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Old 03-11-17, 04:32 PM   #1
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Default Pex loop length & spacing?

I have always read & believed that pex loops off of a manifold should be +- 10% of each other & important for balanced heating of the loops.

It seems that this is not very important.

http://www.pmmag.com/ext/resources/P...0813_Siggy.pdf

See Figure 3


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Old 03-12-17, 03:35 PM   #2
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http://www.uponorpro.com/calculator/

For 1/2" pex at 100*F

1 GPM through 200' = 8.022 feet of Head loss = 3.45 PSI

So the pump is pressurizing the supply manifold 3.45 PSI greater than the return manifold.

3.45 PSI will push .67 GPM through 400'

So when we double the length of a loop, 2/3 as much water flows.



http://www.pmmag.com/ext/resources/P...0813_Siggy.pdf

See Figure 3

Going from 1 GPM to .67 GPM reduces heat from 8,200 BTU/hr to 7,400 BTU/hr = 10% less

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Old 03-12-17, 04:11 PM   #3
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On balancing heating of the loops concept:

After the emitters are installed, you can wait for cold weather & use an IR gun on each loop, then choke off the hotter returns. This is a waste of pumping energy.

Much more heat is lost along the outside perimeter and close to doors & windows than than in the center of the heated space.

A good strategy is to place more pex in those areas, such as 4" On Center vs. 8-12" in the center etc.

Any ideas on how to apply some science to this?
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Old 03-17-17, 03:02 PM   #4
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Reducing flow to only 25% or from 1 GPM to .25 GPM reduces heating to 75%

See figure 2

A New Approach to Variable Flow Hydronic Systems
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Old 03-21-17, 10:11 PM   #5
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Quote:
Originally Posted by buffalobillpatrick View Post
Much more heat is lost along the outside perimeter and close to doors & windows than than in the center of the heated space.

A good strategy is to place more pex in those areas, such as 4" On Center vs. 8-12" in the center etc.

Any ideas on how to apply some science to this?

Yep. And conventional wisdom is wrong, sort of.

First of all you need to understand what you are actually doing.

Naked People Need Building Science

Radiant based HVAC

So what exactly is the HVAC system doing? In reality it is not warming or cooling the air to warm or cool you it is warming or cooling the surfaces of the building. Those surfaces either add or remove heat from you. In all actuality to be comfortable those surfaces have to remove heat from you, If they do not remove heat from you fast enough then you will be uncomfortable, as will removing it too quickly.

The good news is that your wall assembly (as described in your other thread) will likely have very little heat loss so an even floor temperature will likely keep your walls warm enough, as the walls will be absorbing heat from every other surface in the room. Closer tubing spacing will likely raise the response time of the system, But it likely isn't needed.

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Old 03-23-17, 04:14 PM   #6
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My windows & doors U=.25 = R-4

My walls 6" Polyiso foam is R-2 / inch @ 0*F and R-6 / inch @ 70*F
My wall average @ 0*F outside is about R-4 / inch = R-24

So my windows & doors loose heat about 6X faster per square foot, than my walls.

Conduction Heat Loss: Q = Area x U-value x Delta-T

Comes close for walls, windows, & doors, but ignores some BIG factors such as infiltration, wind washing, and Clear Cold Night Radiation.

http://www.healthyheating.com/Radian...m#.WNRBVvkrKUk

http://www.sensiblehouse.org/nrg_heatloss.htm

Radiant slab heat loss down & horizontal is very complex.
Depends on under slab & perimeter R-values, soil type, water content, soil temperature, outdoor temperature, slab temperature, room temperature, slab size, etc, etc.

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Old 03-26-17, 10:31 PM   #7
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Quote:
Originally Posted by buffalobillpatrick View Post
My windows & doors U=.25 = R-4

My walls 6" Polyiso foam is R-2 / inch @ 0*F and R-6 / inch @ 70*F
My wall average @ 0*F outside is about R-4 / inch = R-24

So my windows & doors loose heat about 6X faster per square foot, than my walls.

Conduction Heat Loss: Q = Area x U-value x Delta-T

Comes close for walls, windows, & doors, but ignores some BIG factors such as infiltration, wind washing, and Clear Cold Night Radiation.

Underslab insulation

Calculating heat loss

Radiant slab heat loss down & horizontal is very complex.
Depends on under slab & perimeter R-values, soil type, water content, soil temperature, outdoor temperature, slab temperature, room temperature, slab size, etc, etc.
Yeah, I started to get into all of that, then realized to actually give you all the math and reasoning I'd be writing a book. and it likely wouldn't be done as well as the other books out there.

As far as windows and doors they do have an effect on comfort, but the biggest issues is the percentage of wall area they take up, and their U-factor.

The decreased tubing spacing is just decreasing system response times, allowing a greater percentage of BTUs to reach the walls and windows faster. And remember at design conditions your system should be running flat out anyway. So if it's designed correctly it will still be comfortable when it is most likely to be uncomfortable. That just leaves the shoulder times. During those times the wall will only be receiving a small portion of its BTUs from the point closest to the wall. For relatively high loss walls that extra tubing is a good thing, for lower loss walls and homes it really isn't needed. There are also other ways to get the same effect, such as increasing thermal conductivity, decreasing thermal mass, modulating water flow and temperature, etc...
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Old 03-27-17, 06:19 PM   #8
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The Uponor design guide shows running the supply (hotter) along the exterior walls 1st before moving toward center of room (heat loss is less).

Placing the tubes closer together has the same effect.

As I am doing constant circulation within each zone with an Outdoor Reset Injection controller, the floor temperatures within each zone will tend to even out, even with more heat loss along the exterior walls & doors.

My boiler is a gas cast iron Burnham 3-section (smallest made) & it will probably run at about a 50% duty cycle at Design Temp. (2*F)

The heat is buffered in a 79 gallon tank between boiler and floor system allowing long burn cycles.
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Old 03-27-17, 08:30 PM   #9
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There are smaller boilers than what US Boiler Company offers. If you are running a a 50% duty cycle at design conditions your equipment is way oversized. Size your equipment to your load. Oversizing drastically reduces system efficiency, and reduces component lifespan.
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Old 03-28-17, 09:49 AM   #10
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DEnd

What you are saying is true, "IF" the boiler turned on whenever a thermostat called for heat. Sadly this is how many systems are implemented.

My boiler will NOT short cycle, as there is a 79 gallon buffer tank between boiler and system.

This tank also provides the Domestic Hot Water (DHW) to the house.

My tank is a Vie$$man dual coil indirect. The boiler or Solar heats the tank fresh water via the bottom coil.

An aquastat in the tank turns on the boiler or Solar. The 79 gallon body of tank provides DHW.

The top coil provides warm water to the heating system.

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