We are building a house and air tightness is a top priority. HOWEVER, when I run the numbers in beopt, (climate zone 2 north texas) our house with an ach50 of 3 would use about 1000kwh less over a year than a house with an ach50 of 0.5 with an exhaust fan. Is my data somehow wrong? because if this is correct, why should I spend time and money to make the house airtight only to end up spending more money mechanically ventilating it. MY thought now is to shoot for ach50 of 1 and just use bathroom vents. thoughts?

Put a CO2 meter on the fan and only run as needed.....Have lots of plants in your house.....

What is the rating without the fan in the BeOpt calc?

thanks. thats definitely a strong possibility. I just dont understand why beopt would show such low loss. I have used other calculators that show that in my area the greatest source of heat loss is air leakage.

when you say rating without the fan, what number or data are you looking for?

thanks

A leakage rate of 3ACH/50 is already a pretty tight house, one that needs something OTHER than exhaust-only ventilation. A 1ACH/50 doesn't change that. Exhaust-only ventilation is a generally lousy strategy.

With exhaust only ventilation the path of the ventilation air is undefined, not necessarily going to where they do the most good, and not necessarily along the cleanest path. The biggest air leak in my basement is the seam between the slab and the foundation walls- depressurizing the house would only pull in more soil gases.

Similarly, having a very leaky house doesn't mean the ventilation is going where it's needed most. The location of the leaks matter.

CO2 levels are not a good stand-in for indoor air quality. It's possible (and common) to simultaneously have low CO2 levels and totally unhealthy indoor air.

Dupe.

Below 3, and you really need to be looking at a managed ventilation setup for both intake and exhaust.
You can go nuts and try to come up with a completely passive ventilation setup that works on the Rube Goldberg principle.
Or you can just look for a decent HRV/ERV and control everything in an automated, mechanical fashion.

ok, so really stress testing this from "best" to "worst"

7 ach 50 and no mechanical ventilation,
0.5 ach 50 and 70% efficient erv

the 7 ach 50 house uses less energy over a full year than the very tight house with erv!

Posted By 1201 on 19 Mar 2019 09:46 AM
ok, so really stress testing this from "best" to "worst"

7 ach 50 and no mechanical ventilation,
0.5 ach 50 and 70% efficient erv

the 7 ach 50 house uses less energy over a full year than the very tight house with erv!

A 7ACH house is over IRC code maximum, and can still have lousy indoor air quality.

In most of the US a 7ACH would use more energy than a tight house with an ERV. Where are you?

He is in North Texas, climate zone 2. I'm in North Arkansas climate zone 3. HRV/ERV are rarely if ever installed in my area. Exhaust only ventilation is common.

I could believe the leaky home on the cool edge of zone 2 or zone 3 where both the 1% and 99% design temps are within 35F of room temp might have lower energy use without using mechanical ventilation.

Exhaust only ventilation works best if there are dedicated passive inlet air ports where ventilation is needed the most rather than relying on random inlet paths to do the right thing. In a tighter (=IRC 2018 code compliant) home without creating clean paths in known locations all bets are off.

https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-012-balancing-act-exhaust-only-ventilation-does

ok, good to know. I still want to worry about air tightness, but Im not going to go crazy regarding it, and I will not be looking at an erv or any other complex system

design indoor temp during the summer is 75 degrees (we see highs from 99-105) and winter its 68 degrees and we normally see overnight lows around 38 degrees.

Where can I find the 1% and 99% temps for my area?

thanks

Our house has an extra 4" PVC fresh air intake pipe alongside the 4" PVC pipe for the heat pump refrigerant lines. This 4" fresh air intake pipe terminates inside in the air handler return plenum box and the exterior terminates with a metal screened and filtered down turned inlet. This allows outside ambient temperature makeup air to come in as needed when any exhaust fans are running (bathroom, kitchen, clothes dryer, etc.) rather than pull in higher than ambient temperature air from above the ceiling.

Exactly what we do Arkie. Sometimes you just have to exhaust air and bring in make-up air to keep the conditioned space from significantly depressurizing (e.g., using a large kitchen range hood or wood burning appliance). It is always good practice to bring this make-up air into the conditioned space via a controlled path that has a damper which only opens when the house is significantly depressurized and which then buffers the make-up air temperature before it enters the room. Using a dedicated PVC pipe or the duct work soffits works very well for this purpose. Anything that causes a building to significantly pressurize or depressurize and allow uncontrolled outdoor to enter/exit is bad news for energy efficiency. This is why we prefer HVAC approaches that don’t use air handlers like HR floor heating and passive solar cooling/heating.

Posted By 1201 on 19 Mar 2019 07:38 PM
Where can I find the 1% and 99% temps for my area?

thanks

Design temps of select cities:

https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf

Design temps by county:

https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf

Use less energy?  Maybe.
Now ask yourself why.

