[AC_Hacker]

Originally Posted by Ko_deZ

Hi AC
There is a couple of reasons not to automatically turn down the recovery unit, and more specifically not using CO2 alone. Say you have a hot an humid day, and a cold night, but you are not at home. You would want the unit to run at a normal rate to avoid condensing on your windows and water running down into your window frame. If you come home to take a shower before leaving again you get the same situation. Then you have the situation where you just came home and you make food. To avoid the smell all over the house for hours until the CO2 level rises, you would want it to go a bit faster. Also, if you are like me, and smell like a week old dead cow after a full day of manual labor, some fresh air does not hurt (at least that's what the wife sais :-p ). Of course, you could have both CO2 and a humidity sensor, with a manual override, but now it is starting to become complicated.

Well, I have considered what you have said, and I think you are right that CO2 alone may not be the best parameter to control fan operation. On the other hand, as I have looked further into advanced ventilation approaches, CO2 monitoring seems to be firmly in the development path, as far as very high efficiency goes.

Originally Posted by Ko_deZ

Then, also to consider, is the fact that DC motors don't have anywhere near the life expectancy of an AC fan. It is also slightly less efficient, and if you count the conversion from AC to DC, even more so. More to go wrong also, which would happen when you are out of the house for a long time.

There has been considerable research into Electronically Commutated Motors (AKA: ECM), so the carbon brush thing is not so much a factor. DC motors do offer the promise of inexpensive speed control. When you combine the advantages of ECM and variable speed/power, the advantages are considerable.

Also, as you probably already know, varying the speed of AC motors can be done. But thanks to the development trends in electronics, variable speed AC motors are becoming commodity items... there's one in my Chinese heat pump (outside), whirring economically away right this moment... continuously variable. I just don't know if a variable speed AC motor & controller would be worth it for a home project.

Originally Posted by Ko_deZ

And, by the way, you will need some serious PC fans to get anywhere near the airflow and pressure you should have in a ventilation system. The AC ones used in professional PC racks would be more than sufficient, but I have seen none that is DC, and certainly none with a normal noise level.

Well, that brings up another interesting point, do we really need central air ducting? Running fans to push air through all those many feet of ducting uses up quite a bit of energy just to get the air from one place to another. I am seeing emerging trends to use small HRVs in each room, directly venting stale air to and getting fresh air from the outside, this avoiding duct losses. I have been looking into air volume rates that are required for rooms in my house, and the volume rates are quite small. In fact, my greatest problem is finding fans that are small enough. That is one of the reasons I am looking into computer fans. For a single room, without duct losses, they are just about right. And the tangential blowers are very quiet. That is what I have in my mini-split (inside), and I usually can't tell if it is operating or not, except for the fact that I am warm.

Originally Posted by Ko_deZ

At least here in Norway, you don't get this CO2 control for the HRV. I can see the efficiency benefit, but I suspect that it has more to do with selling point than actual usability.

I don't think that CO2 sensors are common anywhere yet, but I think that the world is about to go over an energy cliff, and a lot of research is taking place now to reduce energy.

Originally Posted by Ko_deZ

If you have a good recovery system there will be very little energy loss anyway. I actually have a problem with mine being too efficient (93% approximately). The air coming in to our bedroom when there is 0C outside (32F?) and normal temperature inside is <2C (4F) below the outgoing air. This keeps most rooms close to the average, which is a bit too warm for me to sleep in. If it is above freezing, we have to open a window a little. Also, air has little energy capacity, so a little change in airflow will not really affect things that much. I would suggest finding other places to save energy than reducing the amount of fresh air. My recovery unit is a medium setting now. I put it in low if we leave for a long time, and high if we have more than 8 guests. it is about 30% up and down in airflow for the different settings.

Well, right now I don't have a recovery system at all, and I need one. I figure if I'm going to do a DIY recovery system, I should do the best one I can.

Regards,

-AC_Hacker