DIY Radiant heat over existing slab

vigilanye08

I have a 1000 sq. ft pole building on my property that I am starting to convert into a 2 bedroom rental. The building has a existing slab what is the best (easy, cost effective, efficient) way to add radiant loop on top of this existing slab. My outdoor boiler will be heating the water. I plan to polish and stain the concrete and leave it as the finished floor. Height is a concern the ceiling height is 8 ft. I don't want to loose to much of that but understand I have to loose some of that. I plan on doing all the prep work and plumbing but having a local mason pour the concrete. My main questions are

1. What kind and how much insulation do I need between the slabs.
2. How thick should the concrete be and what kind of concrete
3. Re bar or mesh
4. Should I frame out my walls first then pour the concrete around them or pour the slab and attach the walls to the slab. If the later of the two how do you attach the walls without a risk of damaging the pex.
5. What size pex and what spacing

Thanks for the help

Ironman

Have you done a heat loss calc based upon an approved method like ACCA manual J? Without that, there's no way of knowing what to size anything.

You cannot hook the outdoor wood boiler directly to slab: the water temp from it is way too high and will crack the concrete. It will also overheat if the water temp is not controlled with a smart mixing control based upon outdoor reset. The in floor piping system will also need to be pressurized so you'll need a heat exchanger between it and the ODWB along with the necessary ancillary items.

Also, IDK of any jurisdiction that will allow a wood boiler as the approved primary heat source.

You will definitely want insulation between the slabs. Your local jurisdiction can tell you what's required. I would think that an R10 would be the minimum. Federal energy code now requires R15 and it's being enforced in many locales.

Proper engineering and design is crucial to making a radiant floor work right and the load calc is the foundation for that. There's a lot more to it than just laying some pipe down.

Gordy

On top of what @Ironman said to get proper insulation 2" xps, and a proper thickness concrete over pour your looking at around 4". Can you lose that much head room?

There are other over the top methods.

vigilanye08

Dose the heating loops really need to be pressurized? I plan on putting a mixing valve. I will have to look into the local codes as far as insulation. Is it really necessary to pour 4 in of concrete over the top. it seams like having 8 in of concrete is a little overkill for a house

vigilanye08

I used loadcalc.net and came up with 16636 btu's heating

Gordy

The 4" assembly I was referring to is 2" of xps gives r-10 and 2" of concrete.

Did you do the heat loss figuring an insulated slab? At what r-value?

The radiant loop will need an operating static pressure of 12-psi.

Is that heat loss for the 1000 SF mentioned?

vigilanye08

Yes that 16636 btu's is for the building after i re insulate it and add double pain windows. Dose this figure seam reasonable. The calculator had me put in the lineal ft of the slab which is 120. It also asked if the slab had edge insulation which the existing slab dose. So i put r-10.

I guess i misunderstood the 4" being a total. That seams reasonable.

Is the xps insulation just the pink foam i am use to getting at the building center?

Ironman

16.6 buts per square foot is not unreasonable for well insulated new construction depending on your locale. What did you use for design temp?
Please note again: a thermostatic (dumb) mixing valve is NOT the correct device for controlling water temp to a slab. You need a smart valve or variable speed injection mixing based upon outdoor reset or else the slab will severely overheat the space. Go to Tekmar's site and read their essay on variable speed injection mixing for a detailed explaination.

delta T

Agree with Ironman, injection mixing is probably the best way to go for a wood boiler. DON'T use a standard thermostatic mixing valve for this application, you will not be happy with the results. Here is a link to the essay referred to above. Proper design is key to making this all work, and more importantly work for years to come.

http://tekmarcontrols.com/images/_literature/e0021_06.pdf

vigilanye08

Thanks i will look into injection mixing.

As far as location Northern MN it can get as cold as -40 around here. The program i used had me put my location and city. I live in the woods so the closest town was 60 miles away. They used inside temp as 70 and outside temp as -21.

any suggestions about these questions they have not been answered yet.


3. Re bar or mesh
4. Should I frame out my walls first then pour the concrete around them or pour the slab and attach the walls to the slab. If the later of the two how do you attach the walls without a risk of damaging the pex.
5. What size pex and what spacing
6. should there be insulation on the outside edge of the new slab.

thanks again you guys are defiantly helping me out.

SWEI

My answer to all but #6 above would begin with "It depends."

You should seriously consider hiring some building design help to go with your heating system design.

Ironman

I concur with SWEI that "it depends". But with a 2" pour, I can't see using rebar or mesh. Use 4000psi concrete if more strength is needed.

I'm a little skeptical of a 16 btu per square foot heat loss in N MN with a -21* design temp. Still, a properly designed in slab system should be able to deliver 30 btus per sq. ft.

40 to 60% of slab heat loss is at the perimeter. Does that answer #6?

A note on injection mixing and your setup: the heat exchanger would go in the injection bridge between the injection mixing pump and the floor loop.

vigilanye08

It sounds like to get the real answers to my questions I need to find someone to design a system for me. The risk of designing the system wrong will not just changing a few parts out it could be ripping up the floor and changing everything. What is the best way of finding someone to help me design the right system for my project?

Gordy

Go to find a contractor on this site, hopefully someone will come up close to your location.

Ironman

P/M me if you want me to take a look at it. I'll need detailed plans.

4Johnpipe

After a heat loss...you can install radiant ceilings...sheetrock is about a .8 R value easy to get through...