Building a radient Floor heat control panel

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Old 11-26-12, 06:00 PM
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Building a radient Floor heat control panel

I am building a garage/cabin in the wisconsin area and plan on putting Pex tubing in the concrete for infloor heating, I curently have a 199,000 BTU ondemand hot water heater that I plan on using for the heating and hot water for the shower and kitchen sink. I was looking at the prices of control panels at the big chain stores and see that the componenets are alot less the the whole unit itself, so i would like to save some money and build myself. I have spent two hours now searching the internet for info on how to build/designe a control panel, what is required, and what the minium is I will need to keep this as inexpensive as posable. Anyone know where i can find more info on building, desinging, and what i need aside from the hot water heater and the pex tubing.
 
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Old 11-26-12, 06:37 PM
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There is so much more that needs to be known and understood before you can jump into this.
You will need heat loss calcs, radiant design etc before you can get started
 
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Old 11-26-12, 07:35 PM
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I have spent two hours now searching the internet for info
Time is money. What do you charge yourself per hour? $25 or more? If so, you are already $50 down and nothing to show for it!

Kinda tongue in cheek in a way, but think about it for a minute and you will see the point I'm driving at. How much time are you going to have to spend to save say $100 ?

looking at the prices of control panels at the big chain stores
What control panel are you looking at? Do you know that it's even what you are going to need?

What TO is saying is absolutely true. There's a WHOLE LOT more to designing a heating system that WORKS than meets the eye.

For starters, before the concrete for the slab is poured, you are aware that a radiant slab needs to be heavily insulated on the bottom and sides, and that you need to keep the ground water out of it, right?

Do you have a close approximation of what the heat loss of the building is going to be when completed?

Do you know how tubing size and spacing within the slab relates to the amount of heat output that will be available to heat the building? In some cases it is impossible to design a slab heating system that has enough output to keep the building warm, so you need to understand the science behind that also.

I don't think you will find a website that is going to give you a 'crash course' in radiant design and come away with enough knowledge to confidently design such a system. People study this stuff for YEARS to get good at it!

It sounds a little like I'm trying to discourage you... and in a way I am, but just enough to make sure you don't make an expensive mistake, because once it's done, it's DONE. There's no 'do overs' with tubing in a slab. I only want to make sure you know what you are getting in to is all.
 
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Old 11-26-12, 07:57 PM
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I have found some info on laying th pex tubing in the concrete, the direction i am going is using the 1/2" pex with the oxygen berior, I have 936 sq. ft. to cover, i will run four loops with one zone, the supply side of the loops will start on the parimeter of the slab going 6" on center for the first 18" then going 12" on center from there, I thought about goin 9" on center but was told that it was not nessary. I am using a moisture protector then 2" pink foam insulation on the bottom, up the sides and 24" out from the edge of the slab just below the surface of the soil, the pex will be layed on to of the foam, I have heard mixed opnion on raising the pex off the foam form 1 to 2 inches or just attaching it directly on the foam. as far as heat loss i do not know how to calulate that. the garage will have 2x6 walls insulated and the celing will be insulated also. double pane windows and insulated doors. from there i am not sure where to go.
 
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Old 11-26-12, 08:08 PM
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was told that it was not nessary.
By whom? What qualifications did this individual have? Reliable? How do you know?

2" pink foam insulation
I am the first to admit that I myself am not qualified to design radiant slabs, but my gut feeling is that 2" is not near enough in your location. Don't it get like really cold in WI ? With only 2" of insulation I would think you would be dumping an awful lot of heat right into the earth.

I think the question of whether or not to pull the tubing up from the bottom depends on how thick the slab is. With it all the way at the bottom of the slab that entire mass needs to come up to temperature from the BOTTOM UP before any heat is delivered into the building itself. I believe the system will be much more responsive if the tubing is closer to the heated interior.

Ceiling insulation at LEAST R40, right?
 
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Old 11-26-12, 08:12 PM
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Heat loss, or "Manual J" calculations

If you have blueprints of how the building will be constructed, you can calculate this yourself.

Go to this link:

Contact Us - P.V. Sullivan Supply Co., Inc.

And download the 'slant-fin heat loss explorer' program. Install on your computer and enter the information it needs about the building as accurately as possible.

This will give you a slightly exaggerated figure for heat loss.

There's a number of guys here who know the program pretty well if you need help, have questions, etc.

Once you know how much heat you need to put into the building to maintain temp, you can begin to determine if the radiant slab will accomplish that goal.
 
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Old 11-26-12, 08:14 PM
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I just quickly googled "radiant slab design" and glanced quickly at this website... looks like some info there might be helpful to you:

Radiant Floor Heat System Installation & Packages | MountainView Supply Inc

and now that I've looked it over, they are suggesting 2" of foam is enough... like I said, I'm no radiant designer, but it still just doesn't FEEL right to me. I'm pretty sure I would go with 4" myself, but perhaps that is overkill?

