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Old 11-19-13, 12:11 AM   #41
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Here's some results to the math.

Assumptions
Attic 85 degrees, Basement 62.5 degrees
Difference 22.5 degrees

Output 4860 BTUhr or about 1424 watts of space heater.
...but, this is ONLY when the temperature difference was that big, which wasn't for a very long time.

"70-80deg F attic air @ 200 CFM"
65 degree basement and 80 degree attic air
3240 BTUhr

65 basement and 60 degree attic air...IMHO, don't bother running the fan.

Once your roof has snow on it, its endgame. If the attic is still holding a temperature warmer than your basement once you have a layer of snow on it, you are better off spending some money sealing it up and insulating rather than pumping the warm air that used to be in your house back into the basement.


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Old 11-19-13, 10:42 AM   #42
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Right now, the programed setpoint temp(of attic air) for turning on the fans is the basement temp+4 deg F.
In the afternoon, the attic has to be 6 deg F warmer than the basement.

With the current setup, there is never any cold air pumped into the basement.
It's always at least 4 degs warmer than the basement air. It's just not making a dent..

Right now, it's sunny and Temperature Sensors show 58.2 F in the basement and 60.2 F in the attic.
The fans aren't on yet.

What I find interesting about the plots, is the absents of effect on the attic temperatures.
I expected to see a drop in attic air temperature, shortly after the fans came on.
This leads me to think there is a lot more warm air available in the attic,
than my 200 CFM fans are pulling down..

I had this idea, the fans would be turning off and on as the attic cools, and re-warms.
But that never happens. Once the sun has been gone a while, it shuts down.

There is residual heat in the attic for hours, depending on the outdoor temps,
it's not long before the attic temp become very close to outdoor temp.
(You can see the plot lines merge on some of the charts).
I don't see much to indicate more attic insulation is needed.
Of course it would help, but the amount of gain for labor required, is questionable.

I understand what snow cover does, I've had a sensor in the attic for years.
During the winter, I still might want to bring in a shot of fresh dry air,
even if it's cold up there.
Since I have a feeling the basement will recover fairly fast..
I may interface one of my Radon detectors into the CAI controller..
That cold air shot will cost me BTUs, but it will make the basement safer.
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Old 11-19-13, 11:17 AM   #43
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I've said it multiple times. If I am going to modify my system (which already has 2 fans), I'd add another fan.
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Old 11-19-13, 04:44 PM   #44
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I mounted a Solar Sheet air to air panal on the side of my house thinking it would offset the cold from my ASHP water heater. Even though it blows 100 degrees sometimes it does not seem to make a dent in the basement temp. My concrete walls are not insulated, I think they suck up the heat. My basement is staying between 56 - 58.
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Old 11-19-13, 05:25 PM   #45
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Mine seems to stay very near the ground water temperature.
Seems like the water table is just a couple of feet under my slab.

I have noticed the basement is a bit more comfortable after the fans have been on.
The warm attic air takes a bit of the chill out of the air.
Maybe the attic air will take some of the sting out of running the A7.
But, when the sun is out, the PV usually makes any A7 run unnecessary.

The lack of insulation makes warming a very hard task..
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Old 11-19-13, 05:38 PM   #46
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Quote:
Originally Posted by Daox View Post
I've said it multiple times. If I am going to modify my system (which already has 2 fans), I'd add another fan.
My system is already a bit too loud. Another fan isn't going to happen.

I may have to be happy with what I've got. We'll see if we can get
some air flow this winter and not bring in a lot of cold air..

I think the slab got down to about 49F last winter..
So far this year, it's at 55 to 60F. And I have a hunch, if I bring in
some 35 F air, it's going to very little effect on the basement air.
It's going to recover it's 55-60 temperature within a short time.
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Old 11-19-13, 05:51 PM   #47
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It sounds like you have 2 things working against you xringer. The first is that your basement and water heater may be acting as heat sinks. The water heater for sure and the basement maybe. The other thing is the attic as source of heat might be insufficient. I'm not saying these things are facts but they are the most important to look into. The need for a lot of fan power seems to me to be a symptom and not a cause of the problems.

The problem with the attic as it is that the heat is diluted by the large volume of the attic. You only have so much radiation energy entering the attic. It might not even be that much in Massachusetts. If you don't restrict that radiation energy to as small a volume as you can reasonably do then the temperature will be lower and the volume of air that needs to be moved with a fan is much higher.

For example you might have 1000 cubic feet of air that is heated by the sun in the attic now. If you restricted that to 50 cubic feet through some kind of insulation then you would need a fan that moves 50/1000 or 1/20th the amount of air down to your basement. That is because 19/20ths of that air would now be cold and not needed to be blown down to your basement.

I just threw those numbers out. So think of it as a principle rather than a respresentative model.
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Old 11-19-13, 07:17 PM   #48
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So far this season, we've had enough sun to provide 90% of our hotwater,
from the 1,300 Watts of PV connected to the water tank's elements.
So, the A7 heat pump isn't a factor so far.
However, there is a little thermal heating in the area of the tank, due to heat losses.

The basement slab is around 65F most of the summer and 55 to 60F during most of the winter.
The upper part the basement walls get colder as the weather gets colder.
40deg F wouldn't be unusual.

My rough calculations for air flow say that all the basement air will be changed about twice an hour.
(when the fans are running).

After looking at the attic temperature plot lines for a few days,
I can't see any change when fans are turned on, or off.
That has to mean the attic has a lot more hot air available, than I'm using.
Otherwise, the attic temperature would drop within minutes of turning on the fans..

It sure seems like I need bigger fans.. Not gonna happen!


I've been thinking this over, and I'm starting to understand what it takes to heat up the basement.
I remember back in the days of cheap oil, we used to turn on the basement
baseboard heat during the winter.. I had a TV down there.
One thing I recall was how fast the basement play room would cool down,
after the heat was turned back down to 50F.. It was almost instant.
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Old 11-19-13, 11:37 PM   #49
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I was thinking your water heater used heat pump technology, which would transfer heat from the room to the water. Did you change over to a regular electric water heater run from solar?

Edit: Never mind. I figured it out.

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Old 11-20-13, 07:19 AM   #50
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Yeah, it's a strange hotwater heater.
During good weather, most of our hotwater comes from the PV arrays.
During bad weather or heavy hotwater use, the A7 heat pump comes on too.
Nowadays, the A7 might get another form of solar assist, from the attic air!

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