A toilet I have in my house keeps getting clogged up so one day I ripped up
the floor boards (it is on a second floor) and I followed the toilet 3-inch
toilet drain pipe line. To my dismay, before taking a vertical drop, the
drain runs 15 feet. The drain is well ventilated after about 9 feet and is
at a steady slope of only 1/4 inch per foot. I replaced the drain pipe, but
there was no damage to the original one so I don't think that will help
much. I built a 1.5 inch pedestal below the toilet to slightly increase the
slope. Even though this is at-or-above the 1/4 inch per foot slope minimum,
is this 15 foot horizontal drain setup "up to code"?

Unfortunately, the house is poorly designed and it is pretty much impossible
to move the toilet to a different location. Additionally, there are no
possible different routes for the toilet drain. The closest vertical drain
drop is only about 6 inches away but it is only a 2 inch pipe for the
shower, and 1.5 inch pipe for the bathtub (which merge a couple feet down
the wall). Why didn't the builder run the toilet drain down the same wall?
Who knows... I cannot run the drain line down the same wall myself because
the drains go into a solid block of the concrete below the house.

Secondly, I've been looking at more expensive toilets which may help reduce
the chance of the toilet clogging up in my current setup. My current toilet
it some generic 1.6 GPF toilet. But I am considering a jet,
pressure-assisted, or power-assisted toilet. Kolher advertises their
power-assisted toilet is king when it comes to flushing -- but will this
help me in my situation?

http://www.us.kohler.com/tech/produc...ushsystems.jsp

*Thanks* for any input.

Bob wrote:

A toilet I have in my house keeps getting clogged up so one day I ripped up
the floor boards (it is on a second floor) and I followed the toilet 3-inch
toilet drain pipe line. To my dismay, before taking a vertical drop, the
drain runs 15 feet. The drain is well ventilated after about 9 feet and is
at a steady slope of only 1/4 inch per foot. I replaced the drain pipe, but
there was no damage to the original one so I don't think that will help
much. I built a 1.5 inch pedestal below the toilet to slightly increase the
slope. Even though this is at-or-above the 1/4 inch per foot slope minimum,
is this 15 foot horizontal drain setup "up to code"?

Unfortunately, the house is poorly designed and it is pretty much impossible
to move the toilet to a different location. Additionally, there are no
possible different routes for the toilet drain. The closest vertical drain
drop is only about 6 inches away but it is only a 2 inch pipe for the
shower, and 1.5 inch pipe for the bathtub (which merge a couple feet down
the wall). Why didn't the builder run the toilet drain down the same wall?
Who knows... I cannot run the drain line down the same wall myself because
the drains go into a solid block of the concrete below the house.

Secondly, I've been looking at more expensive toilets which may help reduce
the chance of the toilet clogging up in my current setup. My current toilet
it some generic 1.6 GPF toilet. But I am considering a jet,
pressure-assisted, or power-assisted toilet. Kolher advertises their
power-assisted toilet is king when it comes to flushing -- but will this
help me in my situation?

http://www.us.kohler.com/tech/produc...ushsystems.jsp

*Thanks* for any input.


There's nothing fundamentally wrong with a long horiz run
at 1/4" per foot.

"The drain is well ventilated after about 9 feet"
What does this mean??
The drain should be vented no more than 3 feet from the
toilet. But I doubt that venting is causing your problems.
Have you determined where the clogging occurs? If you can
plunge it clear, I suspect the clogs are completely within
the toilet .

One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.

Yes, you'd probably be happier with the Kohler.

Jim

Thanks for your speedy response. The clog is definitely in the horizontal
run because I can take a snake and it doesn't get at the clog for several
feet. If I plunge, I can usually get it unclogged as well but you can hear
all the action in the drain pipe and I lose all water in the toilet (etc).

You are correct, the first vent for the horizontal run is at 9 feet (opposed
to the 3 feet minimum as you said).

Anyway, I am wondering about those power flush toilets because it uses even
less than 1.6 GPF (1.0 or 1.4 GPF) so I am not convinced it will help me.
The pressure toilets look appealing however.

Thanks

There's nothing fundamentally wrong with a long horiz run
at 1/4" per foot.

