I live in the Deep South, Fl-Al state line, and have metal roofing on lathe type construction on a late 19th century depot. I've read a few posts here about using PEX under metal roofing in a channel or holder of some type, what about attaching it horizontally between the lathe with aluminum tape and a few strategic braces? 3M makes Al tape that claims good heat transfer and a working temp. range of -65 to 300F(!)... Would 1/2" or larger PEX be more efficient than economical 3/8" using an antifreeze solution in a closed system? I'm thinking 500' with a 275 gallon tank in conjunction with my Paloma on-demand water heater... Thanks for your thoughts!

PEX has a different coefficient of expansion than your roofing- tape isn't gonna hold it.

PEX has a max temp rating of 180F- can you guarantee your stagnation-temps will be below 200F (which is more like it's real practical operating limit)?

In most of the frost-free (or nearly frost-free) parts of the southern US you can get a lot of mileage out of batch hot water pre-heat plumbed in series with the HW heater. If you put 50 gallons of water up there in thin-tank type flat panels it'll provide a large fraction of your summer hot water, and pre-heat your winter hot water, no pumps, no fuss, low maintenance. (150' of 1/2" PEX only holds about a gallon.)

Only if your using it as low-temp hydronic space heating would large storage and large collector area make sense.

For a bunch of HW heating concepts, poke around here: http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

If you look at the SRCC solar ratings for New Dawn Solar, you can get an idea of the thermal performance of this concept. There is a tremendous amount of loss back to the environment from any unglazed solar collector that is trying to make domestic hot water. When you add in the thermal loss from putting something like a radiant floor under the roof, the performance takes yet another hit. An unglazed solar pool collector is about 10X better than a metal roof system system. All that being said, do check out builditsolar.com since there are a lot of other concepts there that will perform well in Florida.

So PEX needs space to expand hence the noise in some radiant heat installations and the 'channels' in the New Dawn system? Could some of the inefficiencies of these type systems be explained by relavtively poor conduction of the heat from the collector (roof) to the piping due to the need for some 'play' for the PEX to expand? I'd thought of the tape as a way to avoid removing/reinstalling the roofing and also make good contact. I've seen some pics somewhere of DIY radiant tubing 'heat fins' made with a press and flashing, perhaps lengths of something similar could be attached between the lathe to hold the PEX in place...

Seems to me the metal roof system's inefficiency would also be it's failsafe in terms of stagnation temps, otherwise we'd see spontaneous combustion around here some Summers! That being said, I've never actually measured the temp of the metal roof...

Basically, I'm trying to do a little with not much and make it tough. I'd hate to see what some expensive panels (especially evacuated-tube types!) would look like after a tropical storm or some hail out of a thunderhead. We've got plenty of heat around here most of the year and I hate buying LPG to heat water. But PEX is cheap, I have a perfect pressure tank, copper to build a heat exchanger, etc... Thanks for the input.

Polycarbonate glazings handle hail etc pretty well (that's what "bulletproof" glass is made of, after all.)

The heat transfer efficiency is cut by a bit more than half if the tubes are not firmly fixed to the metal with large contact area, but if you double/triple the area (cheap to do with an unglazed roof system) that factor is negated.  Most of the inefficiencies have to do with the losses of an unglazed collector during low-temp periods.

Even when it's 90F outside in full sun the losses of a 140F roof are substantial (the minimum roof temp you'd need for making domestic hot water would be ~140F):  It's a 50F/28C delta-T, with copious amounts of convective & radiant cooling!  When it's only 70F out the delta-T rises to 70F/39C, and your collector surface efficiency is down around 50%.


Unglazed collectors work best when the delta-T is less than 15F/8C.  But you can see how making 100F pool water is pretty efficient with cheap unglazed collectors when it's 85F, but domestic hot water, not so much:



So, when it's 70F out, between the sub-50% heat transfer efficiency of the floppy tubing and the 50% hit of the collector surface you'd be doing well to make 20-25% efficiency at DHW temp.  But as pre-heat you can still do pretty well, taking the 60-65F feed water up to 90-100F, letting the water heater take it the rest of the way, up to 120F or so.

Don't store large amounts of preheated water- only about the amount you'd use in a day, or you create a legionella risk.  Storage above 120F or below 85F is safe, but in-between is prime-temp for cultures to bloom.  If you're purging it all at least once/day every day you're safer.  250gallons of 90F water is a hazard- use a lower volume heat exchanger in the tank if you go that route. It takes storage temps of 140F or more to actually kill legionella, but above 120F it won't take hold.

OK, I'm beginning to see the concept more clearly now... I'd really like to have a LOT of capacity with teens endlessly showering, little kid baths and dishwasher & washing-machine each running at least once a day. And I'd like to have relatively efficient setup in cooler weather when the roof system will fall on it's face. Legionella is now a concern, is this a public water supply issue only or are deep wells suspect as well? So much for large capacity pre-heat...

I guess I need to abandon thoughts of a metal roof system and go evacuated tube or flat-plate and aim for temps of at least 120F, back to the drawing board...

In most of the US, and particularly in the SOUTHERN US, you'll get a lot more solar hot water per capitalized dollar out of flat plates than evacuated tubes. Evac tubes make the more sense when you're looking for higher temps or have whole months that barely break freezing.  Even in southern New England it can be a tossup.

Legionella is an issue for public supplies, less so for deep wells (since most well water in the US is signficantly below 85F.)

But don't give up on the large volume pre-heat concept. If the bulk heat storage water is "dead" recirculating solar loop water and the potable water is in a low volume heat exchanger the water never stagnates for days/weeks at temp the way it might in the corner of a tank- 100% of the tepid water is replaced every time somebody makes even a small draw, it's constantly purging.

