Air Conditioning - AC makes "booming" at furnance when compressor come on

I have a Lennox Elite Series Sure Light system. Furnance and A/C installed together about 5 1/2 years ago.

We have been running the fan all the time with and without the A/C. When we run the A/C with the fan on, as soon as the compressor comes on there is a "boom" like noise from the furnance where the A/C coil is located and the fan for the inside of the house.

When the fan is off and the compressor comes on there is no sound other than the air coming out the vents.

There is a two speed fan used in this system and possibly the fan is running at the slower speed and when the compressor comes on the "booming" sound is the fan going to the high speed setting.

Any ideas?

Most likely the sound is coming from the metal duct flexing.

I would guess that it is possible, but when the fan is not running and the A/C comes on there is no noise except for the air coming from the vent.

When the fan is continiously rnning and the compressor stops the fan seems to go to a slower speed.

I am thinking that the fan is running at the slower speed and when the compressor comes on the fan changes to the faster speed.

What is the model # on the furnace?

The model number of the furnance is

G23Q3/4X-100-5

Thanks for the reply

You might look at the wire schematic there on the furnace blower door. Im just out of the box here But it sounds like they wired the G for blower to the heat speed at the furnace and have the high speed for AC tied in to the Y that goes to the compressor. AS I said run the wires down first to see what you have there.

ED;)

Sounds like a plan. I'll do it in the morning as it is still pretty warm here. I'll post what I find in the morning.

Thanks for the reply.

On the PC board there are to cables connecting to a terminal

One comes from the thermostat (5 wires) and the other looks like it might go to the compressor ( 2 wires)

The terminal looks like this with the terminals, thermostat wires and compressor wires

Y yellow red (compressor?)

G green

W white

R red

C blue white (compressor?)

I could take a picture and possibly attach it to the e-mail if that would help.

thanks for your help

Some furnaces have a "continous fan" terminal on the board. This terminal is often wired to the fan's lowest speed & when the thermostat calls for cooling, the fan changes speed.
As a test, turn off the power to your outdoor unit at the breaker box then set your thermostat to call for cooling. I'd bet you get the noise.

As I was reading the post the AC reached the set temp and the compressor shut off. I turned the thermos so the AC was off, pulled the fuses outside, turn the AC to cool, lowered the temp to about 62.

Nothing happed.

Nothing at all happened? No fan indoors, nothing? There could be a built-in time delay, particularly if the thermostat is digital.

I'm sorry, the fan is set to run all the time and I didn't turn it off.

When the fan is set to not run continously and the compressor comes on, there is no booming noise.

It only happens when I have the fan running continously and the compressor comes on.

Talked to a lennox guy. Its like on our Trane. IF you have a V/S blower and you turn it to fan on it will run the blower at 1/2 speed then when the AC comes on then the blower will jump up to top speed if that is what speed its set for for cool. So do you have a V/S blower here???????

ED.;)

I'm out of ideas for now. Maybe Ed or one of the other guys has some.

Ok, The G23 is a basic 80% furnace. Nothing special about it.


You are saying if the fan switch is on "ON", blower runs at a lower speed... then come there is a call of cool, the boom sound comes, and blower runs at a higher speed?

And if the fan switch is at AUTO, and when the blower comes on, the boom don't happen on call of cool?

Thanks for all the replys.

Jay11J has it correct. When the fan is in "Auto" and the compressor comes on, everything is fine.

When the fan is in "On/run" and the compressor comes on, then I hear the "boom"

The fan is two speed, one speed for heating and one the other speed for the AC.

I don't if you are comfortable or not doing this..

Turn off the power, and look at the relay/control board for heating blower speed tap. Remove that wire and "Park" it. then try it this time.

Thanks Jay11J, I'll have to wait until the morning as it is pretty busy around here now and it is very warm outside.

After looking at the wiring and the speed diagram I think I will just leave it as is for now.

I did notice today, I was standing close to the wall where the unit is located, that the fan also makes the same noise when the compressor goes off.

So whenever the fan changes speed it makes that little "booming" noise.

Thanks for all your help

Stand there by the furnace when it makes this BOOM and see if its not up in the duct work some where If you find it just bend the edge of the duct in a little there. It should stop it.

ED;) my .02 cents

I will keep an eye on this and get back when/if I find out anything.