ACH- Air Changes/Hour

Paul Rohrs_4

What are you designing around for ACH? I was poking around the Lennox site looking for some info and came across this statement.

"The average home should have at least 0.35 ACH (air changes per hour) rating. Many newer homes have an ACH rating as low as 0.05.*

Here is the Link: http://www.lennox.com/iaq/ventilation.asp

EDIT: I mean to ask, ideally, what do you design around?

Regards,

PR

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Empire_2

COuld not find posted info, but

@ .5 AC/HR the house would be so stagnant as far as air quality goes. An average house is usually 4 6 per hour. You may be seeing exhaust recovery equipment with this figure. Many will chime in to give their opinions.

Mike T.

Brad White_9

In 20 or 30 years

you will have a 20 or 30 year old home. Meaning settlement, wind loads, cracks, caulking failures, all taking their toll. I take it you are speaking of infiltration for heat loss purposes, not ventilation purposes. On that basis...

The best houses will be in the 0.30 to 0.50 range -when new.

Infiltration is always a judgement call and can add significantly to over-sizing. Here is what I do:

1. Each room gets it's own infiltration rate as part of it's heat loss/radiation sizing. It will see 100% at one time or another.

A. If a room has one exposure (sandwiched between two heated rooms plus heated space above and below), I will take the greater of 0.50 air changes per hour (ACH), or the crack method at 0.50 cfm per foot of operable length of windows. 1.0 cfm if doors.

B. If a room has two exposures, I go to 0.75 ACH or if greater, the crack method. (Assuming windows on any of the exposed walls it is usually proportional). If three exposures or more, I go to 1.0 ACH or crack method. Judgement does enter into this of course.

Remember this is to size the radiation or air supply to each room for it's 100% need. The sum of these is NOT NECESSARILY the boiler size.

2. Once I do the room by room calculations, I take the total aggregate heat loss of the building (transmission only, backing out the infiltration).

To this number I will add either a the greater of the crack method on two sides or the ACH method calculated to the building as a whole and compare this to the total of the rooms including their infiltration rate.

So, you will have three numbers to compare:
1) Sum of all rooms transmission and infiltration;
2) Sum of all rooms transmission only;
3) Sum of all rooms with transmission only plus block total infiltration.

Judgement will determine which is most suited to sizing the appliance(s)

The thinking is, any room will see 100% but the proverbially 4-sided building as a whole will have INfiltration on at most two sides and EXfiltration on the others. So the total infiltration will never be the sum of each room's total infiltration.

GMcD

Radiant systems and ventilation

Assuming the house/building is of very tight, well built construction and infiltration is near zero, the radiant houses I've been involved with (as well as the institutional buildings) usually have the basic dedicated outdoor air systems (HRV's) sized to provide in the range of 1.5 ACH to 4 ACH depending on occupancy. Houses are usually in the 1.5 ACH to 2.5ACH as my design start, with the design logic being to size the ventilation system for basic human ventilation at 30 cfm per occupant, then add more air if it is going to be used for cooling, or second stage heat (in cold climates), or de-humidification (warm, moist climates). This logic also applies to radiant heating/radiant cooling systems where all of the basic room temperature control is from the radiant system - the air system is sized the same way.

Most Codes require a minimum of between 0.35 ACH and 0.5 ACH for residential occupancies as that the is the minimum threshold for preventing indoor humidity to get too high to promote indoor mold growth. They don't have much to do with "human comfort", except for the maintenance of the "minimum healthy air change rate", and most folks meet that Code Minumum by having bathroom exhaust fans controlled from a humidistat. The problem is, where does the make-up air come from? Nobody seems to think about that. The more astute designers/builders are using HRV's and ERV's so the indoor air quality is continually renewed, and as well,to provide controlled, tempered, filtered outdoor air delivered as the make-up air, rather than down fireplace flues, via cracks and imperfect envelope leakage. The Codes are MINIMUM standards and it's OK to exceed them to meet human comfort and additional health concerns.

My personal design ethic is to always exceed Code Minimum - and stay out of the mainstream of mediocrity.

Empire_2

hAT'S OFF TO THE 2 OF YOU.

i GUESS i LEARN SOMETHING NEW EVERY DAY AND i THANK YOU FOR THE CLARIFICATION ON THIS ISSUE. Although I still think the AC/HR is lo,...I will definitely research the standard practice to better educate myself in this matter. ;-)

Mike T.

