Power Vent Installation

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  #1  
Old 06-23-15, 11:51 PM
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Power Vent Installation

My roofer is in the process of installing 2 power vents for my attic. I don't know if he is planning to bring in an electrician to make the electrical connections, but I want to make sure they're wired correctly. They will be side-by-side in the top part of the roof above the attic utility area which has 2 gas furnaces and a water heater. They have thermostatic controls that are designed to be hardwired. Each furnace has a dedicated 20 amp circuit run to a receptacle where the furnace is cord connected (fairly standard in this area). It would be easiest to pull power by wiring into 1 or both furnace receptacles (both fans on one or one fan on each) since they are right there and have plenty of spare capacity (2 amp fans, unsure of amperage for each furnace, but can't be more than 10 amps so total is well under the 80% max). Is this code compliant or do those circuits need to remain dedicated? My plan also includes installing a switch on each fan so power can be removed easily and locally for maintenance. If I can't wire to the furnace circuits, plan B would be to pull power from the attic light, but it's on one of the bedroom circuits which is only 15 amps. Each bedroom has a dedicated circuit so probably wouldn't be a big issue... Thoughts?
 
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  #2  
Old 06-24-15, 12:09 AM
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The power vents need to be on the same circuit breaker as the furnace. They also require GFCI protection.
 
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Old 06-24-15, 04:44 AM
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Are these power vents attached to the furnace exhaust pipes or general attic fans for air circulation to cool the area?
 
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Old 06-24-15, 07:00 AM
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Just general attic fans for air circulation.
 
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Old 06-24-15, 07:15 AM
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Standard operating procedure is to pull power from a nearby light circuit -- usually something like the upstairs hallway light. Since you have an attic lighting circuit, that would be a good candidate too. The HVAC circuits are dedicated to that function so you should not connect the vents there.
 
  #6  
Old 06-24-15, 07:42 AM
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I assume you have done your calculations and decided you need two fans up there, however, the depressurization resulting from attic fans can be a problem as it will also be depressurizing your house. To what extent, that needs to be determined. A gas water heater will tolerate very little negative house pressure before it will backdraft dumping all if its combustion products back into the house.

The path for the air leakage into that very negatively depressurized attic (assuming both fans are in exhaust mode) is through holes drilled for wires, plumbing, and the chimneys. Even if those have been air sealed, the framing is not and it is just covered by drywall, but not sealed.

A normal large attic may have an effective soffit vent area of 2 or 3 ft². To pull that much air in through that area would require a significant pressure. With that much pressure, conditioned air from inside your home will be exiting through the ceiling.

If you assumptions are wrong, just let me know.

Bud
 
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Old 06-24-15, 08:53 AM
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Thanks Bud. All valid concerns, but I have an engineering background and am working with a very experienced roofer who knows his craft well, so we've already done that homework. The house is over 3000 SF and we're pulling air from 2 different connected attic areas. The one where the power vents are located is the top one and there will be no power vents, or roof vents of any kind for that matter, anywhere else. We've already checked soffit area, added an air tight attic stair cover (an actual insulated and sealed door that closes over the ladder...), air sealed all accessible plumbing, wire and HVAC duct penetrations and sealed the HVAC ductwork. The water heater and furnaces are not in the living space, but actually in the attic that's being vented so CO leakage should be insignificant. I've installed new CO detectors on both floors of the home just in case. The plan is to install both thermostats and humidity sensors so the vents run only when needed and aren't pulling cold air through the utility area in the winter unless the humidity is high. If, after all our research, we find we've missed something, my roofer has agreed to adjust the installation as needed to make it work correctly. Now, if I could just get a tie breaker on the wiring...
 
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Old 06-24-15, 09:13 AM
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Just general attic fans for air circulation.
Please disregard my last post, it was concerning power vents for furnace lines.

The water heater and furnaces are not in the living space, but actually in the attic that's being vented so CO leakage should be insignificant. I've installed new CO detectors on both floors of the home just in case.
Have you considered the possible fire hazard of pulling superheated gases through your attic space instead of through their designated ductwork?
 
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Old 06-24-15, 09:21 AM
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I apologize for drifting on your wiring question, but unless those appliances are sealed combustion they cannot meet the mfgs safety requirements being in an attic with that much exhaust capability, especially the water heater as it has a lower threshold and it runs year round.