"The drain is well ventilated after about 9 feet"
What does this mean??
The drain should be vented no more than 3 feet from the
toilet. But I doubt that venting is causing your problems.
Have you determined where the clogging occurs? If you can
plunge it clear, I suspect the clogs are completely within
the toilet .

One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.

Yes, you'd probably be happier with the Kohler.

Jim

By the way, my wife does buy that soft multiple-ply toilet paper from Cosco;
I will try out your suggestion, thanks.


"Bob" wrote in message
...
Thanks for your speedy response. The clog is definitely in the horizontal
run because I can take a snake and it doesn't get at the clog for several
feet. If I plunge, I can usually get it unclogged as well but you can
hear all the action in the drain pipe and I lose all water in the toilet
(etc).

You are correct, the first vent for the horizontal run is at 9 feet
(opposed to the 3 feet minimum as you said).

Anyway, I am wondering about those power flush toilets because it uses
even less than 1.6 GPF (1.0 or 1.4 GPF) so I am not convinced it will help
me. The pressure toilets look appealing however.

Thanks

There's nothing fundamentally wrong with a long horiz run
at 1/4" per foot.

"The drain is well ventilated after about 9 feet"
What does this mean??
The drain should be vented no more than 3 feet from the
toilet. But I doubt that venting is causing your problems.
Have you determined where the clogging occurs? If you can
plunge it clear, I suspect the clogs are completely within
the toilet .

One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.

Yes, you'd probably be happier with the Kohler.

Jim

"Speedy Jim" wrote in message
om...
One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.

Your john may be happier but what about your ................

Colbyt wrote:

"Speedy Jim" wrote in message
om...

One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.


Your john may be happier but what about your ................




LOL Refer to the previous thread on Bidets...

Bob wrote:
By the way, my wife does buy that soft multiple-ply toilet paper from Cosco;
I will try out your suggestion, thanks.


Mine too. Nothing I hate worse that that soft junk. Hold it up to the
light and tear a sheet. There is so much dust created its not even funny.


--
Thank you,



"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16

"Bob" wrote in message
Anyway, I am wondering about those power flush toilets because it uses
even less than 1.6 GPF (1.0 or 1.4 GPF) so I am not convinced it will help
me. The pressure toilets look appealing however.

We had a similar problem at work with a 50' horizontal run. It needs a lot
of water so the solids can float away. A pressure toilet helped. One
advantage was that fact that it had a quick recovery. You can flush half way
through your business, again when you are done, again when you use the
paper. That took care of most of the problem.

Fast forward a few years and add a second toilet. Only difference this time
is venting. The first setup was a hack job and not properly vented. The
addition was a good plumber and he added a vent, put in a standard style
toilet and there has not been a problem in a year of use.

One ore thing. Have the ladies dispose of those cotton things in the trash
instead of a flush. The maintenance guy knew when the secretary had her
period by when the line plugged up.

you may actually prefer more accurate jobsite answers than we can guess
at.
you might have to call a plumber and fix the drain system of your home.

buying a more expensive noisy toilet is not the proper solution.
make sure you have personally inspected the sewer line with mirrors and
flashlights to satisfy your curiosity before you call in a plumber with
a sewer cam you will be at home to watch with the plumber.
but first, with the toilet removed, the floor flange must accept
unlimited amounts of water from your garden hose with no backup,
otherwise professionally have it snaked.
if the line accepts all the water you give it, retest the built-in trap
of the removed toilet by running a closet auger all the way thru it and
pulling a towel back up it to pull out the plastic disposable spoon or
other similar but unsnakeable object somebody dropped in there which is
not presently amusing you. good luck.

pdf 32 pages on venting for venting homework at:
http://www.aspe.org/ASPE_Publication...vol2_chap3.pdf

"Bob" wrote in message
...
A toilet I have in my house keeps getting clogged up so one day I ripped up
the floor boards (it is on a second floor) and I followed the toilet 3-inch
toilet drain pipe line. To my dismay, before taking a vertical drop, the
drain runs 15 feet. The drain is well ventilated after about 9 feet and is
at a steady slope of only 1/4 inch per foot. I replaced the drain pipe,
but there was no damage to the original one so I don't think that will help
much. I built a 1.5 inch pedestal below the toilet to slightly increase
the slope. Even though this is at-or-above the 1/4 inch per foot slope
minimum, is this 15 foot horizontal drain setup "up to code"?

