I’m calculating heat loads for our new house and have a question regarding air infiltration values.
We had a blower door test done on the house, and the results were air infiltration of .05cfm/ft^2 at 50 pascals, or 0.4 ACH. In calculating heat load, would I use these values for calculating heat loss due to air infiltration, or would I want to use a lower ACH figure to account for the pressurization during the test?
In other words, I am wondering how the air infiltration during a blower door test (when the house is depressurized) compares to ‘actual’ conditions on a daily basis…
Thanks!
Shawn
Replies
There was a thread around here not too far back on this. The measurement at 50 pascals pressure difference can be divided by 18-20 to get some sort of average leakage rate under normal circumstances. Bear in mind that "normal conditions" is highly dependent on the house, its location and configuration, current weather, wind speed and direction, etc. In bitter cold and windy weather, leakage is worst, and in mild windless weather it is very little.
You would of course figure some level of leakage into the heat load calcs. From your design temperature, air leakage must be heated up to room temperature. Its heat capacity is nominally 0.24 BTU/lb/deg F. In terms of volume, air at 60 F weighs about 0.0765 lb/cuft.
For summer A/C load, you need to figure in both the air cooling load and also air leakage rate times difference between outside and inside humidity times the latent heat of water (1000 BTU/lb).
A leakage rate of only 0.4 ACH@50 pascals is a very tight house. Passivhaus standard is I think 0.6 @50. So I imagine you'll have a heat recovery ventilator to provide fresh air. In that case, you can use the appropriate ventilation rate in the above calcs, rather than guess at natural leakage, which you can't control anyway.
Edit: I just looked at your profile. You are some sort of engineer, so why are you asking all this stuff you can easily calculate? OK, I reread your post, and you seem to be asking just about how to use the ACH50 value you got, right?
Edited 8/13/2009 9:07 am ET by DickRussell
Edited 8/13/2009 9:10 am ET by DickRussell
So I imagine you'll have a heat recovery ventilator to provide fresh air. In that case, you can use the appropriate ventilation rate in the above calcs, rather than guess at natural leakage, which you can't control anyway.
Okay, I was with you until I got to this statement.
I already have an HRV installed and will factor the efficiency of it into the heat load calcs. However, wouldn't the 'natural leakage' still be a factor in addition to the HRV? Having an HRV doesn't eliminate unwanted air infiltration, right? Seems like I would need to account for both the HRV loss as well as the ACH due to leakage.
ThanksShawn
If your blower door test gave 0.4 ACH at 50 pascals, then the real life air leakage under worst winter conditions is apt to be well within the accuracy of the degree of control you have over what the HRV provides. For example, if you shoot for 0.3 ACH with the HRV, and your worst case natural ventilation is 0.03, can you really dial down the HRV to only 0.27 ACH? At any point, you likely won't know what the HRV is giving you that precisely.If the house already is up, with HRV, what are you doing with the heat load numbers? Calculating theoretical heating cost vs. measured to see how close you come? Changing heating systems (so soon?) ?Maybe you ought to ignore the "natural" leakage if it so low and just use the maximum that the HRV will put out as a "worst case" heat load. If you want to "double-dip," you could add on the nominal worst case natural ventilation rate to the HRV maximum rate.One final thought: if your HRV fans aren't balanced for the actual ductwork, they could be slightly pressurizing the house, in which case the natural leakage would be a bit lower, or slightly depressurized, for a bit higher leakage. In any case, you'll never know what your actual leakage is at any particular time, under any conditions, except when the air outside is still and at the same temperature as inside, and then it doesn't matter anyway.
Edited 8/13/2009 1:23 pm ET by DickRussell
So what you're saying is that the natural leakage for this tight of a house is relatively insignificant compared to the air exchanges of the HRV, correct?
This is a 2 story, slab-on-grade house, which I am still building. I had the in-slab heat running all last winter and was able to monitor energy use with a watt meter so I know what it took. Now I am finishing the upper level and want to add hydronic heat to a couple of rooms... So I want to calculate heat load to compare it with my hard data and also to size the new systems about to go in.
thanks for your help!Shawn
You can find detailed procedures and equations for this in ACCA Manual J, section 8.
