Hello everybody, I am new to radiant floor heat systems. I am in NE Ohio where winters can get pretty brutal. Sometimes many days on end of below zero temps. I spent months researching and talking to different companies about what I wanted to build. Seems like everyone had a different idea of what I should do. Here is a full break down of what I ending up going with and I have some questions about my system. I had a 64x48 shop built with 14ft ceilings. 2 12x12 2 inch thick insulated doors. I installed 3 inch foam board (R22 R value) under the slab. I installed 3000 ft of 1/2 oxygenated pex tubing (300 ft runs) with 10 circuit feed/return manifolds. The concrete is 6 inches thick. The shop has Owens R30 insulation in the walls and R50 blown in the ceiling. Building is full metal finished inside. I basically could not have went with any better or higher insulation in this shop to make it as energy efficient as possible. I didn't have an option of natural gas and I did not want a huge propane tank outside my shop so I went with electric. I went with hydro-smart out of Minnesota for my system. They built a panel with 2 pumps with all the required valves, etc. I went with an 81k btu hydro-shark instant boiler. The building seems to heat fine but it hasn't been real cold yet. (high teens) I have the temp set at 60 and the other night it just ran and ran at 58 degrees and didn't want to hold 60 like it has been on the warmer nights. There are several questions I wanted real world installers, users like yourselves opinions on. There seems to be a HUGE difference of opinion on glycol and what % should be used. I was told I needed 50% ratio.. some say that's way to much and will kill the heat transfer and make the system pumps work much harder, etc. I wound up putting in 15 gallons of cryo-tek glycol and 23 gallons of distilled water in and pumping this into my system. Which left me about 4 gallons left in my fill tub when finished. That ration puts me in at just over 39% of glycol in the system. Is this to much, not enough? I cant seem to get more than 70-72 degrees of water return temp no matter what temp I heat the water to or how long it runs. I currently am sending it at 125 degrees to the system. Does this sound normal to you? Also the pumps have 3 speed settings.. should the water/gylcol be sent through at max speed? Medium? Some say slower some say faster.. Any insight is appreciated. I'm sure Ill have some more questions. Thanks in advance guys!

with 81 kBTU of input you need to push just over 8 GPM to hit a 20 degree temperature drop in a "Design condition"... you might be slightly underpumped for that target but you shouldn't be near 50 degrees drop for the long haul.

That said a cold slab on startup can absorb whatever you can throw at it for a long time sometimes, so if this is still in the first "startup period" you might just have to wait for the slab to warm up.

Faster pumping just uses more energy, in your setup it's unlikely that there would be any other issues. Feel free to try max speed if you haven't yet.

You can figure out your glycol requirement by looking at the ratings for the product you're using. You want BURST protection to something comfortably low, a bit lower than your typical design condition perhaps. There is also a "SLUSH" protection that you shouldn't really care about.. if you're in a condition where you are that cold, the system isn't running anyway, let it slush up. If you're confused Burst protection is the one that uses less glycol of the 2 types of ratings most products give.

Thanks for the reply Rob. The system has only been on just over a week and honestly the day we fired it up it was 50 degrees out for the first couple days which is way out of the ordinary and the concrete from several weeks of 20 degree cold was def ice cold. There is well over 60 yards of concrete there to heat. The 50 degree difference seemed outrageous to me as well.

I have left the pump on the #2 medium setting but maybe I will go ahead and switch to the faster #3 as you recommended.

The glycol numbers on the bucket are vauge.. they list requirements for about every time of pipe but plastic pex. It just seems like there is alot of different opinions on the glycol so I tried to get some what towards the middle of the road with them. I can change the ratio if needed fairly quickly so I figured it wasn't a big deal right away but if it's alot harder to heat a 40% ratio of glycol maybe that's adding to my major heat difference between the inlet and return.

We use a 40% mix for our snow melting systems here in Minneapolis, never in a residence or shop unless the owners plan to turn the heat off and come back on cold weather to turn it on again.

We just spent several thousand redesigning and re-piping a DIY hydro-smart/hydroshark system a few miles from here. One of the most dangerous I have ever seen.

Your boiler is 3 times the size you need.

Before starting any new slab-on-grade steel shop one should insist on an ACCA Manual 'J' heat load. This first step eliminates most of the people who should be selling radiant floor systems.

If they "specify" a 50% mix of antifreeze for interior spaces, they should also be eliminated.

If they suggest an electric boiler for space heating without the benefit of out door reset, keep looking for good help.

If they "require" the use of more than one pump to operate less than 2000 sq.ft. you are working with a pump salesman.

If they want you to use more than an R-10 under a radiant slab in a relatively mild climate like Ohio, tell them to get out of town!

A real radiant floor designer will advise you of the design/supply water temperature (SWT) and let you know that it will take awhile on initial start-up.

My own 40 by 60 steel building will operate at 58°F in the garage area and 72° SWT in the office driven by a EB-MO-10k Electro boiler made right here in Minnesota.

My insulation is similar in R-value to your save the extra inch under the floor.

Since over-sizing an electric boiler is not the end-of-the-world I think you did alright.

Merry Christmas to you and my hydronic hero Robisright!

Merry Christmas and thank you for the responce! Wow.. so what ratio of glycol would you recommend in my area? This system will run all winter and not be shut off unless the power goes out which is rare in the winter. Another wow from me on the size boiler. The ranges from every radiant floor company I talked to for my area of the country all said atleast 50k btu and a max of a 100k btu for this size shop. Most said the SH3-24 hydro shark would be ideal for my application and location. My building is 3072 sq ft and I was told it had to have two pumps. Id like to be able to turn this entire shop up to 70F if I wanted to and not struggle if its 20 degrees or colder out. The R22 foam board insulation under the slab was my idea. There wasnt a huge cost difference involved from the 2 inch to the 3 inch and everyone agreed it would just make things better keeping the cold ground away from the slab/pex tubing.

