radiant heat in small addition, question
Background:
My house was built in 1975 with a hydronic baseboard (180 deg) heating system. The roof and wall insulation are R-19 and R-11 respectively. I am building a very small entryway addition of 70 square feet that will have insulation of R-38 roof (ceiling), R-19 walls, and R-35 under the floor slab. The floor slab will be a 3-1/2 inch concrete slab with a slate tile finish surface. I want to install hydronic radiant heat in the slab and run it on the same zone as the living room that the new entryway will be open to.
Questions:
Is there any compelling reason why I shouldn’t run loops of 3/ 4 inch copper in the slab as an addition to the living room zone (operating at 180 deg) ? My Dad built his home 50 years ago with this type of heating in the floor slab with no problems to date.
How many linear feet of 3/ 4 do you think would be required for this scenario?
Thanks…Tim
Replies
Don't run copper down there... it's way to expensive and corrosion-prone. Instead, invest in Dan Holohans Radiant heating primer and then set about putting in ½" PEX tubing on 12" centers with loop lengths of 250' or less. With 12" centers, you'll need about 1' of PEX per sq. ft of slab. Don't forget to add 10% for manifold runs, etc.
I would run the slab on a different supply temperature because the heating requirements are totally different. Look into the iSeries Taco valves for a simple way to put that zone on a different water temperature without great expense, for example.
I would be surprised if you got a huge benefit from insulating the slab to R35. Most people I know don't go thicker than 4" at the edges with XPS. I used 2" throughout my home, for example. If I'm to believe my heat loss program, the walls, windows, etc. are a much larger source of heat loss than the floor slab in the basement.
I'm still thinking that running 30-40 ft of 3/4 copper in the slab is going to be a lot less expensive than doing a new zone and mixing valve (boiler is 40 feet away). Refer to this PDF research paper on corrosion of copper in concrete, http://www.copper.org/applications/plumbing/techcorner/pdf/corrosion_embedded_material_other_than_reinforcing_steel.pdf
"In general, copper is not corroded in concrete, although it is reported that
thin sheets can become perforated if soluble chlorides are present. Copper
pipes are used successfully in concrete.
Very little systematic work has been reported on the behavior of copper and its alloys in or in
contact with concrete, probably because these metals have given satisfactory
service under such conditions. Rabald [9] says that copper, brass,
red brass, bronzes, aluminum-bronze, and copper-silicon alloys have good resistance
to corrosion in concrete."
Any thoughts on this??....Tim
As Dan Holohan points out in his radiant primer, it's pretty difficult to screw up slab installations due to the conductivity of concrete, the mass, etc.
Nonetheless, if you want a floor without striping (i.e. hot and cold zones), you're better off putting down PEX as described above. Considering how little the stuff costs compared to copper, I'd be surprised if the PEX wouldn't just be more elbow grease to get installed. Besides, you'd probably need to insulate ¾" copper at 180°F in a 3.5" thick floor or you'd develop hot spots capable of burning bare feet.
I have read too many accounts of people in Levitton and elsewhere with Cu&slab-based radiant floor systems having to abandon them for baseboard, etc. because the Cu developed a leak. A research paper from 1966 paid for by the Portland Cement Group and hosted by a Copper Promotional organization won't change that view either.
Anyway, best of luck.
Edited 9/30/2005 4:43 pm ET by Constantin
Hey maybe I should have taken pictures of all the failures in copper pipe installed in a slab that I have had to fix over the last 25yrs. But hindsite is 20/20 I suppose.
& I wonder why ALL my inspectors make us protect copper when we run it in concrete usually with rubitex.
I will add one last thing for Tim it's not just corrosion but expansion of copper when heated is a lot more than concrete---- I wonder which one is going to break first copper pipe or concrete?
Any failures noted when wraping with rubitex? what thickness?
Tim
Hey Tim I have not sean any failures when a pipe was wrapped except one.
It had something to do with some type of adhesive that was used. We couldn't find why or where the adhesive came from.
We only use 1/2" thick insulation & that is mainly so no forces are pressing on the pipe. To be honest the pipe can handle alot of strain it's the joints that usually fail.
What's the downside of running of running PEX at 180 degrees?
Tim
With 180°F hot water running through a shallow, well-insulated slab, I imagine that, at the very least, you'll get striping though burning people's bare feet will be a very real possibility as well. Nevermind the challenge of trying to prevent a slab full of 180°F hot water from over-shooting the setpoint. Or the cracking that may result due to the constant termal cycling.Look, you don't have to believe me. I invite you to go to the Radiant Panel Organization or the HeatingHelp.com web sites and pose the same question in their forums. I predict you'll get the same answer, though the likes of hot rod, et. al. may even tell you at what temperature you'll be burning people's feet when the tubing reaches 180°F at steady-state.After all, in most homes, the maximum necessary supply temperature for an insulated concrete slab is around 110°F, or less.
Edited 9/30/2005 8:21 pm ET by Constantin
ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy...<!----><!----><!---->
84 deg F recommended maximum floor surface temperature.<!----><!---->
Link below takes you to a web presentation on indoor environmental quality, which discusses such things as floor temperatures, humidity, temperature stratification, drafts etc.<!----><!---->
http://www.healthyheating.com/Thermal_Comfort_Working_Copy/HH_physiology_intro.htm<!----><!---->
This is some text from a report you can download from the presentation.<!----><!---->
"In the year 1966 an article, which was published in a magazine of the French Society for Phlebotomies, for example, informed about the dangers of floor heating systems in modern buildings. However, this article also contained information on the technological progress strived for in the area of floor heating systems and the difference between various types of floor heating systems. This critical essay subsequently suggested better cooperation between the heating technology experts and doctors. In the eighties, a new type of floor heating system was introduced in the market, which operated with a relatively lower water temperature (max. 28 'C) and could be controlled for each individual room."<!----><!---->
Almost 40 years ago it was determined that there is a health risk with floors that are operated too hot...<!----><!---->
Well Constantin has it dead on again with great advise.
Although I think copper pipe is the best thing on the planet.
Never NEVer NEVER run copper in a slab.
I spent my child hood & teenage yrs busting up my basement fixing copper pipe that my dad ran in the slab for radiant heat & the swimming pool too but I won't get into that fiasco at the moment.