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# Theory question on hydronic radiant

Member Posts: 166

I think I've hit a wall in my understanding of hydronic radiant heating (it wasn't a long climb up).

I have a (very) basic understanding of the formula:

GPM * Delta-T * 500 = BTU/Hr

So, for example, for any given environment, a 1 gpm flow with a 20* F temp drop means that 10,000btu/hr is being released from the water.

Or, a 2gpm flow with a 10* F temp drop also means 10,000btu/hr is being transferred.

I understand that I can control the flow rate (and the water temperature, which appears to not be a variable in this formula).

And I think I can control the temp drop by using different types, sizes and lengths of tubing, putting the tubing in a metal track, etc. (That's about the extent of my knowledge of these variables.)

I am, however, totally confused about what happens to the heat (the BTU/Hr) once radiated from the tubing.

That is, I have 1/2" hepex in tracks, spaced 8" apart under a flooring of about 2.5" of wood (call it r 3.5). Underneath the tubing is mineral wool (I think r 15) which separate the conditioned from unconditioned space.

From what I gather, that means that the vast majority of the heat is going up through the wood floor assembly.

My question is: If, for example, 10000btu/hr is being radiated, and r 3.5 is above and r 15 is below, does that mean that the vast majority of the heat will be radiated into the living space above? Or, will some of it somehow be lost because of the wood flooring assembly?

That is, if the tubing were above the floor (exposed to the air in the living space above), but with r 11.5 below, would it heat the living space equally (or better)?

Thanks!

• Member Posts: 17,810
It can get confusing...

In part because the hydronic formula is only part of the picture. The other two parts are, first, how does the heat get into the water in the first place and second, as you sort of hint at, how does it get out of the water and into something else.

The first part is easy. That's the boiler.

The second part is a little more complex. One part you have noted -- what is the effect if any of the actual temperature of the water? It is reasonable to suppose it does have an effect -- and, in fact it does. The hotter the circulating water is, the more heat will be radiated from a given area of tube or floor or radiator or what have you. Two take two rather extreme examples: a radiant floor which is about right for bare feet, and a hot water radiator which is really hot. The radiant floor will transfer about 20 BTUh per square foot of surface (floor) into the space around it, running at around 85 at the floor surface. The hot water radiator, however, running at let's say 185 on its surface, will transfer around 200 BTUh per square foot of surface area. Ten times as much.

The control variables for how much heat is transferred into the space are the temperature of the water and the area of the emitter. Radiant floors are designed to have a very large area, relatively speaking, and run at a low temperature. Once installed, the occupant can control how much heat is given off by varying the average temperature of the water flowing through.

Now your insulation question. How much heat is transferred up vs. how much is transferred down in a floor installation is governed by the transfer or insulating qualities of the two paths, assuming that the space temperatures on both sides are more or less equal. But all of the heat given off by the tubing goes one way or the other -- none of it is lost.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,666
Tubing located between your finish floor and subfloor will have LESS Downward loss than a JoistTrak install below the subfloor . The system would be more responsive since the resistance above the tubing is less
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Rich McGrath 732-581-3833
• Member Posts: 7,131
Energy cannot be created or destroyed. Your R3.5 of upward resistance is a bit problematic, the heat will look for weaker paths. I am guessing that you are losing more energy than you think to the great outdoors through the rim joists. Areas like that provide a nice cold surface for the heat to escape through. Do you know what that insulation detail looks like? Do you have access to an infrared camera?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 491
Simply put, you are looking on how to control or look at heat transfer.

Hot will go to cold, always.

The larger the temperature difference, the faster the heat transfer. R-value resists heat transfer. That's why under a radiant floor, the insulation value must be higher than above. Will there be energy transfer downward, yes but a small amount. Will there be transfer upwards, yes but a larger amount because the r-value is lower.

The 10,000 BTU's/hr is the heat loss of the room on a design day, perfect alignment of the planets, cats and dogs play well with each other......Which means this room loses 10,000 BTU's per hour and we need to replace that amount of energy.

Dave H.
Dave H
• Member Posts: 15,413
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 166

Thank you all -- my understanding is improving.

I took a thermal camera to the basement and our suspicions wee confirmed.

The mineral wool technically does a great job, but as installed leaves lots of areas where heat escapes.

I went in to the basement when the outside temp was about 30* F and the basement (with the floor about 7' below grade) was nice and warm. If my understanding is correct, that means plenty of heat created by the boiler and radiated by the tubing (and radiator piping) is being used to heat the basement while the earth continuously works to keep the basement cool.

