During Spring/Fall months, the outside night air can be in the low 60s while our very well insulated house remains in the mid to high 70s. Instead of relying on the AC unit, is there an off the shelf system that would instead pump in fresh night air (presumably through a high quality filter) and exhaust the warm indoor air?

For a DIY solution, I was imagining the following: Two tees inserted into each end of the cold air return duct in the attic, both leading to the outside. When the system engaged, one damper in the middle of the return duct would close preventing the house air from returning through the loop, and one damper in the first tee would open instead exhausting the air to the outside. On the other end, a final damper would open allowing fresh air through a high quality filter to be sucked into the house.

From a control perspective, it seems to me it may be possible to use the wire that usually triggers the A/C system from a standard programmable thermostat. The thermostat would call for A/C, which would first be rerouted through a blackbox (made up of thermostats and relays, or some sort of hobbyist microcontroller) that would compare outdoor air temp to indoor air, and either activate the intake/exhaust system or the A/C accordingly.

Looking forward to your thoughts!

Posted By NathanF on 16 May 2010 02:55 AM
During Spring/Fall months, the outside night air can be in the low 60s while our very well insulated house remains in the mid to high 70s. Instead of relying on the AC unit, is there an off the shelf system that would instead pump in fresh night air (presumably through a high quality filter) and exhaust the warm indoor air?

For a DIY solution, I was imagining the following: Two tees inserted into each end of the cold air return duct in the attic, both leading to the outside. When the system engaged, one damper in the middle of the return duct would close preventing the house air from returning through the loop, and one damper in the first tee would open instead exhausting the air to the outside. On the other end, a final damper would open allowing fresh air through a high quality filter to be sucked into the house.

From a control perspective, it seems to me it may be possible to use the wire that usually triggers the A/C system from a standard programmable thermostat. The thermostat would call for A/C, which would first be rerouted through a blackbox (made up of thermostats and relays, or some sort of hobbyist microcontroller) that would compare outdoor air temp to indoor air, and either activate the intake/exhaust system or the A/C accordingly.

Looking forward to your thoughts!

buy an ERV

I did read up a bit on HRVs/ERVs, and I was generally aware of them before posting, but I was under the impression the point of such units were to *avoid* temperature changes, rather than encourage them. I'm looking to pump in fresh air at outdoor temperatures to cool my home, not send it through a heat exchanger to get fresh air at more or less equal to the present indoor temp.

Thanks.

In homes that don't have ERV/HRV, we have attached a fresh air inlet to the HVAC return air ducts. This allows for some amount of fresh air to be introduced into the home, and it works OK in our moderate climate. We normally use a 4" duct for our purposes, however, you could use a larger duct system and add a damper controlled by a thermostat tied to the outside air temp. Set your HVAC fan to contiuous run, and let it go. Once the outside air hits your target range the damper will open and introduce the outside air into your home. Depending on how tight your home is, you may need to work out a pressure balancing system (ie: crack a couple windows).

I think Nathan's initial post is about right except I'd use an intake fan and an exhaust fan to keep pressures balanced. I'd be a little careful about bringing outside air into a basement on humid days - those 65F walls/floor can get damp when hit by 80F/95% RH air (voice of experience here - had to get the walls cleaned).

I do a similar thing with a whole house fan and opening windows - but it is completely manual (and so doesn't get used nearly as often as it should).

In the commercial world, what you want is called an "outside air economizer".

Nathan

Most exhaust or intake fans are going to have relatively low CFM and you wil not get the volume of air needed. As an example you could have an intake fan feeding air into the return side of your furnace and have the benefit of the furnace filter. I dont think you will get enough volume though.

They make whole house fans. The energy efficient fans have insulated covers that open when the fan is in use and close when not. The insulated models I have seen have an R38. What you have is the thermal mass giving off its heat. I think the advantage of this is that as the air leaves the house it also helps cool the thermal mass of the attic which is giving heat downward through randiant heat.

Although you think your house is energy efficient have you ever had it tested. Have you had the ceiling/attic floor air sealed. I mean throughly sealed and then placed the maximum amount of insulation in the attic.

We typically think of heat rising. But heat flows from hot to cold. So in the summer when the attic is really hot and the house is cooler the heat is flowing down. The attic is the hotest pushing into the top level and it gets cooler as you go down. So the heat flow is from the top down. So in the summer, even on a mild day, the attic is the hottest part of the house. The thermal mass absorbs heat all day long and then when the sun goes down it starts to cool giving a lot of its heat into the house. In the winter the air flow pattern is reversed leaking out the top and in the bottom. The insualtion around the air leaks acts as a filter then when the season and ar flow pattern reverses the air is now flowing in the opposite direction through that filter. Doesnt seem like a good thing for air quality.

