What is my expected air loss (ACH) under normal atmopheric conditions?
I have a leaky old farmhouse that I am slowly over the years gutting and insulating and airsealing. I have a master plan that I plan to arrive at.
I am starting to think about a new heating system and to size my boiler I have created a spreadsheet to do a basic heat loss calculation. Since I am re insulating everything I have a very good idea of what my R values are so the thermal loss part is easy and intuitive for me. Where I get caught up is the air loss calculation. The math makes sense to me. If we break it down to Qi = (.0018)(N)(V)(Delta T) I know my Delta T, I know my volume, but I do not know the number of air changes in the house.
I performed a blower door test before I started on any of this and was not surprised by 17 ACH50.
I can’t perform an exit blower door test until I’m done with the whole building but I need to install my heat pipes before I close things up. Besides, even if I magically knew what the blower door number were, it would be at 50 pascals, not atmospheric pressure.
So: I have no idea how many ACH a leaky house has under atmospheric pressure, how many a reasonably tight house has, what an extremely tight house is expected to have. Any advice?
If it makes a difference I have about 23 K cubic feet and I’m insulating with 9 inches of dense packed cellulose. I have no specific air barrier but I’m planning on being very diligent when I finish with sheetrock. I’m guessing a blower door test will give me a number between 4 and 8 ACH50.
Thanks,
Josh
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Replies
In blower door testing, the only number you can hang your hat on is ACH50. You can use that number to arrive at a rough idea of air changes under natural conditions, but it's only an estimate. You can use N factors and your assumption about final ACH50 to do the math. To what extent can you make the sheathing the air barrier, and do you have access to a blower door to do guided air sealing?
Making the sheathing an air barrier is pretty much out. I'm leaving the outside of the building more or less alone. The sheathing is just rough cut boards so it's pretty much a no-go.
The company I work with owns a blower door. I have been and will continue to use it to find any leaks and close them during various stages of building.
So how do I translate my assumed ACH50 into ACH under natural conditions?
This really doesn't need to be super accurate- after all I am doing this to size my boiler and boilers don't exactly come in incremental size differences. I have have no clue about when I'd even be in the ballpark.
Josh,
Take a look at this post: https://www.greenbuildingadvisor.com/blogs/dept/musings/blower-door-basics . But beware. Calculations of ANHnat are approximate only! +/- 25%? +/- 30%? Dunno. As David says above, You can use that the ACH50 number to arrive at a rough idea of air changes under natural conditions, but it's only an estimate.
I just read up on N factors. I got my answer. Thank you!
Another thought on getting an approximate ACHn... if you are using a Minneapolis door and their Tec-Tite software, it gives you a value for ACHn at the end of the test. I bet there is plenty in the instruction manual on how this is done, and if not, call the company directly and ask them how they make the calculation. I have found them to be very helpful on all sorts of technical questions like this. Colin Genge of Retrotec is also quite helpful and knows a great deal. If you REALLY dig deep, you could probably find technical papers on the differences between projected ACHn from blower door testing and how that compares to tracer gas analysis.
Josh,
It sounds like you have already figured this out. But if any GBA readers are curious, here's what I wrote on this topic in my article, Blower Door Basics:
"Some blower-door technicians estimate a home’s “natural infiltration” or “natural air change rate” (ACHnat). This number shouldn’t be taken too seriously, since it is only an estimate. Natural infiltration rates (and rules of thumb for calculating ACHnat) vary by climate. In Minnesota, ACHnat approximately equals ach50 divided by 17, while in Florida, ACHnat approximately equals ach50 divided by 30. According to Gary Nelson, the president of The Energy Conservatory in Minneapolis, “ACHnat is probably only accurate plus or minus a factor of two.” "
Perhaps you're looking for the less-relevant information. You're trying to properly size the boiler/heating system, which is the right goal, but since you have a heating history with the old boiler, you can use IT as the measuring instrument for determining the where-is-as-is heat load of the house. With a ZIP code or town location (for weather data and 99% outside design temp purposes) and the EXACT wintertime meter reading or fill-up dates & quantities, using the nameplate efficiency of the existing equipment it's possible to reliably determing the upper bound of the heat load (distribution losses included.)
That doesn't help with the room-by-room radiation sizing, and the data can be heavily skewed if you went to Belize for 3 weeks during the billing period with the thermostat set to 50F, or were heavily using auxiliary heat such as wood stoves or mini-splits. Winter periods have the least amount of error from solar gain or other appliances such as water heaters using the same fuel, or inefficiencies due to very-low duty cycles on ridiculously oversized heating equipment, etc.
On older houses of mixed or difficult to figure out construction this method is more reliable than a Manual-J, and takes 15 minutes, not hours.