Here is a pict of my system completed. The steel walls will be finished/installed after I am officially done working any bugs out of this system.

The R22 foam board insulation under the slab was my idea.

What kind of 3" foam board gives you R-22? And, how much of your slab edge is exposed?

I can't rem the products brand name. Local insulation company has it. It has a backing on each side and is extremely rigid. Its stamped R-22 on it. I talked to a couple local shops who have used it under there floors with no complaints before going with it. I used Owens 2 inch foam board around the parameter on the wall edge of the concrete.

Has anyone done a heat loss calculation on your building? I'm wondering what they considered for infiltration.

How about an estimate on how long it takes a 60 yard slab to warm up from "cold" to 70F, was it?

Yes ICF, that would be an interesting and easy calculation to perform. However, we first need to determine all the variables… The slab is heated with ten (10) circuits each having a 300’ length. The supply temp is 125F. The slab is 6” thick. What is the total slab area? What is the initial “cold” temp of the slab? What is the indoor temp profile while we are heating the slab? What flow rate will be used while we are heating the slab?

Is the radiant pump pumping toward the flow meters or the non-flow meter manifold? If you had backward flow that would show up as zero flow in all the loops and would result in huge delta-T and underheating.

Your problem is not glycol. Even at medium speed that pump should easily dump the full output of the boiler at a 50 degree dt, and it should get the concrete up to temp, and bring the dT down to something closer to a design value of 20 to 30 at most.

You have 1500 cu ft of concrete here. An 80k boiler with no load could heat that up on average about 2 degrees an hour. You're not at no load, of course, but no one in america has seen design conditions for long yet so hey... call it a degree an hour as a conservative average value from initial startup. Even if the slab was 30 degrees you should be up to floor temp in 60 hours or less for sure, room temp not too long after that.

What are your flow meters reading when it's running?

Just to be clear I see a black box in the middle. Is that a UPS? Or, on the off chance, is it perhaps a variable speed injection control?

I cant seem to get more than 70-72 degrees of water return temp no matter what temp I heat the water to or how long it runs. I currently am sending it at 125 degrees to the system.

Well, assuming that the boiler "ran and ran" and there's sort of "medium" circulation, it looks as if the entire heat output of the boiler is actually going somewhere.  You just have to find out where.

Well, not necessarily. the modulating electric boiler he's using could be running at much less than its max output if flow is low enough, which is consistent with all his symptoms. All we know for sure at the moment is that its spitting out some volume of 125 degree water.

It's pretty unlikely that he's got a mystery load of 30k+ BTUs/hr consistently under a wide range of load conditions (pretty unlikely his design load is more than 50k.. ). It's also pretty unlikely that glycol (excessive or not) is somehow restricting flow enough in a 10 loop system to limit him to 50 degree Dt with full output for long.

It's likely he's got a flow issue. Flow meters cranked down too much, the actuator caps closing off loops, pump backwards, or air are the most likely culprits I'd say. Or if that is a VSI controller maybe it's configured incorrectly.

Also, once that is fixed, a bare slab in a building like this shouldn't need to get above 100 degree supply water temp under any conditions. 125 is pretty likely to result in room temp overshoot in a properly operating system of this type in a well insulated building, especially with a six inch slab. Maybe it's not a bare slab?

Agreed, use of glycol will only increase the time that it will take to bring this slab up to heating temp. Use of glycol will also reduce the operational efficiency of a hydronic radiant system. One should only use glycol when there is sufficient freezing risk to justify using it.

This is a similar situation to another post not too long ago. 300’ is about the maximum recommended circuit length for 0.5” diameter PEX. The supply/return delta T will be large even at high pump flow rates and this will further increase the time that it will take to bring this 6” thick slab up to heating temp…

its spitting out some volume of 125 degree water.

Maybe he could guesstimate what that pumping volume is. That might help him confirm the output of the boiler.

I wonder if the tubing was stuck right down on top of the R22 slab insulation. 6" down - that's a hefty slab.

Yes, I had the same thought ICF, but I really wanted to avoid a discussion about how dumb that would be, especially if there is any freezing risk as that would also maximize the PEX failure risk too.

It's interesting that the interior temperature got up to 58F. I wonder if he has a way to determine what the temperature of the top of the slab was to get that large building that warm?

And, how much infiltration does a 3000 sf metal building have? Is it better or worse than a home, I wonder?

he's got flow meters on the manifold, no need to guess, he can (and should) read the flow. That will answer a lot.

That pump SHOULD easily be able to push well over 0.5 GPM per 300 foot loop in this case, even with glycol, on medium speed. It should be near 1 GPM a loop on high speed. Problem is not loop lengths, glycol, or pump sizing.

Loops low in the 6" slab would be a no-no, but it also would not explain his Dt if his flow is doing what it should do, that would manifest as "improving" loop DT as the bottom of the slab heated up, and but still a slow startup: btus are not transferred to the slab as quickly due to less surface contact between pex and the slab and the limited conduction path, and come back to the boiler instead of transferring into the slab. Problem is not loop position in the slab.

If the loops are open, pumped in the correct direction and not closed down or airbound, it would not behave like this and he'd be dumping the full output of the boiler into the slab. Obviously that is not happening. So it's definitely a flow problem related to the hardware execution in some way, not design or a lack thereof.

Well, unless he forgot to put up one of the walls or something and he's not telling us, I suppose.

I think he got to 58 because he does have SOME flow and his insulation levels are very high.

Very true, this would be a trivial calculation if we could get answers to allow knowing all the variables at play.