So any air leaks between the house and the attic are going to allow hot air in day and night. If your ceiling /attic floor is not air sealed the warm air is coming in. That air flow through the insualtion is killing its R value. The air sealing will do 2 things, its stops the incoming (hot) air flow from the attic and it improves the efficiency of the insulation. Adding more insualtion makes it that much better. I would recomend cellulose.

I would suggest that the attic floor be throughly air sealed and more insualtion added as a first step. Be sure the attic hatch is insualted and sealed. This will also benefit you in the winter. Atfer that is complete then see how the house reacts. At that point opening windows may be enough to get a cool breeze.

The next step if needed would be the whole house fan. If you push a lot of air into the attic without air sealing the attic floorwill cause some of that air will come back in through the leaks. You will also need to be sure you have adequate eve and roof venting.

You are looking for filtered air and that may make it a little more difficult. If you have a basement window that could be utilized you could create an intake with a filter. For economy you could size it to use a standard furnace filter. The down side is that you would have to make it and it would require manually opening and closing.

What Roberth said (mostly)- it takes a WHOLE lot of air to get much sensible-cooling out of air in the low 60s, and it'll take a whole house fan, not something tiny.

The dew point of 75F/60%RH air is ~60F- lower than 65F outdoor air, so if it's 65F out it's possible to reach unhealthful humidity levels indoors pulling in humid outdoor air for cooling. If the outdoor dew points are ~55F or lower it'll be OK unless you need 50%RH or lower due to dust-mite allergies. Dehumidifying large latent-loads brought in with high-volume ventilation with humid outdoor air can take more power than just air-conditioning the place and using an ERV/HRV for necessary ventilation air. Nighttime ventilation schemes can work well in drier climates (mostly well-west of the Mississippi in the US), but pretty much suck where outdoor dew points stay above 55F most of the summer. For your black-box controls to do the right thing you'll need to measure more than just the outdoor temp, but outdoor humidity as well.

Your sensible cooling load is likely to be primarily direct solar gain, and energy used inside the home, not air infiltration. Exterior shades on windows & skylights can probably cut it by a significant amount. Some homes have a kilowatt or more of power dissipation 24/7 from game machines computers, unnecessary lights (or inefficient lights) etc which adds up to a substantial cooling load in a well insulated house. Cooking with higher-mass appliances can also add up. It's often easier to avoid heat gain than it is to purge the house of excess heat once it has built up.

If someone wants a custom humidistat/thermostat/logger/controller, let me know - I use Arduino microcontrollers.

I had the same idea as Nathan while sitting outside the other night. It was nice and cool but a little loud because my A/C was running. It was warmer in my house with A/C running than it was outside! I thought about having a fresh air intake that was connected to the return before the filter and controlled via the thermostat. When it was cooler outside, the condensor would cut off and the return air would be diverted to use the fresh outside air. That would have been sweet and use less electricity.

But alas, I live in a humid area.

Thanks Robert and Dana for the knowledgable replies! You saved me much research time!

How about a 4" motorized damper ducted to the return, the damper controlled by both a thermostat and humidistat. When temp is below 70 AND relative humidity is below 40% THEN open the damper...would this just be two switches in series with the damper motor?....Maybe a 3rd series switch/relay connected to the "Heat/Cool changeover line"?

Even if the a/c compressor was running but you were sucking in cooler, less humid air, would it cause a problem?

I agree balancing the "new" air in vs. out might be a problem....what about another motorized vent somewhere else, no duct work, just mounted in, say, a basement window, that also opens/closes in tandem?

Doesn't seem too expensive...motorized damper x2: $80x2, thermostat $40, humidistat $40, miscellaneous duct/fitting/wires $20? Would it save $200?

Hi,

I have the same interest. One system I know of is called NightBreeze. I am getting a quote from them on Tuesday. http://www.davisenergy.com/technologies/nightbreeze.php. Nightbreeze requires that the HVAC has a ICM2 type motor (my Carrier systems do not have) and that the coils allow a large amount of airflow.

My opposition to a whole house fan is the outside air seems pretty dirty around here. Danville, CA. As evidenced how quickly patio furniture and cars get dirty sitting outside.

My house has two HVACs, 3,000 sq ft. Huge attic. Built 1989. Have added insulation and started adding radiant barriers (reflective mylar with moisture perforations).

My goal is to continue to improve efficiency of the home before investing in new HVAC units. The other plus for Nightbreeze is it replenishes inside air with filtered outside air. Like ERVs/HRVs but without heat/moisture transfer.

The other term I have have heard use for Nightbreeze type systems is "Airside Economizer" which seems more prevalent in the commercial HVAC space.

Thanks,
...Rich
925-683-6248

Rich

Out of curiousity what did you have to pay for the radiant barrier

Just over $100 for 1,000 sq feet on eBay.

