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Old 12-06-11, 12:08 PM   #1
Xringer
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Default Where to install your new mini-split??

Want to install a new MSHP and you are thinking about where,
the ODU (outdoor unit) should be located?

AND,
You have a large attic space, that gets really warm during cold winter days,
and would like to harvest some of that heat..
(Maybe you don't have enough south facing windows)..


Here's an idea that I had. Insulated duct work, to pump air down to
the mini-split air-intake, using a standard attic fan..
Just need to control the motor, so it's not running when you don't need it.



The sweetest part of this hack, is you are re-cycling some of the heat that was lost via ceiling leakage..
Sorta like a solar space heating + heat recovery system..

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Old 12-06-11, 12:41 PM   #2
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Quote:
Originally Posted by Xringer View Post
The sweetest part of this hack, is you are re-cycling some of the heat that was lost via ceiling leakage..
Sorta like a solar space heating + heat recovery system..
Well, since the attic is attached to the house, and the heat loss from the living space in the house to the attic is influenced by the delta-T between the living space and the attic space, and you will be extracting attic heat from the attic (which will lower its temperature) as an input to your heat pump...

Might this just be an electrically-powered heat loss machine?


Here is a similar idea, but a greenhouse is used as a heat capture device (seems like a waste of a perfectly good greenhouse to me) and it's heat is stored in an underground aquifer, but the source greenhouse is completely separate from the sink greenhouse.

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Old 12-06-11, 01:02 PM   #3
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Well, hopefully your attic has quite a bit of insulation and heat loss from the house isn't that big of a deal through the attic. If not, I suggest insulating the attic first!
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Old 12-06-11, 01:16 PM   #4
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That re-cycled heat is only going to be a small fraction of the heat coming down the duct.

The solar heat 'storage' in my attic is very stratified.
This is caused by solar warmed air flowing upwards and accumulating in the peak.
While the attic floor air temp is sometimes actually colder than it is outdoors.
Because, the attic's thermal mass tends to hold on to that nighttime temp,
and in the morning, it's in the shade, like the north side of the house.

So, if you had outdoor heat sensors and in the peak that would trigger the fan, when it was needed..
A. Mini-Split running in heating mode.
B. Attic air temp a few deg F warmer than outdoor air.

It might even be useful during those early morning Defrost cycles..
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Old 12-06-11, 01:48 PM   #5
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Quote:
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Well, hopefully your attic has quite a bit of insulation and heat loss from the house isn't that big of a deal through the attic. If not, I suggest insulating the attic first!
I did miss a spot when I was putting down the 3rd layer. (I ran out of insulation)!


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Old 12-06-11, 01:54 PM   #6
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"Might this just be an electrically-powered heat loss machine?"

That's the story of my house!!

I could make it better, but my wife wants to keep the windows!!
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Old 12-06-11, 04:07 PM   #7
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Quote:
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Because, the attic's thermal mass tends to hold on to that nighttime temp...
I looked at your photo, and I really don't see anything that looks very "massy".

I think that the continual heat loss through you living space ceilings would make it appear that you had a thermal mass up there...

This would be pretty easy to prove or disprove, since you now have that dandy WebControl board, you could put a string of 1-wire sensors at about every foot, from the attic peak to the insulation, and establish a baseline of thermal mass effect. Then you could shut off your heat pumps and open the windows overnight and see what your thermal mass curve looked like then.

It is a pity that the WebControl board doesn't have a built in data logger... It has everything else!

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Old 12-06-11, 06:03 PM   #8
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Not too big, the mass is in the structure. Mostly wood and roofing shingles.
You can ask any roofer about the weight of those shingles..

I once had a remote La Cross sensor hanging up there about 16" down from the peak,
and I can tell you, it takes a while for the AM sun to heat up the air up there.

Hey! I have not checked that out since we had got the new ridge vent installed. (with the new roof).
It might be interesting to see how effective the mesh type ridge vent is...
And, now we only have one layer of shingles. (versus 2 layers before).

I'm going to go find that sensor..


Yeah, using a dandy WebControl board (with heat sensors etc) should
allow one to tailor the fan motor run time, so it takes out the excess
warm air, without dropping the attic peak's temp down to outdoor levels..
Just harvest the warmest air out of the peak.?.
Yeah, just take the warm air that was going to escape anyways..
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Old 12-07-11, 05:13 PM   #9
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How much of this heat is going to be lost when feeding outside the house, down from the attic and into the back of the ODU?
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Old 12-07-11, 07:32 PM   #10
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My attic peak heat sensor is in place. It's showing 48.8F, my IR pistol shot up there said 47.8F,
my hardwired outdoor sensor (2' off the ground) says 45.1F and the weather guy down the road says 44.3F.?.

It's 70F in the house and it's been raining lightly all day. Yeah, there is some heat leaking up into the attic..

But, I wonder if a lot of it is coming up the basement stairs,
and finding a way into the attic, via the pull-down attic stairs??

The basement & it's stairs are closed off (by a door) from the rest of the house.
So some of the slab heat (50 to 60F) could very well be finding it's way to the attic..


"How much of this heat is going to be lost when feeding outside the house, down from the attic and into the back of the ODU? "


That's going to depend on the insulation around the duct-work.
But, if the air that feeds down is 10, 15 or 40 degrees warmer
than the normal outdoor air, it's going to help efficiency.

During a sunny morning, between about 8AM & 11AM,
the outdoor temperature starts climbing at a good rate.

My guess is, the temperature climb inside the attic is going to lag
behind (outdoor temps) at first, but then the attic will become warmer.
That's the fan ON signal. Warm air forced down to the ODU should
reduce the amount of watts needed to keep the house warm.
(At the same time, sun coming in the windows will be doing the same thing).

I think this scheme would likely work best when the air temp at ground level
was down near the low limit of the ASHP.
If it was around 10 deg F, I'm pretty sure my Sanyo wouldn't mind a shot
of nice 25 deg air coming in..

The Sanyo monitoring system that I'm trying to get running would be very
useful in controlling this kind of attic heat scavenger.

http://ecorenovator.org/forum/applia...ontroller.html

If I can make the CAI board monitor the power (watts) being used by each Sanyo,
factoring in the temperature of the refrigerant, going into and coming out
of the ODU, along with the indoor and outdoor air temps..
It might be possible to come up with some kind of real-time COP data.

I want to use this COP number, to compare the performance of each unit.
Comparing one against the other. If one Sanyo starts working poorly, it should be quickly apparent.

Having a real-time COP displayed on my tablet, might be very useful
for performing little experiments..
Like going outside with a 1200 watt hair drier (4094 BTU) and shooting
some hot air in back of the ODUs..
The effect that hair drier had on the COP display, might tell me if a project
building an attic heat scavenger might be worth the effort...

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