1/2" or 3/4" Pex?

[Cougfan]

I am putting in a radiant heat floor in a 40x70 shop with a 4" slab. I ran the heat load calcs and I will need about 28kbtu. I can't decide if I should go with 1/2" pex or 3/4" pex. If I go with 1/2", I will need 9 zones (12" spacing). If I go with 3/4", I will need 5 zones (16" spacing). The costs will probably be a wash, so that isn't a factor. Does anyone with experience have a recommendation one way or the other.

 

[Highbeam]

Before your question drops off the page, the standard is 1/2" for several reasons. Cost, flexibility, it's easier to lay and it should be a more consistent heat across the slab. Imagine in a perfect world you would have miles of 1/8" pipe so that the whole slab is evenly heated with low flows. Standard manifolds are 1/2" too.

The benefit of 3/4 is that you have less head loss per foot but then you use longer runs anyway so it doesn't matter much.

I vote for 1/2".

 

[Thumper68]

1/2" all the way, so much easier to install. If you are doing it yourself do yourself a favor and lay out all your loops on graph paper with a different color for each loop. Makes the install go so much smoother.

 

[Mr onetwo]

1/2" all the way, so much easier to install. If you are doing it yourself do yourself a favor and lay out all your loops on graph paper with a different color for each loop. Makes the install go so much smoother.

Plus one on that......you do not want to be wrestling 3/4".

 

[dieselgarage]

I had my design done three ways, 1/2, 5/8, 3/4 I'm going with 5/8 in a 3840SQF building.

Ends up being 8 runs.

 

[Cougfan]

Thanks for the advice. I will post pics once I get it all laid out.

 

[powerhound]

I am putting in a radiant heat floor in a 40x70 shop with a 4" slab. I ran the heat load calcs and I will need about 28kbtu.

Thats crazy, my 40X40 with 12' ceiling and pretty heavily insulated, came out to a 40KBTU requirement on the do it yourself calcs as well as the one I had the boiler sales people do for me. I've got 1800 feet of 1/2" in my slab through 8 runs.

 

[Cougfan]

Don't tell me that! Now I am worried . I did mine a couple of different ways and they came out somewhat close. My only thought is that the min temperature that you and I are using may be different. I have been watching your build thread, and it looks like you are putting together a great shop. The only thing that I can think of is that your min temperature up in Bonners Ferry may be quite a bit colder than what I am using in Spokane. Also, I am not designing mine to keep the room at 70 degrees on the coldest possible day, so we may be different there (not sure). I think I am using 10 degrees as the min temp. I know it will get colder than that for a few days out of the year, but I can deal with the shop being cold for a week or two out of the year. I will have a wood stove in there, so I can supplement when I need to. I will double check my calcs and let you know what delta temp I am using.

 

[Highbeam]

Don't tell me that! Now I am worried . I did mine a couple of different ways and they came out somewhat close. My only thought is that the min temperature that you and I are using may be different. I have been watching your build thread, and it looks like you are putting together a great shop. The only thing that I can think of is that your min temperature up in Bonners Ferry may be quite a bit colder than what I am using in Spokane. Also, I am not designing mine to keep the room at 70 degrees on the coldest possible day, so we may be different there (not sure). I think I am using 10 degrees as the min temp. I know it will get colder than that for a few days out of the year, but I can deal with the shop being cold for a week or two out of the year. I will have a wood stove in there, so I can supplement when I need to. I will double check my calcs and let you know what delta temp I am using.

I would never expect two different heat load calcs to be the same. There are so many variables and so much hidden in teh black box computers that you can keep trying different programs until you find an answer you like.

Leakage assumptions, insulation assumptions, temp rise, etc. Then there is the comfort assumptions related to the type of heat. For example, if using a reznor I would likely have a 100k btu unit since people seem to want rapid heating but in a radiant floor application I could use 30 thousand. It's just weird and non scientific. Perhaps it is so scientific that the programs make broad assumptions.

Yes, 30k seems low but you can always add more btus. The only thing you can't do is add more tube in the floor.

 

[BadgerBoilerMN]

All depends on the user. I am designing a 20,000 sq.ft. shop in South Dakota. 5/8" PEX, 24" O.C. < 500' per. Inside design temperature for half the shop is 40°F and the other half 60°F with a total load under 10btuh/hour/sq.ft.

You have to do a proper heat load before you can answer any of these questions. It isn't easy. But over-sizing and over-tubing--read, "over-engineering"--is a lot easier but not necessarily cheaper, than doing the math or hiring a competent designer.

 

[98ssuck]

One thing i have learned is to go with the manufactures of your pipes procedures for measuring heat loss. They all have their own and if do the correct math you do the job once. The information you need may require purchasing a rather large boring book from the manufacture thou. A spacing of 16" for 3/4 may cause distinct hot and cold spots rather than a evenly heated surface.

 

[BadgerBoilerMN]

One thing i have learned is to go with the manufactures of your pipes procedures for measuring heat loss. They all have their own and if do the correct math you do the job once. The information you need may require purchasing a rather large boring book from the manufacture thou. A spacing of 16" for 3/4 may cause distinct hot and cold spots rather than a evenly heated surface.

That is a funny thing to learn.

Whereas there are accepted differences between radiant heating and warm air, the heat load of a building will not change with the choice of tube.

There are several software programs available that have dedicated modules for radiant floors and ceilings. The loads typically determined room-by-room, the radiation type selected according the the load and tube length, spacing, flow and average water temperature all chosen by the designer within the accepted parameters of the particular program he has chosen to use.

Residential heat loads, commonly used for DIY garage work, will use ACCA Manual 'J' standards.

 

[Cougfan]

So here is how I calculated my heat loss:

(Surface area of each component) * (U Value of Component) * (Delta Temp outside to inside)

Then I added up all of the heat loss from each component which gave me a total heat loss

These are the components that I took into account: Walls, Floor, Ceiling, Rollup Doors, Man Doors. I did not take into account loss due to air drafts (maybe I should have).

I am not trying to over-simplify things, but this seems pretty straight forward. What did I miss?

 

[powerhound]

Did you get her done before the winter season?

 

[Cougfan]

Not yet. I am scheduled to pour during the week of Oct 28th (assuming we don't get crazy cold weather). I still have to dig a floor drain, grade, foam, pex, rebar... Dang, there is a lot to do.

I will post some pictures when I make some more progress.

 

[powerhound]

Any chance you're using Boardwell Concrete for your contractor?

 

[the spyder]

Good to see there is someone else here in the NW pouring so late in the year. I ran 1/2" (6 runs) using a kit from the blueridgecompany in my 30x48x6" slab. I am meeting with my contractor this week to squedule the pour. I'm hoping for the next two weeks.

 

[Cougfan]

PH - No, I am using a friend of mine that does some great flatwork in the area.

Spyder - It's getting a little late, but I think we will be okay. It has been getting down to about 28 at night. Hopefully it doesn't get too much colder, or I may be in trouble. Being that the shell is already done, I should be able to keep it warmer inside the shop during the pour.

 

[anthony666]

you're way over thinking this .. it's a garage, with a wood stove to pick up the slack, not a hospital full of preemies .. your heat loss calculation is good

my professional opinion is to use 1/2" he-pex, stop worrying and unclench that sphincter muscle

 

[Cougfan]

HaHaHa, you are probably right. Have you been talking to my wife...she said something just like this.