Another good fresh air web site is http://www.alpinehomeair.com/viewproduct.cfm?productID=453055571

Posted By NathanF on 16 May 2010 02:55 AM
During Spring/Fall months, the outside night air can be in the low 60s while our very well insulated house remains in the mid to high 70s. Instead of relying on the AC unit, is there an off the shelf system that would instead pump in fresh night air (presumably through a high quality filter) and exhaust the warm indoor air?

For a DIY solution, I was imagining the following: Two tees inserted into each end of the cold air return duct in the attic, both leading to the outside. When the system engaged, one damper in the middle of the return duct would close preventing the house air from returning through the loop, and one damper in the first tee would open instead exhausting the air to the outside. On the other end, a final damper would open allowing fresh air through a high quality filter to be sucked into the house.

From a control perspective, it seems to me it may be possible to use the wire that usually triggers the A/C system from a standard programmable thermostat. The thermostat would call for A/C, which would first be rerouted through a blackbox (made up of thermostats and relays, or some sort of hobbyist microcontroller) that would compare outdoor air temp to indoor air, and either activate the intake/exhaust system or the A/C accordingly.

Looking forward to your thoughts!

It sounds like a bit of a Rube Goldberg Machine.  Unless I'm missing something, couldn't you just open a window and/or use a fan at night?

Rich

I am glad you didn't pay a lot. In most cases radiant barriers are of little value. Dealers talk about how they were developed for NASA and used in space. They either don't know the true benefit (if any) they have in a particular application of they will not tell you. What a person should know is that they work best with high temperature differentials and little or no insulation.

Therefore if you home is not insulation you will get some benefit from the radiant barrier. The more insulation you have the less benefit you get from the barrier till you get to the point that you will not see any real difference.

If you have inadequate insulation and a leaky ceiling then where you should start is sealing the leaks and adding insulation. When you do that you have eliminated the need for the radiant barrier and you get the better benefits of the air sealing and a good insulation

It is difficult to get information on radiant barriers other than the biased information from the sellers. Oak Ridge National Laboratory has a calculator showing the effect of radiant barriers. If you play with the calculator you will see that the more insulation you have the less savings you get from a radiant barrier. Basically if your home is insulated near the minimum recommend level for your locale a radiant barrier is not going to give much benefit. If you spend the same money on adding insulation or better yet air sealing you will see a better return on your investment.

One last thing radiant barriers lose much of thier benefit as they get dusty as they would in an attic.

Jerkylips

Think of how heat flows HOT to Cold. At the end of the day where are the hot and cold barriers. Attic and roof are hot with lots of stored energy. Inside the house is cooller. Where is the heat trying to go - to the inside. We think of heat rising but it is really moving to a cooler place. In the winter the attic is cold and the house is hot. Heat is moving up to meet cold. Lets reverse that for summer and the heat is trying to go down.

Can it be forced to go the other way - yes with a fan. Now we are making the cooler inside air go to the hot air and forcing out the hot air through the vents. Cooler air is pulled up to replace the air sent to the attic.

Hi Roberth, I have heard this before and other issues which put this project on a lower priority. My attic does get quite hot and holds heat in the cement tiles. I did blow in additional insulation and added gable fans. I have not found any leaks in the ceiling and have sealed the ones I found in the floor. The white carpet showed black where there holes in the floor. Someday I will tackle leaks in the ductwork but see that as a difficult task since the crawl space is not great and the existing insulation would have to be pulled back. I laid the thermal barrier on the existing insulation and placed a thermometer on both sides. The difference was pretty significant though it did not seem to improve the rooms it covers. My next thought is to place it under the roof and see if the attic improves. I am reluctant to do that since I heard it could damage the roof. Will ck out Oak Ridge and in finding this all informative. Thanks, ...Rich

Rich

If you know where to look there are lots and lots of small leaks. Every house has them unless special detail was paid during construction and that has only occuered recently and in a low precentrage of homes but it is growing.

How do I find them? I have scanned my ceiling with a temperature sensor (Cen-Tech Infrared Thermometer). An insulation guy scanned with his tool a number of years ago. All my sockets have foam behind the plates.

There are a lot of small ones and some big ones. If you take wherever a wall meets the ceiling on the Attic side is the top plate- interior and exterior walls. That seam needs to be sealed. Wherever you have a light fixture. Above every wall switch the wire runs through the top plate. Where the plumbing stack goes up. Where the flue goes up. If you have a masonry chimney there are big gaps that require fire safe sealing. If you have soffits or canned lights.

Recently I was at a house that was being remodeled. An interior wall was removed. I could see the bottom of the insulation that sat over the top plate. It was absolutely black from the air leaking out. Dirty insulation is a very good indicator of air leaks. The problem is that the dirty area may be at the bottom and not visible until the insulation is lifted up.

My list above will tell you were the common leaks are unless you have been up there with can foam. Some tricky ones may need the help of a blower door along with a thermal camera. Infared thermometers look at spots while cameras look at whole areas and pick up subtle differences.