If you have a full basement where the drains run through, you can cut the heat used by teenage endless showers with a drainwater heat recovery heat exchanger on the drains:



http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13040

There's one manufacturer of a horizontal version more suitable for slab-on-grade, but there isn't (yet) 3rd party independent testing available on it:  http://www.ecodrain.ca/

Natural Resources Canada has standardized the testing of various models for apple-to-apples performance comparison:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45

But there are others not yet on the list.

Financial analysis shows that you should always buy the biggest-fattest version that acually fits- the present-value of the increased performance more than offsets the additional cost:

http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf

If it reduces the size & cost of the solar heating necessary to support the load by 20-40% it pays for itself pretty much up front. (Wouldn't dream of doing solar HW without it.)

So use the 275 gallon tank as a container for the working fluid from the panel or roof system, with a heat exchanger coil for potable water suspended in it? That might eliminate the need for propylene glycol, an expansion tank, etc. too if it were a drain-back system... But it seems like it would take a lot of capacity in the heat exchanger to have much hot water and/or a really efficient type exchanger, like large I.D. copper maybe.

Greywater heat recovery would be great but might get complicated getting the heat back ahead of the water heater with distant bathrooms... Are there thermostatic valves or some device to allow the water warmed by the drain to flow back to the water heater?

Some people have done OK using a coil of PEX or HDPE as the potable HX in the atmospheric-pressure drainback tank:

http://www.builditsolar.com/Experimental/PEXColDHW/TankConstruction.htm



With copper you can do with a much lower volume due to the high conductivity.

Multiple small-ID coils work better than an single large diameter coils due to a more optimal surface area to internal volume ratio, but it complicates the fabrication. 

This multi-coil sucker was complete overkill:




http://www.builditsolar.com/Projects/WaterHeating/Matt1KSystem/SystemPhotos.htm

http://www.builditsolar.com/Projects/WaterHeating/Matt1KSystem/solar%20panel%202-1.pdf'

The coils in a commercial Ergomax DHW heat exchanger tanks are nowhere near this big, and 4-5 coils crank out 5+ gpm of 120F+ water out of a 130F tank just fine.  (I have one of them on my heating system as DHW for my house in MA.)

You don't need valves or anything else on the greywater HX- they're plumbed in-line with the water feeds and need no controls. They are low volume on the potable side, and the output temps don't typically exceed 75-80F with 100-105F drainwater. Potable-side plumbed in-line with the cold feed to the entire house (which is by definition ahead of the cold feed to the HW heaters), and drain-side at the main drain exit of the whole house is the preferred method from a total efficiency point of view.  It does mean that long cold water draws from other taps will be room-temperature-ish and never gets cold while someone is taking an endless shower.  3-5 gallons after the shower has stopped you'll be at street-water temp again. You'd be looking at something like this:

Well/street water> drainwater HX (output T to house cold)>solar tank HX> Hot water heater

If your normal late-summer water in from the street/well is already 75F you may want to T off the cold to the kitchen ahead of the drainwater HX, but feed the rest of the house with it's output.

For variations on plumbing configurations for drainwater HX see:

http://www.renewability.com/uploads/documents/en/home_retrofit.pdf

http://www.ecoinnovation.ca/en/residential-solutions/connection-types.html

The vertical type drainwater HX really need to be as close to perfectly plumb to max 'em out. Laid on their side the performance is degraded by 80-90%(!).   But they're completely passive & no-maintenance, should last and perform for decades. I suspect with the horizontal Ecodrain you'll be spending some plunger-time on a regular basis (TBD).

My problem with the drainwater HX is the vertical orientation, only one place in the whole drain system carries the flow from all three bathrooms vertically and that is 30+ feet from the water heater.

I'd found the heat-exchanger in a tank write up at builditsolar.com, it's simple and efficient. I've been researching flat panels and need to get a better understanding of sizing everything, 60 sq ft of panels looks to be in the ballpark.

Even 40' of 3/4" pipe from the drain to the HW heater is only a gallon of water, a sub-30 second delay @ 2.5 GPM flow. That's "simultaneous enough" flow to reap the lion's share of the benefit in even a 5 minute shower.  I presume the tie-in to the cold water feed to the house is a similar distance (and similarly not a problem.)

If individual horizontal tie-ins at the showers seems preferable, Ecodrain makes a horizontal unit.



I'd expect to get familiar with the use of a plunger with one of those (TBD), despite the proprietary no-stick coatings they talk about.  They're not NRCan listed yet- don't have much in the way of independent performance data on 'em, but even if it's only 30% it's way better than zero.

This looks like a safer "better mousetrap" variation on batch solar HW heating that might be appropriate here:

http://www.harpiris.com/howsuncacheworks.html

The stored heated water isn't the potable water, rather, the potable water is pre-heated as it passes through an internal heat exchanger.  There's 50 gallons of "dead" water thermal mass in it, but only a gallon or so of potable water, so it's purging 100% of it's potable water with every substantial draw, so risk of legionella from tepid stagnation temps is near zero.

The advantage to batch heaters is their simplicity & reliability- no pumps to control or service- all flow is provided by the water pressure.  Unless you're just DYING to do a DIY active solar setup, one of these is likely to be very cost effective.  Too bad it's not more freeze-resistant, or I'd bite:

http://www.harpiris.com/faqs/freezeresistance.html

You could still use drainwater heat recovery ahead of the batch heater to good effect.  Keeping interconnecting plumbing as short as possible and insulating it reduces the distribution losses to reasonable levels.