With your engineering ability and the blower door information, you can nail it down very precisely. The leakage rate will be more for winter, less for summer in your climate.
Also bear in mind that, if your house has ducts in the unconditioned space, these may have been closed off during the test, and doing so might result in an inaccurate (too low) infiltration rate.
I'd still like to hear the answer to the original question, which was basically how does the air infiltration at 50 pa during the blower door test compare with actual natural infiltration. I searched around some and could not find the answer.
I would guess the infiltration during the blower door test is a lot higher?
Gary
Air infiltration at 50 pa is much higher than what you would normally experience. But it's still hard to quantify how much.For instance, a stormy winter night with 40 mph gusts and a Delta T of 100 (F) will be a much higher rate than a calm spring day with a Delta T of 2.I think the difficulty in quantifying the effects of air infiltration is one of the main reasons why the average person hasn't paid too much attention to it. Anyone can look at their insulation and say "I have an R-19 wall" (although, we know that may or may not be entirely accurate) but not many people can guess the infiltration rate of a house.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
There is no direct corrleation. Blower door tests at "standard" conditions serve to provide relative comparisons only. I have been calculating the heating and cooling loads on buildings, simple and complex. Accounting for air infiltration is never an exact science.
Hi Tim,
I hear what you are saying, but surely having a blower door has to be of some use in setting the level of infiltration you would use in the heat calculation? If you were doing a heat loss on a home, and the blower door test showed 3 ACH, what would you use for the infiltration level in your heat loss calc.Gary
I do heat calculations on a room by room basis. Performing this manually, rooms with one exposed wall are calculated, with "average" construction (2x4 framed walls, batt insulation) at 1/2 ACH, two exposed walls at 1 ACH. Northern and western exposures in the midwest get an extra 10% because the cold winter air is coming in at those exposures. These numbers are incorporated into the load via a "heat up" factor (volume x the inverse of the ACH x the design temperature difference). For better than average I would half those numbers, for "very tight" maybe 1/4. Unless construction is really poor or there is a great deal of volume and/or exposure, this doesn't account for a large percentage of the heating load.
Using software, like Trace 700, you assign a positive, negative or neutral pressure and loose, tight or average contruction to each space and let the computer do the work.
I emailed the contractor that performed our blower door test and his reply was consistent with some of the others in this discussion:
Generally, I hear people say that dividing ACH50 by 20 gives you an answer that is close. Although I did just find an old Energy Star publication that says for our climate a 2 story house should divide the number by 15.
Shawn
Shawn,
There is a table of values in my Residential Energy Book
the values for 2 story range from 1/11.2 to 1/23.5
Is your site well shielded, normal or exposed?
.. assuming North west minnesota
and 2 story....the range would be
1/14.9 for well shielded
1/12.4 for normal
1/11.2 for exposed
Edited 8/19/2009 10:38 pm by homedesign
Thanks for the numbers.
In regards to my air sealing measures, I wanted to pass along an observation during the blower door test.
As i mentioned earlier, I had caulked all double stud seams in the wall framing. When the house was depressurized during the blower door test, we were walking around with the thermal imaging camera to look for leakage. The only leak we found in the entire building was in the vaulted ceiling, along the seam between two parallel chord trusses (the trusses were sistered together on either side of the entryway to the silo). I had not caulked this seam, thinking there was no way the air was going to leak through 10" of SPF (which completely covered the outer chord of the trusses) and another 10+" of cellulose...but it did!
I would not have figured this caulking to be very important until I saw the result of not doing it.
I suppose this step would be unnecessary if an interior air barrier is installed and properly sealed. But for those relying on the SPF to do the job, it would be wise.Shawn
Hey Shawn,
I really like what you have done... and your comments...do you have any photos of construction posted on a photo sharing site?
I also had a comment about those numbers I posted... they were probably developed based on more conventional homes.. not a home like yours.
They may not translate at all to a house like yours.
Actually the book I referenced..."Residential Energy..cost savings and comfort for existing buildings" has a lot of data and conclusions that I believe do not translate to the kind of homes that we are talking about here.
