design help

knel

I have an old 20x20 detached garage turned workshop. (No garage door and lots of windows.) It has natural gas supplied to it. I'm having a new floor poured soon and would like to place pex in the slab. I've never done this before but am handy enough to DIY. I realize it is probably an unconventional setup for hydronic heat, being so small of a space. Hence, some basic questions:

At 1 ft spacing, should I run one 400 ft loop of 5/8" or two 200 ft loops of 1/2"?

If I also add a sink to the room, closed or open system?

Boiler or tankless water heater?

Also, what are ways I can keep the system efficient in terms of energy consumption? Model numbers to look at for the mechanicals?

Thanks in advance.

STEVEusaPA

You should do a heat loss. That will tell you pipe size, loop length, flow rate and temperature.
You’ll never do a 400' loop in almost any scenario.
You must insulate underneath the slab and the edges.

knel

Thanks for your reply, Steve! I calculated design loss at 10,665 Btu/hr.
Any pointers on tube size, loop length, spacing, flow, temp?
Pump recommendation?
Heater/boiler?

SuperTech

Use a boiler.  It has a heat exchanger designed for your purpose.  A water heater is not designed for space heating, flow rates and delta T requirements are completely different.  Always use a closed system, a combi boiler might be a good option if you have hot water requirements.  
The design process does start with figuring out heat loss of the space. 

Zman

If it is just basic heat for a little garage, in my mind you can justify going cheap if you like. It's not like anyone is freezing if it goes down occasionally.
A 40 gallon residential water heater will handle the job easily for a small fraction the cost of a full boiler system. I agree with the folks that say that boilers are the "right" way to do this, however.... if it was my money and all I was trying to do is keep a little shop warm, I would spend the money on insulation (especially under the slab and slab edge) and quality tubing and then put in a simple heat source.

Ironman

Good O2 barrier pex usually comes in 300’ or 500’ rolls. Why not just get a 500’ roll and run two 250’ loops at about 9” OC? Make sure you use O2 barrier PEX, PERT or PAP, not common plumbing PEX.

Use the smallest mod/con boiler you can get (probably 50k btus) as even that will be oversized. Get the ODR curve setup properly for a radiant slab.

Get a good boiler trim kit like a Caleffi and a good manifold.

Make sure it’s well insulated under the slab and around its edges. 60% of a slab’s heat loss is at the perimeter.

Have the boiler setup and commissioned by someone who has a digital combustion analyzer and who knows what they’re doing.

Proper design and setup is the key to getting a radiant floor that properly performs. Anyone can lay down PEX.

knel

Thanks all, I can appreciate the different approaches as it gives me more options to look into. Definitely not skimping on insulation under slab and perimeter since I'm in IL. Kinda wish there was a low modulating boiler for maximizing efficiency.

For now, it seems I'm safe to run a couple of 250' loops with closer spacing at the walls, given so many windows. When the time comes, I can decide on the rest of the setup and have a pro dial it in.

Ironman

Rinnai has a nice little 50k btu combi.

knel

Looks like maybe Rinnai discontinued the 50k in favor of a 60k.

So, if I were to find an older model which is 47000 BTU DOE, does the 6.25:1 turndown actually put the minimum at 7,520 BTU? (=70% of design day conditions)

Does this sound like a reasonable solution? Are there other low modulating boilers out there?

Alan (California Radiant) Forbes

Lochinvar used to have a Cadet CDN040 boiler (8,545 BTU-37,600 BTU), but it's no longer in production.
I'd go with a water heater. There are too many arguments against a boiler.
These guys have some kits for small jobs:
https://www.janesradiant.com/hot_water_radiant_systems.htm

hot_rod

With X amount of dollars I'd spend the most on a good slab and edge insulation detail, 2" minimum. That is an investment that pays back for the life of the building.

Next I would tube it at 9", maybe even 6" on center for a small job like that. That allows the lowest possible supply and quickest response, for maybe an additional coil to tighten spacing. Both these are a one shot opportunity. 3- 300' 1/2" loops.

I too would use a water heater, they are self buffering, trouble free, maybe upgrade to a mod con or combi if you want DHW, later when $$ allows.
With tight spacing and low SWT, maybe an A2WHP is in your future when the government starts giving them away under the fossil free programs

knel

Thanks Hot Rod, I'll be placing 2" under slab and 2.5" high density EPS at the edge.

Never thought to space at 6", but for not a lot of added cost (and with future-proofing in mind) it seems like the way to go.

Any other immediate considerations to make a later A2W retrofit work?

hot_rod

We have been beating the low SWT designs for some time now in Idronics. It stared with the solar thermal tie in and optimizing the low winter time solar temperatures. It plays well with future heat sources like A2WHP.

hot_rod

Also, keep the tube up into the pour as best you can, that will also lower required SWT
https://www.pmmag.com/articles/87540-depth-perception

knel

Hot Rod, Again, thank you. I've read more than once that raising the tubing doesn't make a difference, but those numbers speak for themselves. Thanks for the resources!

knel

A follow-up and a question...

I went with a three-loop spiral counterflow layout, given the high heat loss on all sides. Loops are on 6" spacing at just under 250' in length. Ended up using 1-1/2" plastic mesh-ups every 1.5 ft to lift the remesh and tubing up into the 4" slab.

