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michael 03-20-13 12:07 PM

Residential Energy Compliance State Regulations
I recently submitted plans to the local (Northern California) planning department for the construction of a new 2 bedroom house. They came back from the plan checker with some necessary corrections, but the one that has me stumped details requirements for whole house ventilation. I'll describe below my understanding of this, but I wonder if anyone on this board can help me make sense of it. The state energy agency ( requires that a house of 1200 SF such as ours have a continuously running 40 cfm exhaust fan or in-blowing fan so that whole house air changes happen regularly. The regs state specifically that opening windows is not a sufficient plan, but that a mechanical system must be in place to continuously exchange the air in the house. It can be done with intermittent fans such as a combination of kitchen and bath exhaust fans, but the formula, in the end, requires either such large fans in those locations or that they be on for such a long period of time that a single, moderately sized fan seems like a better choice. The federal government documents on this subject admit that nearly all forms of whole house ventilation result in higher heating and cooling costs. I understand the concern that some folks have regarding trapped, stale or toxic air inside new, energy efficient, tightly sealed dwellings, but I don't understand the sense of insulating a house, reducing infiltration to a minimum, and then pumping the heated or cooled air to the outside all day long. Logic tells me that in some places, the in coming outdoor air will be just as polluted as the air in the house. All that aside, it still seems like an affront to all the folks who are trying to build safe, energy efficient houses, to be paying instead to pump their heated or cooled air out of the house daily, continually. Can someone help me see some sense in this plan?

Michael Moreland
Mendocino, CA

Daox 03-20-13 12:25 PM

The solution is simple, don't pump all the heat/cool out, but still bring in fresh air. :) This is what a heat recovery ventilator (HRV) is for. Its a heat exchanger that transfers the heat/cool from the outgoing air into the incoming air so that you don't loose as much heat/cool. Yeah, they aren't 100% efficient, so you still loose some heat/cool, but not nearly as much.

We have a good DIY thread about them here:

randen 03-20-13 01:45 PM


It may seem like a silly idea but the HRV's are a great idea and yes they are not 100% efficient at recovering all the heat energy. But for a new tightly built home is a great idea for energy efficiency and if you check with some of AC Hackers work on C0 2 concentrations you will be suprised at how fast and much it can collect. This can lead to health problems and feeling tired while in the house.

I remember going to my grandmas house and the lingering oddors of cooking smoking and a-little stale air when alot of people visit the small house. Can't be good and this was by no means a air tight house.

We installed a very good HRV system its completely quiet and it speeds up for a moment as someone uses the bathroom. But what I'm very pleased about is the outdoor fresh air smell when you enter the home. Even right after cooking within a half hour you can't smell anything. If you feel the air thats exiting the home it is cold from the heat exchange that occured so it does work

HRVs, Worth every penny and good for your health. Get a good one and make sure the contractor places a return duct close to the troubled spots bathroom, kitchen, Sofa for hockey season.


Exeric 03-20-13 02:28 PM

Well, I guess a good discussion thread begins with more than one point of view. So I'll represent that different point of view.

Stale air with higher than recommended CO2 buildup always happens in houses occupied by humans. I live in northern CA also so I'm aware of your dilemma. I live in neighboring Lake county. There are two main aspects for deciding on whether to get an HRV, besides the high expense of a non DIY unit. The first is the total amount of whole house air exchange your new house will have with a 50 pascal blower attached to the door. A PassivHouse rating is about .5 per hour or thereabouts. I forgot the exact number. However many people think that airtightness under around 2 air exchanges per hour has diminishing economic returns. A house with a rating of 2 or better is colloquially described as a "pretty good house".

That brings up the second determining factor. We don't live in a real cold climate, though its colder in northern california than most easterners realize.
It makes more sense to pay for an extremely tight house in colder climates. For one thing, the temperature differentials inside and outside are bigger which means more temperature is being lost in winter. If you are just building a pretty good house, which makes economic sense in pretty much all of California except the high Sierras, then you will have enough random leakage that a Panasonic whispergreen bathroom fan will make a lot of sense. They can run all day at 30 or 40 cfm using only 3 or 4 watts. So you're not wasting a lot of electricity. With an HRV you are going to be using a lot more power than that and because the temperature differentials inside and outside are not as extreme here the energy reclaimed in the HRV will not be as good as when it is used in colder climates.

