regardless of the mechanism: slabs are often started before they are truly cured. the initial startup often is a much more intense process than any startup afterwards. good, bad, or indifferent, that's the way modern building rolls.

Concrete cures and then dries.

http://www.concretenetwork.com/vapor-barriers/water-vapor.html

"After the curing period, the slab begins to dry. At this point there is a lot of liquid water in the concrete pores—in fact, the slab is saturated. This liquid water begins to evaporate from the surface and if no additional water gets into the concrete, within about 90 days for normal-weight, 0.5 w/c concrete, the slab will be dry enough so that most floor coatings won't delaminate."

The case in point is not wether, but how. Many novices are educated by their peers and persist in heating slabs before cured. This is a mistake.

Take heed my warning about learning from your peers - no matter how desperate the circumstances - lest you go from frying pan to fire.

http://www.cement.org/basics/concretebasics_faqs.asp

Your scare tactics are not worth the time it takes to read them.

There are many peers out there as informed and educated and often with more knowledge than some so called professionals. There are many very good professionals and "peers" on this site. And there are those that have not a clue. I am not including you in that group but admit that you don't always have all the answers and you can learn along with the rest of us.

It has been proven time and time again on this very site that someone calling themselves a professional does not make it so.

All points well taken.

Of course, everyone knows who I am. My credentials and experience are posted. Many DIYers are misled in the beginning (say by self-serving advocates of "open" systems and other such nonsense) and soon find themselves at the mercy of perhaps good intentioned but still ill informed volunteers.

I only raise the question for the wise man to consider.

 I actually agree with part of that statement. I agree that certain suppliers push systems( the open system) to DIY's as an easy fix and to sell their product and bypass the installer. I agree that they do this most often to the detriment of the DIY who does not have all the facts and does not do their homework. It is a disservice to the industry.

We all know that I don't agree about open/closed but that is not  a factor in this thread.

We might be friends yet:).

Like, I have said before, I bet my honest and up front personel experience with radiant systems sends more people to the installer route than the DIY route!

all right you two, knock it off...your worse than the kids...LOL...thqanks a bunch for your input. and you are right, theres a lot of things you need to know about how my system operates before we can make judgement. I poured my concrete about 18 months ago, so I was sure it has cured.

But the professionals that designed my system told me I could heat it right away, in fact they preferred it that way. I didn't have the water hooked up, so I didn't instead I ran a propane heater and never let the heat inside get below 50, of course the slab and the ground are going to be a lot cooler. the major conceren was the way it heated. 2 lines running the system at a time heated fast and great, however with 4 lines running I could never get more than 70 degrees out of the system, even after running it for more than 4 days straight. I live in a doublewide next to the house, so I didn't really "cycle It" any different than it would cycle itself...

My other gripe is the profesionals that designed it, although they said they could be in contact by their tech support line, really didn't and couldn't help me out. I've gotten a lot more info from you guys, and I do appreciate it.

I have the system running, and again I have gotten the water temp up to 72 degrees so far, but its been running for 3 hours. after a couple of hours I turned the input heat down to 118...instead of 131...only because the output temp, seemed to be holding steady at 72.

J, with your takagi...how warm does the water get coming out of the slab after running a while? by the way, the flow rate is 2.8gpm, which seems like it is right where you guys told me it should be.

thanks again!!!

My return temp ranges from 65 to 95, depending where it is in the cycle of heating. You need to worry less about what the return temp is and more about the room temp. Let me ask a few more questions that will help us all help you. Is the slab completely isolated from the walls? Can you take surface temps on the walls and floor in different areas. I am not sure what kind of thermometer you will need but it would help to know if there are cold spots and what temp the surfaces are. Also the heat sink wall. The others can speak to the flow rates better. I have 4 loops on two zones that are each about 300 feet in length. I would not run only part of the loops on a zone. My flow per Zone in 1.1 gpm.( I did learn all that stuff when I installed it but it is gone to the far reaches of my mental filing cabinet at this time) My slab is thinner as well. Jill

Horrible situation. I was excavating my bathroom floor and broke one of the plastic tubes that are part of the floor heating system. Does anyone know how I can mend it? It started leaking and I turned off the water main, but there are a couple of holes in it. Is there a repair kit for the tubing, could it be mended with copper fittings? Any help would be greatly appreciated! Zara

Thanks for the responses.

