[Greenbuilding] distribution of radiant heat

Corwyn corwyn at midcoast.com
Sun Jan 9 07:59:33 CST 2011


On 1/8/2011 7:34 PM, JOHN SALMEN wrote:

> I'm a little confused. I had no problem with the term fireplace as here they
> are typically sealed units and the term is used to differentiate from
> freestanding wood 'stoves'. I have not seen an 'open fireplace' built (that
> is the term that used to be used here) for over 20 years (and it was a
> rumford).

Lucky you.  I still see brand new homes with them.  (we call wood stove 
inside fireplaces, 'fireplace inserts'.)  No matter, we are all on the 
same page now.

> What I am confused about are what the calculations are in reference to? (I
> don't really understand them) and what the point is of making a comparison
> to propane? I get the point about embodied energy and financial cost but the
> question here is about distributing heat from a central source.

The point is, if you are putting materials with 5MBTUs of embodied 
energy in to a house, to avoid burning 20 gallons of propane over 20 
years (for 1.8MBTUs) then you are engaging in Green Bling.

> I think using a fan or some design to distribute heat from a stratified
> ceiling area is not unreasonable as a long term provision. The comment that
> heat will migrate eventually in a well insulated building is not really
> reasonable based on the comments we have had here and most experience.

Really?  I have an existence proof.  At this moment, with the fire going 
on it's 4th hour, the floor temp is 65.5°, the peak 25 feet above is 
70.7°.  That is while the stove is running, I will be glad to check 
again in the morning, if people think that will make a difference.

[Edit: It's morning.  Floor 61.4°, Peak 66.5°, so about the same.]

Perhaps someone will calculate the stratification in a room with perfect 
insulation and surfaces with emissivity of 0.95, and we can work 
backwards from there.

> The stratified air in a cathedral ceiling can easily be 10deg higher
> (especially over the heat source).

What insulation levels are you talking about here?

> All it is doing is warming the ceiling
> mass and migrating out regardless of how well insulated so it is not
> unreasonable to look at ways of improving air flow and distributing that
> heat a little more effectively warming a few more surfaces.

It is also warming the floor below through radiation.

> The fantech 125(cfm) inline is rated at 18watts (.18 max amp) - 125 cfm will
> throw a lot of warmed air into a small bathroom space quite quickly. It
> takes .018btu to warm 1 cu.ft. of air 1degF. So if the stratified air temp
> is 78deg (which is think is more realistic over a fireplace) then 1 cuft. of
> air is holding about 1.4btu.

You are assuming the bathroom is 0°F?!  If the bathroom is a more 
reasonable 58°F, then you can only drop the temperature of the moving 
air to a bit over the temperature of the bathroom so there is only 20° 
of temperature difference, so 0.36 BTU / foot^3.

> the fan moving 120cfm can theoretically move
> about 10000btu/h with an energy consumption of roughly 60btu/h. The room
> probably only needs about 1500btu so an actual fan requirement can be worked
> out from that.

Actually it is only 2600 BTU/hour. How much of that 'theoretical' is 
lost pushing that heat down and across what, 50 feet of ducting(?).  How 
well insulated does that duct need to be in the crawl space, in order to 
actually be warming the bathroom rather than cooling it?

Not sure what you mean by 1500btu.  If you mean the amount to warm it up 
from the claimed <60°, I disbelieve.  That would require a thermal mass 
below 187 BTU/°.  If you mean 1500BTU/ hour then I don't think you can 
do it.

Let's take the most optimistic view, just try to maintain the bathroom 
at a comfortable level, and believe 6 impossible things before 
breakfast. 1) That we have an infinite supply of heat at 78° at the 
ceiling, 2) That the fan can deliver 120 cfm through 50ish feet of duct 
with at least 180° of bends 3) That there are 0 thermal losses along the 
entire duct including where it travels in the crawl space 4) that the 
bathroom only has  600 BTU/°F of thermal mass. 5) The propane generator, 
battery system, etc. all work at 100% efficiency.

So heat losses are 1500 BTU/Hour; Temperature difference between 78° 
incoming air and comfortable temp (68°) is 10°; Each cubic foot of air 
therefore brings (78-68)*0.018 = 0.18 BTUs; 120 cfm makes it 21.6 
BTUs/Minute, or 1296 BTUs/Hour; So the bathroom LOSES 204 BTUs/hour (1/3 
of a degree).

In fact, you could probably maintain at almost 67° (1425 BTUs/Hour 
input, 1450 BTUs/Hour losses), but then I would ask that you start 
remembering the impossible things you did to get here.

Heating up after cooling is only going to be worse.  But let's consider 
what happens.  Let's be generous and say the bathroom is 58° in the 
morning,  we fire up the fan, in the first hour we are getting 20° of 
temperature difference, so 2600 BTUs, losses are only 1000 BTUs (or so),
1600 BTUs net, 2.5 degree increase.  2nd hour, starts at 60.5° 2268 BTUs 
in, 1125 BTUs loss, 1143 BTUs net, 2 degrees increase.  Third hour 
starts at 62.5°,... well you get the idea.

The sixth impossible thing?  In my opinion, that the clients are going 
to be happy with it.

If you are going to use propane fired generator to produce electricity, 
to store in batteries, to run a fan, to push air across half a house, to 
under heat a room, just to avoid heating directly with propane, I am at 
loss.

My suggestion?  Cork flooring, and a heat lamp on a timer switch to keep 
just the occupants warm.  (and, of course, smaller house, more 
insulation, appropriate windows...)


Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.greenfret.com/
topher at greenfret.com
(207) 882-7652




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