I realize when building a new house it feels best to go for the tightest house you can build. If you get to PassivHouse standards you probably will need an HRV because their won't be enough random air leaks to provide the return air for a simple exhaling ventilation fan. At .5 you are likely to get stale air, and high indoor CO2 and high humidity that only an HRV can control.

But as I said, its overkill in our climate. If I was in your position and you do a blower door test I would try to get a house that leaks less than 2 but greater than, say 1, at least ideally. You can then get away with just a simple Whispergreen fan in the bathroom, assuming its a simple single floor building with a somewhat open floor plan. If it ends up being in PassivHouse territory then you might be forced into an HRV. A needless expense in California (in my opinion).

Exeric 03-20-13 04:37 PM

I guess I should add that one really can't retrofit MORE leakage into the house after the house is completed. If you did everything right in sealing it and you got your house passivehaus tight you can't go back in and create a balancing vent hole without creating a drafty point in the house. Rather than create a balancing vent it might just be best to go with an HRV at that point.

The whole idea of having a house that's not extremely airtight, but is consistently airtight, is that there is no big area that leaks more than all the others. There won't be a drafty point in the house then and it will be a happy accident type of thing where a ventilation fan works just fine. It's probably difficult to purposely engineer that "happy accident" into a house but it happens often enough by accident that you would be considered unlucky if it didn't happen in the process of building your house. But if you were that unlucky then you are basically screwed and would have to go with an HRV unless you can cope with at least one big drafty area.

michael 03-20-13 08:55 PM

I stayed up late last night with energy form CF-6R-MECH-05, and in the end settled on a Braun bathroom exhaust fan that can be tuned down to 40 cfm, draws 4.9 watts and is rated at <0.3 sones. It will run 24/7 and take care of compliance. I'm intrigued with the concept of an HRV, and if I were to go in that direction, I think I'd like to DIM, but for now, I'll stick with the 40 cfm fan and a "pretty good" job on the house. mm

Ryland 03-21-13 07:59 PM

Just because you have a system installed doesn't mean you have to have it turned on all of the time, the inspector might want it hard wired in but you'll need a service switch and you might as well replace that $0.39 switch with a timer so the vent fan comes on when you want it to instead of being on all of the time.
heat recovery systems do an amazing job and if a bath vent fan will move enough air to comply with code then you might as well install a bath vent fan with a heat recovery heat exchanger in it because it will keep 80%+ of that heat or cool that is inside of your house inside instead of venting it all outside.

michael 03-22-13 05:13 PM

Thanks, Ryland, for your suggestions. I've spent some time looking at heat recovery systems, and they look to be good solutions to the problem of losing heat through whole house ventilation. I have to take Exeric's comments above seriously, however, such as the fact that the difference between indoor and outdoor temperature where I live is not extreme. Also, I've been thinking about our heat source which is a radiant floor (pavers over 1.5" of concrete) which holds plenty of heat, and the air in a building such as ours doesn't appear to stratify. Losing some of the slightly heated air to the outside and having it replaced by slightly cooled air from the outside won't do much to alter the essential heat of the house which resides mainly in the mass of the floor. I'm not convinced that there would anything better than an energy trade-off if we were to employ an HRV. I don't reject the idea, but I am questioning its efficacy in our situation. mm

Ryland 03-23-13 09:18 PM

Just because you have a large heated mass doesn't change how much heat it takes to heat your house if you are venting heated air out, you are still loosing BTU's that have to be made up.

The mild climate does give you a great advantage, but why be wasteful if you have a choice?

MN Renovator 03-24-13 07:48 AM

I think this is the correct math, let me know if I'm off-base.

1.08*40CFM*40 degree difference(70 inside 30 outside)=1728BTU/hr*24 hours=41,472BTU per day.
That's 8 hours of an electric 1500 watt space heater running or about $1.20 at 10 cents per kwh.
About .4 therms or $.40 if your total gas costs are a dollar per therm. So if your average outdoor temperature for a month is 30 degrees outdoors your rough number would be $12.40 if you are paying $1 per 100k BTU of natural gas or for electric resistance heating it would be about $37, about a third of that cost with a heat pump.

The bigger issue that I see is the moisture content that you might be dragging into the house in the summer if your dew point gets over 55 degrees.

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