I worry so much about the water temp coming out of the slab, because that is the only thing I have to gage the room temperature by. I know that if the water temp doesn’t get above 7o degrees theres no way I’m going to have a warm house. If the water goes in a 120 degrees, then I feel it should come out at 90 degrees after a while. If the water coming out of the floor is 90 degrees then the radiant heat will warm the room. But, as in my case, if the water coming out of the slab is only 76 degrees after 8 hours, its not going to keep the room comfortable.

My initial worry was the flow rate, after reading your posts, I realize my flow rate is fine. So the puzzling part is why I can run lines A, B, C, or D individually, or as pairs, and achieve high temperatures (87 degrees after 25 minutes). However, when I run any 3 or all 4 lines together, my highest temperature is 76 degrees after 7 hours.

Common sense tells me that if the flow rate is fine, and it doesn’t change (it remains constant at 2.7) then I should see a steady increase in the temp of the water coming out of the slab. After 6 or 7 hours of continuous run time, the slab should be a lot warmer than 76 degrees. I realize I have a large heat sink I must warm up. And I realize I shouldn’t be cycling the system. However I just can’t bring myself to let it run for long periods of time with an input temp of 131 degrees and an outlet of 76 degrees…that seems to high of a delta temp. Basically it doesn’t make sense that something is robbing that much heat from me.

I started the system yesterday at 2:30…I had 2 of the 4 lines open and circulating, the output water temp started off at 55 degrees. After 25 minutes of operation I had a temperature reading of 131 input, 87 degrees output, and 2.7gpm of flow. So I opened all 4 lines and waited. By 6:00 the temperature was holding at 76 degrees. 131 degrees input, 76 output, 2.7 gpm.

I decreased the input temp to 118 degrees and left it, I came back at 9:00 it was still running 118 degrees input temp, 76 degrees output temp. 2.7gpm.

Now from what everybody is telling me, I should just have to wait and it’ll start coming up to temperature. So that’s what we’ll do, but it seems to me that there should be a more gradual rise in temperature.

By the way, this isn’t a slab on grade. I’ve gone down 3’ to the footers and started pouring concrete and stone continuously up from there. The slab is poured within the footer/wall, about grade elevation….the floor is insulated, and I also insulated the perimeter of the slab where it comes in contact with the outside wall to isolate it. I have a thermometer in one of the rooms so I can monitor the room temp, and maybe it would be a good idea to get a few thermometers around on the floor to see if there are any cold spots.

Have you done a manual-J type heat loss calc on the structure?  Somehow 4 days of 131F water in and 70F out @ 2.8gpm seems like a LOT of heat:

2.8gallons/minute x 60min/hr x 8.34lbs/gallon x (131F-70F)=

        85.5KBTU/hour

which would be MY heat load when it's about minus 90F outside.

I don't care how much that slab weighs, it can't be big enough to soak up 4 days @ 85kbtu/hr without being able to fry an egg on it.

You say the slab is insulated?

BTW: Return temps of 72F are fine if it's keeping the space warm enough- is it?  The whole efficiency point of heating with radiant is that with such a large radiator it can deliver sufficient heat for comfort at lower water temps, and the cooler the return-water the higher the combustion efficiency.  The water temperature at the heater's output has an almost inconsequential effect on combustion efficiency as compared to return water temperature.  At 70-75F return water your Takagi is probably running about 85% efficiency, (compared to something like 82% with return water of ~110-115F.)  The amount of heat you deliver to the slab-radiator is a function of flow x delta-T (the arithmetic above).  Turning down the Takagi to 118F lowers the heat delivered to the slab, but doesn't increase the efficiency unless it stops the burner from cycling on/off.   The effect of turning it down may even be slightly LOWER efficiency, since below some fire modulation point the effectiveness of the heat exchanger begins to drop, and the stack temperature rises, meaning a greater percentage of the heat of the flame is going up the flue.  This is controlled to some degree by the smarts inside the Takagi that control the blower & modulation level, but the lowest-modulation probably isn't the efficiency sweet-spot. (In larger copper-tube boilers it never is, but their cousin water heater heat exchangers are designed slightly differently in order to tolerate colder input water temps without destructive condensation.  The efficiency sweet spot with water heaters is proabably somewhere in the middle-third of firing rate, whereas with modulating copper tube boilers it's when it's going flat-out high-fire.  Haven't measured it- just an educated guess...)

