheet metal mechanic puts the flat pieces into his bending brake at a 345-degree angle and makes a partial bend. He then reverses the piece 90-degrees and puts in another partial bend. This had the effect of creating a large "X" when viewed. The term 'cross brake' means to literally cross or "X" the large metal area. Hence the term. Plenum are notorious for 'oil canning' if they are not cross braked. Just like wood, light weight sheet metal has a 'grain', which causes it to move into a particular position when there's no pressure on it. Depending on whether the position is inward or outward is the greatest factor on whether the 'oil can' effect will be prevalent. A plenum side that measure 17 x24, for example, has 408 square inches of area. If when the blower comes on it exerts just .2 psi on the plenum side, that's a total pressure of 81.6 pounds. So if the natural lean of the metal is inward, it gets pushed outward with 81.6 pounds of force when the blower comes on. Some where, some time, after the blower turns off, the metal will 'oil can' back to its 'at rest' position, probably making the same noise it did when the blower came on.

[ACFIXR]

That was an excellent explanation,thanks skippedover.