A few weeks ago I posted a problem with too much negative air pressure
in my house caused by my attic fan. The motor I have on the fan is
similiar to this.
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I wanted to get somekind of speed controller to slow down the speed of
the fan. I've seen them on Broan's website, and they are rated for 6
Any issues if I put the motor on a rheostat? Are these motors designed
to work on low voltage?
If you have negative air pressure in your attic, why don't you simply
add a few more vents? There are probably eaves where a vent could be
added reducing the pressure.
As far as controlling the speed of the fan motor, I think DC motors are
better suited for such control. Of course you could reduce the AC to an
AC motor, but it can present problems for the motor.
On Mon, 22 Aug 2011 06:54:32 -0700 (PDT), Mikepier
I already asked about this, but I'll try again.
You just put in the water heater, right?
You posted about venting, but I don't know what you did.
Did you change the venting?
Did you have a problem with the old water heater pilot?
If not, you should look at your venting.
I've got no idea how you let that water heater chase you up to the
I thought I answered this last time, but the water heater is vented
through an exterior brick chimney. Its not going through the attic at
all. And the water heater is right underneath the chimney vent, so
total length of ducting from the water heater to the chimney is maybe
1 - 1 1/2 feet
On Mon, 22 Aug 2011 08:18:28 -0700 (PDT), Mikepier
You don't say if you changed the HW vent, either location relative to
the furnace vent, or angle of entry into the chimney, or went to a
And whether you had the problem with the previous tank.
I'm no expert on venting HW tanks.
I've always left them as I found them.
But you were talking about changing the venting.
If you did, that's where I'd look, not the attic.
Besides blowing out the pilot, wrong flue draft is a safety issue.
Vic, the venting is the same, I changed nothing. It is a 3" rigid duct
(per water heater spec) that leaves the water heater on an upward
pitch and goes directly into the chimney. It is not tied into the
furnace vent, which is on a seperate duct run.
The water heater was installed in January. I first noticed problems
when I started using the attic fan in the summer. If I do a match test
by the draft hood with the attic fan on, the flame is blown out. After
I shut off the attic fan, the flame test is fine, no issues.
In my situation however, I think the attic fan helps with the A/C on.
My house was originally installed with force hot air heat, with the
supply ducts low to the ground. Like many other people in my
neighborhood, I added on Central A/C using the existing ductwork.
Because the return vent is low on the floor along with the supply
vents, I think the attic fan "draws" that air upwards, making it feel
cooler. I could be wrong though.
On Mon, 22 Aug 2011 12:02:55 -0700 (PDT), Mikepier
Mike, I read up a bit on drafts, and found your attic fan issue isn't
unique. But I didn't find any simple solutions.
Your old water heater had a horizontal diverter, which probably did a
better job of keeping downdraft from the pilot.
Dumps any downdraft away from the heater.
These were common pre-1992, if not the norm.
That explains why the flame didn't blow out before.
Besides that, newer water FVIR heaters might use a smaller pilot
I don't know if horizontal diverters meet code now, but if they do,
putting that back on solves your flame-out problem.
I see no reason they won't meet code, since the new diverters are no
better that I can see.
Shouldn't change heater operation at all since you keep the supplied
hood. Just dumps the downdraft elsewhere.
I found one place selling horizontal diverters on the net, but can't
find it now. Very little can be found about them.
I saw the one on the pic you posted and remembered I tossed a couple
of them when I put in the new style heaters, but everybody else who
knows about them must have died before the net got going.
You should turn up the heat on the water heater to go on when the
attic fan is running to see how quickly a draft is established,
Just for kicks.
If you don't put a horizontal diverter back in, I'd check to see if a
chimney cap can improve draft. I don't know about that.
I'd do all that before messing with the attic fan.
You should have CO detectors if you don't already.
There is no way his house is so well sealed such that his roof fan is
pulling interior house air into the attic, causing a negative pressure
that can only be relieved through the water heater flue pipe.
His water heater exhaust is vented directly into his chimney. Does the
chimney pass through the attic?
Why isin't anyone telling him to install a water-header draft motor?
How do we know that his furnace fan isin't causing negative pressure
around the general area of the furnace / water heater, causing reverse
air flow through the water heater flue pipe?
Is the furnace air-handler properly sealed?
Are there any return air vents/ducts near the furnace that can be
Is the furnace filter clear, or clogged?
Does the furnace have a humidifier? Is the humidifier housing sealed,
A lot of people say "attic fan" and the answers seem to indicate
I have to wonder if most of these situations that you're finding on
google search are really just whole-house fan and NOT roof fan.
I'm thinking it's the HVAC fan.
Anyone with a conventional furnace will have enough convection heat from
the furnace combustion exhaust to establish a flow up the chimney to
pull the water-heater combustion exhaust. That's why most codes don't
call for water heater draft motor when the water heater is vented into
the chimney with existing conventional furnace. When you have a high
efficiency furnace, I think most codes will call for water heater draft
In the summer when the furnace is not on, you might have more of a
problem with chimney convection.