Bottom line, don't focus too much on the return water temp.   Is it the system the space warm?  If not, turn the output temp up, which will modulate the fire higher.  But if the burner is running at moderate to high-fire 24/7 without heating up the space, that heat has to be going somewhere, it doesn't just dissappear (the first law of thermodynamics) so figure out where.  Uninsulated slab on wet soil can be a real heat sink.  Get an infra-red thermometer, poke around and see if there are any cold spots on the slab, particularly the slab-edges.  You'll find that where the 131F water enters the slab the slab itself is well over the 70F return-water temp, and that where the return legs of the loops are the slab is well below 70F.  The amount of heat the slab can deliver to the space is a function of the average slab temperature, not the return water temp, not the heater output temp.  The amount of heat delivered to the slab is a function of the difference in temp between the entering & return water, and the flow rate (again, as calculated above).  If the heat is going into the slab but not the space, it's going somewhere else or the return water temperature will rise and the slab will get hot. Low return-water temps means that heat is being delivered SOMEWHERE.

I don't understand why you keep running less than all four loops?

Are they in zones? Do you have a layout?

I have two zones. Each zone is two loops. The two loops in a zone work together to heat one part of the slab.

Is your system not hooked up to a thermostat?

Zara- this really should have been posted as a new thread, but I'll jump in anyway. (Feel free to respond by posting a new  thread however.)

Assuming the tubing is crosslinked polyethylene (aka "PEX), there are systems for splicing & repairing it, eg:

http://www.wattsradiant.com/pdf/PEXFieldRepairs.pdf

If you're not sure what it is, have a pro look at it- they'll have a better idea as to what it is, and are more likely to have the necessary fittings, crimping tools, etc.  Last thing you probably want is an improvised DIY hack that fails in 2 years.

Bottom line, almost anything is repairable- just try to do it in such a way that it only needs to be repaired once.

I'm concerned that turning off the water main stops the leak. Is this an "open system", with potable water circulating in the heating system?  Or does the boiler have an automatic filler-valve to keep it from ever running dry?

Kenney, to address you point about your professional design, you never had a system designed. Design involves things like calculations and design fees. If you do not have actual calculations (or at least the results of those calculations) of heat loads, flow rates, and frictional losses, the system was not "designed". it was ballparked. While the professional design or lack thereof on this system is not really the issue, I just wanted to make sure it's clear: this system was not "professionally designed". In your case, you might not have needed one, I don't know.. this is a fairly simple setup, ballparking may have been defensible in your case (in others, not so much). But the failure here was not a failure of "professional design".

I would say the slab is warming up and the heat is radiating out the rest of the space / home.

I know you mentioned the two zones or four zones running. The reason the water is warmer running only 2 zones is the water is moving faster through the two zones so it doesn’t have time to give up its heat. When you’re running all 4 zones the water is a bit slower so the tubes have time to radiate the heat out in to the slab.

The only way to know is to get the floor up to a temperature you want with the space at the temperature you want and let it sit. I think the sticker shock or amount of heat of taking the slab from 55F to 75F is just that. There is a lot of energy there. As soon as the slab gets above the temperature of the air it will start to lose heat very quickly to the space above it. So if the slab is at 60F and the air is at 55F a good portion of the heat going in to the slab is "leaking" out to the air and warming the entire space.

Number wise this is a lot more like our garage setup. The garage is insulated as well but the floor sits about 45F all winter (unless the kids leave a garage door open). If I run heat out there it takes at least 6 hours to get the return above 55F, there is a real lag between the heat and running the zones, it’s a large mass. When it was really cold (-15F or -26c) I would run all 4 zones for about 4 hours, until I saw the return lines about 55F and shut it all off. The garage air would be about 30F and the next day it would be close to 40F.

Hello All,
Well this is interesting, here is my observation,
you said "heating 1000' of 3/4" (4 lengths of 250' each) pex tube in my 1300 sq feet"
I think you are light on pipe and heavy on concrete, I guess pipe in on base of slab on about a 15 inch pattern. what is sealing hight? windows, walk out basement, heavy stone walls around, you have a large mass to over come,
ball park btu 20 BTU sq ft X 1500sq ft= 30,000 btu hr 98,000 BTU gallon prop-20% for the stack on a flash water heater = 78,000 BTU per gallon burned @ 30,000 BTU hr = 1 gallon prop per 2.6 hours on normal operation after slab is heated.
Mind you this is not a scientific calculation, more a late night passing look,
Better turn it on for a couple days and see what the real damage is,
woops, its late
My error on sq footage, same formula on 1,300 sq ft will provide similar numbers,
Dan