Putting speed control on attic fan

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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.
(Amazon.com product link shortened) 1&pf_rd_i00BKQGDE&pf_rd_m=ATVPDKIKX0DER&pf_rd_r3R2NTM0Q87VFEGEJM8
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 amps. Any issues if I put the motor on a rheostat? Are these motors designed to work on low voltage?
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Mikepier wrote:

(Amazon.com product link shortened) 1&pf_rd_i00BKQGDE&pf_rd_m=ATVPDKIKX0DER&pf_rd_r3R2NTM0Q87VFEGEJM8

    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.
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That's the problem, I think I have adequate vents. I have 2 roof vents, and a gable vent. I'm not sure if adding another vent would fix it.
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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 attic.
--Vic
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wrote:

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
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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 different size. 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
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wrote:

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.
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On Mon, 22 Aug 2011 09:27:04 -0700 (PDT), Mikepier

Okay, just checking. Now I'm stumped too.
--Vic
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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.
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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 flame. 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. Strange. 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.
--Vic
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Vic Smith wrote:

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 closed?
Is the furnace filter clear, or clogged?
Does the furnace have a humidifier? Is the humidifier housing sealed, or leaky?
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You can plug water heater downdraft attic fan into google and find other cases. Probably only happens when the chimney draft is just marginal without an established flow.

What do you mean by that?

I don't know if you can retrofit them. Seems you can't, but I might be wrong. Got a link?

He never mentioned if his A/C was running, so that might be worth looking into.
--Vic
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Vic Smith wrote:

A lot of people say "attic fan" and the answers seem to indicate "whole-house" fan.
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 motor.
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 to have.

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 than that?

Not just the A/C, but is the HVAC fan running? (sometimes you just want air circulation in the house, without the AC running)
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You're probably right. I didn't look close at first. But Mike has said his attic fan causes a downdraft, and said it's just an attic fan. Pretty clear from his description.

You'd have to figure how to turn it on and off automatically. Don't know if downdraft would still blow out his pilot. Seems the horizonal diverter is the easiest path.
--Vic
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Vic Smith wrote:

Actually, no.
Mike said this in his second response:

http://www.electricmotorwarehouse.com/fasco/hot_water_heater_blower.htm
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.
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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.
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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 problem here.
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" snipped-for-privacy@optonline.net" wrote:

What the fuck is wrong with you?
If he really does have a reverse airflow through his flue caused by his roof fan, then a chimney liner is going to do fuck-all to help that situation.
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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.
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" snipped-for-privacy@optonline.net" wrote:

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 flow.
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 problem.

Again, you're completely disregarding the whole point that started this thread.
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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