Unfortunately, the house is poorly designed and it is pretty much
impossible to move the toilet to a different location. Additionally,
there are no possible different routes for the toilet drain. The closest
vertical drain drop is only about 6 inches away but it is only a 2 inch
pipe for the shower, and 1.5 inch pipe for the bathtub (which merge a
couple feet down the wall). Why didn't the builder run the toilet drain
down the same wall? Who knows... I cannot run the drain line down the
same wall myself because the drains go into a solid block of the concrete
below the house.

Secondly, I've been looking at more expensive toilets which may help
reduce the chance of the toilet clogging up in my current setup. My
current toilet it some generic 1.6 GPF toilet. But I am considering a
jet, pressure-assisted, or power-assisted toilet. Kolher advertises their
power-assisted toilet is king when it comes to flushing -- but will this
help me in my situation?

http://www.us.kohler.com/tech/produc...ushsystems.jsp

*Thanks* for any input.


code in my are is 1/8 per foot.so that water will not run away and leave
solids behin..You can actually have too much slop on a toilet drain.

"digitalmaster" wrote in message
.. .

"Bob" wrote in message
...
A toilet I have in my house keeps getting clogged up so one day I ripped
up the floor boards (it is on a second floor) and I followed the toilet
3-inch toilet drain pipe line. To my dismay, before taking a vertical
drop, the drain runs 15 feet. The drain is well ventilated after about 9
feet and is at a steady slope of only 1/4 inch per foot. I replaced the
drain pipe, but there was no damage to the original one so I don't think
that will help much. I built a 1.5 inch pedestal below the toilet to
slightly increase the slope. Even though this is at-or-above the 1/4 inch
per foot slope minimum, is this 15 foot horizontal drain setup "up to
code"?

Unfortunately, the house is poorly designed and it is pretty much
impossible to move the toilet to a different location. Additionally,
there are no possible different routes for the toilet drain. The closest
vertical drain drop is only about 6 inches away but it is only a 2 inch
pipe for the shower, and 1.5 inch pipe for the bathtub (which merge a
couple feet down the wall). Why didn't the builder run the toilet drain
down the same wall? Who knows... I cannot run the drain line down the
same wall myself because the drains go into a solid block of the concrete
below the house.

Secondly, I've been looking at more expensive toilets which may help
reduce the chance of the toilet clogging up in my current setup. My
current toilet it some generic 1.6 GPF toilet. But I am considering a
jet, pressure-assisted, or power-assisted toilet. Kolher advertises
their power-assisted toilet is king when it comes to flushing -- but will
this help me in my situation?

http://www.us.kohler.com/tech/produc...ushsystems.jsp

*Thanks* for any input.


code in my are is 1/8 per foot.so that water will not run away and leave
solids behin..You can actually have too much slop on a toilet drain.

sorry about the spelling errors..was in a hurry

"digitalmaster" wrote in message
.. .

"Bob" wrote in message
...
A toilet I have in my house keeps getting clogged up so one day I ripped
up the floor boards (it is on a second floor) and I followed the toilet
3-inch toilet drain pipe line. To my dismay, before taking a vertical
drop, the drain runs 15 feet. The drain is well ventilated after about 9
feet and is at a steady slope of only 1/4 inch per foot. I replaced the
drain pipe, but there was no damage to the original one so I don't think
that will help much. I built a 1.5 inch pedestal below the toilet to
slightly increase the slope. Even though this is at-or-above the 1/4 inch
per foot slope minimum, is this 15 foot horizontal drain setup "up to
code"?

Unfortunately, the house is poorly designed and it is pretty much
impossible to move the toilet to a different location. Additionally,
there are no possible different routes for the toilet drain. The closest
vertical drain drop is only about 6 inches away but it is only a 2 inch
pipe for the shower, and 1.5 inch pipe for the bathtub (which merge a
couple feet down the wall). Why didn't the builder run the toilet drain
down the same wall? Who knows... I cannot run the drain line down the
same wall myself because the drains go into a solid block of the concrete
below the house.