Then there's this article about an install gone wrong...

Mistakes to avoid when installing radiant heat in a concrete floor slab - set proper depth of radiant heat tubing in the slab

Sometimes it's good to read about the tragedy too!

Comment from this article:

Place the radiant heat tubing at the industry-recommended depth down from the surface of the slab. Typically the maximum depth that tubing should be placed in a concrete floor slab is 2" down from the finished floor surface.
 

Last edited by NJT; 11-26-12 at 08:31 PM.
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Old 11-26-12, 09:13 PM
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We DIY'd our 1000ft slab-on-grade (plus attached garage of 600ft).

Everything i read at the time said to put it about 2" down, as Trooper quoted. We tied ours to the mesh in the slab (which you pull up into the concrete as you pour it). We did not super-insulate the underside, rather we poured a 5ft deep foundation wall (clay here, dont want this 2-story to move).
So the idea was to allow enough heat to 'drain' into the foundation hole. It warms up some, Id imagine, then seems to stop sucking the heat. Our oil usage is quite reasonable.

Some notes..
-get the proper expander tool for the pex ends
-consider just 2 6" spaced runs on the perimeter..we did 3 and it seems that a little too much heat blows off along the perimeter. I know its needed for windows/loss.. but the floor feels too unevenly heated. This could vary based on your liquid speed.
-fab up something to air-pressurize the tubes when you pour.
-use pex metal pipe elbows to form the tubes coming out of the slab, they form the pipe to a nice 90deg curve.

For the control/interconnect you could use a large plastic electrician's bin.. contractor shops have them. They should also have the wiring connection strips etc. I have a medium sized one holding my large transformer and relay, but Id like to make one to fix up the mess of t-stat and zone valve wires.
 
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Old 11-27-12, 06:02 AM
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thanks for all the usefull info. i am going to talk to a HVAC company here in town this week.

I am still wondering if anyone found info on desinging the control board? It seems simple EX. pump, Tstat, pressure reliefe valve. Just move the water through the tube. But again i am no expert and looking at the pre made systems at home depot there is alot more to them. They sell the individule components and they come up to around half the price of the unit itself. I could copy one of theres but at this point i would like to educate myself a little on what i am going to build. EX. how to use the ondemand heater for heat and hot water. I am a controls enginer in the manufacturing feild and work and design insturmentaion control systems, but i can not find any info on these so i can sit down and put something together. EX. water flow, pressure, tempature. A friend that has this simelar setup designed by a contracto is able to use a single cell ondemand heater to do both infloor and hot water, the only thing is when you are using the hot water the heating loop is not working.

I already have an 199,999 btu ondemand water heater that i want to use, trying to keep this enexpensive as it is only a garage/weekend cabin.
 
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Old 11-27-12, 04:01 PM
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I am still wondering if anyone found info on desinging the control board?
I'm still a little confused by your use of the term "control board".

I think you are talking about one of those 'pre-packaged' deals that you hang on the wall, hook up the tubing, hook up the boiler, and done... is that what you mean?

Yeah, buy the parts and build it yourself...

So what information are you looking for in that regard? Like a system diagram or something?

This isn't for radiant, but is this what you are talking about designing?

 
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Old 11-27-12, 05:23 PM
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Yes that is exactly what i am talking about. I have the ondemand hot water heater, after the pex tubing is in the floor i will need to hook the ondemand to the pex tubing and make a working system. Yes the systems i was talking about are the prebuilt units at Menards that can cost around $1900, I look at the components that are used and they did not add up to $700, so i would like to build what you have in the picture but i would like to educate myself on what is needed and how the system works so i can determine what i need and how to put it together.
 
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Old 11-27-12, 05:52 PM
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Depending on how much you want to spend... a pricey book by John Siegenthaler is the 'bible'.

Modern Hydronic Heating: For Residential and Light Commercial Buildings, (1428335153), John Siegenthaler, Textbooks - Barnes & Noble

Make sure you get the THIRD edition, there are still plenty of 2nd edition for sale out there...

BUT, there is LOTS of tech info in these PDF files that you can use! for FREE!

The Radiant Store installs low temperature radiant heating systems

Being from an Engineering discipline, you will appreciate this info!
 
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Old 11-27-12, 06:46 PM
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Thanks NJ, this is the info i was looking for. Appreicate the help.
 
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Old 11-27-12, 08:00 PM
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Keep us posted on your progress... run proposed designs past our 'review board'... we're always most happy to criticize, but always in a constructive manner!

Seriously though, I'd be interested in hearing how you are doing.

One more 'minor' (or maybe not?) point I would like to mention... it might be wise to check with the manufacturer of your water heater and find out if radiant heating is an application they recommend, or NOT.... ESPECIALLY if you have any kind of WARRANTY concerns.

Water heaters generally need to be certified for space heating use and should carry the ASME "H" stamp if they are.



