Radiant Floor Heat; Maybe Not a Good Idea For Highly Energy Efficient Homes

[MountainDon]

In looking for some solar heating info I ran across this interesting article. For many homes radiant in floor heating is a wonderful choice. It's hard not to like warm floors in winter. However, this article points out that for many of the new 'green', very energy efficient homes radiant in floor heat makes less sense, maybe no sense at all.

http://www.oregon.gov/ENERGY/CONS/RES/tax/Radiant.shtml

To summarize the new energy efficient designs may not lose heat fast enough to make the large investment in the in floor radiant system worthwhile. One or more through the wall gas furnaces or a split mini heat pump, at a lesser investment cost may be better suited to highly energy efficient structures.

[JRR]

Good info.  But I'm a believer ... if the structure is so well insulated ... why not just downsize/downcost the radiant system ... go totally electric elements ... or use water "heater", rather than water "boiler"(article in Fine Homebuilding last fall covered this concept)?

[Shawn B]

Considering that for a average radiant heat system for a "average" size home can cost upwards of $20 thousand that makes sense. Plus if you live in a area that needs A/C you still are stuck installing either mini splits or a full ducted system and that adds more $$$$. Radiant makes sense in cold climates, like the Rockies, Pacific NW, inland NW, northern tier States etc.

For small homes and apartments I have serviced systems that used a decent high eff water heater (tank style) for the heating source and it worked fine.  Remember radiant systems are designed to keep the house a little cooler, but because of the radiant heat properties they usually keep the heated areas a more even temperature, thus it is not as noticeable to the occupants.

Also part of the "green" movement has brought forth a geothermal (heat pump) boiler for radiant systems. The output water temp is only around 95F-100F. The cycle time is long, by no "evil co2 emissions". The jury will be out on these units for several years, unfortunately many State and Federal grants are available and pushing this "green" technology, that is not yet extremely proven.

Also just wanted to add that if you go with a simple control system, no snow melt, no pool warmer combo, not too many extra zones that will save a bunch on the cost of a radiant system.

[MushCreek]

Maybe I'm missing something, but radiant in-floor heat looks very DIY friendly, unlike geothermal, which requires deep wells and sophisticated equipment. All radiant is is PEX tubing inset into the sub floor looped back and forth. A reflective surface is put behind it, and your finish flooring goes on top. Articles I've seen make it look pretty simple. Same with solar collectors; in areas with generous sunshine, a copper-piped collector is going to make a lot of hot water, fast. I'm not afraid to try it- the materials don't cost much, and if doesn't work well, it can just live under my flooring, and I'm the only one who will know it's there.

[Jeff922]

I put 600' feet of 7/8" pex in my slab.  It's very DIY friendly.  The cost of the PEX and the manifold with two zones was only $700.  Although I don't yet know the cost difference between a whole house hot water heater and a conventional one.

As a sidebar:  Radiant heat systems do have some problems.  They will work best in a well insulated home with few windows.  I know someone who had a radiant system put in there house and the system is costly and wasteful.  The reason is they have a huge home (in VT) with lots of windows.  Outside air temps in new england fluctuate greatly during the course of a day (ya know the old expression about running your furnace at night and the AC during the day?)  Anyway, the system struggles all night to heat the large thermal mass of the floor.  It is finally up to temp in the early morning just as the outside temp starts to rise.  It soon becomes too hot in the house and windows are opened to dump the excess heat!  

Truth be told, my Victoria's cottage has WAY too much thermal mass in its 6" thick concrete slab.  There is no way it will be able to adjust to the outside temps during the course of a day (see why good insulation and few windows is important?).  But my plan is to maintain the slab at a consistant low temp and make up the difference in the evening with my wood stove.  Thermal Mass has become a buzz word that is always viewed as a positive benefit, but caution must be used when designing a radiant heat system.

[Don_P]

I'm certainly not a pro on these. I have one out there that is a hog, it is on a reflective radiant bubble type insulation. It was sold as having an R value of about a gazillion. Pouring a slab on a radiant barrier I'm convinced is R zero, there is no reflective space, the insulation is the bubbles. Many systems want insulation around the perimeter claiming the ground underneath is part of your heat sink. I suspect feeding the mass of the planet at 54 degrees is not efficient either. My leaning at this point is to count on insulating under the entire slab well. The piping has been in our slab for 25 years, I'll probably try it out when I get a round tuit but we burn mostly wood. We've gone through 200 gallons of fuel over the last three winters so for us its more a matter of when I get tired of cutting wood which option to use. Even if we don't hook it up I do feel it was money well spent.

[MountainDon]

Re slabs...... from the article quoted.....

A key requirement for most radiant-floor heating systems is adequate insulation beneath the heated slab or beneath the tubing (when tubing is attached to the underside of a subfloor). Most manufacturers recommend a minimum of 1" (25 mm) of extruded polystyrene (XPS) for concrete slab-on-grade radiant heating systems, but significantly higher levels are justified in cold climates.

