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Simplified Radiant Design Nomograph & Worksheet: Heating only
For advanced designs see
ASHRAE Design Graph for Sensible Heating and Cooling with Floor and Ceiling Panels

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Radiant heating design graph

This simplified nomograph can be used for:
Poured floors such as concrete slabs or lightweight cementitious floors
Sub-floor systems with a maximum 9" o.c. tube spacing where solid aluminum heat transfer plates are applied throughout the entire length of pipe (no ventilated/perforated aluminum fins!)
Prefabricated grooved heat transfer panels with aluminum heat transfer fins
Minimum R-8 insulation below the radiant panel (use Type 3 or 4 rigid, blown, spray or batt...no bubble foil)
Rooms designed for 70 deg F

This simplified nomograph can not be used for:
Sub-floor system where aluminum heat transfer plates are applied intermittently along the pipe
Sub-floor system where pipes exceed 9" O.C. with or without plates
Sub-floor system where ventilated aluminum fins are fastened intermittently along the pipe
Staple up systems of any type without solid and continuous heat transfer plates
Suspended tube system of any type
Tubes fastened to sides of joist

Notes on flooring characteristics:
Low resistance (high conductivity) flooring would be anything less than 1.0 ft2hr F/Btu; examples would be tile, slate, bare or stained concrete, terrazzo etc.
Medium resistance (medium conductivity) flooring would be anything between 1.0 ft2hr F/Btu and 1.5 ft2hr F/Btu; examples would be engineered flooring, light weight carpeting etc.
High resistance (low conductivity) flooring would be anything greater than 1.5 ft2hr F/Btu; examples would be solid hardwood floors on top of subfloors, engineered floors with throw rugs, thicker carpets and underlayment's etc.

A word on heat pumps and flooring:
Heat pumps are limited to operating conditions under 100F return temps assuming a 10F rise. This means you'll want to avoid high resistant  (low conductivity) flooring in all but the most high performance home.
 

Radiant design worksheet to be used with the simplified radiant design nomograph

line

item

unit

action item 

Values

1

target operative temperature

F

specify

70

2

heat transfer coefficient

Btuh / ft2F

specify

 

3

fluid density

lbs / US gal

input

 

4

specific heat

Btu / lb

input

 

5

zone area

ft2

input

 

6

heat loss

Btu / hr

calculated

 

7

floor R-value

F ft2 hr/Btu

state for record

 

8

panel surface covering*

type

state for record

 

9

average fluid temp (from graph)

F

input

 

10

loop length tail pieces

ft

input

 

11

spacing (designer choice)

inches

select

 

12

# of loops

----------

input

 

13

delta t, fluid (designer choice)

F

select

 

15

inside pipe diameter

inches

select

 

16

head loss - radiant manifold

ft of head

input

 

17

heat flux

Btu /hr ft2

calculated

 

18

surface temp

F

calculated

 

19

supply temperature

line 9 + (0.5 (line 13))

F

calculated

 

20

return temperature

line 9 - (0.5 (line 13))

F

calculated

 

21

gross active footage

ft

calculated

 

22

length of active loop

line 21 / Line 12

----------

calculated

 

23

length of active loop with tails

Line 22 + line 10

ft

calculated

 

24

flow per zone

line 6/(60 pw cp line 13)

US gpm

calculated

 

25

flow per each loop in zone

line 24/line 12

US gpm/loop

calculated

 

26

velocity in each loop in zone

ft/sec

calculated

 

27

ft of head per 100 ft. of PEX

ft of head/100 ft

calculated

 

28

head loss - radiant loops

ft of head

calculated

 

29

head loss in tube & manifolds

ft of head

calculated

 

* for radiant cooling projects it is important to know the flooring absorptance value.

Additional reading:

  1. Sample radiant cooling calculation procedure.

For additional support visit our visitor services page.

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