Issue at hand, building will be 150L x 80W.  One end, 80W X 50L will be sectioned off with offices.  The office are will also be running a heat pump system so the radiant will be provide about 80% of the heat BUT the heat pump is designed to carry the full load if needed. 

The issue is the mechanical room is at one corner of the building.  People keep saying "keep the loop at 300ft or so".  Well, though I will fully design the thing with proper thermodynamic design, it is looking like My runs WILL be longer plus I really need to design in a fashion where I can add or remove heating from the office area so if the boiler cannot keep up with the whole building, we will throttle the shop area so the office stays comfortable.  That means circuits only in the office area yet are at the opposite end of the building. 


Do you guys "remote" heat or "manifold" a system under ground?  I am not wanting to have ANY joints under that slab...

remote manifold. 300 foot loops is a guideline, real lengths depend on pipe size, heat load, temperature drop requirements. I regularly use 500 foot loops if conditions allow, without upsizing pumps. but conditions do not always allow...

How would i do it? I'd hire NRT.Rob to do the design work.

heh, nice. checks in the mail jonr

You have a large space, you will need multiple zones.
We design and provide product to this type of structure often.
You will need some type of supply return to the office end of the building or vice verse.
In general we would design a 3/4/500 foot type system with multiple manifolds offering some zoning options.
Perhaps the office areas may be 1/2 pex. It all depends on the building and insulation.
We would need building and R value information to provide heat loss and system design quote.
Glad to help should you need,
Dan

I have not worked up an entire manual J for the building yet but building will be R19 walls and R40 ceiling with corrugated metal all around. Wall height to be 18ft with 3.5/12 pitch. Office will be sheetrock with 9ft ceiling and ceiling trusses will be cellulose insulated as well to separate upper and lower floor.

I was trying to avoid a separate manifold at the other end of the building but sounds like there is simply so way around that fact. Do you guys use HDPE or PEX for the manifold feeders as well? Off the cuff, 1.5" pipe might be needed here which could be a real bear to install.

Yes pex can be used but it is very common to run insulated copper piping overhead to different manifold arrangements.

you would use PEX. no way will you need 1.5" pipe. but the devil is in the details. depending on your heat source, pumping and electrical setup, the feeds could be done with anything from 1/2" (injection) to 1"... all depends on what makes sense in this case.

I was referring to the remote manifold feeder line, not the radiant diffuser tubing on the floor. I will have to look at the numbers, but probably run 5/8" due to the size of the building. I would also like to keep the head loss down. I am not sure what kind of head loss you guys are typically designing a system at but it sure seems like even 2gpm/circuit would be too much to get acceptable loss. That is just energy out the window to move the fluid around.

5/8 is fine in 500 foot loop, but the 3/4 will have considerably less head loss and a larger capacity to carry heat, more efficiency on pump size epically if you are putting in any kind of glycol. Given the overall size of the building and the fact that you likely will be tying off to a re bar structure elevated in slab 3/4 starts to look better. If you are stapling to a 2 inch foam insulation 5/8 is a option and is easier to work.
All depends on the building lay out.
Dan

I was referring to the manifold feeder line as well and I stand by my statement. 2 GPM/circuit would a ridiculously high flow rate. I typically design for 9 ft of head or less typically so as to allow the use of alpha ECM pumps on good settings.

No offense, but you are obviously not experienced with this kind of design. a 12,000 sq ft building should probably have someone with some experience involved.

and every guess in this thread on pipe size/flow rates is a full on guess without room by room load calcs and an idea of what you're doing for a heat source.

I am just looking for general install and design notes. Have no fear, the calculators WILL come out. We are an engineering firm so we just need to start running numbers.

I know in the prelim data for the geothermal loops, it seems like optimal design will have to take a back seat to just working with feasible tube sizes and head losses. Looking at the conductivity of PE and soil, it seems you can tie your self in knots trying to get where you want to go..


One thing we are still looking at is "where" the tubing will work best for us in the slab. There is a lot of data published that indicates a large efficiency penalty for placing tubing towards the bottom of a pour. Because we certainly would be installing anchors in the floor at some point as well as have very heavy machines on the floor, I think placing tubing towards the top is out. Off the cuff, I would want to install tube on the top of our wire mesh that will sit about mid point in the slab on a 6-8" pour. We have also considered installing a trace wire on each tube so we can locate them if needed.