Secondly, I've been looking at more expensive toilets which may help
reduce the chance of the toilet clogging up in my current setup. My
current toilet it some generic 1.6 GPF toilet. But I am considering a
jet, pressure-assisted, or power-assisted toilet. Kolher advertises
their power-assisted toilet is king when it comes to flushing -- but will
this help me in my situation?

http://www.us.kohler.com/tech/produc...ushsystems.jsp

*Thanks* for any input.


code in my are is 1/8 per foot.so that water will not run away and leave
solids behin..You can actually have too much slop on a toilet drain.

There we go! This is most likely the problem's cause. The old codes dealt
with those old 4.5 gallon flushes and not having enough water to wash the
solids away were not a problem, getting the water out and gone was. There is
much debate going on right now that addresses the realities presented by low
water toilets and the codes are s-l-o-w-l-y changing to reflect that
reality. We need less pitch, not more! The idea is to float the buggers out
like a raft, not like a kayak in whitewater. Your run of 15 feet is
problematic because of the low flush. As the poster said: the water is
running out faster than the solids which get left behind and stick wherever
they stop. If you train everyone to triple flush every time they leave a
"package" you would solve the problem. NGH (Not Gonna Happen). You can
Gerry-rig a modern toilet to flush more that 1.6 gallons per flush. That may
be a good solution for you. I've even seen a black market for old 4.5 and 6
gallon beasts around my parts.

While a poor vent can cause drainage problems in certain configurations, the
primary purpose of venting every fixture is to keep the other fixture's
traps from being sucked dry. If you ripped everything out and moved the vent
to the proper location you would probably not see a significant difference.

I just wonder if you could fit a 4" line in there, if it would help;
any opinions?

Thanks for your speedy response. The clog is definitely in the horizontal
run because I can take a snake and it doesn't get at the clog for several
feet. If I plunge, I can usually get it unclogged as well but you can hear
all the action in the drain pipe and I lose all water in the toilet (etc).


That last tidbit pretty much guarantees that it's a venting issue,
doesn't it?

If you've got a solid plug of matter moving down the pipe,
and no way for air to get in behind it, of course it's going
to stop moving.

The old flushers are still sold in canada and can be purchased on e
bay, thats a option

Bob,

A toilet I have in my house keeps getting clogged up

Most clogs are due to excessive paper use and/or flushing items that
should be thrown in the trash.

ripped up the floor boards (it is on a second floor) and I followed
the toilet 3-inch toilet drain pipe line. To my dismay, before taking
a vertical drop, the drain runs 15 feet. The drain is well ventilated
after about 9 feet and is at a steady slope of only 1/4 inch per foot.
I replaced the drain pipe, but there was no damage to the original
one so I don't think that will help much. I built a 1.5 inch pedestal
below the toilet to slightly increase the slope. Even though this is
at-or-above the 1/4 inch per foot slope minimum, is this 15 foot
horizontal drain setup "up to code"?

The last code I dealt with specified 6 feet as the maximum distance
between the closet flange and the vent. This was for UPC, I believe IPC
allows up to 12 feet. So, I doubt the 9 foot distance to your vent would
be too much of a problem.

If you have room in the joist space, you could add a "Y" in the drain
line to add a vent closer to the toilet that runs horizontally above the
drain line and tie into the regular vent at the other end. Probably not
necessary, but it may be an option if you're worried about it.

I wouldn't go any steeper than the 1/4" per foot slope. The liquids will
rush down the pipe faster, potentially leaving solids behind to cause a
clog. Many of my drain lines are about 1/8" per foot slope, and they
drain well.

If you had 4" pipe, I would have suggested switching to 3" pipe with a
low-flow toilet. The lower quantity of water can spread out too thin in
the bottom of a 4" pipe to carry away the solids. But, it sounds like you
already have 3" pipe, so that shouldn't be a problem.

Assuming the toilet is the only fixture you are having trouble with, I
suspect the drain is fine. You may want to focus your attention on the
vent. Go up on the roof and check the vents. Run the garden hose down the
vent and/or run a snake through the vent. Sometimes they get clogged with
leaves, birds nests, bee hives, or whatever. Trying to drain a toilet
without a vent is like trying to pour liquid from a can with only one
hole. It'll go "glug-glug" instead of flowing smoothly.

