PEX Pipe in Residential Subfloor Thermal Mass

  • Thread starter DoctorLem
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Summary:
I am installing a Thermal Mass in the center of our home. We are looking to figure out how much PEX pipe and what diameter for proper thermal transfer.
Hello friends!

We are installing a future provision for a Rocket Mass-type woodstove in our home.

This is a future project but realized that we could take advantage of the open area between the footing and finished floor by adding an insulated block of concrete with PEX pipes for Heating and Cooling Thermal Mass. The intention being to charge the Mass with heat from outside boiler, plate collectors, evac tube collectors, ext. Assuming water temp would be around 100ºF entering Mass. Summertime use would be for cooling with a buried loop system.

The Mass material will either be concrete or rammed earth, isolated from the stem walls and footing with 2" rigid foam, capped with stained concrete at finished floor level.

The volume measures around 22" high, throughout, 20ish' wide and between 3-6' deep, depending on where you measure in the "T" shaped foundation. This will be around 2.7 cubic yards of mass plus the top slab (the eventual fireplace will rest on the top slab).

The main questions being:

-What size PEX would be preferable?
-What amount of linear feet would I use to get maximum thermal transfer?
-What water velocity would I need to plan on achieving for water flow?

Thanks for any assistance in finding resources!
 

Answers and Replies

  • #2
jrmichler
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Thanks for any assistance in finding resources!
It might be too late, but a better approach is to insulate under and outside the footings and basement walls. My own house has 4" foam insulation under the footings and outside the crawlspace walls.

Resources:
Search hydronic heating system design to find a number of good hits. Amazon also has a number of books that look good - search hydronic heating.

Short version is that you select the total length of tubing based on the amount of heat transferred in BTU per hour, then select the tubing size and configuration to match the circulating pump. The configuration is the mix of series and parallel in the tubing layout. You will need to study resources from the above searches to learn about pipe pressure loss (AKA head loss), and pump curves. It's good practice to design for minimum pump power. Good search term is pressure loss in pipe. Lots of good hits with those words.

A very rough guess is that you will need at least 100 feet of tubing in the concrete, and possibly as much as 500 feet. More tubing gives more heat transfer and requires more pump power, unless you make some parallel loops. There is an optimum tubing length, but it's a broad optimum, so you do not need extremely detailed calculations.

Any water velocity will transfer heat. Higher velocity has a higher heat transfer coefficient, but requires more pump power. Lower velocity will require more tubing, while allowing a smaller pump. Pump power adds up. It can be a surprisingly large portion of your electric bill. My personal preference would be for more tubing and a smaller pump to save energy.
 
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These are all great considerations! As much material as it will take to complete this, I'm hoping that we can have all of the considerations in view.

I'll keep on the rabbit hole of internet research, adding your search terms.

I'm thinking about 4-5 circuits, for redundancy, if there is a failure, as well as to have gradient heating, and a circuit at the under floor level.

I'll attach a few pics for more conceptual reference. The Thermal Mass will be within the [yellow] stem wall and [cyan] existing foundation and on top of the [orange] footing. This volume will isolated with 2" rigid foam.

Thank you kindly!
Screen Shot 2021-06-02 at 5.26.19 PM.png

Fireplace Foundation- Iso View.png
Fireplace Footing- Dimentions.png
 

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  • #4
berkeman
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Summary:: I am installing a Thermal Mass in the center of our home. We are looking to figure out how much PEX pipe and what diameter for proper thermal transfer.

adding an insulated block of concrete with PEX pipes for Heating and Cooling Thermal Mass.
Sorry for the dumb question (I'm an EE, not an ME), but PEX is plastic with low thermal conductivity so why wouldn't you use metal piping for this thermal project? I'm not sure what type of metal piping is most compatible with embedding in concrete, though.
 
  • #5
jrmichler
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Not a dumb question at all. The total heat transfer includes the film coefficient from water to tube, through the tube, contact coefficient from tube to concrete, and heat transfer through the concrete. The heat transfer through the concrete is a function of the thermal conductivity and thermal mass. Concrete has relatively high thermal mass, so the tubing needs to be spaced fairly close together. When the tubing is spaced closely enough, there is enough heat transfer area that the thermal resistance of the PEX has little effect on total heat transfer. Related factors include low cost and smooth interior for better flow.

It's a system optimization situation.
 
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