Please Explain

Mike T., Swampeast MO

I see the different outputs as being EVIDENCED by delta t, but don't see how delta t is actually driving the output.

This is not about higher output during the heating phase of a digitally controlled environment.

It is about the MAINTENANCE between a large, warm object and its cooler surroundings when the available BTUs are a constant.

Why are those available BTUs "available"?

Mike T., Swampeast MO

Two rooms. One internal, another with exposure. Nearly identical size. Radiant floor heat with copper tube in thermofin under 3/4" plywood, cement board and tile.

Identical amount of plates, tubing and even number of bends connected reverse-return to a single supply/return.

Average surface temp of both floors nearly identical in operation under constant circulation. Nearly identical air temp in the space as well.

Delta-t on the exposed loop about double that of the one that is not exposed.

It seems completely obvious that a different amount of heat (in IDENTICAL constructions) is being transferred via conduction from the water in the tube to the copper to the aluminum to the wood to the cement to the tile. HOW is this possible?

The ONLY real difference between the spaces is an exposed wall and glass in one and of course such are cooler than the other surfaces.

I keep arriving at the conclusion that the only way this is possible is if the amount of radiation is DRIVING the amount of conduction... I seem to find no other way to arrive at the high variance of delta-t but low variance in surface temperature.

Is this just a peculiarity of a peculiar construction (copper in fin) or an exaggeration of a process that occurs with any large, low temperature radiating surface?

Why do I imagine that were these identical ELECTRIC systems embedded in the mortar that the unexposed room would have a MUCH higher surface temperature than the exposed?

Sorry to get so deep, but if this really is what is happening than it means such can be manipulated to advantage--even if it can't be perfectly calculated.

Dave H_2

As Dan once put it

The BTU's are jumpin off the train.

In this case, the platform is bigger in one room compared to the other.

Most times, our circs sized for radiant floors are oversized, so the the only change we can see here is different Delta T.

Are there different stats in each room? Or one controlling both?

Mark Eatherton1

MRT...

Nah, not the clown with the gold bullion hanging around his neck... MRT is Mean Radiant Temperature. THat is, the temperature of those surfaces surrounding the emmiting element. In the case of the exposed room, the flow of energy is greater because of a greater difference in MRT.

Many times, we assume an output of 2 btu's per square foot per hour per degree F difference (convective loading) in surface temperature versus air temp, but what REALLY loads a panel is the difference in MRT.


ME

Mike T., Swampeast MO

No thermostats. Just constantly circulating water in the tube whose temperature varies with the outdoor reset.

Mike T., Swampeast MO

Understood. A floor of a constant temperature will give off more heat via radiation if the MRT drops. But, that additional heat must be available, right?

What I am not understanding is how if the same number of BTUs are available in the supply water and the construction is essentially identical how the surface temperature winds up being the near constant with both the amount of heat extracted from the water (via conduction through numerous "layers" and materials) and the amount of heat liberated (mainly radiation) being the things that are most variable?

Why isn't the unexposed floor significantly warmer when the same amount of heat is available to conduct? The radiation seems linked to the substance of the water itself.

Again, why do I imagine this would not occur if the heat source was electric resistance embedded in the mortar because radiation couldn't establish this "link" with the electricity?

doug_10

Maybe...

Because this is in constant circulation, any variance tends to even itself out. In other words, if you started both systems "cold" you would see a different floor temp. in each room for a while, but because they are linked and constantly running you don't?

To possibly further what ME said; you have a greater potential for heatloss in the exposed room, therefore you must have a greater delta T (all else being the same). Just because the floor temp is the same in both rooms doesn't mean that the exposed room isn't doing more "heat" work...it has to in order to maintain the same relative temperature as the other room.

If all factors are the same except for delta T then the Delta T must be higher if there is a higher heatloss in one area....sorry, but sometimes stating the obvious in another manner might help.

If you had a fan blowing on the floor in the exposed room, it would drop the Delta T even more, right? The convective losses would increase, therefore the water would shed even more BTU's...

Hope this helps. Take Care, PJO

Mike T., Swampeast MO

There is a difference from a cold start. The exposed room takes MUCH longer to reach maintenance than the unexposed, but again both floors wind up with nearly identical average surface temperatures.

Diagram below shows piping.

Construction is copper tube in Thermofin, attached to underside of ¾" plywood, ½" cement board, ceramic tile.

Both loops are nearly identical in length. Both have the same number and type of bends. Reverse return attached to the same branches from the mains. As I understand flow, it should be nearly identical in both loops.

Yet, the delta-t on the exposed loop is about 30°; the internal about 10°! The underside of the plates in the exposed loop is VERY carefully insulated, but the unexposed is UNINSULATED. Logic tells me that the uninsulated plates MUST be loosing some of their heat via radiation AWAY from the floor while the insulated looses very little heat via radiation. Yet it's the uninsulated version that is extracting significantly less heat from the water!

I completely understand that a floor surface 82° in temperature surrounded by a room with a MRT (mean radiant temperature) of 60° will liberate more heat than with a MRT of 65°.

What I DON'T understand is how the increased radiation from the floor in the space with exposure translates to more BTUs being drawn from the water itself. BTUs that arrived at the floor surface through conduction--through multiple layers and materials.

This is why I keep bringing up the electric thing...

If I embed electric resistance cables that will produce 480 watts of heating, it will ALWAYS draw about 4 amps at 120 volts. Right? So, it's always liberating the BTU equivalent of 480 watts to the floor. Changing the MRT won't change the amount of heat added to the system. Am I missing something?

See the difference?

Mike T., Swampeast MO

Please excuse my stupid mistake. 480 watts draws about 4 amps at 120 volts.

I'm prone to that sort of stupid error--I actually see it properly on quick review then discover my error when proofing later...

Steve Ebels

Gut level intuition

I have always observed that btu output of a floor surface is dependent on two things. The surface temp of the floor and the ability of the room that the floor is in to lose the btu's to the outside.

Surface temp is dictated primarily by one thing and that is the water temp. If you put X degree water in a floor of a given design you will arrive at a surface temp of X degrees. The charts that are generaly seen associate the btu output of a floor with room air temp only and I have always felt that this is 1/2 of the equation. That half would represent the conductive and convective output of the floor. The other 1/2 of the scenario is the radiant portion of the floors output.

I have noticed the same phenomena that you are describing Mike and invariably the difference is the amount of glass or exposed wall that the floor is "seeing". If you take a floor running at 75-76*s in a 70* room it will have a vastly different delta T than a floor of the same size that has a big increase in exposed wall or window area. The heat is leaving the tube at a faster rate due to the room with the exposure's ability to dispense with those btu's.

Again, this is just a gut level feeling.

Steve (not an engineer) Ebels

Mike T., Swampeast MO

If that gut feeling is correct then it certainly seems to imply that radiation is greatly influencing the conduction of the sytem--essentially "sucking" heat out of the water itself--RAPIDLY. While the surface temperature of the panel remains quite constant the BTUs conducted through it depend on how much radiation is going out of the surface of the panel.

In other words, radiation is providing direct, proportional feedback that can be used to regulate air temperature VERY accurately without measuring the temperature of the air itself!

I'm really not seeing a reason that this "super-simple radiant" wouldn't work with much larger spaces. Entire smaller homes on a single zone, added zones in larger, fancier construction. Combine with panels, radiators or even baseboard (TRVd of course) in spaces where radiation is inadequate or undesired.

Nothing but a "warmer-cooler" dial to slightly shift the reset curve of a reset control that controls either the boiler or a temperature mixing device. Use the nice "balancing" modular manifolds, set, forget and dispense with zone valves, a great many circulators, proportional flow devices, flow checks, pressure bypass, etc.

Cast iron boilers should appreciate the constant circulation and reduced potential for condensation--even at lower than "normal" return temperatures. Copper tube boilers should appreciate the nearly constant flow rate.
Condensing boiler should absolutely thrive and modulating/condensing boilers should be singing Wagner.

Any conduction-based radiant panel should be a candidate and something tells me that this would work VERY well in the most difficult spaces like kitchens.

When I start playing with my five-heat (electric radiant, hydronic radiant, forced air, TRVd iron radiator, hydronic towel warmer) bath this winter I should be able to somewhat disprove/prove this idea. Methinks though that I have FINALLY determined how to heat my kitchen--and it's the SIMPLEST possible method!

Mike (certainly not a trained engineer either) Thies

Tim_9

MRT:delta T

I believe the physical connection between the MRT and delta T is the average temperature of the floor. The system will tend towards equilibrium and alter any variable it can to satisfy equilibrium. In this case it is delta T. What causes delta T to increase is that a larger portion of the slab is at a given temp to satisfy equilibrium. So I believe that if you take a few slab measurements from supply to return that you will find that the average floor temp has increased in the room with the lower MRT.

Tim

hr

Some old articles

I have written by Joe Fredrich explain how the radiant floor output is higher near the windows, due to the cold surfaces. Really no need to tighten tube spacing there, as the delta t will drive the output higher than in the center of the room.

At least that's how I understand it.

Imagine how a slab in a basement put out 15-20 BTU/sq ft. Now take that exact same slab outside on a 32 F day. Maybe 100 BTUs? square foot.

This is noticable, also, when you start a large commercial indoor slab in cold weather.

As ME described it's the delta tee that drives the different outputs.

You can watch these output #s on any design calc software as you change the indoor air temperature. Run a room at 72 air temp and look at the output, then bump the air temperature to 65 and see what happens.

Not sure I have answered you question. Somehow I think you have a deeper question

hot rod

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Steamhead

Those extra BTUs

move between the constant large warm object and the cooler object.... because they must.

I see this as similar to the maxim "High pressure goes to low pressure.... always". It is a basic principle that pressure or temperature must equalize. This is why we have heating systems in the first place- to keep inside temperature from equalizing with outside temperature.

High temperature goes to low temperature..... always.

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