Yea, I think that using the
Yea, I think that using the economizer for temperature control would be relatively easy, but also controlling it to get the right ventilation airflow rate is another story for the reasons you identified. I think that commissioning ventilation flow rates on the order of 100 cfm is always going to be problematic, especially when the ventilation is tied into the HVAC system.

Re: humidity not an issue…
Re: humidity not an issue…
Ok, so the variable blower & damper would react in concert based on ODT? I can see how that would work: max ventilation flow if ODT < threshold, otherwise min ventilation. Threshold could default to [setpoint - 20] or thereabouts to ensure boost mode doesn't introduce excess moisture. Boost mode would be disabled if t'stat not in cooling mode AND indoor temp < [setpoint - 5] or thereabouts. The challenge of course would be maintaining the min ventilation rate when ODT > threshold. Setup and field commissioning has been the Achilles heel for CFIS. Moreover, if source equipment is multi-stage or variable capacity and/or duct system is zoned, the ECM and vent damper controls would have to be smart enough to maintain correct flow. One way to accomplish that would be to maintain field-selectable static pressure drop across the vent damper.

I would buy that.

I am familiar with CFIS and
I am familiar with CFIS and you are right, this would be an “amped-up” version of it.

If you do not switch to economizer mode until it is around 55 F or cooler outside, then the outdoor air is plenty dry (dewpoint at or below 50 F) even if the if the outdoor air RH is 100%. Thus, there is no reason to know outdoor air humidity.

I understand your skepticism about manufacturers selling an “economizer”. It could happen for one of two reasons: 1) a significant market develops for it based on its own merits (doubtful), or 2) it is required by code or law in a significant market (e.g., California). My main point is that HVAC manufacturers can do “accessories” better and cheaper than 3rd parties if there is sufficient market to justify the development, especially in terms of the required controls.

Sorry, I put this aside and
Sorry, I put this aside and it fell through the cracks. I just wanted to clarify that the reason the NightBreeze 3-way damper assembly is ‘rather large’ is because it has to handle the full system airflow when the outside air path is closed. I wasn’t talking about bringing in anywhere close that that volume of fresh air (attic fans are another matter). Of course, there are other ways to design an integrated economizer.

What you’re proposing is more akin to CFIS as proposed by Armin Rudd at BSC (see http://bit.ly/2z9P0X1 and check out Armin’s comment and mine), except yours would be amped up beyond the prescribed ventilation rate to take advantage of favorable conditions. But why do you say outside RH wouldn’t be an issue?

I remain skeptical that any domestic unitary equipment manufacturer would go after this market. But you lived in that world for 25 years so you would know that better than I.

David, I have a little
David, I have a little experience in this area too. I have probably been dabbling in this field almost as long as you 😉 My point about the variable-speed blower is that it will soon be effectively free on new systems since it will be standard. I am not interested in looking at economizer applications that require high air flow rates when there is a small temperature difference between the indoor and outdoor air because I still think that operating the air conditioner is a better alternative for us lazy people (you and John can open your windows and run your fans). I am more interested in high-performance homes that may still need some cooling to cover internal loads when it gets down to 50 F or lower outdoors. Of course this still requires dampers, but the airflow rates would be much lower, and outdoor humidity levels would not be an issue. So we are now talking about kind of a hybrid economizer/ventilation system since the economizer airflow rates would be somewhere between normal heating/cooling airflow rates and ventilation rates. I have a sneaking suspicion that a large residential AC manufacturer could do this much cheaper than a 3rd-party add-on component supplier. They just need a market. As for controls, that is practically free now since the hardware is already there and we just need the additional software logic.

It takes more than a variable
It takes more than a variable speed blower. In order to integrate 100% outside air cooling with main HVAC system (as opposed to ventilation mixing with compressor-cooled air), we need a rather large** 3-way damper assembly at the return plenum, a differential enthalpy sensor (compare indoor and outdoor dew point) and a thermostat designed specifically for this application. In particular, the t’stat would ideally support a low-limit set-point to allow some over-cooling to further delay the need for compressor-based cooling. Utilities like that.

** sized to accommodate full system flow, ideally without adding any static drop

Such a system was developed more than 10 years ago (http://bit.ly/2wXeLYT). The reason it didn’t take off was the high cost. Even in a dry climate with large diurnal swings, payback is elusive for a system like this, especially for homes built to today’s codes and beyond with already low cooling costs. Forfeiting integration and automatic control greatly simplifies things.

I agree with John’s point re: whole house fans. In my case, I’m only interested in cooling the master bedroom. We like the bedroom “cold” when sleeping (73F in summer). Otherwise, we keep the stat on 77F or 78F, aided by ceiling fans. As it turns out, any time the house gets warmer than that, it’s usually too hot to use outside air for cooling.

I agree with John and David
I agree with John and David that opening windows makes sense under some conditions, especially in drier climates. I guess that I am just too lazy;-)

However, I am interested in looking into residential economizers. AC and furnace manufacturers will eventually be using variable-speed motors on all of their indoor units, so adding a cost-effective and well-controlled economizer should not be hard to do, assuming that there is a market for it. Hopefully, this market will grow everywhere for new houses since higher-performance envelopes result in lower outdoor balance points, thus increasing the opportunities for economizers. These economizers could also be designed to provide the year-round mechanical ventilation requirement in tighter homes.

I wonder if we can come up with an economizer that is cheaper to build and install than operable windows and whole-house fans? If not, there might still be a market for lazy bums like me.

Roy, I live on 8 acres in a
Roy, I live on 8 acres in a very rural area. But I routinely opened the bedroom window at night when I lived in town on a 1/4 acre lot. Never gave it a 2nd thought, in terms of security. I always make sure the windows are locked when we travel, but that’s it. When I’m home, I have other means of security 🙂

Where I live, dust is a fact of life. Even when windows are closed up tight for weeks at a time (mid-summer), we still get a lot of dust inside the house. As I mentioned, I use a filter with the window fan, which pretty much eliminates any *extra* dust load from the fan, relatively speaking. My next home will be super tight, so hopefully that will cut down on dust! And my ‘economizer’ setup will have a high MERV filter. Our pollen season (mostly from grasses) typically occurs after it’s too hot to cool with outside air.

As for noise, where we live now, we occasionally awake to coyotes howling, sometimes within yards of our bedroom window, so we hear them even when the windows are closed. In town, our neighborhood was quiet. I didn’t hear any sounds over the white noise created by the fan. Oh, I forgot. About six years ago when we lived in town, a local defense contractor was testing a Predator (UAV) with a new diesel engine developed to simplify supply chain logistics. The open ended test stand was close enough (about a mile) that I had to keep the windows shut for about 4 weeks during nice October weather.

I spent most of my adult life in North Carolina, where humidity limits the ability to substitute windows for A/C during the shoulder seasons. But I monitored the dew point at the nearby airport. I would always open the windows when the outside dew point was lower than about 55F.