1/2" or 7/8" PEX tubing????

[tdkkart]

Found a us a house with a large enough lot to build a shop, probably going to go 32 x 48. Unfortunately due to a restrictive ordinance it will only be 8' walls......

Anyway, gonna do the Pex in the floor. While doing resarch most seem to be talking about 1/2" line in 300' loops,spacing approx 8" apart, making no real mention of flow rates or temp drops between supply and return.

One place in particular advocates the use of 7/8" tubing, 400' loops, spaced approx 16" apart, and makes specific mention of moving the fluid fast enough to only have a 10-15* temp drop between the supply and return sides of the loops.

The temp drop deal makes sense, more even heating, plus does not make the water heater work so hard because it only has to recover 10* or so.
If you put 100* water and run it slow enough to **** it down to 50-60* the water heater temp will soon drop so far that it will never recover.

Comments??

 

[Red'n'WhiteRebel]

The supplier I'll be dealing with is doing the system design from my plan as well as supplying the material. I'm sure there are cheaper ways to go but there's no re-do's with in floor heating and I want it done right. A friend of mine in the business will help me install it so there's some savings there. I think that the other variable in this is how many zones you plan on splitting the system into. Initial cost will be higher but you end up with better control and lower operating costs. I'm hoping to get my design done within the next few weeks.

 

[sneezer41]

1/2 ID

There is no real cost in having too many loops. Anyone trying to sell you on out of the ordinary setups is getting you in trouble. Hell, you could have 20 100 foot loops and you would just pay for a lot of manifolds.

A big temp drop is efficient, the water heater doesn't care

The most efficient heating system on the planet would have a 0 degree K return temp!

 

[sixty4]

I use 5/8" Wirsbro. I keep my loops 250' max 12" space. Everyone has a different school of thought. I put the radiant blanket down first/or blue stryroform, and tie wrap the tube to the wire mesh. This has always worked for me! I also like the Taco mixing blocks to help regulate temps.

 

[MrCrewcab]

Don't know what the weather is like where you live but i was told to keep the spacing 6" for the first 3 feet against the walls then goto 12". I kept with 300ft 1/2" lines and layed them out with the supply and return next to each other. works great. our winters get down to -35F

 

[HoosierBuddy]

1/2" is the commonly recommended size. It's what I used, and it works great. 1/2" fittings are going to be easier to come by too.

Temperature differential betweeen inlet and outlet water is a function of the effectiveness of your heating loop. The better your heat exchanges between the tubing and the slab, the bigger the temperature drop will be. If you don't have a temperature drop...then you aren't heating anything.

Phil

 

[Chris J]

There is no way to apply a simple rule to selecting tubing size and the length of the runs. No matter how you slice it, somebody has to do some actual design work if you want a system that works well and doesn't cost you a fortune to operate.

7/8ths" is a HUGE tube. Assuming you don't have a truly exceptional case, the decision will probably be whether to use 1/2" or 5/8". If somebody is recommending 7/8", you can probably assume that they don't understand what they are doing.

There are many, many considerations here. Tubing size is principally dictated by a basic requirement to keep flow rate through the tube at between 2 and 4 feet/sec and the amount of heat you need to carry in btu/hr. To maintain this flow rate in 7/8" tubing means you have to pump a LOT of water, and to keep from overheating the floor, the system would have to operate with a very small temp differential between the inlet and outlet.

It is easy to see that this is not going to be very efficient. The pump will be doing a fair amount of work to transfer only a small amount of heat to the slab. i.e. you are go to pay for a lot of electricity to pump the water that is accomplishing very little for you. You'll get nice heat out of the slab, but being paying way too much for it.

I also get the impression that you may be planning to use a typical water heater as the heat source. While this can be done, it is a short term solution only. Standard water heaters are not designed to generate the total amount of heat that a slab will require, and will frequently operate almost continuously during cold spells and dramatically shorten the life of the unit. You want a "boiler", not a heater, if you want the equipment to survive for any length of time. Your concern about the heater never being able to "catch up" is indicative of a heater that is way too small for the application. If the garage needs 15K btus/hr to maintain a stable temp, no heater or boiler that generates less than 15K btus/hr is going to be able to keep up.