If you live in a cold climate, your roof vent should be at least 3".
Smaller vents can ice over when the moist air leaves the vent, eventually
blocking the vent. If your vent is smaller, you can replace the current
vent with larger pipe and transition to the smaller pipe in the attic
(the closer to the heated space, the better). I'd also install a
"mushroom cap" over the large vent opening to keep out leaves, squirrels,
birds, etc.

You may also want to check the float level in the toilet. It may be set
too low and not giving you the full 1.6 gallons with each flush. Or the
flapper may be closing too early, causing the same problem.

I've been looking at more expensive toilets which may help
reduce the chance of the toilet clogging

We bought inexpensive off-the-shelf American Standard toilets from Lowes
when we built our house 2 years ago. I made sure to pick models with a
large trap diameter, but otherwise we just chose a style we liked. We
haven't had a single clog in the year or so we've lived here, and they
flush better than the 3.5 gallon toilets we had in our last place.

Unless the toilet was one of the first 1.6 gallon models that came out, I
wouldn't worry much about the toilet. If you would feel better replacing
it, I'd just pick up a new toilet, and not worry about all the pressure
assist stuff. They'll just cost more and be a future maintenance issue...

Anthony

The last code I dealt with specified 6 feet as the maximum distance
between the closet flange and the vent. This was for UPC, I believe IPC
allows up to 12 feet. So, I doubt the 9 foot distance to your vent would
be too much of a problem.


In theory, you can get away with any value that's fewer
pipe-diameters than the inverse of the slope.

Meaning less than 48 pipe-diameters for a 1/4" per foot
drain. Which means you can draw a horizontal line
from the lip of the trap to the vent-openning.

SInce most codes permit anything from 1/2" to 1/8" per foot,
that would work out to 8' from trap to vent,
in a 4" pipe, but only 6' for a 3" pipe.

In practice, you want to vent as close to the trap
as you can, as long as you're far enough away to
keep flushing action from lobbing **** into the
the vent and plugging it. (Which I think is
supposed to be around 1.5 to 3 pipe diameters.)

"Sev" wrote in message
oups.com...
I just wonder if you could fit a 4" line in there, if it would help;
any opinions?

I don't know enough about it to answer that. It could create a flatter
surface to float the **** on. But it could also reduce the slug of water
behind the **** that keeps it moving!.

"HerHusband" wrote in message
...
Bob,

A toilet I have in my house keeps getting clogged up

Most clogs are due to excessive paper use and/or flushing items that
should be thrown in the trash.

ripped up the floor boards (it is on a second floor) and I followed
the toilet 3-inch toilet drain pipe line. To my dismay, before taking
a vertical drop, the drain runs 15 feet. The drain is well ventilated
after about 9 feet and is at a steady slope of only 1/4 inch per foot.
I replaced the drain pipe, but there was no damage to the original
one so I don't think that will help much. I built a 1.5 inch pedestal
below the toilet to slightly increase the slope. Even though this is
at-or-above the 1/4 inch per foot slope minimum, is this 15 foot
horizontal drain setup "up to code"?

The last code I dealt with specified 6 feet as the maximum distance
between the closet flange and the vent. This was for UPC, I believe IPC
allows up to 12 feet. So, I doubt the 9 foot distance to your vent would
be too much of a problem.

If you have room in the joist space, you could add a "Y" in the drain
line to add a vent closer to the toilet that runs horizontally above the
drain line and tie into the regular vent at the other end. Probably not
necessary, but it may be an option if you're worried about it.

I wouldn't go any steeper than the 1/4" per foot slope. The liquids will
rush down the pipe faster, potentially leaving solids behind to cause a
clog. Many of my drain lines are about 1/8" per foot slope, and they
drain well.

If you had 4" pipe, I would have suggested switching to 3" pipe with a
low-flow toilet. The lower quantity of water can spread out too thin in
the bottom of a 4" pipe to carry away the solids. But, it sounds like you
already have 3" pipe, so that shouldn't be a problem.

Assuming the toilet is the only fixture you are having trouble with, I
suspect the drain is fine. You may want to focus your attention on the
vent. Go up on the roof and check the vents. Run the garden hose down the
vent and/or run a snake through the vent. Sometimes they get clogged with
leaves, birds nests, bee hives, or whatever. Trying to drain a toilet
without a vent is like trying to pour liquid from a can with only one
hole. It'll go "glug-glug" instead of flowing smoothly.

