Staple-up hydronic heating: air cavity or aluminium exchanger?
I’m getting ready to install the radiant tubing in the floors (staple up from the bottom to the sub floor in between the joist bays). I was planning on getting aluminium heat exchangers, as I imagine they are more efficient for heat transfer. However, I don’t like the added cost of the material and labour. I spoke with my local Upanor rep, and he mentioned that without them, I’d need to do an air cavity at the top of the floor joist bay.
Then I found this article by Big JL stating that the air cavity method (without radiant) makes for warmer floors. https://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance
Not sure what the best option is, obviously not adding aluminium is cheaper and quicker for install, but getting some conflicting information here as to what actually more efficient and I’m curious.
Does anyone have any comment or experience with this?
Jamie
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Replies
Jamie,
Heat transfer plates rely on conduction to carry warmth to the floor, while suspended tubes rely on convection and radiation through the air space. This is less efficient at transferring heat, and will require hotter supply water - in the neighborhood of 160 to 200 degrees F.
Both staple-up and suspended tubes traditionally use an air gap (with insulation and a radiant barrier installed below), so the discussion from the building science paper you mention applies to both.
You also need to support the tubing every 24 to 30 inches, which involves additional labor and material costs.
One of the best sources for information on radiant systems is "Modern hydronic heating: for residential and light commercial buildings" by John Siegenthaler. You can pick it up used for less than $20.
I dont know about efficiency, but last radiant house I did was staple up tubing for the main floor, basement was regular embedded pex. Prior to that hVAC guys pushed the plates so thats what I used. I was skeptical of the staple up tubes, seemed like the poor mans radiant but the homeowners love the floor and said bills were not out of the ordinary, and if these people love them, it must be good cause a texture booger and this lady would be complaining. It was like a rubber hose and they stapled them up with a wide crown staple gun.
Extruded aluminum heat transfer plates do not require nor should they have an airspace . All the heat that the tubing emits should be concentrated to the plates and into the floor through conduction . Conduction is king .
Higher temp water will lower your boiler efficiencies also . The colder the return water , the higher your efficiencies go . Insulation below should be a minimum of R19 and should be installed tight to the bottom of the sub floor .
If cost is an issue and high temps and efficiencies lower than 90% do not bother you , you should probably consider economical radiant ceiling design which uses higher temp ( 169*) and a very small portion of the ceiling as the emitter , that portion is on the perimeter and is about 10 - 20 % of the available ceiling , also done with extruded aluminum plates fastened to the bottom of beams or rafters and 1" strapping or 3/4" running parallel with the plates and tubes , R 13 would be sufficient above . If you chose to go with 60 - 70% of the ceiling it would be less expense than floors , lower water temps that are reasonably conducive to very good boiler efficiency and floors , walls would stay within 1-2 * of room setpoint temp .
Ceilings are actually more comfortable than floors . Not new technology , on the contrary , just a lost art that served many well for decades . until the available labor pool no longer knew how it worked . Don't ask if heat rises , what do we think the odds are that the top of the sun is always facing Earth or maybe it turns over in the winter twice a day .
Celings are also in Uponor's CDAM and in Siggy's MHH3 as mentioned earlier . You'll also find reference to them in Robert Beans healthy heating site , and I have done dozens of successfull ceiling jobs in homes ranging from 1,500 to 10,000 square feet . All end users are in love with them .
The BSC article is about unheated floors that aren't being used as radiators. Radiant changes everything.
For the same rate of heat delivered, suspended tube requires the highest water temperatures, followed by simple staple-up (no heat spreaders), then suspended tube with fin-type heat exchangers, then sheet metal heat spreaders, with the lowest being extruded aluminum heat spreaders. (Some of the better sheet metal types do somewhat better than the narrowest extruded type heat spreaders, however.) Only the suspended tube variants need any sort of air gap, to allow convection loops to operate within the cavity.