I'll recommend a book you should consider. Since you are still in the "design" stage, you still have time to educate yourself. Take advantage of it.

"Modern Hydronic Heating, 2nd Edition" by John Siegenthaler. ISBN-13: "978-0-7668-1637-4"

There is also a link to a fairly comprehensive .pdf on the subject elsewhere on this board (perhaps somebody will post it again). However, after reading it, it is not nearly so comprehensive as the above book (though still a darn good place to start).

I came to the conclusion that most plumbing contractors have their way of approaching a hydronic system, and apply that approach to all types of systems. Most that I have contacted want to treat a slab with a high thermal mass the same way that they would handle a low-mass residential floor. They ALL want to use a tankless, "on demand" boiler that outputs heated water at a fixed 140 degrees (F) and pump this directly into the slab. In most cases this can be done with a low-mass floor because it responds fairly quickly and an air thermostat will sense the heat fairly quickly and turn off the flow of heated water so that the floor (and room temp) don't badly overshoot the desired temp.

If you try this with a high-mass floor, the floor will take on a huge amount of heat before the heat reaches the surface of the floor. By the time the air thermostat senses that the temp is reaching the desired point and shuts off the flow of heated water, there is so much heat in the floor that the air temp will continue to rise long after the desired temp was reached and ultimately overshoot the desired temp by many degrees and will take hours to cool back down. This same thing applies when the thermostat calls for heat again, but by the time the thermostat calls for heat again, it will take hours for that heat to migrate out of the floor, so the air temp undershoots the desired temp too.

The point is that you should take advantage of the opportunity to get an education of your own. DO NOT rely on a plumbing contractor to get you the right system.

 

[Milner]

There is also a link to a fairly comprehensive .pdf on the subject elsewhere on this board (perhaps somebody will post it again). However, after reading it, it is not nearly so comprehensive as the above book (though still a darn good place to start).

http://zurn.com/images/pdf/ZPM02101.pdf

 

[Maulerman]

To add to Chris J's reply you do not want a convential water heater or boiler to get to the point where the water vapor in the flue gases condenses. That will kill the heater. I recommend a condensing boiler for slabs because the discharge temp can be run down to 100F without harm. 140F water is pretty high for slabs. With a condensing boiler you would not need a tempering loop.

 

[MrCrewcab]

Where are you located and i can give you a quick heat load calc.

 

[haggis]

I installed a 1/2 pex system in my house. I wondered how it would work if I put it in a garage I'm going to build.

With my home system (using a propane water heater) I set the desired floor temp and then leave it alone. I don't turn it off and on. It would take a couple of hours for the floor to completely cool down or heat up doing that.

It seems I would waste alot of power by leaving a garage system on all the time, but it would take too long to heat up If I switched it off and on.

What do you guys do?

 

[70pcuda]

Isn't the 7/8" PEX actually 3/4" ID and 7/8" OD? So it's not really as big as it sounds? I got a quote for a radiant system for a 2000 sq foot building from radiantcompany.com and they recommended 6 300' loops of 7/8" PEX. (if I remember correctly.)

I'm not to that point yet, still in the planning stages.

 

[5wndwcpe]

I just finished up laying the tubing in my shop and here are my thoughts. I used 6 loops of 1/2" at 300' per for a 1568 sq.ft. floor. If I were you, I'd consider something like 8 loops of 1/2" at 300' each. Reason being that 7/8" tubing is going to have a nasty bend radius. Of course, without knowing your heat load, any design is just an uneducated guess, but the point I'm trying to make is 1/2" is going to be much easier to work with and the closer spacing will tend to reduce hot spots.

 

[Red'n'WhiteRebel]

We need some pics 5wndw. You gettin' close to bringin' the Mopar collection home?