Below is a link that includes the limit for a naturally drafted water heater, -2 pascals, page 14.
http://www.bpi.org/Web%20Download/BP...5nNC-newCO.pdf

Despite your air sealing efforts your whole house still leaks air at a rate that replaces all of the inside air in your home every 3 to 4 hours and some of that leakage connects your house to your attic.

I'll go away now, since (pardon the pun) your roofer has you covered.

Bud
 
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Old 06-24-15, 01:34 PM
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Thanks again! Always good to hear top comments in case something has gone over my head . According to Mfg. specs, we erred on the side of caution and slightly under sized the installation. Based on attic SF (2500) and soffit vent area (600 SF), we should be installing 3 power vents so I wasn't overly concerned about sucking all the conditioned air out of the house or "super heated gasses" through my attic space. This a very common practice around here and I haven't seen too many reports of death or homes that have gone down in flames because of it, but I will now be properly cautious (read paranoid) and take some measurements with the fans running once everything is complete. I have seen a number of different wiring methods, most of which I didn't like, so was most concerned that I get that part right from the start. Not crazy about taking 4 amps off one of the bedroom circuits, but if it's code, it's code. Final question: Do these need GFCI protection? I hadn't thought about that until Justin mentioned it, but even if the rest of his reply wasn't relative, it makes some sense that this part might be since the vents are exposed to the elements.
 
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Old 06-24-15, 02:22 PM
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GFCI protection is not required. You can pull off a bedroom circuit. You could also install a new circuit if you really want. Or, pull off a bedroom circuit and if that breaker ever trips then worry about putting in a dedicated.
 
  #12  
Old 06-24-15, 02:32 PM
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On topic for a change. If you are branching off of another circuit, check all connections all the way back to the panel. Back stab connections on a receptacle are often weak spots and in some wiring, the point at which you connect may have gone through several of them. Move those to screw terminals and check all screw connections to be sure they are snug.

Now, 600 ft² of soffit vent area sounds like a lot, or a decimal point out of place. If we give a 2,500 ft² attic floor (45 x 60) a 210 linear ft perimeter and allow a 50% net free area, your soffits vents would be over 5' wide.

Once you have the actual net free area, calculate how much air can flow with a 2 pascal pressure differential and I suspect it will be far below what those fans will be moving. Thus the pressure will be greater than the max 2 pascals and the water heater will be subject to flame roll out.

As for many other homes having been built this way, the codes have changed and for good reasons. In fact, they will soon, if not already, be eliminating naturally drafted water heaters.

Bud
 
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Old 06-24-15, 04:16 PM
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Perfect Ben. Thank you! Excellent advice on the backstabs Bud. House was built in 99 and the backstab problems were known by that time so all connections should be on the screws. So far all the switches and receptacles I've had the chance to mess with in the last 16 years have been, but I'll double check. Electrical and tile cement seem to be the only two areas where my builder didn't skimp. I'll also see if I can double check my roofers math on the intake vents. There's a lot of roof area and no gables so a lot of soffit. Hopefully he's right and there is enough.
 
  #14  
Old 06-24-15, 05:24 PM
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The guidelines the roofer will go by are dividing the attic floor area by 300 (double that is not well air sealed). 1 ft² of vent area for every 300 ft² of attic floor. 2,500 divided by 300 gives us 8.3 ft² of total vent area. That number is then split, half high and half low (approximately). If you have 40' of ridge vent at 18 in² per foot that is about 5 ft² of high vent. If that 600 is square inches instead of square feet then you would have 4.2 ft² for low vent.

Using a very simplified equation, at 2 pascals our expected CFM would be 6.3 CFM. Now, most air flow equations are way beyond my interest, so I will defer to you to calculate how much air can enter the lower vent area while maintaining a pressure differential of 2 pa or less. But my 6.3 CFM seems on the low side, never the less, the flow would be nowhere near the 2,000 plus CFM those fans will be exhausting.

Here's my point, unless that is a sealed combustion furnace and water heater, DO NOT install those fans. I'm more than willing to accept correction, but 2 attic fans with both the furnace and water heater up there looks to be outside of consideration. Let's see if some of the HVAC pros here will comment.

Bud
 
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