Climate Control - Partial Recirc?

[gschoen]

I read this about recirc/heater performance once and can't find any more information about it:

Concern for the environment spurred automakers to create more efficient engines. But more-efficient engines mean less heat for warming vehicle interiors. Heating units have had to become even more efficient to compensate for lack of the waste heat that conventional systems use.

We have developed a system for the Prius that gets the same performance as conventional systems despite the lack of engine heat available for the system. Ventilation loss--heat that escapes from the cabin as fresh air comes in--accounts for 60% of the heating load placed on air-conditioning systems. Our two-layer flow air-conditioning system recirculates warm air in the cabin to raise heating efficiency.

By using a 50/50 ratio of fresh and recirculated air we were able to reduce the ventilation load 50%. At the same time, we were able to achieve the same cabin temperatures possible with conventional systems. Warm cabin air has a high moisture content. So, simply recirculating the cabin air would fog up the windows. The new system sends recirculated air out at foot level. Drier fresh air flows near the windows, which prevents moisture from condensing on the glass.

Found in this thread: http://www.priuschat.com/forums/airconditi...ht=presentation
Original Source: http://www.denso.co.jp/AR-e/1998/TW/climate.html

What I can't figure out:

-They use the term "air conditioning" - does this mean the system requires a/c to be on to do a partial heating recirc? Or is "air conditioning" just a generic term for climate control, like the Prius controls use?

-Does turning off your a/c in the winter disable this system since it can't modulate the humidity? If that's the case, this suggests keeping your A/C on while heating is important both for heating performance AND maximizing MPG to allow engine to shut down.

-OR is partial recirc a default that occurs regardless of your a/c setting? If so, what triggers it? anytime you choose vent and floor, or is floor always recirc? If so, when cooling, does the system do partial recirc if you have it on split or floor level?

-There was a long thread about poor heater performance in VERY cold climates. Does keeping the A/C button on for these people help improve heating performance? (Didn't post to the thread b/c it's long and a couple months old now - http://www.priuschat.com/forums/7-vt6039.h...html?start=120)

I suppose this is more of a heating question and a little unseasonal, but has been nagging me since I read it and couldn't find any more info. Thanks!

Gregg

[Tideland Prius]

Well for one thing, I think they're using the term Air Conditioner as a general term for climate control (esp. since our climate screen says Air Conditioner). Turning off the A/C (regardless of AUTO or not) will turn off the A/C like it says. But if the A/C is "on" in AUTO mode, it isn't necessarily on all the time. The computer will figure out when to use it just like how it'll auto-recirc for quick heating/cooling.

As for the partial recirc, the Prius' climate control is capable of a two layer system. It'll blow fresh air to one layer and recirculate the other. If I remember correctly, it recircs the lower layer (foot) and blows fresh air to the upper layer (face). It'll do so in AUTO mode.

[Tideland Prius]

Here it is from pg 184

2–way air intake control
To improve fuel economy, OUTSIDE AIR
mode control changes when the air intake
selector is set in OUTSIDE AIR on
the following conditions:

- When the maximum heating is needed
with the air flow in “Floor” or “Floor/
Windshield”, fresh air flows in the upper
compartment, while air is recirculated
in the lower compartment.

- When cooling is needed, both fresh
and recirculate modes turns on.

[jayman]

Variable recirculation has been used on jet transports for over 3 decades, and much more recently in commercial buildings. The primary reason is for economy.

One problem you encounter is excessive recirculation in an enclosed space with high particulate and VOC levels. Remember when they used to allow smoking on airplanes? The way my chest felt after a 3 hour flight I only had to wonder if that is how smokers feel 24/7.

Another problem is if the system isn't maintained properly and the filter media clogs. I've seen high density media and HEPA media in commercial buildings that became so clogged the blower suction was able to suck the media right out of the frame, wrapping the media around the blower and causing a fire.

If the system is routinely inspected and cleaned, you can also expect mold to grow on evaporators and humidifiers. As always, the weakest link in the chain is the human one, the "wetware."

[DanMan32]

It is a shame that A/C and HVAC were used interchangably, as it makes it difficult to differentiate between conditioning the air, to cooling the air.

However, most of the time, the compressor will be used even during cooler climates mainly to control humidity. The system can measure cabin humidity as well as cabin temperature. It can also measure radiant solar heat pouring in from the windows. That's what that sensor located left side of dash near windshield is for.

The HVAC is pretty sophisicated and I agree, difficult to understand fully. Like we say for the hybrid system, just drive it. Set the temp you want and leave it there in auto, possibly turning off the AC compressor when temps/humidity are moderate and you want to be sure of HVAC economy.

[TonyPSchaefer]

In a building automation system, the dampers accuate accordingly to maintain a mix of Outside Air (OA), Return Air (RA) to maintain acceptable Supply Air (MA). Described in the original post is similar.

If RA has already been conditioned (temp and humidity) it would be a waste to discard it and condition fresh air so we reduce the OA damper and recirc the RA. (Calm down, there are laws enforcing minimum fresh air percentages)

So on days when it's really cold or hot outside, the smart thing for any system to do (be it in a highrise or a car) is to draw in a minimum of OA and maintain the temp and humidity of the same air. Just like brisk acceleration followed by general coasting, once the air is conditioned, it requires far less energy to maintain it's condition.

So, to answer a few questions:
For those "in the know" the term Air Conditioning refers to the humidity, temperature, pressure and flow control of air being supplied. Any of those variables can be controlled up or down and it's still called air conditioning.

If the Prius truly is smart, it would use the measurements of internal air and outside air to determine whether it's more efficient to recirc or not.

[DanMan32]

I believe it does, as it does display it. And the solar sensor works good too.
Yesterday the HVAC quieted down to about 3rd fan position after a period of some cloud cover blocking direct sun. Cloud moved, sun shone, fan speed (and probably compressor speed) increased a bit. AOT could have not gone up that fast.

Some building systems extract heat from exhaust and return it to intake (or vice versa as appropriate) using heat exchanging.

[jayman]

<div class='quotetop'>QUOTE(TonyPSchaefer\";p=\"91213)</div>

In a building automation system, the dampers accuate accordingly to maintain a mix of Outside Air (OA), Return Air (RA) to maintain acceptable Supply Air (MA).  Described in the original post is similar.[/b]

Tony:

I'm very impressed with the networked systems from York (ISNConneXsys) and Trane (Tracer/Summit), as well as the open BACnet. Most use or allow the use of particulate and VOC sensors to blend based on IAQ.

However basic system maintenance is still important. If the media cabinet is clogged to the point the media collapses and is sucked into the blowers, then it really doesn't matter how smart the system controller and scheduler is.

Jay

[jayman]

<div class='quotetop'>QUOTE(DanMan32\";p=\"91226)</div>

Some building systems extract heat from exhaust and return it to intake (or vice versa as appropriate) using heat exchanging.[/b]

Dan:

Especially in cold climates. For energy efficiency the building envelope is now so tight that IAQ issues are becomming a serious concern.

HRV's (Heat Recovery Ventilator) allow heat transfer as you describe, without mixing the two streams. As you would expect, the heat exchanger core will ice up in very cold temps, especially below 0 F.

Most of the "smart" HRV's have a sensor on the core that will close a damper and engage recirculation to defrost the core. The older HRV's depended on an electric strip heater to defrost the core:

http://www.lifebreath.com/pr_hrv01.htm

http://www.venmar-ventilation.com/Vent/Eng...g/hepa3000.html

Jay