Radiant loop length question

[oldbones]

Ok, here goes.

I am one week into my new shop/garage project, and we started setting forms today. I ordered the in slab materials for a radiant heat system from a company I found through this site. It's here, and we will be ready to start laying tubing later this week.

Here's the rub... Everything I have heard or read makes me wonder if the tubing runs these guys have spec'd out are too long. He sent me 3 500' rolls of 1/2" PEX, and is planning a 3 loop (1 zone) system for my 1440sf slab (monolithic) on grade building. I asked about the long loops, and he went to some length to assure me that they've got it all figured out, and with the right pump a 500' loop of 1/2" is just ducky. I had a plumber over today to look at the rest of my project, and found out he has a good bit of radiant install experience, and he just about had puppies when I told him the supplier was calling for 500' runs.

Ok, let me have it!

 

[chevelle64]

I'm no expert by any means. I am going with Radiantec. They set me up with 3 300 FT runs of 1/2" pex. This is for an 832 sq ft building. I haven't started laying tubing yet. Building is being built first. That's all I know. Good luck.

 

[BooUrns!]

There is a loop calculator program you can search out online and install. I've never seen it draw up a loop longer than 250'. There are forums directly relevant to heating, plumbing and radiant applications. Try posting your question on there and ou will quickly find soem expert advice. Just don't discuss prices there, they're more secretive than the illuminati when it comes to how much they gouge us simplefolk...

 

[Ezzie]

My system designer said more shorter length loops is better than long runs. He recommended keeping them in the 250' - 350' range max. so I did 6 loops of 300' each for a 28' x 32' area. I haven't got it fired up yet so can't give you any feedback yet on how it works.

 

[tdkkart]

so I did 6 loops of 300' each for a 28' x 32' area.

Hmmm??? 1800 ft of tube in a 900sqft area?? A tube every 6"??
It'll damn sure be even heating if done right, but I wonder if you won't end up with cold water coming out the far end?? Seems like you'll have to pump the fluid really fast to keep the temp drop down.

I've been studying up for my future project, seems like ther's 2 schools of thought. Use smaller pumps on short runs, or use bigger pumps on longer runs.
Also 1/2" vs 3/4" tube, and close vs wider spacing.
Hard to say what the best way is.

 

[5wndwcpe]

Were it me (and I just did a 1568 sq.ft. floor) I'd cut those rolls in half and run (6) 250 ft. loops. With the configuration you currently have, I'd be concerned about the temperature differential between the supply and return sides. Think about how cool that water will be in the last 100' of tubing compared to the first 100'. You may very well end up with hot spots and cool spots depending on your layout. True, if you kept the loops at 500' you could use a pump with alot of head, but then you're using more energy (electricity) to push the same amount of water with a greater delta T. IMHO.

 

[sneezer41]

shorter loop lengths are good

why do you think that cold water out is bad? colder return water means you got the heat out. Design the system to deal with the old return water[ie primary secondary] if you need to

 

[Ezzie]

Hmmm??? 1800 ft of tube in a 900sqft area?? A tube every 6"??
It'll damn sure be even heating if done right, but I wonder if you won't end up with cold water coming out the far end?? Seems like you'll have to pump the fluid really fast to keep the temp drop down.

Yes, the tubing is fairly close but to provide even heating, I did a spiral in/out pattern so that the in/out of each loop are beside each other. Here is a pic of the layout before we poured the concrete. It will be driven by a single Grundfos UPS15-58F/FC (pump used in the Triangle Tube Prestige Excellence 110 boiler).

 

[kvom]

My 26x36 slab will have 4 200' loops.

 

[tdkkart]

why do you think that cold water out is bad? colder return water means you got the heat out.

Again, another vast difference in the way companies spec their systems, and I think it will make a large difference in the efficiency of the systems, and I believe that it is a big part of why there are groups of people that are happy with their systems and others that are not.

Theory #1 is to use short loops and move the water slowly enough to pull all or most of the heat out of the water. You end up with a large temp differential between supply and return water, and you require the heat source to reheat the water much more on each pass through the heat source.
You have to be careful in how you lay out the tubing, if not you will end up with cold spots in the floor.

Theory #2 says that you move the water fast enough to have a temp drop of approx 5* or so in each loop. Recovery time of the room temp takes longer because the slab does not gain heat as quickly, but the heat source does not work as hard to reheat the water. Tubing layout is less critical, and there are less variations in the slab temp.

Which is better in the end??

 

[sneezer41]

The heat source doesn't 'work hard' AFAIK, it just works. The longer it runs[assuming a fixed firing rate] the more energy it uses. Keeping the boiler too cold [via very cold return temps] might lead to rotting of the boiler, so setting up a primary loop feeding secondary loops keeps the return to the boiler stable.

 

[tdkkart]

We often hear of folks who's tank-type water heater source will not keep up, can't recover fast enough. We've all had this happen in our homes, 100* water out, 55 degree water in, and the water heater can't catch up agains the 45* temp differential, which results in a downhill slide and the heater runs continuously.
However, it may be able to keep up if the temp diff is only 5* between in and out. I would certainly think a tankless would be able to do it easily.

 

[oldbones]

My radiant provider tells me the electro-boiler system they have designed for me should be running 125-130 degrees boiler out temp with 15-25 degrees temp drop in each loop being the target.

 

[Ron Lombardo]

My garage is 24 x 24 and has 3 floors of radiant I broke each floor up into 2 zones. 288' per zone . Basically it does depend on the pump GPM and head pressure ... if you go too many feet per loop the pump will dead head ..meaning the restriction in the pipe and system will exceed the ehad pressure the pump can pump or the pump curve ..menaing the gpm will decrease to a point where it will take for ever to circulate.

Also you dont need piping in a garage every 6"... 12" is more then enough ... remember radiant IS NOT like warm air or hot water base board .... the first time you fire it up ..your going to think it isnt working ... its going to take a day or two to come up to temperature. Then your going to expect the floor to be warm ... and all that ISNT happening ..if it is warm the temp of the system is TOO HOT ... 100 -120 MAX and if the temp is that on the floor your TOO HOT and defeating the purpose.

With that said the pump HAS TO BE flowing at least 4 gpm - 8 gpm ... on the return. Balancing Valves and a flow meter will tell the tale.

ROn