heat ducting

 

I have a big lean to conservatory on the front of my house. It stays
quite warm even in winter.

Whilst the front of my house is warm, the rear is much colder and it
seems a good idea to suck some of the heat out and duct it through the
loft space and then down to the ground floor at the back.

I'd like to find out how to calculate appropriate duct and fan sizing.

 



Say you have 200 ft^2 of US R1 single-pane south-facing glass with 90%
solar transmission that collects 0.9x200x250 = 45K Btu/h of full sun
(in the UK? :-) as a current source, with a 200 Btu/h-F thermal
conductance to outdoors...


With minimal undesirable thermal mass in your sunspace, you might have
something like this, viewed in a fixed font:

    45K Btu/h Ts        Tr
       ---    |   1/Gf  |   1/Gr
Ta ---|-->|---*---www---*---www--- Ta
       ---    |
              |
      1/200   |
Ta ----www----

If the outdoor temp Ta = 30 F, you'd have a Thevenin equivalent circuit
like this:

              Ts        Tr
      1/200   |   1/Gf  |   1/Gr
   ----www----*---www---*---www--- 30 F
  |   -------------------------->
  |                I
  |
  |   30+45K/200 = 255 F          
 ---
  -
  |
  -

If the rear part of the house has a thermal conductance to outdoors of
(say) Gr = 875 Btu/h-F and the rear temp Tr = 70 = 30+I/Gr, I = 35K Btu/h.
We can make the sunspace temp Ts = 80 F if I/Gf = 80-70, ie we move roughly
Gf = 3500 cubic feet per minute of air from the sunspace to the rear and
back into the sunspace. Moving it without lots of noise and fanpower means
keeping the air velocity low, eg 500 linear feet per min inside a 3500/500
= 7 ft^2 duct, eg a rear closet or room connecting the attic and basement.

Now how do we translate all this into English? :-)

Nick

 

On 29 May, 18:52, nicksans...@ece.villanova.edu wrote:

I will study this with interest. The thing is I was rather hoping I
could do all of this in, at the most, 6 inch ducting. Once I get above
this, it all begins to show and gets a bit ugly.

My plan is to enter the main house inside at the top of the
conservatory roof. It goes in through the top of my wardrobe then up
into the loft, across to the back of the house, and down into the
bathroom airing cupboard. There's just room for a duct in amongst the
towels and the hot water cylinder. Then down to the room below where
there would be a vent in the ceiling. I also want to be able to divert
the flow straight out of the roof in very hot weather. I suspect from
what you say that it would need a quite powerful fan to move useful
amounts of heat and this would be noisy.

 



You might move 3500 cfm through a 6" duct at 202.56 mph...

("CAP'N, THE DILITHIUM BOOSTERS CANNA TAKE MUCH MORRRA THIS!")


Sounds like you may have 2 bottlenecks, the wardrobe and the duct in
the cupboard. Perhaps you can use the whole cupboard as the "duct," or
the entire bathroom. And you need to move the air 16' in the vertical
direction? How about a $35 1984 Dodge Omni automobile radiator with its
12V fan in the upper part of the conservatory, and another one in the rear
of the house, with 2 garden hoses connecting them? Continuing this theme,
you could put some tire planters in the conservatory, an engine block
coffee table in the living room, some steering wheels in the bathroom,
windshields in the kitchen, and so on.

Nick

 

On 30 May, 11:23, nicksans...@ece.villanova.edu wrote:

What a wag you are Nick. I shall enjoy sorting the fact from fiction
from your responses. Dodge Omnis are a bit thin on the ground here. I
wll see if an Austin Maxi will do.

My wife will love the tyre planters.

Bertie