D107

Energy-wise structures go for MAX of .5 ach

Got the link below from a prior wall post. On new construction, they use panel systems and count on ventilation systems to provide fresh air.

On old uninsulated clapboard house blower test revealed 1 ACH. Now with 4" blown in cellulose,(R13) then 3/4" polystyrene rigid insulation (R3.8) over clapboards under vinyl--if I do vinyl--I should have close to R-19. (Plus sealing wherever I can.)


http://energywisestructures.com/sci_air.html

Weezbo

consistenly constructing better homes based on previous models

would bring much finer results. each year our tact is to do some things a little better .twenty five years ago or better it was fairly apparent that things were in need of change,that bought about requirements in building practises that taught everyone a few lessons on the need for controlled ventilation. sealing methods bought over from Europe produced super tight buildings,.035 became "normal" when the tests came back even lower from a blower door we wondered where is the air comming from if we dont have controlled ventilation. in other words,not all buildings are constructed by the same practises these days. our current thought is that our home raters need to get on board and make it clear that there are homes being built and have been consistently been built that demand a better rating than a 5 star plus.

it is common practise to use .35-> .05 even though our practises do not currently live in that century.
there are builders across the world that have made super tight construction "normal" so it is an opinion buh if quality matters to the contractor and the blower door tests have a track record of super tight construction you may consider those previous numbers as indicative of their current practises. sounds strange maybe that there are contractors out building recognized codes, buh they do exist in nearly every community all over the world.

GMcD

Confusion

I think some of us are talking about two separate issues:

Issue #1: the natural infiltration/exfiltration rate of a building envelope - the un-controlled outdoor air that the heating (and cooling) system needs to allow for in the building load calculations.

Issue #2: what is the design ACH that a radiant system designer would use to select a powered home ventilation system.

My post above is in reference to Issue #2. The Energy-wise website (and some of the other posts) are referencing the infiltration rate of a house, rather than the powered ventilation rate one would use to design a powered ventilation system.

It is not uncommon for a tightly built house to have less than 0.2 ACH worth of infiltration, which is a good thing, but it clearly indicates that a powered ventilation system MUST be used with a hydronic heating system in order to provide healthy, and comfortable ventilation in the house.

Mi39ke

How Low Can You Go?

The low numbers are consistent with what Energy Star is seeing in quality envelope performance.

In Wisconsin, they tell me that the average ACH is .15 to .16 for new homes in the Energy Star Program. This is tested after construction is complete. We cannot design for over .50 ACH in my State, I believe. That would oversize everything incredibly.

The ICF people are now doing in-house heat loss calculations because they are having a hard time finding people who are will to design around their "bag tight" construction methods.

At a recent conference, a couple diehard data people presented a paper on the issue of whether or not a tight home is too tight (not enough fresh air). They datalogged around a dozen homes of various types. C02 levels were monitored, exhaust and ventilation rates, how the people lived within the home, etc. I believe they found that even the tightest of homes didn't require the HRV or ERV that most people are recommending for air changes.

The ventilation for the removal of outgassing VOCs or humidity control might be another matter, but the CO2 levels measured convinced them that maybe we are selling too many ERVs, etc. for the wrong reasons. Anyway...that was the drift I got out of the presentation. Still haven't read the report, so reader beware!

When they presented the paper to the brainiacs in California, they were not well received, they said. No one in the ivory tower likes to be confronted with reality, these days. [chuckle]

Michael Ward

Unknown

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Unknown

for heat load purposes, for good stick frame construction I use .35-.45

For houses with blown insulation, SIPs, ICFs, or exterior foam sheathing I use .2 ACH. This is probably a little high for natural infiltration, but it would give a little wiggle for a good ERV with a modest flow rate.

Log homes, .75-1 ACH. Yeah, I've been burned.

Never come up short yet (except on that log home, darn it)...

Geoff, those exchange rates seem awful high for purely IAQ ventilation. with continuous operation, why would you ever need rates that high? ASHRAEs standards have been steadily dropping for twenty years... and overventilation is a big fat energy waster, no?

Larry Weingarten

Michael...

...I'd really like to get my hands on that paper. What is the source? Thanks!

