DIY Hydronic Floor Heating

[AC_Hacker]

Mr. Bean,

Thanks for joining the discussion.

We have a thread on Exergy HERE that I started, where I tried to introduce some of the aspects of Exergy. Sometimes it's a bit of a tough sell. If you'd care to contribute to that thread, consider yourself invited.

Also, the issue of exergy, and how to design a low exergy radiant floor is certainly appropriate to this thread.

Having looked at quite a large number of radiant floor web sites, I have concluded that most of them assume that fossil fuel will be the heat source, and then all the high-exergy rules of thumb flow from that, wide PEX spacing, under-subfloor installation, etc.

I looked at your page on Low-Exergy Energy • Efficiency • Entropy • Exergy • Efficacy, and it's great, just great.

If you would be so kind, I'd be interested in hearing what you have to say about designing & building a low-exergy radiant floor, as that is my main DIY project this summer.

My current thinking is to use 1/2" PEX on 6" centers, and use extruded plates (flange up) over high-density rigid insulating board (which I have yet to locate), with 1/4" Hardie-backer over that and topping that with linoleum.

Your thoughts?

-AC_Hacker

[Ko_deZ]

Constand decay rate is not a real world situation, is it? Using dt/L and claiming it to be real world example would demand that the material surrounding the pipe drops in temperature exactly like the water inside all the way along the circuit, which would be a very odd situation imo.

For a counterflow heat exchanger I could possibly expect something similar to that result, but I am pretty sure it will never happen in a real floor.

As such, I am now even more firmly sure that the example is not a real world example, but a simplified model.

The part 2 paper does not as far as I can understand, measure repeatedly along the tubes to verify or disprove that a fixed dt/L is very close to, or equal to, a variable dt/L as a function of dt between water and air. Part 1 is more a theoretical model from what I understand. Unfortunately I cannot find a free version of it, and I will not pay for it just to see what it sais.

[Robert Bean]

AC_Hacker,

Re: Thanks for joining the discussion.
My pleasure.

Re: Exergy… bit of a tough sell.
No doubt…it’s a hot topic even amongst the academics and practitioners; and those from the Fraunhofer Institute and other highly regarded think tanks seem to be doing a nice job leading their peers to the not-so-new principles...now if the rest of the world would step up...

Re: If you'd care to contribute to that thread, consider yourself invited.
Thanks perhaps after our ASHRAE meetings at month end.

Re: high-exergy rules
Yes and more so in some countries than others…Denmark appears to be one of the exceptions by choice with its energy policy leadership whereas Iceland the choice was already made for them. Besides exergy you can also look at the effectiveness coefficient (Φ ) of various heating system using:

Φ = (ts – tr) / (ts – tis)
where,
ts = fluid supply temperature
tr = fluid return temperature
tis = desired space temperature

North America represented by its traditional use of high temperatures and narrow delta t’s for space heating has one of the poorest coefficients where once again Denmark has one of the highest using lower temperatures and wider delta t’s…a by-product of its co-gen/ district energy systems, high performance housing and low temperature heating systems.

Re: E^5 …just great.
Comment: Thank you - it was fun writing it…

Re: hearing what you have to say about designing & building a low-exergy radiant floor
Comment: setting aside discussion for onsite CHP, matching load temperatures with source temperatures i.e. the narrowest spread possible is an exergy objective; that eliminates combustion leaving you to geothermal if the power is generated by hydro/wind/photovoltaics; or you could use solar thermal as part of a hybrid – all of these options make the cash register ring…only you know what you can afford and what is practical for you…at the end of the day for some people in some climates it’s better to invest in insulation, caulking and appropriate window strategies and focus on getting your building loads down below say appx. 25W/m2 (8 Btu/hr ft2) and then put in the simplest system you can regardless of the exergy efficiency…for these cases (setting aside other factors such as house size ) this person is likely a better earth steward than another trying to solve a thermal problem with a high tech complicated mechanical solution that they become a slave to…let’s just say to obtain the highest exergy efficiency takes a wherewithal beyond most peoples grasp but if its within yours then by all means go for it.

Your strategy of spacing, plates and flooring will give you a high fin efficiency and with a low building load a resulting low mean fluid temperature. This will enable the highest efficiency from what ever plant you choose.

You should be able to get a fairly close idea what the return fluid temperature would be from your system from studying these two pages.

http://www.healthyheating.com/How-to...ant-panels.htm

http://www.healthyheating.com/Radian...sign_Guide.htm

Cheers,
RB

side bar: Keep in mind whatever mechanical system one builds…one will have to someday sell...and just like on the TV “Fix this house and sell it” programs - what is important to the seller is rarely as important to the potential buyer; who will undoubtedly hire someone to evaluate the system being sold as part of the home being sold.

Many a home is devalued by its HVAC system and the more outdated and more complicated it is in the eyes of the buyer the less its worth to them – even if it means the world to the creator.

In general, given the current housing market and it's likely extension into the future - selling a home with a mechanical system having a serial number 00001 is harder to do than buying a home with a simple standardized system having serial number 10,000.

