PEX with high thermal heat transfer

Ivo Renkema

Why would a sandbed be a bad idea?

Ivo Renkema

PEX with high thermal heat transfer

I am working on solar heating; and the temperature at which my collectors operate is crucial: the lower the temperature, the better.


I understand that if I use PEX to heat a floor (in my case a sandbed under a floor), that I will pump water at 95 F and it returns at 85F to get my floor at 68F. That is because PEX is a good insulator.

In this case, good insulator is bad for me. I need some tubing that has a high heat transfer, so that my water returns at the temperature of the floor.

What do you suggest? If there such a tubing out there? Or should I work with aluminum or copper heat exchangers (with the likelihood of those corroding or breaking down)? Or fins?

Because it is a sandbed, it will not be accessible after construction. The tubing will have to be right the first time, and then last -say- thirty years ...

Thanks for your help!

Andrew Hagen_2

Insulator

You are embedding the tubing in sand? The sand is a much better insulator than the pex.

kpc_23

I concur...

sandbed it a REALLY BAD idea!

Paul Pollets

Don't!

PEX tubing needs to be embedded in concrete to work properly for a slab application. Embedding in sand is incorrect for the application.

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Unknown

What room temp are you trying to maintain? 68 degree floor temp would only maintain a room temp of something less than 68 degrees.

At any rate, the floor is likely to be able to dump far more BTUs than your solar will provide, whether you design for a 10 or 20 degree drop. Unless you're in some very marginal climate with a lot of solar panels.

You are ultimately limited by the output of the floor. That is, once the mass is heated up, limited by the heat loss of the space unless you are willing to overheat. In any case you are limited to 40 BTUs/sq ft before your feet start getting too hot, 30 BTUs under most wood products.

PEX can do this, if you reduce your resistance in the flooring (use tile) and use an appropriate installation method (not a sand bed). whether it "insulates" or not, you will plenty of exchange area which will outstrip your panels ability to collect heat in the vast majority of cases.

Bob Gagnon plumbing and heating

instead of the floor

think about radiant heating an inside wall or ceiling. All heat will be within the heated envelope, downward heat losses would be pratically eliminated.

Thanks, Bob Gagnon

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Greg_41

Radiant in sand

is a bad idea, i even don't like to attach the tubing to the eps insulation below slab (last job had 8" below and 4"@frost walls) i like to attach the tube circuits to the expanded mesh that is in the matrix of concrete. the entire tube is suronded by concrete, and the recovery is much faster having the heat closer to the surface. i use rehau tube and manifolds, it's awesome stuff, the tools are expensive and good, and it's been around 30+ years, no leaks with everlock fittings

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hot_rod

sand bed storage

has been promoted for years by a solar guru up Wisconsin way. He does have a point about storage-ability, cost, installation ease, etc.

On the downside, sand is not a very good conductor, especially when dry. Adding a few bags of portland would be a nice, not very expensive addition. This would provide a much better contact to the tube and increase the conductivity. Think concrete and gypcrete.

My biggest concern was lack of control-ability. In the Nov/ Dec. issue of Solar Today magazine "Warm radiant Comfort in the Sand" two points seem to jump out.

"It takes about a month to get the sand bed saturated with heat, then the temperature inside the building is regulated by judicious opening and closing of windows."

Opening and closing windows may not be a good thermostat for all home owners??

Also the author notes "Sand, on the other hand, has spaces between the granules, so heat transfer is much slower. However heat can travel in a concentrated path rather quickly through a sand bed towards cold."

I'm not quite sure how that all works, seems it changes from a poor conductor to a good conductor based on heat travel direction???

A radiant surface will emit approx 2 BTU per square foot for every 1 degree temperature difference between the slab surface and the surrounding room and surface temperature.

So a slab at even 78F surface, in a room of 70F will put out approx 16 BTU/ square foot. In a tight well constructed home this could easily present uncomfortable over heating conditions. In fact 16 BTU/ square foot could very well be design day load conditions!

Imagine the overheat potential if the sand bed were to reach 82- 85F or higher, especially in a well insulated structure.

I emailed the author/ owner for additional info and bought his book, which I like. I never did get a response, but I am on their e-mail yoga list

My thought would be an additional foam insulation layer on top of the sand, then radiant tube in the upper 4" slab pour. Then you could control the rate of transfer from the sand bed storage to the slab via wall or slab thermostats. Use the heat from the sand bed as needed not at the masses terms!

A lot depends on your climate. if you live in an area where it gets cold and stays cold, high mass can be a friend. in my area rapid and frequent temperature changes leans more toward fast responding, low mass or panels on slabs like Quik track, etc.

With so many choices in products and methods you should be able to find one ideal for your climate, needs, wants, and desires. it can be a large investment, and not easily modified, spend so time finding the best application for your project.

Check out March/ April Solar Today for more about the unique home they built on a sand bed radiant concept. I love their home, just not onboard with the sand box Although it seems to work great for their condition and lifestyle.

Will it work? No doubt.. But one of the biggest features of radiant slabs is ultimate comfort, and unlimited zone-ability. Use of a "double hung" thermostat doesn't seem to fit in the picture well for all homeowners. Maybe it will for you?.

hr

Annabelle

Radiant heat tubing in sand

So now in 2012 what is the feeling about putting tubing in the sand versus in the concrete slab for radiant heat? I know it needs to have foam insulation under tubes and up the sides. I understand that the heat seems to last longer in the sand than concrete. My builder likes concrete since this has been a proven method for years. Anyone have suggestions, preferences, experience with the tubing in the sand? Good or Bad?

kcopp

it was bad....

then...its just as bad now. Concrete is a MUCH better plan. Who told you that sand holds heat longer than concrete....? that is simply not true.

duramax

geothermal

I live in up state ny  and I am going to put closed loop tubing in the ground for heating and cooling  I am ready to start diging , but it is very rocky , iam going to try to go 8 foot deep because it does get pretty cold here ,my question is I no they say a sand bed is bad, but is it bad if I put a little under and over pipe to pertected the pipe