Yours, Larry

GMcD

Ventilation rates

Depends on what and how the house is being ventilated. Anybody here calculate the make-up air requirements for fireplaces and especially range and dryer exhaust make-up as part of the system?

The HRV air volumes I use are based on cycling the units from humidistats, temperature, and make-up air requirements - somebody turns on the range exhaust hood, or the dryer, and the HRV goes on to supply make-up air. I always try to get direct make-up air to fireplaces (woodburners for sure) unless they are direct vent gas units. In many of the projects I've designed, the radiant floor or ceilings haven't had enough capacity for those peak summer or winter design days and so I add the "second stage' heating or cooling capacity into the HRV, so if we get that one week of cold weather (or hot weather), and the radiant can't keep up, the air side can provide that 10% to 15% more second stage capacity when needed. That's because I keep having architects and glazing suppliers going with cheaper glass and leaving me dealing with poor mean radiant temperatures along some perimeter zones.

So, no, the ACH ranges I design for the HRV capacities isn't "just" IAQ only. Don't forget that the Code minimum ventilation rates are primarily for minimum "health" and that doesn't necessarily mean "minimum comfort".

If I finally get a project with the best glass and solar shading so that the radiant heating and radiant cooling can do 100% of the room temperature control at peak conditions, then the HRV would only be sized for the most make-up air use in the house- if they had an electric range and a 250cfm range hood, then size the HRV for 250 cfm, and a touch more for safety (somebody might be running the dryer and taking a shower at the same time as breakfast is cooking). If the home has a gas range, then the range hood should be at least 400-500cfm, and size the HRV for that, plus your safety factor. That usually ends up resulting in a higher ACH in a small volume home compared to the McMansions.

Paul Pollets

Geoff

We think from the same box!

WA. State requires a min of .35ACH for residential units.

Most builders utilize bath fans on timer with no make-up air. Some use an economizer package with timed damper on furnaces.

Rather skimpy approach, I'd say.

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Mark Hunt

Curious


Have any of you ever done blower door testing after the construction is complete?

If so, what have you found?

My testing has shown that nearly all of the homes that were supposed to be "tight", leaked like sieves.

Most of the builders I talk to have no idea what ACH is.

Mark H

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Unknown

Actually, Mark Hunt just raised a good point that reminded me;

If i'm doing a load for a regular old GC'ed "high end" (read: large) home, my infiltration estimate rise to .5 minimum.

In my experience, the larger and more expensive the home, the worse the workmanship.

edit: also architects going crazy with wood framing members influences this.

GMcD

Stick frame- yes

Yes- the policing of the envelope for wood frame buildings is abysmal and they mostly leak badly. In a well sealed house with a curmudgeon like me looking over the Contractor's shoulder, we might see an infiltration/blower door test result in the 0.15 to 0.25 ACH range, and most of that air is the stuff leaking out via the bathroom exhaust fan, range hood duct, and fireplace flues due to the pressurized space. I haven't tested any spaces where the "built-in" air leaks at the ducted exhausts have been taped over which would be a more true indication of the "envelope only" leakage.

Mark Hunt

So I'm not the only one then


That is EXACTLY what I have noticed.

Mark H

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D107

new home going up next door--no tape on exterior plywood wall

seams, just Tyvek over it. Should be done right?

Unknown

Yeah, i forget about that because frankly, most of my jobs are for people who are very concious about their envelopes. Heck, I'd say a small majority of my jobs are SIPs/ICFs or blown insulation. I keep forgetting that doesn't represent the building industry as a whole. Hard to see past your own perspective sometimes I guess

man.. I walked on to a high end project down here with lots of windows on one wall.. and whatever wasn't window was wood, practically. even if they installed the insulation well, there was nowhere to put it! The whole wall must have averaged about an R5. Maybe. On a beachfront point with CRAZY wind.

Makes me shudder.

GMcD

Inside vapour barrier

It's not so much the exterior sheathing- it's the interior vapour barrier if you are in a cold climate, and then sealing around the bottom and top plates, and all penetrations at the indoor vapour barrier- plugs, lights, wires, ducts, pipes, etc. Also around the window frames, and exterior door frames, and try to get some exterior insulation to get a thermal break from the exterior wall studs to the exterior finish, too. Then it's policing up the specified weatherstripping on openable windows, exterior door edges, sealed and insulated exhaust ducts with backdraft dampers that really work.