Comments on: Why I’m Not a Fan of Positive Pressure Mechanical Ventilation

By: Marvin

Marvin — Sat, 08 Mar 2014 02:51:46 +0000

Live in Canada with cool
Live in Canada with cool winters. Have wood and a gas fire place. Sometimes there is a gas exhaust smell coming from the gas fire place. The house is only 3 years old and has a air exchanger. Large exhust for stove. I believe that we are having a negative air in the house sometimes and is pulling air from gas fire place. How can I check for positive or negative air pressure.

By: havalandÄ±rma

havalandÄ±rma — Thu, 17 Jan 2013 14:55:29 +0000

Thans for information
Thans for information

By: Judy Roberson

Judy Roberson — Tue, 31 Jul 2012 22:29:03 +0000

Allison,

Allison,
The title of your article is misleading. What you’re “not a fan of” is NOT positive pressure (supply) ventilation, but using a central AHU to provide it. I agree that the system you describe – and object to – is a worst case scenario in terms of energy efficiency, insufficient outdoor air supply, and intermittent (vs continuous) operation. Fortunately, there are better – more efficient and effective – ways to provide supply ventilation. Using a small, right-sized, efficient (high cfm/W) fan & ventilation-only ducts that are independent of the central FA system provides these advantages: the ability to pull outdoor air from a known (unpolluted) location, pull it through a MERV 7-12 filter, deliver small amounts (20-25 cfm each) reliably to bedrooms and living areas (even when doors are closed), run continuously (non-stop), and slightly pressurize the home. Because of these advantages, I consider “independent supply ventilation” to be the “better half” of balanced ventilation, which doesn’t always make sense due to budget and/or climate. The reason most people are not aware of this option is that no major manufacturers promote it. Of course, intermittent exhaust fans are always needed in bathrooms and kitchens, to remove moisture & odors, which is what they do best.

By: Tim O'Brien

Tim O'Brien — Fri, 29 Jun 2012 00:45:49 +0000

Whole house dehumidifiers can
Whole house dehumidifiers can provide positive pressure ventilation without the problems listed above. An efficient whole house dehumidifier will ventilate using a fraction of the fan motor watts of the air handler and the dehumidifier will remove water vapor before introducing the mixed ventilation air into the house. The more efficient whole house dehumidifiers are more efficient at removing water vapor from the air in a house than the AC system.

Efficient whole house dehumidifiers are more expensive to install than the simple positive pressure ventilation system described above (or a bath fan negative pressure system), but they will provide a very comfortable house at a low operating cost.

Maintenance is the main challenge for positive and balanced ventilation systems. Many homeowners do not maintain these systems and some contractors do not check the condition and performance of the ventilation systems during the seasonal check-up. If the outdoor air grille and filter are not maintained clean, the system will not ventilate as designed. With a dirty outdoor grille and/or filter, the simple positive pressure system will not provide adequate ventilation, but will provide some reduced amount of ventilation.

With a dirty outdoor grille and/or filter, the balanced ventilation system will become a negative pressure ventilation system. ERV and HRV systems offer a good payback for houses in extreme climates, but a much poorer payback for houses in moderate climates - they use more fan power with their two fans than a simple positive or negative pressure ventilation system (that does not require the AHU fan to operate). I have seen balanced systems with clean filters in the ERV/HRV, but a nearly plugged insect screen on the outdoor duct. A marginal or non-existing payback for much of the US climate with a tendency for poor maintenance can leave many houses with a negative pressure ventilation system that was expensive to install and is expensive to operate.

By: David Butler

David Butler — Thu, 28 Jun 2012 03:15:51 +0000

I caught up with Brian Coble
I caught up with Brian Coble earlier today. He confirmed that Advanced Energy stopped using air cyclers in Systems Vision homes due to comfort complaints, as well as other reasons. But he clarified that it wasn't because the supply air was too cold, but rather an issue of excessive air movement, which I think we all know can create discomfort when it's cold outside.

So I read too much into Scott's comment, perhaps because Allison framed the comfort issue in terms of distributing unconditioned air.