If the furnace is near the water heater, in a small enclosed space, I
can tell you for sure that the HVAC fan will definately try to pull air
from the closest source if the air handler plenum or air filter housing
is not sealed or if there are return air intakes nearby. A clogged air
filter will make this worse.
Never mind. I was thinking that a leaky chimney might be the cause of
his problem, but in fact it would actually help. If his chimney does
pass through the attic (on it's way out and above the roof) then the
roof exhaust fan would only help pull air out of the chimney if there
was a "leak" in the chimney. That would create a negative pressure at
the water heater flue intake - not a postive pressure like he's claiming
I don't see why it wouldn't be easy to get ahold of a draft motor and
stick it on top of the water heater and connect it's ducting in-line
with the water heater exhaust stack and the chimney flue stack. It's
just a matter of "plumbing". Why do you think it's more complicated
Not just the A/C, but is the HVAC fan running? (sometimes you just want
air circulation in the house, without the AC running)
Mike said this in his second response:
Well, they probably sell these things where-ever you can buy a gas water
heater (home despot around here at least).
I don't know how they're controlled when you've got a hot water heater
with a standing pilot. Maybe some sort of thermostatic switch that can
detect when the burners are turned on.
These draft motors pull air up out of the flue - they don't
"down-draft". They up-draft.
He needs to put a piece of paper to cover the flue opening at the top of
the water heater and see if the paper is really being pushed down / away
from the flue opening. Or go on the roof and put a piece of paper or
cardboard over the stack opening and see if it gets sucked or pulled
down. If it is, then something in his house is pulling air down
through his chimney, and it's not his roof-mounted ventillation fan. A
kitchen or bathroom fan, or his furnace fan is doing it. Maybe he's got
a window-mounted fan (or A/C unit?) somewhere doing it.
I have confirmed multiple times its my attic fan.
Also my Central air has no effect on the downdraft. If the A/C is on
and the attic fan is off, the draft is fine.
My water heater has a sealed chamber for the pilot, so I don't know if
it would blow out that easily.
You said that the water heater is on it's own chimney. What
is that flue size? If it's a large size flue, typically used with
an old furnace, it's likely that the flue is too large. That presents
a problem of another kind. If it's too large, the exhaust gas
will cool off on the way out and condense back into water.
Natural gas exhaust is acidic and over time, that condensate
eats away at the mortar joints, causing the chimney to fail.
With a furnace on the same chimney flue, that was not a
problem because the chimney could only get cold enough
in winter to cause this to happen and then the furnace is
also running. That prevents it from happening. But with
no furnace, you don't want a gas water heater venting into
a chimney that is too large, unless the entire chimney is
surrounded by heated living space.
The solution is to put in a chimney liner, which is easy
to do. Having a chimney that is too large also means that it
will not draft properly, which could also be part of the
Excuse me, ignoramus, but first, if you look at that in context,
which you did not, the problem I was specifically referring
to the chimney liner solving is the case where the chimney
is too large for the water heater. That problem can exist
regardless of any backdrafting issue and it can lead to long
term damage and failure of the chimney.
And second, having the flue correctly sized to the appliance
helps create the correct draft. If the flue is sized correctly
the hot gas stays hot and rises. If you dump a smaller
exhaust load into a chimney that is too large, the natural
draft action does not occur. So, yes the chimney liner
can help with a water heater that is having problems
with the exhaust backdrafting.
So, have another beer, maybe you'll feel better. More
likely though, it will just result in more profanity.
You can't say that his chimney is too large unless you know if he lives
in a northern climate and if he has a high-efficiency furnace.
In a northern climate, you can have condensation inside the chimney if
there is not enough total combustion flow (furnace + water heater).
Normally for a regular furnace, it's putting out enough exhaust to keep
a good convective heat flow going and "help" the water heater flue stack
In the summer, there really isin't a problem with needed help from the
furnace (which you won't get anyways because the furnace doesn't run in
the summer). The water heater exhaust flow should have no problem
getting up and out the chimney in the summer. Now if you have a reverse
air-flow in the chimney, then nothing you can do to the chimney
(including using a liner) is going to help with that.
So unless you know what type of furnace he has, and just how cold it
gets where he lives, then you can't say that he *needs* a liner for his
water heater exhaust.
And like I said before, even in cold climates it's standard to have a
non-lined chimney and have furnace and water-heater exhaust run
passively up and out the chimney. It's only when you have a
high-efficiency furnace do you start to look at using a draft motor or a
liner to help the water-heater exhaust.
Fuck the draft, and the sizing. This isn't rocket science.
When you've got something creating a negative pressure in the house,
causing reverse air-flow through the water heater flue, then you put
down your books and graphs and tables and you fix the reverse air
Again, you're completely disregarding the whole point that started this
This boob thinks that he's getting a breeze flowing into his water
heater exhast flue and blowing out a match that he holds against the
draft intake vent (presumably the gap between the water heater and the
flue intake above it).
None of your correctly-sized shit is going to make that breeze go away.
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