In some respects, I want the tubing on the bottom of the mesh simply for protection during pour. You guys that have been on concrete jobs know things get fast and hectic which is no time to dance around tubing.

OK,
some numbers, but the building heat loss, insulation pack none figured in, these are just head numbers for argument sake.
Assume zone A has 8 loops
3/4" 500' 7.95 feet of head / 12 GPM so a Taco 3 speed or 008 variable speed pump is just fine, Variable would be preferred as flow will slow based on temperature. Pipe spacing may be in the 18 " range, (heat loss dependent)

5/8" 500' 17 feet of head / 12 GPM so a Taco 001 or 0011 variable speed pump is just fine, Variable would be preferred as flow will slow based on temperature. Pipe spacing may be in the 16 " range, (heat loss dependent)

The main point is here is the head differences, a 4,000' foot block of pipe on one manifold will cover area X determined by heat loss and spacing,
Dan

for a thick pour like this you'll want the pipe in the middle. there is probably rebar/mesh there already. tie to that. it's the only way to go in thick slabs.

if you're doing geo then you can't do 1/2" feeds to the remote manifold, oh well

I do 500 foot loops of 1/2" with less than 9 feet of head all the time. all depends on the heat load of course.

Could a guy reasonably get away with 2 manifolds? I prefer not to even have two but I do not see any way around that fact. Keep in mind that the shop area will be wide open and not ultra critical on temp variation as long as it is not a mile off. I was thinking of designing in a manifold specifically for the office area (4000sf) and the rest on the main manifold in the mechanical room where the boiler and such will be located.

I am trying to think how I might over head install the feeds for the remote manifold. Sure seems like underground would be easier but either way, it would need to be insulated.

Just for info
Head loss for 1/2 pex pipe 500'
100% h2o 100 degrees 1 gpm = 20.1 feet head 500 foot loop
That's a lot of head. looking like a big pump to push much,
100% h2o 80 degrees 1/2 gpm = 6.3 feet head 500 foot loop
1/2 gallon @ 80 degrees is not much on a 6 inch warehouse slab.
Add glycol?
Dan

that's why the math should be done. 1 GPM is a ridiculous amount of flow for a lot of applications. do a 30 dt and that's 15,000 BTUs/h. and in some cases it won't be enough flow. both can occur in warehouse situations, depending on the heat load, available supply water temp and tubing on center.

I can say outside of snowmelt situations I don't remember the last time I found that it made any sense to do larger than 5/8" loops and even that is rare.

If you are selling tube to people who don't value their time, short 1/2" loops and lots of manifold makes sense. Output is obviously not an issue as heat loads for most steel buildings is lower than the average residence.

We use 3/4" pex regularly (for snow melting and space heating) but would use a matt for commercial radiant floors over 20,000' as professionals will likely install the system. The matts are 1/2" PEX by the way, but pre-assembled so the manifold cost is offset.

Larger diameter pipe, longer runs at the same head. Spacing can also be widened depending on the load. As for tube depth, the effective placement of rebar is more important as performance takes many forms. Deep tube will effect response time and nominal design water temperature, but both are highly dependent on the envelope and intended function of the building. The "response" is really in the mass of the concrete floor rather than the proximity of tube to surface.

Best response is with tube 2" from the surface. As for protection, the tube is very tough and under preasure during the pour (we supervise every commercial radiant floor concrete pour). Standard practice in our shop is to set rebar on chairs to spec. and tie tube to the rebar. If the radiant panel is tighter than rebar, flat wire can be had in 6-6x 110 pattern.

We also go around machine anchor points and put the pipe on the bottom where shallow anchors can't be avoided. The higher the tube the easier it is to find with IF or infrared, though with a cool ambient and hot floor tube is pretty easy to find if not too close e.g. radiant matts.

I like 3/4" PEX on 18" centers for most of my working garages and pole barns...if the heat load allows of course.

Unless you're running 1000 foot 3/4" loops, I doubt you'd find my 1/2" length specs much longer than what I see most people running 3/4" loops for. such is the magic of high dt applications.

High delta T's are pretty rare, but I am with you and even use them on old gravity conversions and new panel radiators. It takes some getting used to, but easier to use 3/4" than to fight with engineers and building officials ;-).