In-floor heat scheme for economy
Guy we know is building his own small house, and will heat most of it with in-floor hot water tubing.
For the above-ground main floor deck, instead of using gypcrete atop a wood frame, or doing under-the-subfloor tubing, he beefed up the joists and beams, did the tubing within a 3.5″ slab poured atop the 3/4″ T&G subfloor, and will do a leather-look stain finish on the slab.
The cost of the crete and finishing, plus the extra beef to the structure (about 40 psf more in live load), is offset by the elimination of aluminum plates or fins for below-deck tube runs, the required below-deck insulation when doing below-floor radiant, and the elimination of floor finishes such as tile or wood or vinyl or linoleum above.
Whaddya think?
Replies
I think he still needs below-floor insulation, unless the space below the heated floor is conditioned and he's none too picky about zone temp differences. The slab will still want to radiate down as much as up.
Mike Hennessy
Pittsburgh, PA
That is for sure the ideal way to go for homes heated continuously. That added thermal mass goes a long way to moderate temperature swings between the tubes being fired up and deadens sound like no other.
Beer was created so carpenters wouldn't rule the world.
I wonder if he should have poured the slab over rigid foam over the subfloor?
I should have made it clear. The deck being heated thus is above the fully finished and conditioned space below, the walkout lower level, also heated via its slab, this one on grade with rigid foam beneath. Floor finish the same: stained.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
The difference between gypcrete and the slab you describe? Gypcrete is lighter and thinner? And your slab is more like a normal concrete slab? Did he pour the slab before erecting walls at that level or how did he detail wall framing and such?
Cannot gypcrete be acid etched/finished also? I've not thought about that as an option and I really have no idea.
3 1/2" slab on a wood floor structure is some stout stuff ... a lot more structure, it seems (not that it is bad).
I put my radiant floor on a plywood subfloor, then I cut 3/4" plywood to hold it in place (my own custom radiant trak). then wood floor floated on that. My main floor slab I acid etched and really liked doing that!! I like the finished concrete for the same reason ... eliminates another material to purchase and install.
Slab all done before framing. Here are details.
A "sub plate" arrangement of two thicknesses of 2x went down everywhere there will be a wall, exterior or interior. Sandwiched between the two thicknesses is a layer of 1/2" ply, to yield your 3.5" screed height.
Boxouts at same thickness go wherever needed for things like the kitchen island which will need electric and plumbing, plus under sinks, toilets, etc. Let's make wiring and running piping easy. No boring of crete.
Sitting on 1-1/2" chunks of broken landscape paver blocks (cheap chairs) goes a grid of 3/8 rebar, 12x12, and the tubing is tied to that so as to be centered in the slab.
Gypcrete might be not right for either finishing with the steel whirlybirds, or for staining, or for abrasion durability. This mix is pea gravel crete.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
Edited 9/7/2008 6:23 pm ET by Gene_Davis
We've done this except we stapled down a tarpaper slip sheet between the plywood and the slab to allow it to shrink separate from the ply. ran it 3/4" thick and strapped the tubing to the ply 2x12's 12" oc per engineers specs black finish, worked great. ------------------
"You cannot work hard enough to make up for a sloppy estimate."
I pour concrete over wood floors all the time vs gypcrete... you get a fire rating... u get a finished floor... all the things you listed... rare i do one 3.5" thick i'm more in the 1 3/4 to 3" range... mine are old buildings and i do it to level out a 150yo floor so that i can frame on top of it... i break the slab under every wall to stop sound travel and i use lightweight concrete and fiber and wire...
mass is good...
p
I'm not so sure you are doing it for "economy", though. You pay your money and take your choice ... ultimately there is NO free lunch (even if you take me out to lunch).
Generally, concrete construction is way more expensive than wood. But we are talking about more than just walls/floors ...
