Pls excuse 2 things. 1, I can't search here for some reason. 2, in an earlier post I suggested a certain person design a poster's floor, not knowing (DUH!) that there would be other pros commenting on this site. I have no opinion/information on ANYONE'S credentials, here or anywhere, so take that reference of mine as blissful ignorance. I like what I read here. 3, Now to my question. A couple forums seem to say "heavy alum plates and staple-up are married. EOS" while others say "plates are not needed". Knowing zip, I am then stuck in the middle. Is there some point in the btu/hr/sf range where plates are not necessary/recommended? My house is projected to lose about 7 btu/hr/sf at -50 F, which is why I wondered if the "don't need plates" crowd is correct (for my situation). Thanks. john

http://www.ngml.ksu.edu/research/project_archive/project_archive_publications/ashrae/1036_final_report.pdf

"Don't need", depends, but you can pretty much double your heat transfer to the room at a fixed temperature with 'em, as opposed to without 'em.  (Meaning you can run lower temperatures, yielding higher system efficiency.)

If you jam a huge wad o' tubing in the joist cavities and run it all at 200F you can heat almost anything, I s'pose, so by that measure I guess you don't really need 'em- ever! :-)

But if efficiency counts for anything, you need 'em.  Bet your heating fuel dollars on it.

BTW: Is "...-50F.... " your design day temp?  (In which case you've got superinsulated R100 clear wall construction and quadruple pane windows?)

Dana: Thanks. Somehow, that report appeared on my desktop just after I emailed this query. I don't remember downloading it, but the numbers are impressive. I will undoubtedly use them, and the heavy ones, as per folks here say to do. They certainly have visceral appeal, regardless of the numbers. Unless they are outrageously expensive, why NOT use them? As for design temp, -51 is what I have been told, and read. Allegedly, that is the temp above which 99% of your days will be; something like that. I would not be surprised if all those degree days get revised up sometime soon, as Earth warms. But for now, that is what folks publish. Our degree days number is about 14,000, way above most normal places. My walls won't be quite R100, but will be R42; double 2x4 w/ batts in between; lid about R50; windows as small as I can get away with, but much larger than I would have if I had more than 10% of the vote.

at 7 BTUs/sq ft, you need about 100 to 105 degree water in a suspended tube to meet load assuming regular subfloor, wood floor, good downward insulation. and that's at -50!

With heavy plates you are around 85 degree water.

Hard to say that is worth it in your case, unless you have access to a large amount of free 85 degree water or something.. Then again: pretty hard to say radiant floor is worth it in your case at all. I would use a panel radiator or maybe some radiant ceiling and save some bucks, but suspended tube should be quite adequate for you if you do decided to go with radiant floor. In this case though you are very unlikely to ever be able to tell that your floor is heated, unless you are over outside air temperature space (vented crawlspace, for example).

Rob: Thanks. I have been wondering about all that. We have lived here for 35 yrs, doing fairly well, though at -40 it is not perfectly warm. Upstairs is 958 sf over 958 sf downstairs (daylight basement). The upstairs has 34' of baseboard, all in the W side of the house; E side (kitchen, bath, hall, foyer) is heated from below as much as from the W half, I surmise. 25 yrs ago we added a SOG addition, 14x42, along the E side, level w/ the upstairs. With kids, we burned about 850 gallons of oil/yr. For 6 yrs w/ just two people we burn about 750 gal of oil/yr, incl heating dom water. The old 14x42 addition is coming off and a new, 38x46 is going on, SOG again. I can absolutely see radiant in the new slab, but am wondering if it is worth putting it in the upstairs floor of the original house. The biggest reason I want to add radiant to the original house's upstairs is that I will be installing a wood boiler in the new addition, and could heat more of the house w/ wood if I ran radiant upstairs. The floor is easily accessible. The design for the staple-up has two loops running per floor joist space; floor joists are 12" OC. To the untrained eye, me, that seems like a huge amount of tubing (approx 2000 lin ft) to replace 34' of baseboard, esp since the staple-up will cover the entire floor and the old baseboards are all on one side of the house. QUESTION: Do you think I need two loops/floor joist space, or is having the 1/2" pex staple up on 12' centers OK, esp if I were to use those alum plates. I don't know if the heat would be "striped" w/ 12" centers, but I have a hunch that even that would beat the baseboards we now have. That is just a wild guess on my part. Anyone have an opinion/suspicion? BTW: I will be adding the staple-up after the addition is all done and we move out of the basement. So, if this fails, we'll just use what's been working. Thanks much. john

two runs of pipe would be cheaper than one run with plates. I would stick with that plan if I were you.