Because you have a home that is leaking like you left a screen door sized hole in a wall someplace.

So yes.  You're not paying for the energy on a climate control system.  Because your indoor climate is completely out of control.

So, if your outdoor weather is PERFECT year-round, yes.  You'll spend less.
If you require ANY climate control whatsoever, you've going to spend orders of magnitude more.
Because you'll have to produce at a level that overcomes the effect of that massive leakage.  And you'll have to more or less do  it CONSTANTLY because if it shuts off, you then have the leakage quickly render the structure uncomfortable again.

And yes, you can do as some of the people here are saying, and try to deal with it passively.
It's a very careful balancing act though, and if you get it wrong...

Me?  I'm a control freak.  And lazy to boot.
I have a marked preference for an automated approach.

thank you Dana for the info.

Dilettante. I guess I dont understand.

If I can get the house to breathe naturally, Like houses have done for millennia, and save money over a complicated ventilation system that will require servicing and maintenance, what is the issue?

I dont understand how I'll spend more with climate control if the erv is introducing outside air at a rate equal to or greater than a "leaky" envelope

Has anyone ever installed a manometer in there house that they monitor regularly. A lot of this pressurization/depressurization seems like guessing and old wife's tails....

Just wondering. Will have to look for one that measures very low.... We are installing a wood burner now and don't care for the idea of putting another hole in the house if it is not needed.

Thanks and sorry if it is a side track but seems to fit some of the conversation.

Posted By 1201 on 20 Mar 2019 03:52 AM
I dont understand how I'll spend more with climate control if the erv is introducing outside air at a rate equal to or greater than a "leaky" envelope

The ERV uses electrical power that adds up to at least something over the course of a year, the leaky envelope does not. A highly efficient low-wattage exhaust fan in a leaky house uses about an order of magnitude less than a balanced ventilation system like an ERV (due to much lower static pressures.)

The rate of natural "ventilation" (no fan) even in a leaky house is highly variable. With dedicated inlet ports along clean paths and an exhaust fan there is at least a guaranteed minimum rate.

You use a magnehelic to measure air pressure differences:

Magnehelic

A magnehelic is often used to balance HRVs and other air handler systems. If you don’t have any air handler systems operating, you will find that your house is slightly depressurized in most rooms. This is largely the result of stack effect (the house acts like a chimney). Things like bathroom exhaust fans, central vacuum systems, kitchen stove hoods, and wood burning stoves/fireplaces will significantly depressurize a house.

When there is significant wind, the rooms on the side of house that the wind is hitting will be pressurized and the rooms on the opposite side of the house will be depressurized. You can’t do much to prevent that beyond well-sealing the house during construction and minimizing this effect. Air handlers (e.g., your HRV or hot air furnace) can be configured to pressurize or depressurize a house and should be configured so the house is neither when none of other aforementioned electrical/mechanical things that significantly depressurize a house are operating and when there is insignificant wind. We have a port to the outdoors located in our utility room (it's normally connected to one side of the pressure switch used to open/close an electrical/mechanical damper that allows outdoor air to enter our buffered make-up air soffit whenever the house is significantly depressurized) and we occasionally use this port and a magnehelic to check on the overall state of our house.

Posted By 1201 on 20 Mar 2019 03:52 AM
thank you Dana for the info.

Dilettante. I guess I dont understand.

If I can get the house to breathe naturally, Like houses have done for millennia, and save money over a complicated ventilation system that will require servicing and maintenance, what is the issue?

I dont understand how I'll spend more with climate control if the erv is introducing outside air at a rate equal to or greater than a "leaky" envelope

Simply because the house "breathes" doesn't mean you have control over the climate.  Nor that the interior climate is comfortable to live in.

This is like opening all your windows in a blizzard.
Someone complains that the house is freezing.
And you're going "Yeah!  But the house is BREATHING!"

Now, if this is just a vacation shack or something, where you're not living full-time, and you can deal with some discomfort, fine.

If this is your HOME?  You're going to want to be comfortable in there.
Warm in the winter?  Yeah, but the house is "breathing", and you're allowing heat to escape through the leaks.
Cool in the summer?  Yeah, but the house is "breathing", and you're allowing cooled air to escape through the leaks.

This means your HVAC system (the heating and cooling elements, not necessarily the ventilation elements) has to work harder and longer just to MAINTAIN a given temperature.

Another thing you have to work to maintain in that environment?  Humidity.

Basically I'm saying you've either borked your calculations or there are things you're simply not taking into account.

And yes, houses have "breathed" naturally for millennia.
Now, how many millennia-old homes do you have examples of?

Are you going to be building in concrete and stone?  Or using something with a wood component?