Energy wise, you probably aren't buying much if anything w/ e.g. the concrete vs. gypcrete, vs all wood ... although it depends heavily on the specifics of the application ... i.e. thermal mass in itself does not buy you greater efficiency or energy savings (that is a fallacy).
You are putting a lot into pouring the slab ... and the structure to hold it.
Don't get me wrong ... not putting you down ... just trying to add my 2 cents to help keep the realities in perspective. Lots of ways to do things, and they often all have their good/bad aspects.
Sure would like to hear more on that Thermal mass fallacy.
For thermal mass to provide benefit (Btu wise), it stores energy that is later retrieved to be used. If you have to pay money for the energy that it stores, you don't gain anything. If you are able to store excess available energy or 'free energy' (e.g. solar energy), then it begins to provide you a positive benefit.
Placing thermal mass in your average/conventional house does not magically create energy or make things more efficient. Certainly the house reacts differently to temperature variations and subsequently your heating system responds in a little different manner ... but the therms going into the furnace will not change appreciably.
There are several conditions under which thermal mass can have a positive benefit including: storing solar energy, storing excess energy from internal loads (e.g. people, lights, etc)(which is a factor primarily in commercial buildings), and under certain weather conditions where the daily outside air temperature rises above AND falls below the thermostat set point. All three have something in common ... a 'free' source of energy.
The analysis of the affects of thermal mass can be a very complex endeavor. The principles are relatively simple though. You can't get Btus out of mass that you haven't put into the mass.
Mass in a floor is like a heavy flywheel. It evens out the start/stop loading attributed to 'lightweight' construction. And while the building reacts a little differently to loading changes (outside temp), you still have to put the same energy into the building to keep the flywheel going. It still has to follow the basic laws of thermodynamics. I've seen a lot of sales pitches that mass construction is much more efficient (e.g. log or heavy wood houses) and generally, this simply isn't true.
Interesting.
But the mass would slow the "cycling" thus the furnace
can run more efficiently no?
Plus the mass (storage) being on the floor would help
in the inevitable rise of heat. Again slowing the the cycling. I don't claim to know any of this just thinking out loud.
Now you're getting into the dynamics of the mass a little more. The answer (isn't it always?) is ... it depends. Short cycling a furnace means it fires inefficiently ... so you through away more btus each time it fires. Cycling ... depends a little more on thermostat characteristics ... the deadband (temp at which it kicks on vs temp at which it kicks off. If the deadband is 1/2 degF ... the furnace will cycle more frequently than if it had say a 4 degF deadband. Given a steady state (stat setpoint and outside temp), a mass house probably won't affect this much differently than a light house.
But mass is a material that does better with dynamics than steady state ... that is variations in load ... e.g. when the outside air temp drops like a rock ... the light house will drop more quickly than the mass house (assuming equal insulation levels). The furnace in the light house will kick on sooner and stay lit a short time ... then off ... then on. The mass house mass will lose Btus to the space and probably not cause the furnace to kick on for a while ... then when it does, the furnace may stay on somewhat longer (at the higher efficiency).
If you go to bed at night and turn the system off/down, the mass releases btus to the space so the temp doesn't drop like a rock like the light house. The light house reaches setback sooner and cycles the furnace for short periods to maintain say 55 degF. Morning time ... both furnaces kick on; both at same efficiency (big loads). Lighthouse space temp rises quickly and kicks off furnace. mass house is different ... it warms slowly ... now you are charging the mass back up (like cycling the light house furnace all night) AND you are raising space temp. This takes longer, however, so you may feel compelled to bring it on at 3:00 am rather than 6:00 am. Remember, the mass house you can 'shut down' earlier at night as well.
It's a lot like you running ... do you like and are you more efficient doing a lot of quick short fast sprints ... or do you like to idle along more at a steady pace ... going the same distance either way? Kind of like diesel engines ... built for comfort, not for speed.