John Brooks
We have a blog which covers the construction of the barn-house as well as other happenings around here at http://www.greengateguesthouse.blogspot.comShawn
Hi,
On doing a little more looking, found this page on the Infiltec (blower door maker) site:
http://www.infiltec.com/inf-btst.htm
It suggests that the ACH at 50 pa is about 20 times the natural infiltration rate. Excerpt from link above:
ANALYZE DATA
* There are several ways to analyze data and here is a simple one:
* Calculate house volume in cubic feet (example: 1500 sq ft * 8 ft = 12000 cu ft)
* Inside and outside temperature can be used to correct data with computer.
* Use 50 Pa pressure and flow data (example: 50 Pa and 2000 cfm)
* Compute 50 Pa Air Changes/Hour (ACH50)
* Example: ACH50 = 2000 cfm * 60 min/hr /12000 cu ft = 10 ACH50)
* Use "can't reach" table if house is too leaky to get to 50 Pa
* Compute natural infiltration by dividing ACH50 by 20 (approximate)
* Example: 10 ACH50 / 20 = 0.5 ACH natural infiltration
* Minimum Infiltration: ASHRAE recommends a minimum Infiltration = .35 ACH
* This suggests a minimum ACH50 = 20 * .35 = 7 ACH50
* Houses that are leakier than 7 ACH50 should generally be airsealed.
So, on a (say) 2000 sf home, with an outside temp of 30F, the heat loss for infiltration on an ACH50 = 0.4 ACH would be:
(2000sf)(8Ft)(0.4/20 ACH)(0.075 lb/ft^3)(70F - 30F)(0.24 BTU/lb-F) =
230 BTU/hr -- almost nothing, but the HRV to bring it up to minimum reqd ventilation would add some more losses.
Gary
Shawn,
Building an enclosure that exceeds the Passivhaus airtightness standard is no easy task.
Kudos to you.
I would like to hear how you achieved such a good air barrier.
What is the make-up of your wall and roof construction?
Thanks for the kudos, here is our barn-house:
View Image
Other than some remodeling to our old farmhouse, this is my first experience in housebuilding. I think the success of the building envelope was due to keeping it small, simple (other than the attached 'silo' I built!), and putting our money in the right places. With a budget of only $50k for the entire project, I have done all the work myself and used alot of reclaimed/salvaged materials so that we could channel our funds into the best quality purchased items.
Foundation is slab on grade with 2" XPS underneath and as 'wing' insulation 4' out.
Wall construction is 2X6 framing on 2' centers. Insulated headers and 2-stud corners. Thermal-bridging was eliminated by a layer of 1" XPS all around (and 2" on the silo). I also caulked all of the double-stud seams (around windows, top-plates, etc) to prevent air leakage between them. The arched roof consists of deep parallel chord trusses. Windows are all H-windows (made in Ashland WI), and primarily on the south and west sides for solar gain. These are triple-paned gas filled windows, and many of them are fixed sash to minimize leakage potential. They were flashed with window tape and sealed with spray foam. We spent about 30% of our total budget for the windows!
Once it was dried in, the entire shell was sprayed with Icynene- completely filling the wall cavities as well as 10" in the roof (this is a hot roof design, with arched ceiling on the inside). The remaining truss cavities in the roof were then filled with dense-packed cellulose. The walls are about R26 and the roof around R70. The insulation was 20% of our budget, and we had the SPF done by a VERY detail-oriented contractor, and it obviously made a difference based on the blower door test.
I wanted to minimize penetrations in this envelope, hence the hot roof design and FPSF. Utilities were brought in underground, through the slab. Heat is hydronic in-slab with a very small electric boiler (plan to add solar next year or two, though). Appliances are electric, so no combustion venting was required (other than the wood pellet stove I installed for backup heat.
I installed A Venmar HE HRV for the required air exchange.
While the house meets Passivehouse standards for air tightness, thermal bridge free construction, and window quality, we are way off the mark for annual energy input. We live in Northern MN, about 15 miles from the coldest city in the country- typical winter lows of -40F and it can go days on end without ever seeing the temp get above 0F...not a good place to try to build a Passivehouse!
That said, I wish I would've increased the R-value of the walls (something I may do in the future), and the perimeter of the slab.
Thanks for asking!