If you live in a cold climate, your roof vent should be at least 3".
Smaller vents can ice over when the moist air leaves the vent, eventually
blocking the vent. If your vent is smaller, you can replace the current
vent with larger pipe and transition to the smaller pipe in the attic
(the closer to the heated space, the better). I'd also install a
"mushroom cap" over the large vent opening to keep out leaves, squirrels,
birds, etc.

You may also want to check the float level in the toilet. It may be set
too low and not giving you the full 1.6 gallons with each flush. Or the
flapper may be closing too early, causing the same problem.

I've been looking at more expensive toilets which may help
reduce the chance of the toilet clogging

We bought inexpensive off-the-shelf American Standard toilets from Lowes
when we built our house 2 years ago. I made sure to pick models with a
large trap diameter, but otherwise we just chose a style we liked. We
haven't had a single clog in the year or so we've lived here, and they
flush better than the 3.5 gallon toilets we had in our last place.

Unless the toilet was one of the first 1.6 gallon models that came out, I
wouldn't worry much about the toilet. If you would feel better replacing
it, I'd just pick up a new toilet, and not worry about all the pressure
assist stuff. They'll just cost more and be a future maintenance issue...

Anthony

"Trying to drain a toilet without a vent is like trying to pour liquid from
a can with only one hole. It'll go "glug-glug" instead of flowing smoothly."

What I find interesting though is the fact that you must vent a toilet
*downstream* of the fixture. Totally counter-intuitive if you think that the
purpose is to provide air for draining. The slug of water has to run a few
feet before it gets to the vent. My plumber told me we can't put the vent
behind the toilet (upstream), but he didn't say why. (probably doesn't know
the why). I'd love to know why that is.

"Bob" wrote in message
...
Thanks for your speedy response. The clog is definitely in the horizontal
run because I can take a snake and it doesn't get at the clog for several
feet. If I plunge, I can usually get it unclogged as well but you can
hear all the action in the drain pipe and I lose all water in the toilet
(etc).

You are correct, the first vent for the horizontal run is at 9 feet
(opposed to the 3 feet minimum as you said).

Anyway, I am wondering about those power flush toilets because it uses
even less than 1.6 GPF (1.0 or 1.4 GPF) so I am not convinced it will help
me. The pressure toilets look appealing however.

Thanks

There's nothing fundamentally wrong with a long horiz run
at 1/4" per foot.

"The drain is well ventilated after about 9 feet"
What does this mean??
The drain should be vented no more than 3 feet from the
toilet. But I doubt that venting is causing your problems.
Have you determined where the clogging occurs? If you can
plunge it clear, I suspect the clogs are completely within
the toilet .

One of the major causes of clogging in low-flush toilets
is the use of "soft" tissue. This modern stuff wads up
into great clumps which defy flushing. As an experiment,
try switching to old-style tissue for a few weeks.

Yes, you'd probably be happier with the Kohler.

Jim



Since when did they redesign toilet paper? How do you know it is the old
kind?

I agree though, replacing the pipe without a firm diagnosis was premature.
A new toilet would have been much easier. Just running a hose down the open
pipe would have shown if it was sufficient to remove the waste.

Regardless of toilet you choose, the diameter of the trap seems to be the
key to troublefree flushing. 2" trap works quite well. You may in fact
have a partial clog in the trap of your present toilet which contributes to
the clog

What I find interesting though is the fact that you must vent a toilet
*downstream* of the fixture. Totally counter-intuitive if you think
that the purpose is to provide air for draining. The slug of water has
to run a few feet before it gets to the vent. My plumber told me we
can't put the vent behind the toilet (upstream), but he didn't say
why. (probably doesn't know the why). I'd love to know why that is.

I think it's because when you flush the toilet, you get a sudden rush of
water that would go upstream and downstream. Since there's more than just
water in there, you could get debris pushed upstream, which could
potentially block the vent.

By placing the vent downstream, every time you flush, you're washing away
any debris left behind from the previous flush, thus keeping the vent line
clear.

This is the same reason codes do not allow horizontal venting from the
drain line. The vent has to come off the drain by at least 45 degrees so
there's less risk of it being plugged up.

Of course, assuming you have space in the joist bay, you could vent off the
drain "downstream", then run the vent back towards the wall to go up and
out.

Anthony