If you have a source of free heat (no, firewood is NOT free ... but it does heat twice! :) ) ... e.g. solar and allow the temp to float into late morning and have lots of south window, voila! You have taken advantage of free heat by storing solar energy ... that can be used that next evening (in addition to heaing your house during the day).
Generally, yes, you are right ... but my guess is that GENERALLY, the small difference in efficiency will not show up in your heating bill.
I'm going to rest, now ... until I get more food for thought (this feeding is a two way street, ya know?).
Take your rest.
And thanks for the food for thought.
And when he comes back tomorrow, let's hear the analysis of how outdoor reset controls, as typically used in today's systems with hot water boilers, affect the whole equation.
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"A stripe is just as real as a dadgummed flower."
Gene Davis 1920-1985
the only thing that might need mass is the heat source. if it's low mass and you have small zones, mass can help. heat sources with more mass do not benefit from mass emitters.A $400 buffer tank is a lot cheaper than a mess of concrete, however, and does not introduce any kind of thermal flywheel effect in the living space. If mass is helpful, that's how I'd do it. Or some intelligent controls use zone sync features to manage cycling effectively as well.that said, floor sensors and outdoor reset are generally enough to control the thermal flywheel in the living space. indoor feedback is even better.any way you slice it, 3/4" of plywood and some lightweight or even heavy gauge plates are likely far cheaper than this 3.5" suspended slab. a 3.5" suspended slab with no finish floor however is the cat's meow for performance... you can't be (edit: beat) practically zero resistance between your emitter and your room. whether that's a major benefit or not depends on his heat load, however.then again, maybe he's designing for passive solar. in that case, he's absolutely done the right thing.
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-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Edited 9/8/2008 10:22 pm ET by NRTRob
eh ... got me started again ...
Yeah, hot water reset is a VERY GOOD thing to have ... The biggest problem w/ radiant floor slabs and space temp thermostats is that the thermostat senses space temperature, not the affect of the slab. When the stat says 'enough', it is usually 'too late' ... you've spent hours pumping Btus into the slab only for the stat to 'get it' about 4 hours too late ... then you overheat.
Night setback w/ radiant floor heat is tricky ... you have to 'anticipate' this lag of space temp and slab heat. Sometimes you just play w/ it and tune it.
A good way to minimize problems and temp swings because of this lag is to reset your hot water supply temp with respect to the temperature of the outside air. Now you edge your flywheel along with moderate/continuous help. When it is cold outside you put hot water in the floor ... you need it. As it gets mild outside, you back that temp off. This way the pump doesn't cycle (and the boiler, too) frequently trying to deal with the lag/stat thing.
Ideally, you want your pump running continuously ... just satisfying your stat ... just barely. That means you've balanced your load and the heat system output. The vast majority of your heating hours are at substantially reduced loads (e.g. say maybe 30-40% of your peak). By reseting the temperature, you don't have the start stop problem and the potential for overheating.
Outside air reset controllers can be expensive (I've no idea why). I put in an adjustable 3 way valve that I simply tweak a few times a year to accomodate the current general trend of weather.
Reset tailors the btu consumption to the need at any particular point in time ... nice smooth running of the flywheel/energy machine. Like just keeping my flywheel going with a regular, but light kick of my hand ... flic flic flic. Efficient AND .... comfortable!!
Good thing about radiant floors ... it is typically the ONLY part of a building you are routinely in direct body contact with ... and if you can make that warm rather than cold, it is like standing in a hot springs at minus 20 degF (naked) ... you can do it easily ... I know, I've done it. In case you didn't pick up on it ... this means a lower stat setpoint will keep me comfortable ... which reduces the heat loss on my house.
Now there will be a couple that may jump in here and say ... yeah but people don't do that. I understand that, but I respectfully disagree that they are necessarily the 'vast majority'.
food for thought ... I'm going to bed ... I'm fried.
the heavy flywheel is a great way to explain it
Beer was created so carpenters wouldn't rule the world.