Well that's how I see it anyhow...

Originally Posted by AC_Hacker

The styrofoam beads will reduce the thermal conductivity of the concrete...

Last summer, I was testing the thermal conductivity of various aggregates in concrete, and one of my test aggregates was aluminum chips. Made sense, beings as how aluminum is light and a wonderful conductor of heat. What I didn't count on was that the alkaline nature of concrete made the aluminum fizz, and the concrete+aluminum puffed up with a ka-jillion little bubbles and was a worse thermal conductor than plain old concrete.

The rest of your ideas sound really great.

Ha, fizzy bricks! I learned the same thing recently when building a hydroxy booster for my jeep. Lye and Aluminum produces quite a bit of hydrogen.

Not sure if someone already mentioned this or not... But, nobody that actually has radiant heat calls it "hydronic", at least not around here. It's just radiant heat. I don't have to start saying "hydronic" now do I? It sounds like "moronic". If radiant heating is too much of a mouthful, surely we can come up with something better than hydronic? Actually neve rmind, it's not important. Just blabbing to get my post count past "lurker", that's sounds creepy.

[Robert Bean]

American Concrete Institute, CHAPTER 6 ACI 318/318R-65, BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE, p. 77, section R6.3.2 — "The code prohibits the use of aluminum in structural concrete unless it is effectively coated or covered. Aluminum reacts with concrete and, in the presence of chloride ions, may also react electrolytically with steel, causing cracking and/or spalling of the concrete."

[Drake]

Radiant heat can come from several sources e.i. hydronic, electric, solar, fire. So saying radiant heat doesn't always mean hydronic.

[Exeric]

"Step 4 - Floor Coverings (ft2●h●˚F/Btu)

Contrary to popular misunderstanding, the heat flux is not affected by floor coverings as long as the floor is not a highly polished mirror like surface. Carpet or concrete have for practical purpose identical abilities to emit radiant energy the only difference floor coverings have on the design is in the tube spacing and fluid temperature. The greater the sum of R values of the floor assembly above the tubes, the closer the tubes and or the hotter the fluid temperature must be. The hotter the fluid temperature the greater the back loss and thus the higher the insulation required below the tubes."*

This quote from Mr. Bean's well informed website explains a lot. Many pages back on this thread Student 07 and AC Hacker had some disagreements about using underhung vs overhung pex installations. The disagreement centered mostly on the use of aluminum channels for underhung installations. Student 07 said they weren't required and AC has maintained that they were essential in all installations (except perhaps in concrete), but especially in underhung installations because of the thickness of subfloor and and underlayment heat had to move through. However Student07 used a radiant barrier 1.5" below the stapled pex and a layer of Insulation and 2" foam board below that in his installation. The pex was VERY WELL insulated from below so I think these Student07 and AC's approaches are essentially equivalent as far as delivering heat to the living space.

So I thinker perhaps the decision to install above or below subfloor pex is dependent mostly other circumstances. Those circumstances would be primarily 3 things:

1. What climate you live in. As S07 said, I think in Portland where AC lives it might be better to go with a system not optimized for heat retention. I think in his location it would be wise to be able to lose heat quickly when a warm spell hits. I can identify with that requirement as I live in Northern California.

2. If one has access to a crawl space. If one does then that points to an stapled up system. If you go that route and don't want to install three different layers of insulation as S07 did, then by all means use the aluminum channel system as well. If one doesn't have access to a crawl space then above is the way. But I myself would worry about what an above subfloor pex installation would have on hardwood floors. From my reading here it can be done but you have to do it right with lower temperatures and not a lot of cycling. I don't know if he was right about too hot spots imprinting on wood floors but it sounds logical to me.

3. What the aftermarket sales effect would be. I think from this consideration the underhung system has an advantage. If there is carpet or linoleum installed over an overlaid pex layout the new owner may be asking for trouble if he decides remove it and install wood or tile. If the current system is too hot and cycling on and off a lot it won't be compatible with new floor covering. That's not to mention the increased chance of springing a leak and causing damage during any alteration to the floor covering.

Just my 2 cents after taking 2 days to get through this entire thread. I learned a lot from it!

[AC_Hacker]

Originally Posted by Exeric

Contrary to popular misunderstanding, the heat flux is not affected by floor coverings as long as the floor is not a highly polished mirror like surface. Carpet or concrete have for practical purpose identical abilities to emit radiant energy the only difference floor coverings have on the design is in the tube spacing and fluid temperature. The greater the sum of R values of the floor assembly above the tubes, the closer the tubes and or the hotter the fluid temperature must be. The hotter the fluid temperature the greater the back loss and thus the higher the insulation required below the tubes."

Can you please tell me the URL where you got this?

Mr Bean is usually fairly well informed, I'd like to give him the benefit of the doubt by reading his own words in context.

I think many of your statements which you attribute to Mr Bean, are not fully informed.

-AC

[Exeric]

Here it is. It is the second reference he gave. Go to step 4 on that page.

Originally Posted by Robert Bean

AC_Hacker,


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