Underfloor bathroom BTUs

Andrew_4

What is the accepted maximum number of BTU's you can get out of a radiant floor? What about tile, with the tubes touching the bottom of the plywood underneath the cementboard under the tile? What if the PEX was in aluminum transfer plates? What about if the tubing was just suspended in the joist space? I realize that the maximum will occur with a floor temp of around 85F, but can all the different methods get you to that floor temperature, assuming sufficient heating below the tubing?

Thanks

Andrew

Andrew_4

What is the accepted maximum number of BTU's you can get out of a radiant floor? What about tile, with the tubes touching the bottom of the plywood underneath the cementboard under the tile? What if the PEX was in aluminum transfer plates? What about if the tubing was just suspended in the joist space? I realize that the maximum will occur with a floor temp of around 85F, but can all the different methods get you to that floor temperature, assuming sufficient heating below the tubing?

Thanks

Andrew

Mike T., Swampeast MO

Around 30 btu/hr/sqft in a "typical" space maintained at an air temperature of about 70° is generally regarded as the upper limit for a radiant FLOOR panel. This equates with an approximately 85° panel surface temperature.

This is a highly qualified statement about the FLOOR and its relationship to the SPACE ONLY! It says NOTHING about the ability of the system to actually TRANSFER heat from the heating medium (like water) to the panel!

Make the space not so "typical"--say with 3 exposed walls and lots of glass--and that 85° floor will be putting out MUCH MORE than 30 btu/sqft/hr--that is IF the transfer method can move the heat quickly enough to the panel. At the other extreme--say with no exposed walls and no glass--and that same 85° floor will be putting out MUCH LESS than 30 btu/sqft/hr. (The mass of the panel seems to be the determining factor in how much less before the air becomes significantly warmer. I'm trying to work on a simple equation to roughly compute this, but for now can only supply the reasoned logic further down in this message.)

Of the generally used floor panel heat transfer mechanisms, those that utilize a heavily conductive method near the TOP of the panel (like Warmboard) seem to be the best ("best" meaning fastest/greatest potential) at drawing BTUs from the transfer medium. Bare tube PEX seems to be the worst ("worst" meaning slowest/lowest) potential.

Similarly if you change the general air temperature, output changes drastically--say a warehouse maintained at about 62°. In this case (and if you for some reason needed it) that same 85° panel would be giving off a whopping 50 btu/sqft/hr! Again though, your transfer system would have to be capable of moving this much heat to the panel. Were this a greenhouse with an air temp of around 80°, the output of your 85° panel would drop to about 7 btu/hr/sqft.

The ability to reduce output (given the same surface temperature) with an increased mean radiant temperature but without affecting air temperature to a high degree is not as clear-cut. Again, I believe it is influenced most highly by the mass of the panel. For evidence, I parapharse some statements commonly published here and elsewhere:

With an extremly high-mass panel like tube-in-slab, air temperature in the structure remains remarkably steady among the spaces with the SAME panel construction despite the fact that the tube layout has little or nothing to do with the layout or exposure of the individual spaces--i.e. you don't have to pay extraordinary attention to keeping tube runs in the same room in structures with "typical" exposures. With a product like Warmboard however (this is NOT bashing the product in ANY way so don't get your dander up), it is generally and commonly stated that you pay HIGH attention to the interior walls and avoid runs that "share" different spaces lest you wind up with air temperature balance problems.

There's also the little matter of my bathroom radiant experiment where two spaces are essentially identical except for outside wall/window exposure--one has "typical" the other none. Despite an essentially identical number of BTUs available for transfer the output difference is either 100% or 50% depending on if you consider it "half" or "double". This with nearly identical air AND surface panel temps.

Andrew_4

Thanks, but

Mike,

Thanks for the reply, but of course, without a heat-loss anaylsis for my space, I can't really say how much heat I would be losing. I have two baths, one in a corner of the house with an exposed alcove with knee walls, and the other with only about one quarter of the exterior wall as the first bath. Both rooms are basically gutted, so will have R-13 in the exterior walls, and I'll fill the alcove walls and ceiling with cellulose. So heat loss will be different
for the two baths, even though, since they are both small, I will be putting them on the same loop. But do you think that with a 70F air temp, and an 85F floor temp, you could suck 30 BTUs out of tubing suspended in the joist space, even if the water temp is 180F?

hr

Andrew

why not just do a heat loss calc? Then none of us would have to guess as to "if" it will work. A free, simple heatloss calculator is listed just to the left of this post. Just a click away

Nail down some knowns, then we can advise as to some workable options.

hot rod

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Andrew_4

Heat loss

I tried to get the SlantFin software, they sent me a postcard stating that they are working on a new version, and will send me one when they are done.