Shawn
Edited 8/17/2009 3:22 pm ET by shawncal
"Appliances are electric, so no combustion venting was required (other than the wood pellet stove I installed for backup heat."Did you do anything particular to provide combustion air for that pellet stove, or do the HRV fans adjust themselves when you have stove venting going on?
For the pellet stove, I installed an outside air kit during the building rough in. The Icynene was sprayed to seal around this penetration.Shawn
Any detail on that inlet air supply routing? I've heard elsewhere of running the ductwork as an inverted "U", up through the wall cavity and back down and into the stove. The idea of that is to make a "trap" for the cold air, much like under a sink, to reduce air infiltration when the stove is not in use.
I've also heard suggestions of this type to reduce unwanted cold air flow into the house. The one I hear most often is to use a flex duct that runs down into the basement by the furnace or water heater, and then turns up for a couple feet or more to "trap" the cold air in bottom of the U.
I don't believe such a "trap" works in the situation described.
To use the plumbing p-trap comparison, remember that a p-trap uses water to stop air--if the water were not present, the air would move freely through the trap.
In the combustion air situation, whichever end of the duct is subject to greater pressure will flow air toward the end with lower pressure. Ups and downs, twists and turns, will only increase friction slightly.
There will always be a pressure difference between the inside and outside--sometimes negative, sometimes positive, depending on many factors, like exhaust fans, height of house, location of the opening to outside air, etc. Such pressure differences are almost always more powerful than the difference caused by temperature differences between inside and outside on a combustion air duct.
Your thoughts?
Well, certainly if the stove vent were not closed, then the stack effect would overcome the density difference between warmer air at the top of the inverted U and the colder air trying to enter. And during operation of the stove this is desirable for good draft.If the house is tight, but the stove isn't necessarily tight to the same degree, does an inverted U in the supply pipe help? I'd like to know, because I need to do this if it does help. With a straight-in pipe full of cold air stopping at the stove, that will be a cold pipe during the winter. When the stove is not operating to compensate, that pipe is a heat leak. It would be nice to think that if the cold air part of the pipe is surrounded with insulation then the cold is kept outside better.
For me, it helps to try to visualize what is actually happening when an appliance drafts.
We often hear that "warm air rises"--well, it does, but it is actually being pushed upward by the cooler, denser air at the bottom of the stack. I think of the air in the stack as if it were a long balloon filled with warm air. Such a balloon will rise, but will stop rising when it reaches a height where the surrounding air is no longer pushing it up, or, stated otherwise, when it reaches a height where the surrounding air is the same density.
This helps to answer your Q: If the house is tight, but the stove isn't necessarily tight to the same degree, does an inverted U in the supply pipe help? No. The column of warm, less dense air whose lower end is the stove and whose upper end is the termination at the roof, will be pushed up and the displaced air will be replaced most easily through the combustion air duct. The cold combustion air will flow whether or not there is a "trap" in the line, just as water flows in a pipe when it's pressurized, so long as there's positive pressure on the supply end.
Insulating the duct will reduce heat transfer to surrounding air, but will not slow the draft effect of the stove to the outside via the flue or chimney pipe.
If there is just the internal damper in the stove and when the stove is not in use the damper is pushed "closed" (however tight that might be), I would expect cold outside air to settle down into the flue pipe, but be warmed to some extent by being surrounded by house. there would be some convective currents going on within the pipe as the warmer air next to the pipe wall rises up, buoyed up by the colder air settling down the middle. This represents an ongoing heat loss when the stove is not in use, in addition to heat losses directly to the supply pipe filled with cold air and entering down low.Given that there is apt to be somewhat cold air in the flue pipe, down to the level of the stove, especially if it is double walled up to the ceiling and insulated vent pipe from there on up, I still have to wonder if it might help to have an inverted U air supply pipe buried within the wall cavity's insulation and with the top of the U above the stove, perhaps up near ceiling level. Without a substantial flow of air for the siphon effect, why wouldn't the inverted U have some benefit, behaving much like the water-filled trap under a sink?If this reasoning still is flawed, then what air supply arrangement would address heat losses with a wood or pellet stove in a very tight and well-insulated house and where that stove is used only occasionally?
Edited 8/18/2009 8:48 am ET by DickRussell
I believe the only way to stop unwanted airflow via the combustion air duct would be to have a tightly-closing damper in the duct--which is acceptable code-wise only if the damper is motorized and interlocked with the stove to assure its opening whenever the stove operates. Not a very practical solution for a home.