The thermal camera aimed at the basement ceiling shows that where the mineral wool meets the floor joists there is a route for heat loss.

There doesn't appear to be a rim joist problem, as I stuffed plenty of mineral wool in that area.

But, the house's insulation detail is somewhere between crappy and non-existent. And, the first floor footprint of the house extends at various locations past the borders of the basement, so some rooms only have electric heat (not the hydronic radiant).

Then I took the thermal camera outside and see that I'm doing a great job of heating the shingles on the roof (3 stories).

The house has a new roof, and renovated interior with top floor ceiling in many places flush against the roof rafters. Basically, no easy (or even difficult) way to get the roof insulated.

So, with complete access to the basement, I'm thinking of lining the underside of the floor joists with 2" or 4" green insulation board. My thinking is that this will direct more heat up (instead of down into the basement).

Here's some pics.

• Member Posts: 166
More pics

• Member Posts: 7,131
The rim joists look good. You have tons of downward heat loss. I am thinking that you have a very continuous R-3.5 up and very loose and inconsistent insulation underneath.
Great pictures!
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 4,352
The simple answer is more insulation above means the tubing system radiates less heat and the return water temp rises/the delta t over the emitter falls.

The output will be less, you can reduce the swt to keep the boiler in the condensing range if that output is still acceptable, but the output is also a function of the temp difference between the tubing system and the air so decreasing the swt will decrease output further.
• Member Posts: 526
I really want one of those thermal cameras.
• Member Posts: 166
Zman said:

The rim joists look good. You have tons of downward heat loss. I am thinking that you have a very continuous R-3.5 up and very loose and inconsistent insulation underneath.
Great pictures!

That is a great understanding: I think the r-3.5 up is nearly perfectly continuous, with various places being higher r value; and the insulation downward is loose and inconsistent.

I'm thinking I could cover cover the entire ceiling (bottom of the floor joists as well as joist bays) with 2" XPS board (r-10). That should keep the heat going up, right?
• Member Posts: 166
mattmia2 said:

The simple answer is more insulation above means the tubing system radiates less heat and the return water temp rises/the delta t over the emitter falls.

The output will be less, you can reduce the swt to keep the boiler in the condensing range if that output is still acceptable, but the output is also a function of the temp difference between the tubing system and the air so decreasing the swt will decrease output further.

You've put in words exactly what was gnawing at my mind: With enough insulation both above and below the tubing, the heat radiated by the tubes will not enter the space to be conditioned. Further, the heat radiated by the tubes (into that closed space) will be greatly/vastly/or-even-possibly-entirely reduced.

Is my understanding correct?

Thank you so much for this!
• Member Posts: 166
JakeCK said:

I really want one of those thermal cameras.

It is both fun and useful.

I have the Flir C5. I don't have any experience with other models and very little experience with this model, so I can't make a recommendation other than I like it.
• Member Posts: 182
The true net effect of delivered btu's to any emitter is measured for both flow and temperature at the input and output from the emitter. This true emitter flow rate and temperature drop/GPM can verify the engineering, or lack thereof for each emitter. Now it is usually impractical to put flow meters on each emitter. And knowing flow rates are subject to change due to zoning pump flow changes or variable mixing control fluctuation, some approximation of performance can be determined provided flow controls can be manipulated and compared. With some system designs that are individually valve controlled and approximate flow rates are known from design troubleshooting on each circuit it can help verify installation quality. Because if not, there is nothing worse than getting all the measurements not knowing that a flow rate is compromised or unbalanced because the means to isolate the circuits for flow measurement was not built in. But who buys hundreds of dollars of more controls or tools to verify flow rates before connecting each emitter except the high bidder or the US Gov't? If it leaks we find it, but what if its just kinked a little or a hidden valve partially closed? Our problems do not come from properly designed, built, commissioned and operated systems; they come from the improperly designed, built, operated systems.
For me the best radiant systems are not temperature limited, unfortunately they can't be applied everywhere. Tip: Just because a pipe is open on both ends doesn't mean it cant be clogged.
• Member Posts: 7,131
edited November 2021
@Lance
The OP has multiple posts on the same system (makes it a PITA to follow). He does have flow meters on each emmiter and plenty of flow. https://forum.heatinghelp.com/discussion/185583/3-gpm-on-hydronic-heating-help#latest
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein