DIY Hydronic Floor Heating

[AC_Hacker]

Originally Posted by Daox

Wow, that would indicate that in slab would be MUCH more efficient. Especially in a heat pump situation I'd imagine.

As I mentioned, the hydronics guy is referring to conditions around here, and the weather is much milder here than in many parts of the country and world (must remember we have an international audience here!). The feed temps would be higher in areas where the heating demands are greater, but the ratios would stay pretty close.

Nicolas Léonard Sadi Carnot

Heat pumps, in their present state of development, have difficulty making feed water temperatures much over 120. And the Carnot theorem indicates that efficiency is increased as Delta T is decreased. So heat pump + staple up hydronic heating is not a good match.

Here's a table of Thermal Conductivity of common materials...

The reason a slab is more efficient is that concrete is a pretty good conductor (k=0.42 to 1.7), at least compared to wood or MDF (k=0.12 to 0.17), etc. Another reason favoring concrete and gypcrete (AKA: 'the wet system') is that the slab is poured over and around the PEX, so conduction from the PEX to the concrete is assured. The aluminum (k=250) plate idea , even with a U-bend in them are not in 100% contact with the PEX... conduction needs contact.

But weight is a big factor with the slab on a suspended floor, and I have been working and testing and researching this problem for nearly a year. I think I may have come up with a way to keep cost and weight down and performance at an acceptable level.

Stay tuned...

-AC_Hacker

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[Daox]

One of the things I really like about the poured option is increase in thermal mass. I wouldn't try to get rid of too much weight as it'll detract from that.

[Xringer]

After reading, 'When It Does—and Doesn’t—Make Sense', I've come to the conclusion
that my plan for warmer feet in the den is just a pipe dream..

I'm a little disappointed, but I need to take a winter break anyways.

Cheers,
Rich

[AC_Hacker]

Originally Posted by Daox

I wouldn't try to get rid of too much weight as it'll detract from that.

There's a lot to be said for thermal mass...

• It's generally cheap.
• It doesn't use any power.
• It has no moving parts.
• It's very quiet.
• It's extremely reliable.

There are also some things to be said against it...

• It is bulky.
• It is heavy.
• Its weight must be accounted for in structures that support it.
• It must be correctly sized to enable it to absorb and release heat at desired rates.
• Thermal inertia can be your friend and your foe.

So in a retrofit, things are more challenging. I did a calculation on what is to be my 'test room' (A=144 ft.sq.)and determined that a 1.5 inch slab floor would weigh 2610 pounds (Density of concrete = 145 lb/ft.sq.) 2610 pounds.

That's a lot of weight.

I also did a calc to determine how much water would be required to have the equivalent thermal storage, it came out to being close to 80 gallons. so, I could pick up a used 80 gallon water heater and put it in the basement, where it would be out of the way, and not stressing my floor supports, and pump heat into the tank and draw the heat out as required.

There are down-sides to that plan

• If the floor 'leaks heat', it will leak into the heated space, but if the water storage tank leaks heat it will leak it into an empty basement.
• I have to contend with pumps, controls, etc
• I am dependant on electricity.

There's also the angle of phase change materials. I tried some tests with Glauber's Salt and was not so thrilled with the initial results, however this guy had great results. They have been tried in floor tiles, drywall, etc. Personally, I have yet to try but hold out great hope for calcium chloride hexahydrate (AKA: snow melt salt) which has a page at BuildItSolar.

Here's an awsome list of PCM papers.

But this one, Carl Vener's Dissertation, gets cited more often almost any other PCM paper.

...so the right choice is really an array of choices. It's a matter of choosing the right alternative, given all the convening elements.

So much to consider, so little time...

Regards,

-AC_Hacker

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[Christ]

So what I"m getting from that little graph there is that I can store heat in thermal tanks (like I had planned) but the best thermal fluid that's easily usable is probably going to be a salt brine, right?

Unfortunately, that means I'll have to custom fabricate the tanks so that they're resistant to salt corrosion...

I'm going to have to take some time out and read those papers soon, before I even start digging my plot out. I'm planning to start digging on the nicer days of winter, because it's all shale anyway... freezing temps don't make it any harder to dig up.

[AC_Hacker]

Originally Posted by Christ

So what I"m getting from that little graph there is that I can store heat in thermal tanks (like I had planned) but the best thermal fluid that's easily usable is probably going to be a salt brine, right?

The graph is saying a couple of things...

When you compare concrete and water as heat storage mediums, water has a greater capacity, per mass unit to store energy. So to store a given amount of heat, you would need less water... less than 1/4 the mass of water to store energy.

The graph also looks at the energy storage potential of calcium chloride hexahydrate, a hydrated salt. The graph indicates that this salt is about 50% better than water, and over 6 time better than concrete. What the graph doesn't make clear is that it is particularly looking at the phase change characteristics of of the salt, in other words the large amount of energy that is stored and released when the salt is heated and cooled between its liquid and solid state, which occurs at around 85 degrees F, which is very convenient for heating purposes.

The links I put in the previous post go into more detail about the interesting properties of PCM materials.

[NOTE: I just came across a PDF that does a good intro to PCM.]

Regards,

-AC_Hacker

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[AC_Hacker]

Here is a link to download a trial copy of HVAC-Calc Residential 4.0. This is a program that runs on the PC and will calculate the heat loss (or gain) for a house.

They also have a commercial version, but the residential is more suited to homes.

This is a timed version, but the time that they allow is very generous.

It is a great way to try various "renovations" and calculate how much they will reduce heat loss.

* * *

And here is another free heat loss program, in the form of a spreadsheet, from Canada, called RetScreen.

I have seen plenty of interest in RetScreen from lots of professional people. With some going so far as expanding the usefulness of the program and making their version available also.

I have seen discussions on this site (EcoRenovator) about making spreadsheets, but if they are being posted, I have missed the posts.

At any rate, RetScreen is an impressive contribution to a world running short of energy, and it is wonderful to see what a generous nation can do.

Best regards,

-AC_Hacker

[Daox]

Wow, I didn't know there was THAT big of a difference between water and concrete.

Thanks for the chart AC Hacker.

Now, I need to find an elegant way of storing water in rooms...

[AC_Hacker]

If you can combine structural requirements with thermal mass, it's a win-win.

BuildItSolar has lots of pages illustrating work that has actually been done on thermal mass being used for heat storage.

Originally Posted by Daox

Wow, I didn't know there was THAT big of a difference between water and concrete.

Thanks for the chart AC Hacker.

Now, I need to find an elegant way of storing water in rooms...

Yeah, water is a really good medium for heat storage. It has the added advantage that it can be pumped through pipes, so it may be possible to locate your heat storage tank somewhere else.

So water storage is used as a 'heat battery', and energy is stored when there is too much energy (sunny day) and withdrawn when there isn't enough energy (cold night). One of the problems in making this work is insulating the 'battery' so the heat energy isn't lost. There are water storage tanks available in the US with claimed loss rates of a few degrees per day. There are water storage tanks available in Europe with claimed loss rates of a few tenths of a degree per day.



BuildIitSolar has several pages illustrating progress that has been made along these lines, regarding water storage of heat.

In the 70s there were experiments with water filled tubes for heat storage:

Nobody ever said that they didn't work, but the idea didn't gain public acceptance.

Currently there's still work being done along this line. Here's a guy who is using water-filled windows:

YouTube - Trombe Wall? Window? best of both worlds

And you could also get a very large aquarium. Goldfish are able to survivie a wide range of temperatures.

Then there is the Phase Change Material thing... Here is a graph that I made to illustrate the sudden increase in the heat storage of water when ice is warmed from 27 degrees to 37 degrees:

What this illustrates is that it takes about a BTU to move a pound of water (ice) from 27 to 28 degrees, and from 28 to 29 degrees, but to move it from 32 degrees to 33 degrees, it takes 144 BTUs. This is huge! it's over 100 times increase, no battery, no moving parts. It happens on the molecular level and is completely reversable. The water doesn't wear out.

What's not so useful here is that it's happening at 32 degrees, and we're trying to stay warm.

But there are other materials that will do a phase change at a point nearer to our comfort level:

• Glauber's Salt
  
• Calcium Chloride Hexahydrate (AKA: driveway salt)
  
• Paraffin (A special form of the stuff in the grocery store)

So there have been problems with the reversibility (AKA: incongruous melting) of glauber's salt incongruous melting, also with Calcium Chloride Hexahydrate, but for Calcium Chloride Hexahydrate the problem appears to have been solved. Paraffin doesn't seem to have the reversibiity problem, and is being micro-encapsulated and put into things from gloves to sheetrock. BASF of Germany is moving ahead in this.

Here is an interesting paper, where PCMs are combined with a flat plate solar collector to store and release heat on a daily basis.

We live in very interesting times...

Best Regards,

-AC_Hacker

[Daox]

Backing up a bit, do you have the 1st or 2nd edition of Modern Hydronic Heating for Residential and Light Commercial Buildings? The first edition is a bit cheaper some places, but I'd wonder how much updating has been done for the 2nd edition.