Thanks. I had no idea what the plates cost. Done deal. j

plates, i added the plates as i installed the loops. i did not use heavy plates (maybe i should have) but the plated area:
at the same temp,
same flow,
as unplated 3/4 hardwood..5/8 plywood..3/4 chiplap....WAS WAY WARMER

i didnt read how you were heating the water but payback seems to simple to even calculate (lower water temps)

For a "typical" condensing boiler the difference in combustion efficiency at 105F vs 85F return water is still 3-5% (if only half as dramatic as dramatic as the kick you get going from 125F down to 105F, it's still something.):

http://www.rmcotton.com/_mod_files/ce_images/compeff2.jpg

But it depends on the boiler, it's heat exchangers, and whether it's in is most-efficient level of firing modulation:

http://hpac.com/fastrack/f1.gif

Standard cast iron oil boilers run in the flatter part of the curve, topping out at ~86-88% combustion efficiency- and the system running low return water temps needs already needs to be designed with boiler-bypassing to keep the temp of the return water entering the boiler HIGH enough to protect it, in which case your only efficiency gains are in the lower distribution losses. (Figure on 5% at the absolute most.)

I haven't a clue where you can run your wood boiler for optimum efficiency & no damage, but there are very few indeed (SFAIK only available in Europe) that run in the condensing temperatue zone. Assuming that Rob's numbers are in the right ball park (a good assumption, since he designs this stuff every day) the actual efficiency gains of even lower temps will be very slight indeed (probably not easy to measure in a wood boiler system.)

ever see the math on a GEO for lowering the water temperature? i am working with climate master 036 water to water. it seems from the charts and learn so far lower the better in these units. i will be getting presure / temp flows soon on my system

You're looking at return temps Dana. We're talking supply temps. 105 supply is likely an 85 to 95 return temp to the boiler (I should have specified in my above post I was talking supply temps not average temps). Going from that to an 85 supply and a not lower than 70 degree return is a pretty small increase.

with wood boilers, about the only way to run them is full burn dumped to a storage tank, IMHO. water temps pretty much only affect how long you can cruise between burns, and what kind of parasitic losses you have elsewhere.

well, if you do geo, get the temps down as far as you can, and damn the cost.

I've got to agree, and with a wood boiler you'd never run it that cold through the boiler so it could be an infinitesimal (or perhaps merely theoretical) boost in system efficiency from lower distribution losses.

It's also not a very warm floor, eh?   (Not the cushy toe-warming cruisin' in yer socks sort of thing most have come to associate with radiant staple-ups using 140F supply water.)

well even with a mod/con that kind of water temperature drop doesn't help you that much.

Much more good info since I last checked in. Thanks. Re: mod/con boilers, we apparently should not run them here, as our fuel is not refined enough. So said one of my ex-students, who is a local guru on boilers now. I plan on running my wood boiler at WOT and dumping the heat into storage, even if I get a boiler w/ a large mass of refractory cement. That seems to be the "best" way according to the folks on hearth.com that do this a lot. I am a little concerned about not getting any real benefit from radiant floor heat in my original house ("cruisin' in your socks at -53") but I am also willing to try it. Hell, the pex isn't that spendy, and I can run one loop uninsulated at first, letting some heat radiate to the basement. Maybe then I can run hotter water and have warmer floors??? If that sucks, I'll then insulate a bit at a time. Too, I suppose if I set the thermostat high enough I'll HAVE to run hotter water. My wife said to crank it up to 78 so she can sew naked. If I get that first loop working satisfactorily, I can hook up more loops. If I never do, I'll bag it and do what's been working OK for 30 yrs.