The damper in the stove itself that you close manually will have much the same effect, even though the damper will not be really tight, as you mentioned. There will still be a small loss of heat due to a small flow of cold air entering the combustion air duct and leaving up the flue.
I can't think of any arrangement of the combustion air duct itself that would stop the convective flow. The p-trap analogy does not apply because it uses water to stop the flow of air or gases. The inverted U would not create enough resistance to counterbalance the stack effect of the vent system as a whole.
It's like siphoning water from a bucket: as long as the bucket is higher than the outlet of the siphon hose, the water will flow regardless of any ups or downs of the hose along the way. The same rule applies: fluids always flow from an area of greater pressure to an area of lower pressure.
So, as long as the rooftop termination of the the flue sees lesser pressure than the inlet of the combustion air duct, the air will flow, unless a damper stops it. This flow will happen regardless of any intermediate rise or fall of the combustion air duct.
How about routing the insulated vent pipe up through a larger diameter pipe, with combustion air coming down the annular space and then to a smaller pipe down to the stove's air inlet. Once the fire has been started to establish draft with the door open, filling the flue pipe with warm flue gas, the door would be closed and the flow of cold air down the annular space would commence. When the stove is not in operation, both the annular space and the flue pipe itself would fill with cold outside air, with no density difference.
Good idea.
It's similar to what's called a direct vent in the USA, but in England, they call it a balanced flue, which is a good description of how it works.
I see these all the time on gas-fired stoves and fireplaces. I suppose some manufacturers of wood stoves do the same thing.
(Apologies to OP and others for hijacking the original thread: this is only vaguely related to original Q about the effect of air infiltration on a load calc.)
No apologies required, as I am following the thread with the same problems in mind.
I installed our pellet stove with horizontal venting through the sidewall. After attaching the venting and intake air to the stove, I sealed these points as best as possible. I have not insulated the intake line yet, but plan to before winter. It is definately a problem.
Interestingly, the dealer had told me to put a 6" loop in the intake duct to create an air trap, as you've been discussing in this thread. He went on to say, "this will completely eliminate the cold draft when the stove is off." Yeah right. I never did it, and would like to figure out a real solution.
I was wondering if there was any type of inline fabric damper that might work for the intake side...something that required the pull of the combustion fan to open it up, but would collapse to a closed position when the stove was turned off...?Shawn
I went looking for info on the subject and found this:
http://www.woodheat.org/outdoorair/outdoorairmyth.htmInteresting reading. What does the info on the pellet stove blower indicate for cfm air movement? Can your HRV be tweaked to adjust the fans to avoid depressurization when the stove is in operation?
VERY interesting reading.
Although the author provides no references to any studies that he claims have been done, all that he says is reasonable and very much in line with my own experience over the last 16 years as a mechanical inspector.
I investigated a house with two 8-inch combustion air openings where the gas water heater located in the basement was still backdrafting. The cause had nothing to do with either the flue or the combustion air--it was caused by a poorly-designed duct system that de-pressurized the basement.
As the article points out, the forces that can cause imbalances in the house pressures are usually greater than the buoyancy pressure differences created by the stove, whether gas or wood-burning.
In other words, the big fan wins.
In regard to shawncal's pellet stove, it is another type altogether since it has a powered exhaust and is vented horizontally, and the combustion chamber is effectively isolated from the room atmosphere. I'm also guessing the the exhaust outlet and the air intake are at about the same elevation, therefore in the same pressure zone.
So, the action of the exhaust fan is what causes air movement more than any stack effect. The system is self-balancing in this case, and there will be very little heat loss through convection flow during times when the stove is not burning, and still less infiltration of outside air into the house.
I believe the blower door test results should account for the local 'average' wind speed for your location, so the 0.40 value should be good for the peak heat load calc. many days it may be below that. Some above that. The blower door overpressurizes, yes, but one factor to get the calc is an average winter wind speed.
You will likely oversize a bit anyway in the end (available equipment sizes are usually always chosen larger than the peak calc'd load). You don't get much better than the blower door test. Most times, the air leakage values is at best a total WAG.