Friday, August 31, 2012

Mechanical Ventilation


Mechanical Ventilation

Mechanical ventilation of your home is a good idea. That means a fan running 24 hours a day 365 days a year. It provides a means of reducing indoor air pollutants and improving IAQ. It’s good for you and your whole family. We have a cute saying “dilution is the solution to indoor air pollution”

But aren’t you supposed to show me ways to save money and use less electricity? I don’t get it

True our goal is to save you money and we will show you how to use less electricity. Remember our goal is to address your safety and comfort and using methods of performance contracting and building science and our end result will be a more efficient home. Even though a fan will be running at all times these fans are very efficient and sip electricity rather than guzzle it. The benefit more than outweighs the cost.

Can’t I just open a window?

Sure you can it’s a good idea to open your windows.  We still want to mechanically ventilate your home to ensure it has fresh air at all times. We want to have control over our fresh air and not simply guess. You can open a window whenever you want but we still need ventilation.

Ok so now we have established you need ventilation. What kind and by what standard. The logical answer would be to use ASHRAE 62.2 2007 here in California s a guideline.  It is what the current California new home construction standard uses. There are some rebate programs that require other versions but we will stick with the referenced code.

Let’s take a 2000 square foot four bedroom home as an example
The formula is pretty basic (sq ft * .01) + (bedrooms + 1 * 7.5)
(2000*.01 =)20 +37.5( =5*7.5)

So we need 57.5 cfm for our home.
So let’s get a 60 cfm fan right? Uuuummmmm probably not

The thing is that we want that amount delivered not close. The type of material used for ducting is important. The length of the run is important. How many elbows does it have is important. Proper sealing of ducts is important. Quality of the product is important.  Not accounting for these factors will have our 60 cfm producing 40 cfm or even less.

I would normally recommend that we take that size and add 15 percent and round up to the next available fan. I would recommend a 70 cfm fan in most cases for proper ventilation.

What about using more cfm would that be better?

Yesnomaybe is the answer.

Yes              It is better for IAQ
No               It is wasting Energy
Maybe       Every home is different and occupants are different. If you have 10 100 gallon fish tanks we might want to ventilate a bit more. There are other reasons as well. We call them when we see them.
For the most part we want to size according to the standards that is why they are there. We want size and install the system correctly and test it to make sure it’s providing you with what you need.

What kind of fan?

This is a good question and the answer can vary given the any different factors.

Best
Balanced system’s which brings fresh air from outside and exhaust stale air from inside. Cost is the biggest issue with this style

Better
Supply only ventilation which brings air from the outside. It puts the home under positive pressure (see does my house suck?) and ensure the air is fresh

Good
Exhaust fans. These are already set up in many homes. The install and parts are more cost effective. However this air will be made up from wherever. Studies have shown that even though there is no control of where the air is pulling from that the air that it pulls in is better than the air in your home. This is the most common strategy not due to performance but due to price.

OK lets seal it tight and ventilate it right!

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