PEX & steel corrosion??

Royboy

on a job where I'm set to install PEX tubing in a sand lift next week told the builder that they are no longer allowed to tie PEX to steel. some concern about corrosion.

I've not heard of this, nor has my PEX supplier.

have any of you and what's this about (if anything)?

Roy

Ken_40

Unsure of what a...

"sand lift" is, I never heard of any issues in the use of steel and PEX before. Considering it is the most common way to tie a boiler manifold to the attached circuits/zones and pumps: Pex attached to brass adapters threaded to steel near boiler piping, one must wonder if he confused steel pipe or fittings used in conjunction with NON-barrier tubing?

Or was he just running his mouth as more than a few ready-mix drivers are prone to do...

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Royboy

by a sand lift

I mean the sand under a slab. in this case there's 2' of sand under the slab, with insulation below the sand.

anyway, the issue seems to have been about concern for rusting/corrosion of the steel over time in the sand (or in concrete, if the pex had been there, and that somehow degrading the pex.

but by the lack of response, I'm gathering that this is not an issue that anyone else is aware of ...

Roy

Jim Franklin

Sand radiant

Maybe it's also because a sand bed radiant system is so inefficient that hardly anyone uses that type of system as well. Why aren't you installing the tube in the concrete?

Royboy

first off - I didn't design the system

though I am doing the install. the pex in sand part is a 2' deep sand bed storage under a radiant slab. the sand bed is charged from an active solar system (320sf flat plate collectors). the idea is that because of the large storage capacity of the sand bed, the solar system can start charging it mid to late summer and thus utilize more of the abundant summer sun to contribute heat during our long northern WI winter. seasonal heat storage for solar systems is a currently cutting edge concept, it seems. (see the "solar heating ideas & questions" thread - or the article in the current issue of Energy Design Update).

this system has a passive delivery mode which I'm not yet convinced is the best way to go, though it does have simplicity going for it. the designer has a couple dozen houses running these systems and he seems happy with the overall approach.

there is a back-up forced air system (using air conditioning ducts).

I'm doing the solar system on this job.

I was going to be tying the pex to rebar to position it in the sand before it is covered but actually realized that PVC electrical conduit is cheaper and easier to transport than rebar, so I'm going that route.

still curious about the steel/pex concern though. I'll ask the concrete guy about it when I see him on friday.

Roy

hot_rod

The rebar or remesh

will only rust if there is water present. I doubt that would be the case in you application? If so the source of the water needs to be addressed as it will wick away heat from a slab, or subgrade quickly.

I agree with Geoff, sand is a very poor material to store heat in. It leans more towards an insulator.

At best it will be a warm haven for insects throughout the heating season.

Hard to get good compaction on dry sand also. I wonder that it would pass for a suitable under slab fill material?

Question authority! And write a disclaimer into you contract addressingthat design.

hot rod

hot_rod

thermal conductivity chart

This would show how much better gravel or rocks would be for storage. Better for a subgrade material also

Google thermal conductivity of sand



Range of all reported values for soil

0.15 to 4

Saturated soil

0.6 to 4

Sand perfectly dry

0.15 to 0.25

Sand moist

0.25 to 2

Sand saturated

2 to 4

Clay dry to moist

0.15 to 1.8

Clay saturated

0.6 to 2.5

Soil with organic matter

0.15 to 2

Solid Rocks

2 to 7

Tuff (porous volcanic rock)

0.5 to2.5

Royboy

my guess ...

is that the value listed for "solid rock" is for the rock itself - not accounting for rock or gravel with air spaces between - which is what sand is on a smaller scale.

as far as I know, rock & gravel are more used in air charged storage than fluid charged and I don't think that's too popular anymore with air-quality concerns.

my understanding of the rationale for this storage design is that sand is not the best storage medium, but is cheap, available, and easy to place and insulate. my sense is that the amount of tubing is increased in order to compensate for the reduced heat transfer into sand. I think that though sand is not the best storage medium that it is perceived to be the most cost-effective and practical.

I'm aware of at least 3 designers actively working this high-mass sand bed solar storage concept - and I'm sure there are others. One, Bob Ramlow, who did the design for this project and has done many similar ones, primarily in WI. Met another guy just the other day in Duluth, who's starting a project that will have a 4' deep half-sand/half-taconite bed (surrounded by a foot! of foam on all sides). This one is insulated from the building and will have a pumped delivery system into a radiant slab delivery system. It is going to be heavily monitored and over-built in terms of valving and flow options. And then the lead article in Sept Energy Design Update, entitled "A Basement Full of Hot Sand," is about another system with active delivery, this one utilizing a ground source heat pump to pull heat out of the sand. This latter system has 5000 cf of insulated sand and is estimated to have a storage capacity of 200,000 BTU/degree-F.

HR - appreciate your encouragement to question authority. I'm happy to be involved in this project and have recommended significant changes that I feel will improve it (such as a drainback solar system rather than the spec'ed antifreeze system). My gut sense is that an active delivery mode would be preferable to the uncontrolled passive mode in the design - but that's just my instinct. The latter two systems mentioned above are taking that tack. I brought the idea up to the general and the architect and we agreed to just let this one happen as designed. If the passive delivery does work out, it is certainly a simpler system.

I'm intrigued by the idea of seasonal heat storage as a means of getting over that hump that plagues solar heating of - at least in this climate - having the least available sun during the heating season.

Other than high-efficiency wood boilers - which intrigue me with wood as a pretty close-to-nature solar storage medium - I find the concept of seasonal or even annual solar thermal storage to be exciting.

What do you think?

Roy

GMcD

Now it makes more sense

OK, I can understand why the sand and radiant tube system is being played with. The approach above has merit, but I hope the sand bed is well insulated to keep that stored heat in place. Back to the original issue- PEX tubing and steel corrosion - never heard of that unless there is water and oxygen available, and if the sand is kept dry, it should be no problem. Now wet and warm sand = primordial ooze factory....