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Kestrel
Member Posts: 102
Hello again - I'm hoping all the brain power here can come to my aid again!
I'm working on the calculation for my first floor. House is 105 year old in Seattle. Basement presently torn open for remodel, and joist space accessible. Plan is to ditch forced air and switch to gas-fired hydronic - tubing in floor joists for first floor, radiators on second floor and soon-to-be finished basement.
Based on heat loss calc at design temp of 20'F, total for the first floor is about 18500 BTUH (see below). The layout is very open, with entry/living/dining and kitchen pretty contiguous; a single small bathroom is the only really separate space. Significant floor area taken up by kitchen and bathroom cabinetry, and spaces for up and down stairs (two different locations) I also calculated for heat loss at the much more frequent (relentless?) ambient temp around here of 40'F:
Room..........BTUH20........BTUH40........Total area........Available area.....BTUH/avail
Living 8250 4950 300 280 29.5
Dining 3655 2190 140 140 26.1
Kitchen 2775 1665 125 45 61.7
Back Entry 3045 1825 100 57 53.4
Bath 785 470 71 57 13.8
total BTUH20/total area = 25
total BTUH20/avail area = 32 = q(up)
(I think I'm going to have to use a kick plate n the kitchen at least)
From Modern Hydronic Heating I adapted calc for in slab tubing (I[m using the first edition):
q(up)=32
q(down) = q(u)(Rff+Rair)+T(above)-T(below) / R(down) = 3.8
I ignored R(slab), and for Rff used R for 1/2 inch subfloor (fir, measured) and 3/4 inch jatoba, and got 1.34.
R(air)=0.61
It's above a heated basement, so T(above)=67 and T(below)=67, and they cancel.
Thus, q(down)=3.8
T(ave) = T(air) + q(up) x (R(ff) + R(air)) = 67 + 32 x (1.95) = 129.4
If deltaT is 10, then T(in) must be about 135.
a = [ 1 / (Rff+Ra) ] x (1+ [q(down) / q (up)]) = 0.57
Looking at the tubing layout (may need work), Im using 4 lengths on average 240 feet each (vary 200-260, 0.5 inch)
Total BTUH = GPM x deltaT x 500, and design delta T is 10,
total GPM=3.7, split among 4 circuits, ave=0.925 GPM each
therefore, b = a x L/500 x f = 0.57 x 240 / 500 x 0.925 = .293
So at this point, Solving for T(out) = T(room) + (T(in)-T(room)) x e^-b
and T(in)=135, and T(room)=67
I get T(out) = 117
This gives me a delta T of 18 and seems very wide of the mark of delta T=10 that I was aiming for.
Can anyone help spot the mistake(s) I'm making??
I'm working on the calculation for my first floor. House is 105 year old in Seattle. Basement presently torn open for remodel, and joist space accessible. Plan is to ditch forced air and switch to gas-fired hydronic - tubing in floor joists for first floor, radiators on second floor and soon-to-be finished basement.
Based on heat loss calc at design temp of 20'F, total for the first floor is about 18500 BTUH (see below). The layout is very open, with entry/living/dining and kitchen pretty contiguous; a single small bathroom is the only really separate space. Significant floor area taken up by kitchen and bathroom cabinetry, and spaces for up and down stairs (two different locations) I also calculated for heat loss at the much more frequent (relentless?) ambient temp around here of 40'F:
Room..........BTUH20........BTUH40........Total area........Available area.....BTUH/avail
Living 8250 4950 300 280 29.5
Dining 3655 2190 140 140 26.1
Kitchen 2775 1665 125 45 61.7
Back Entry 3045 1825 100 57 53.4
Bath 785 470 71 57 13.8
total BTUH20/total area = 25
total BTUH20/avail area = 32 = q(up)
(I think I'm going to have to use a kick plate n the kitchen at least)
From Modern Hydronic Heating I adapted calc for in slab tubing (I[m using the first edition):
q(up)=32
q(down) = q(u)(Rff+Rair)+T(above)-T(below) / R(down) = 3.8
I ignored R(slab), and for Rff used R for 1/2 inch subfloor (fir, measured) and 3/4 inch jatoba, and got 1.34.
R(air)=0.61
It's above a heated basement, so T(above)=67 and T(below)=67, and they cancel.
Thus, q(down)=3.8
T(ave) = T(air) + q(up) x (R(ff) + R(air)) = 67 + 32 x (1.95) = 129.4
If deltaT is 10, then T(in) must be about 135.
a = [ 1 / (Rff+Ra) ] x (1+ [q(down) / q (up)]) = 0.57
Looking at the tubing layout (may need work), Im using 4 lengths on average 240 feet each (vary 200-260, 0.5 inch)
Total BTUH = GPM x deltaT x 500, and design delta T is 10,
total GPM=3.7, split among 4 circuits, ave=0.925 GPM each
therefore, b = a x L/500 x f = 0.57 x 240 / 500 x 0.925 = .293
So at this point, Solving for T(out) = T(room) + (T(in)-T(room)) x e^-b
and T(in)=135, and T(room)=67
I get T(out) = 117
This gives me a delta T of 18 and seems very wide of the mark of delta T=10 that I was aiming for.
Can anyone help spot the mistake(s) I'm making??
0
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