But Brian raises an important point. In general, it's a bad idea to operate the blower off-cycle in winter. However, in a high performance homes with a low design airflow, say 0.25 CFM per SF (about the upper limit of the homes I work on), running the fan at half speed, especially if registers are properly designed and adjusted, shouldn't be an issue.

Although I've never had complaints from the handful of clients with PPV, I acknowledge that it requires careful attention to design by someone who understands the issues discussed in this thread. But I also remind folks of numerous reports of poorly installed HRV's and ERV's.

By: David Butler

David Butler — Sun, 24 Jun 2012 22:52:46 +0000

Allison wrote: <
Allison wrote:
> there are ERVs that are far less expensive than $3000...

In retrofit, probably not much less than that, especially when you account for commissioning. In new construction, the lowest ERV quote I've seen was $1800, but the contractor retracted his bid when I prompted the builder to inquire about commissioning.

By: Allison Bailes

Allison Bailes — Sat, 23 Jun 2012 16:24:15 +0000

Nate A.: I
Nate A.: I don't have a good answer for you right now for retrofits. The $300 Panasonic Whisper Comfort ERV has been popular, but I don't know how well they perform.

John N.: Sounds like you're on top of the situation in your home, but not every home is a HERS rater.

Mark J.: There are ERVs that are far less expensive than $3000, but yes, maintenance is definitely an issue. Martin Holladay wrote about that very issue a couple of weeks ago at Green Building Advisor. Your statement that, "Any ventilation system which does not supply sufficient air DURING the AC cycles, is just wrongly designed," isn't really true. With 1.5 tons being the smallest standard AC you can get, it's easy for the AC to be oversized, especially in high-performance homes.

Scott S.: I don't see how just the normal runtime of the AC can provide enough ventilation air to meet ASHRAE 62.2 in a small, efficient Habitat home. With 1.5 tons being the smallest AC you can get and the cooling loads on some of those homes being less than a ton, I'm sure, it seems like it'd be difficult. I'll definitely talk with Bryan to see what they're doing because I haven't seen positive pressure, central fan systems that work well yet.

By: David Butler

David Butler — Sat, 23 Jun 2012 08:49:31 +0000

Here’s the blower table for
Here’s the blower table for one of the models I referenced: http://bit.ly/NjDSYg 

A blower table will show the entire static range for each speed. I chose 0.5″ for my examples. Perhaps a bit conservative for low speed, but again, my point was simply an argument against generalizing (which admittedly I’m sometimes guilty of).

By: Bob

Bob — Sat, 23 Jun 2012 05:49:09 +0000

The 500W/400W ratings came
The 500W/400W ratings came from me testing my own system. It's rated for 3.5ton with a 1/3HP blower.

IMHO it's odd that Lennox uses 0.5" at the LOW speed tap as a baseline to determine motor wattage on a system that's rated for 0.5" on high. If they used 0.5" on HIGH speed the motor wattage would read considerably higher.

By: David Butler

David Butler — Sat, 23 Jun 2012 05:04:38 +0000

Bob wrote:

Bob wrote:
> A typical 1/3 HP blower motor uses 500W on high, 400W on low.

Perhaps for the largest models, but those typically have 1/2 hp motors. Here are some examples of several Lennox models (based on 0.5" WC at lowest speed tap). Lennox is one of the few majors that publish watts on blower tables.

air handler (CBX32M)
1.5/2.0 tons - 190W (1/5 hp)
2.5 tons - 190W (1/3 hp)
3.0 tons - 255W (1/3 hp)
3.5 tons - 310W (1/3 hp)

furnace (G51MP/GM51MP)
2.0 tons - 230W (1/5 hp)
3.0 tons - 325W (1/3 hp)

The point is, without doing a life-cycle cost analysis, it's impossible to generalize about the trade-off between cycling a PSC (positive pressure ventilation) vs. forking out thousands for an ERV or HRV.

But I agree it's better to increase vent CFM and reduce the cycle time, especially with PSC.