Mike T., Swampeast MO

Heat Loss

Oh crap, am I ever going to get static for this, but here goes...

The actual heat loss doesn't matter! Let me qualify that by saying that it only doesn't matter if you have a reasonably massive, low-temperature panel under constant proportional control with a reasonable amount of exposure AND you design for a known surface temperature at design conditions with a predicted space air temperature.

Unfortunately, I've yet to see the heat transfer data that would allow this calculation with bare tube, so even if all of the rest of the criterion were met I'd have nothing but a true guess of the actual surface temperature to expect when using bare tube.

If any of you want to have some fun, compute losses of:

Outside design 8°.

8' x 5' space. 9' high walls. Occupied space above and below. 8' of exposed south wall (½" plasterboard (the tile/cement board ½ way up walls shouldn't matter...); 2x4 with fiberglass; 7/8" shiplap sheathing; 40# felt; asbestos-cement siding.

2-45" x 24" single-pane, double-hung, wood windows with aluminum storms. Very well weather-stripped, "average" air infiltration.

Temp in space 73°. Remaining surfaces (including ceiling but forget floor) considered "cold" partitions with 68° on other side. Uninsulated partitions save the ceiling with 1" of styrofoam & 7½" of cellulose.

------------------------------------------------------

2nd space identical in area but NO exposed walls or windows. 8' high walls. Same space temp and same "cold partion" conditions EXCEPT consider that one 6' cold partion is about 64° instead of 68° and that ceiling (40 sq.ft.) has 64° on other side as well but NO insulation. (This due to lowered ceiling and large unheated closets with some exposure adjoining.)

-------------------------------------------------------

BOTH have 22 square feet of heated floor area out of 45 sq ft total floor area. BOTH should be receiving the same number of BTUs circulating through the floor plate/tube assembly. BOTH produce an average surface temperature of about 83° at outside design condition at/or extremely near the predicted air temperature. Air temp remains amazingly constant with changing outside temp and panel temp drops slightly with dropping outside temp.

-------------------------------------------------------

The BTUs/sqft should be RADICALLY different!

-------------------------------------------------------

Explanations???? I've looked and looked and looked for the heat loss calculations (even rough ones) for these spaces. I never made them! Exposed has an electric b/b (in case the floor heat didn't work as designed) and the other space has essentially zero exposure...

hr

That's a big IF, Mike

"The actual heat loss doesn't matter! Let me qualify that by saying that it only doesn't matter if you have a reasonably massive, low-temperature panel under constant proportional control with a reasonable amount of exposure AND you design for a known surface temperature at design conditions with a predicted space air temperature"



I feel it is important for homeowners, and contractors alike to run some calcs. You will find cases where radiant floors, especially in rooms with little panel square footage, are not the best choice. And also with modern construction you will se some unbelievably low BTU numbers spit out!

We sure don't want folks pulling numbers out of thin air. Hard enough for contractors to "fix" improperly installed or calculated systems as it is

You tend to take this radiant thing to higher levels than the average bear. Not a bad thing, by any means. And your home may be a test bed. But we don't want the consumer getting guesstimate systems installed

Andrew, try www.hydronicpros.com there you can demo a nice, and simple to use design software. I think you should be able to calc your system with the downloadable demo version. Give it a try!

hot rod

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

What ever posessed me to say that?

Maybe three pots of coffee and no lunch.

I definitely agree that a thorough, accurate heat loss calculation is the first part of any job--without it your whole system is based on nothing more than a guess. You still need to know the size of the heating plant. For the vast majority of emission devices and systems you need to accurately know the proportionate loss of each space.

But there does seem to be something else going on that's essentially independent of the loss from the space...strange.

Could have sworn that I did a heat loss calculaton on those baths but can't find anywhere. Found all of the calculations for surface temperature but nothing else. It started as just something to keep the chill off of the floors and I wasn't even sure if it could do that because of low velocity.

Radiant Wizard

EXcepted BTU's

There are MAX Btu's for certain applications. Excepted BTU's is determined based on the project.

1. How many BTU's per sqft are required to heat the space?

2. What are the floor R-Values?

The anwser to your question is where I see more and more people get themselves into trouble. Why? They tend to not get a heat loss and radiant design done and just run a so called "acceptable water temp". They never take solar gain into issue, they tend to due thermostatic mixing and overheat rooms. Then they call on the experts to figure out what's going on.

I guess my point is, there is no accpetable number although there are some surface temps and water temps that as designers we look out for when doing a project.