[Shawn B]

Don P,  The last few systems that I have observed going in used a insulated tarp like blanket, (sorry can't recall the technical term) or foam board. It appears that most contractors have stopped using the foil/bubble rolled insulation. 


Mushcreek,  There are several factors involved in radiant pex systems.  In particular length of loop runs, number of loops, number of manifolds, pex size, pump size, depth that pex is buried, water temp, heating source used (boiler).  Tubing layout is critical as is total feet of tubing.  By changing pex size and length...that effects pump size.....which effects water temp....etc.

Most pex systems buried in concrete run around 120F and no more than 140F or the concrete can start cracking. Which usually requires a mixing valve. So the actually work of installing the tubing is DIY friendly.....knowing how to do it correctly is not without some sizing, layout, and control system help.

Jeff92, 7/8 pex seems really big too me.....did a contractor help you size the pex?

[Jeff922]

You're right Shawn, 7/8" is really big.  I guess most people use 1/2".  The 7/8" Pex I used is a proprietary product of the Radiant Floor Company in VT.  This Pex has the same wall thickness as 1/2" pex so it's somewhat easy to bend.  Essentially, you can have the same volume of water in a system as 1/2" pex, just fewer loop (spacing between loops is larger).  It's less length and less hassle tying it down and whatnot.  This company specializes in DIY radiant.  I've mentioned it elsewhere, but here is there extremely good design guide:

http://www.radiantcompany.com/manual/index.shtml

I'm also a little skeptical about leaving the middle portion of a slab uninsulated as a "heat sink".  I did 2" rigid under the whole thing.

[Shawn B]

Jeff,   Most applications use 1/2 or 5/8 pex, but there are several ways to set up these systems.Your system is a little unique and should work well with your well insulated slab. I think the key for DIY and radiant systems is to have some very good support in tubing layout and pump sizing and control system design. I would recommend that someone always work with a contractor and one that specialized in radiant.  In fact the better Radiant heating companies usually specialize in them and don't due many forced air systems.

[Jeff922]

Radiant Heat system need extra care in design.  SOOO many of these systems have been improperly installed and work poorly.  I researched this extensively (I would never rely on a single contractor's word) and there were more than a few issues that varied greatly in opinion.  For example:  How high in the slab should one place the pex?  The middle? The top third? Doesn't matter?    It depends greatly on who you ask, and all of these people would describe themselves as experts.  As Don P pointed out, insulation under the slab is another contentious issue. My advice is to skip the contractor opinions altogether and conduct your own research.  There is a lot of information out there, and I trust the information provided by engineers, and design specialists way more than contractor opinion.  Contractors very often don't get any feedback on the projects they do.  The assumption is that if they don't hear any complaints, that they must have done everything right. The problem is who is qualifying a contractors opinion.  I can slap a sign on a pick-up truck saying I'm "Jeff the Radiant Heat Guy" and there is nobody to stop me.  Be very careful folks.  And if you are using contractors, GET REFERENCES, and ask to see examples of their work (and talk directly with the homeowners).  If a contractor seems put out by this, move on to another one. 

[ben2go]

Radiant floor heat is what I am planning for my rehab house.I got $600 figured up to do the 1/2 inch pex,manifold,and plumbing.I was debating on a small propane whole house boiler,wood fired,or solar.The house is roughly 1100 to 1200 sqft.A good many windows.Those will be upgraded to low E,and some will be smaller or eliminated.Also, all insulation is being replaced,siding,all new dry wall,doors,roof,ceilings,sub floor,and flooring(DUH).Water damage is aaaaaa,well, you know.

I'm in an area that sees high 30's to 40's overnight and 80's through the day, right now.We aren't currently using any heat or A/C,but soon we will need a little heat overnight.I live in the foot hills of the mountains,on a hill, in a small valley.Cold air pours in from the mountains after sunset,and the sun starts beaming in,in the early morning.So we have quick cool downs and quick warm ups outside.Did I mention that we have no trees close by?No shade or wind block.

Should I be concerned about using radiant floor heat for my situation?I know the temps will be a lot more stable in the house we are rehabing, and planning to use radiant floor heat in.I should mention that the ceilings are just barely over 7'.Any thoughts or concerns I should consider?

[NM_Shooter]

Ben, I suspect that you won't like the radiant heat very well.  Any climate that warms up to 80 deg during the day is too warm to use the long cycle time of radiant heat. 

[ben2go]

Ben, I suspect that you won't like the radiant heat very well.  Any climate that warms up to 80 deg during the day is too warm to use the long cycle time of radiant heat. 

Any suggestions?Outside central heat and air units get stolen around here for the copper.So I prefer to stay away from those.I know I could build a cage around it.Since I am in an area that hasn't took to "green energy", like onsite solar or wind,I am stuck with the grid.So I don't have to be as energy conservative.I do plan on a small pot belly or wood stove for those times when we get snow/ice storms and loose power.