Getting very close to the pour. My question has to do with in-slab sensor location. I've put together a "sleeve" from leftover pex with a few-inch scrap of copper tube with cap and the end for quicker response. I know to place it between the tubes, but should it sit between the supply or return end of the circuit? Same depth as the tubing? And how far from the wall?

Paul Pollets

At least 4-5 ft. into the room, and not next to the manifold. It doesn't matter which tube it's next to. The overpour will spread the heat output.

GroundUp

Do you have time to tear it up and start over? That is a very poor layout in terms of temp balancing. The outermost (red) loop will be diminished of heating ability before the fluid is even halfway through it due to all the exterior wall exposure, and be reheated by the other loops as it goes further into the slab which reduces the output of those loops as well. Now you will have hot and cold spots across the slab. The exterior exposure should be balanced across all loops.

Zman

Ideally the sensor would be between the blue and blue dashed lines as far as you can from the manifold, and near the surface. A copper sleeve will work nicely.
The 6" spacing and being elevated in the slab will make for nice even heat. You should be able to get a delta of < 10 degrees which will also help even things out. Your graphic does make me a little dizzy when I stare at it too long.

BTW, this will be one of the most carefully designed garage slabs of all time. Nice work!

TAG

I have to think about that layout as well -- would have done a counterflow. With ODR and constant flow my guess is it will not matter .... otherwise -- it will respond unevenly.

I have had the best luck moving a tube away in one spot and getting the sensor alone. My first slab in a sun room was centered between two and it was still too quick to heat ..... I used it as a minimum setting sensor.

knel

Thanks all for the feedback and tips!

@GroundUp genuinely curious how you would have done it. Keep in mind slab edge insulation of R-11.5.

My understanding is that counterflow offers the most even distribution, esp. when heat loss is even on all walls. And running warmest part of three loops around the perimeter first seems equivalent to closer tube spacing applied along outside walls/windows to deliver more heat where there is the most loss.

Eager to learn.

GroundUp

knel said:

Thanks all for the feedback and tips!

@GroundUp genuinely curious how you would have done it. Keep in mind slab edge insulation of R-11.5.

My understanding is that counterflow offers the most even distribution, esp. when heat loss is even on all walls. And running warmest part of three loops around the perimeter first seems equivalent to closer tube spacing applied along outside walls/windows to deliver more heat where there is the most loss.

Eager to learn.

The idea with this type of radiant is to have an equal heat output from each loop. The exterior wall is always going to be the coldest spot in any slab, so essentially you'd want to balance the exterior exposure evenly between the loops. The way you have it, that red loop has now taken the full brunt of the cold exterior and has already given up all its usable heat by the time it leaves the perimeter and has nothing left to give as the rest of the loop is run. For example, let's say you have a 100 degree supply water temp to the manifold. By the time that red loop reaches the end of its perimeter duty, the water temp may be only 60 degrees while the other loops are gradually warmer as they head toward the center of the building. Now as your red loop progresses toward the center, the water in it is already cooler than the surrounding slab being heated by 90 degree water from the other loops so it is slightly reheated by the other loops before returning to the manifold but hasn't actually given off any heat since 75ft into its journey. The entire path of your red loop after it leaves the perimeter is a cold stripe in comparison to the other loops, getting closer to temp matched as it gets closer to the return manifold.

To balance it evenly, you would have added up the linear footage of the perimeter and divided by the number of loops. If the slab is 20x20, you have 80 linear feet of perimeter and would then divide by 3 to get 26.67. Ideally, each of the 3 loops would serve 26.67 linear feet of the perimeter instead of one loop serving 75 feet.

You'll probably never notice the difference, but it should have been done differently in order to be ideal.

Alan (California Radiant) Forbes

I like your layout and think it will work fine.

I would have done three independent, counterflow spiral loops because that is how I learned to do it. Your way would never have occurred to me and I think it's great.

knel

Thanks, @GroundUp. It makes a lot of sense on splitting up the perimeter equally between the loops. The configuration would seem pretty straightforward with two loops. However, I can't envision what that would've looked like with three loops, and still keeping the loops the same length.

Was curious to see just how cold the edge of the slab will get in my situation, compared with the rest. Ten degrees colder? 20 degrees? Here's modeled with 25 °F outdoor and 70 °F indoor in a convection-type heating scenario.



There's a 4+ degree difference from the first loop to the second loop. And a 2+ difference between loop two and three. I'm not sure how much this will throw off the balance, but would only be emphasized on design day.

What this did make me realize is with my current setup and first tube centered over footing, I'll be unnecessarily heating the bottom of the 2x6 sill plate.

So, while I won't be tearing it up and starting over, I'll be moving the first lap of each loop inward a bit. An easy fix. At the new placement, there is only a couple of degree difference between loop #1 and #2.

hot_rod

I like that tube layout, it certainly takes some thought to get it to fit in. you have your 3 supplies going to the highest load area of the slab. i predict a nice even floor temperature with very little striping effect.
Pex with a copper end for the sensor, bring the pex to where it is easy to access that for installation and sensor replacement.
I used to take the sensor pex tube up to an electrical box in the room so the sensor ended up at the thermostat location.

knel

Great idea, @hot_rod and one I'll implement.

The pour went well. Glad I came here and didn't go with my original intentions for tubing.