 

[5wndwcpe]

We need some pics 5wndw. You gettin' close to bringin' the Mopar collection home?

I posted some today and for whatever reason they didn't upload. I take another crack at it tomorrow. Hopefully the floor will get poured this week and I'm hoping to be able to bring the cars home by August or so. I'm pretty much working 3-4 hours a day by myself so like Uncle Joe, it's moving kinda slow.

 

[JaxGarage]

Has this issue been resolved. Most older systems ive seen are using 1/2" but the mentioned companies are still insisting that 7/8 is better.

 

[tdkkart]

Has this issue been resolved. Most older systems ive seen are using 1/2" but the mentioned companies are still insisting that 7/8 is better.

I don't know that it's been "resolved", but I built and am now running a system with 1/2" tube that works just fine.

I can tell you that working with 1/2 tube isn't the easiest, I'd hate to try playing with 7/8". I think it would be near impossible to work with in the scales that the typical shop would work with. As someone mentioned, the bend radius is gonna be huge, making looping on 12-18" centers near impossible.

 

[JaxGarage]

Is anybody using the 7/8 tube like these companies are suggesting or is everyone still using 1/2"

 

[ktm010]

It's been awhile since I installed mine 21 years ago. 7/8 might be more work but is possible, I ran some in between floor joist 16" on center, and some in concrete can't remember 16" or 24" centerd.

 

[PBrazelton]

It's been few weeks since this thread died, but I thought I'd chime in for posterity.

I installed about 2000' of 7/8" PEX from the Radiant Floor Company about four years ago. I actually created my account on this board when researching doing this. It's an open direct, runs off of a Takagi TK-3. The system has run through four Minnesota winters. Here are some thoughts:

Pros: Tech support from the company has been awesome. The premade manifold and mixing valves worked well from day one, though I had an issue with installing the system with a water pressure regulator (my city water comes in at 90 PSI). Everything shipped complete, and I found the directions very helpful.

Cons: I've worked with a lot of PEX since then, and I have to say 7/8" is a huge PITA. You have to drill substantial holes in your joists to run, and it's pretty much impossible with just one person. It's also tough to work around obstacles.

I've read some dissent on whether the larger diameter PEX gives off more heat, but I can't for the life of me understand why it wouldn't. If you need the additional heat, it will probably work well for you.

 

[BadgerBoilerMN]

This is a great string I have to keep going.

PB has it right. There is no possible reason you would need 7/8" tube, it is a painful gimmick. The "open" system is not legal in MN or any other state for good reason.

7/8" PEX as the potential to "give of more heat" but you don't need it unless you are melting snow.

Chris is close and has some good points but mixes up (fixed temperature) tankless water heaters with the infinitely superior wall-hung (temperature modulating) condensing boiler that thrives on radiant slab temperatures (normally, well below body temperature). You can't go wrong with a properly size condensing boiler (with built-in temperature sensitive controls) for any radiant slab installation.

Find a competent, experienced designer and follow their advice, which should start with a proper computer generated heat load analysis.

 

[Mr onetwo]

Man oh man, where is all this **** about 7/8" tubing coming from!? Only an idiot (not you guys...just any company that recommends this)would think that there is an advantage to using 7/8"...it is a HUGE PITA in an aircraft hanger let alone a small slab!1/2" tubing in 250ft loops is the way to go.The Delta T( diff between supply and return) of a properly designed hydronic system is 10 degrees.Any more than that and something is wrong.

 

[Mr onetwo]

Find a competent, experienced designer and follow their advice, which should start with a proper computer generated heat load analysis.

Well said dude! Don't skimp on the design or install!Once the concrete is poured, there's no goin' back.

 

[IGOTWUD]

One nice thing I like about 7/8 pex is it is spaced 16" oc soooooooo..... when you want bolt down equipment , like a lift or 2, machinery, air comp. ect. you have half the limitations you would with 1/2" pex spaced 8"oc. In my living areas I would use 1/2" pex, the shop is a work area always expanding!