If you are planning an 'permitted and inspected' installation (a wise decision), the inspector may look for this mark. Probably a good idea to find out before committing totally to make sure they aren't going to shoot you down after all is said and done.
 
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Old 11-28-12, 05:54 PM
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So i ran the soft ware for heat loss and it shows 38000 btu/hr for a heat loss, it recomends a 90,000btu unit. The ondemand hot water heater i have is a 199,000btu unit, am i correct that this will be plenty, actualy overkill? Will it work?
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Old 11-28-12, 07:30 PM
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You will NEVER get 199,000 BTU from an on demand hot water heater.
There is just no way to get enough flow thru the boiler for heating delta T loading.

Domestic water can enter the boiler anywhere from 35 to 70 or higher degrees F, it might leave the heater at 120-130 F. There is quite a high delta T here, therefore the flow is much less than heating.
Heating for infloor might have a design temp of 120 F and might have a 10 degree delta T, therefore the flow rate needs to be higher.

You would be ok with your heating load as the design flow rate might only be 4 to 8 GPM, which would flow well thru a tankless heater.

Will you use a heat exchanger, or are you going to run your potable water thru the floor. If this is a garage, then what about freeze protection. Would you need a gylcol solution in it. ?

So many factors to doing the job and doing it right
 
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Old 11-28-12, 07:41 PM
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The ondemand hot water heater i have is a 199,000btu unit, am i correct that this will be plenty, actualy overkill? Will it work?
It sure is enough BTUs!

Have you had a chance to look for that ASME rating yet? Not that it's any guarantee of anything, but it would be nice if it were rated by them for the application.

But now here's the 'rub'... you said:

I have 936 sq. ft. to cover
AT BEST, you might be able to get say 35 BTUH per square foot with a radiant slab. This is probably going to be with closer spacing than you had planned (and been told by some person?).

So, let's round up to 1000 sq ft and multiply that by 35... hmmmm... only 35,000 BTU of heat emission.

This is the caution I made early on that you need to be sure your slab can put enough heat into the building to counter the heat loss.

Yes, the program DOES add some 'padding' into the total numbers, so it's 'OK' to cut it close, but you will be cutting it VERY close.

Fact is that once you start with floor coverings and adding furniture and things, you begin to limit the heat output.

Personally, I would not rely on 35 BTU / sq ft... I think 25 is a more realistic figure, so now you are down to 25,000 BTU of emitters to counter a calculated loss of 38,000.

Check your wall and ceiling 'factors' carefully. Make sure they are realistic.

I notice that the floor factor is 0.69 ... is that what the program suggested for a radiant slab?

Also, make sure you are using the correct 'design temperature' for your location.

You might consider some supplemental fin tube baseboard selectively placed under the windows and such to make up the deficiency.
 
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Old 11-28-12, 07:45 PM
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Here's a 'trick' you can do...

The program specs the feet of fin-tube based on the water temperature you enter in the 'setup' screen for the job.

You can go back and diddle that number experimentally.

For example, fin tube is rated around 550 BTU with 180 entering water temp. Since you are running a lower temp for the radiant slab (the water shouldn't be above 120 for radiant apps like this) if you change the water temp to 120 you will see the recommended feet of fin tube change.

Next, figure on the 'theroetical' 25 BTU/SF for the radiant slab, and take that away from the amount of fin tube recommended for each room.

The REMAINDER of fin tube is what you would have to add to supplement the deficiency in the radiant slab.

Did that make sense?
 
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Old 11-28-12, 07:54 PM
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NJ, you can have higher BTU outputs than 35, but it's the surface temp that becomes the limiting factor. Should you allow the surface temp to get to 90 or 95 F then the outputs can be quote high. A garage is an area where one can let the surface temp get higher as you tend not to walk in a garage barefoot or with light foot wear.
Also, tube spacing tends to determine design water temp, the tighter the tube spacing the lower the water temperature. I have designed entire floors at 6 or even 4 inch OC spacing. This drives the water temps way down, perfect for heating with geothermal heat pumps.

But you may have already known that :-)
 
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Old 11-28-12, 08:54 PM
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but it's the surface temp that becomes the limiting factor.... walk ... barefoot or with light foot wear.
Right... good input... which is why I'm questioning the 'authority' that told GP he didn't need the closer spacing in the field of the slab. I believe he DOES need the closer spacing to get the output he is going to need and not have his feet swell up like baked potatoes!

He needs to be able to heat the home with floor surface temps no more than 85.

I grew up in the 50's in one of the early radiant heated homes built after the Korean war... a Levittown development but not in Levittown.

Let me tell you, that was the most uncomfortable home as far as winters went. Some rooms were so freekin hot on the floor that you couldn't keep your feet on the floor for more than 10 minutes. Other rooms were downright frigid.

I don't like to see people make the same mistake they made... again and again...

So I'm hoping that GP will go with a tighter spacing and lower water temps... but I'm never overly optimistic about heat output!
 
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