We have 2 hvac units, located in the garage, connected to 1 common pvc condensation drain pipe. The drain pipe for each unit enters the wall behind the units before connecting to this common pipe which then bends down to the concrete floor where it is imbedded in the foundation slab and exits to the side of the house. Each hvac has an Aprilaire whole house humidifier connected to it.

Because there is a clog somewhere in these drain pipes(s), water from the humidifiers, when running, quickly backs up and flows out of the overflow pipe on each unit.
Hvac servicemen have twice vacuum pumped the exterior end of the common drain pipe and pulled some water and sediment out on each effort. The sediment appears to be minerals from the very hard water we have here.
The amount of water pulled out is much less than the volume of water we believe remains in the pipes.
One serviceman applied a nitrogen blast into 1 unit's pvp pipe where it first exits from the unit but accomplished nothing but possibly making the clog tighter.

I ran a plumping snake 15 ft. into the exterior end of the common drain pipe and found no obstructions, concluding the clog must be somewhere between where the snake ended and the 2 individual unit pipes connect to the common pipe inside the wall. (This pipe is longer than 15 ft. before it bend up to where it connects to the 2 individual drain pipes.)

The hvac servicemen say they do not know what else to do.

Would a long snake possibly work?

I thought of pouring some Lime Away mineral liquid into the drains to dissolve the clog but question where that might do more harm than good?

We are really stuck on this problem, what else can be done to clear these pipes?

1 - you could pour a full gallon of undiluted bleach into the pipe from the a/c end - may, or may not, clear the clog

2 - then you need to plug ONE of the a/c connections and use a 'kinetic water ram'



this tool operates by introducing kinetic energy in the form of 'water hammer' to dislodge and 'blow away' any stoppage present - a REAL AC service company WOULD know what to do

3 - a cheaper CO2 version is available at the big box stores

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I've read suggestions regarding the use of bleach but the possible pipe/joint damage that were mentioned were more than I wanted to risk since the clog is apparently mineral based and the pipes are mostly inside the wall or slab. Then again, I'm not an expert.

The nitrogen blast that one serviceman applied to the a/c portion of the pipe would seem to be similar to your water hammer, is it not? You mentioned plugging up the other nearby a/c outlet pipes. This was not done by the serviceman; I felt, as you implied, that it was ineffectual because the force would just blow out the nearest unplugged pipe - which I believe it did. This guy was not the most experienced, and readily admitted he didn't know what else to do.

How do you plug the pipes tightly so that the applied pressure does not blow them off? Just a pressure fitted pvc cap would just blow right off, right?

Where can I get the tool you pictured?

the tool I pictured is 300-400 dollars

I have no idea about possible rental

the 'big box stores' sell a 'similar' tool powered by CO2 cartridges for 40 bucks

the principle is:

the clogged pipe is filled with water
(there is a side hose port if necessary for back filling)

the air chamber is manually pumped up to between 20-100 psi

the possible outlet tappings in the affected piping are capped off using wet rags stuffed into place

the 'charged' tool is pressed tightly (using appropriate adapter) against opening

trigger is pulled

air chamber 'pop discharges' into piping WITH SIGNIFICANT RECOIL/BLOWBACK

resulting water hammer dislodges obstruction

you then need to dry off


it is NOT the pressure, but the SHOCK WAVE which clears the stoppage


nitrogen may also work IF, repeat IF, all possible outlets are capped/plugged

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

That tool and technique sounds like it should really work for most clogs, but only a professional plumber could afford to have one.

Out here in the hard water desert, most homes do not have humidifiers so virtually no hvac techs claim any experience dealing with the type of obstruction caused by mineral scale like one find on shower heads and faucet spouts.

I'm concerned that the obstruction we have is like that hard scale only larger.

What do you suggest for that type of clog?

What should I look for in an experienced, licensed plumber that can fix the problem?

you could try pouring a bottle of CLR

and/or



and/or

call in a GOOD RECOMENDED AC TECH


good luck

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

IT is probably more expensive than you want to pay. I would use my CO2 tank to blow it clear, either after capping one of the two lines or from the outside connection. Either one would be effective.

There are 3 drain pipes on each hvac unit - drip pan, pan overflow, humidifier - connected to 1 common hvac drain on each of the 2 hvac units. Each of these 2 drain pipes enters the wall behind which they connect to 1 common final drain to the outside.

When the service man applied a nitrogen blast to the humidifier drain on the 5 ton unit he did not first block the other drain pipe ports before blasting. My guess is that the blast just went out the other unblocked ports rather than blasting the clog farther down the drain.

I believe the situation is beyond the hvac guys capabilities.

Is there a possibility that a very long, thin plumbing snake inserted into the final outside drain port could reach the clog and clear it? Do plumbers still use snakes? It would be more tedious and time-consuming, but the other quicker approaches have not worked.

The snake would have to be able to pass through 1-2 90 degree bends in the pvc pipe. Is the possible?

snaking is not practical unless a 'straight run' in such small piping

find an actual HVAC 'factory tech' who actually has experience

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

quote; would have to be able to pass through 1-2 90 degree bends in the pvc pipe. Is the possible?

NO!!! It would not even go through a single PVC condensate elbow.