Heat exchanger plate and snowmelt combo
| Posted in Energy, Heating & Insulation on
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If anyone has an installation mentioned in the subject line, I would appreciate hearing about the PEX brand used for this application.
(I’m looking at Wirsbo).
I would like to add a seasonal loop (to our gas boiler)for the patio and driveway that will be heated via a heat exchanger plate.
Thanks
Alan
Replies
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Some rules of thunb are as follows:
100 to 150 BTU per sqaure foot (that adds up fast for a driveway). I have a snow-melt sidewalk running off of a hot water heater. But it is a total of only 300 sqaure feet (30,000 BTU/hour).
Spacing of 6" on center is recommended.
I insulated under my sidewalk with 1" of blueboard. In a driveway, often sand is used as a normal sub-base for concrete. It also provides a bit of insulation from the unlaying ground. Running the tubing into the sand base at expansion joints allows for movement of the slabs relative to each other.
140 to 180F outgoing temperature. That is higher than indoor RFH because the heat delivery is so much greater and at 3 gpm in 1/2 PEX or 7 gpm in 3/4 PEX, you need either a bigger temperature differential or more loops with a larger manifold. However, more loops operated at a lower temperature is more efficient (why does any of your slab need to get to 140F?). But long loops are easier to install and require less manifold piping. 140-180F will be hard to do with a HX. But lower temperatures can work fine (and are more efficient).
If you have 140F going out and 50 F coming back, you deliver 72,000 BTU/hour per 1/2" loop. For 6" OC and 100 BTU/hour/sq ft, that gives a loop length of 1440 feet. For 3/4" tubing, 251,000 BTU/hour is delivered and that could work for a 5,000 foot loop. At those extreme lengths though, you'd need a pretty high pressure pump.
Operationally, you can either 1) look outside, say, "Darn, it snowed", and flip a switch to melt it all. Or 2) shovel or plow the fluffy stuff off and let the system melt and dry the remaining icy stuff. I've done both. And for a sidewalk, it is so quick to run a shovel 100 feet out and back, I usually do that first. -David
*Thanks so much for the reply, David.I have only baseboard loops in two zones, so the water temp is at 180.There will be a problem with the return temp being too low, however, so the return loop may need to be tempered back up a bit with the supply water before going back into the boiler.Also, the boiler is rated for 93,000 btu and if I try to do a driveway it may be necessary to run only the snowmelt loop when the need arises.That shouldn't be a problem.I do appreciate the specific numbers you gave me and that will be very useful in figuring out the loops, etc. and also the practicality of it all.I don't mind shoveling and it doesn't snow like it does in Alaska.The driveway pavement will be paver bricks on sand.Thanks for your help,Alan
*"baseboard loops at 180." So the HX output will be about 130-140F, which is fine."problem with the return temp being too low" Yes, you can run the return loop through a tempering valve. The valve output would be set to 120 or 130 or whatever the boiler wants. Its inputs would be the return from the driveway and essentially another "zone" off of the pump. No solenoid valves on that "zone" though. The tempering valve will only take water from the circ pump when it needs the hotter water to temper the return water. This may dictate a larger pump (e.g. 6 gpm of 50F water would need 9.6 gpm of 180F water for a tempered return temp of 130F at 15.6 gpm). So in that hypotheotical case, the pump needs to move 16 gpm at enough pressure to get it through your longest loop."boiler is rated for 93,000 btu" which would handle 1000 sq feet of driveway. More at a lower heat delivery rate which would work, just take a bit longer. "The driveway pavement will be paver bricks on sand." That will degrade preformance of the snowmelt system compared to tubing imbedded in concrete. Tubing imbedded in concrete has great thermal conduction into and then through the concrete. Tubing placed at the top of the sand below pavers will only be touching the pavers on one side, not encased in concrete. Also, the 1/8" sand gaps between pavers will make each row of more distant pavers distinctly cooler. Ideally, tubing would go under every paver (by running perpendicular to the pavers' length). In one sense, all the heat eventually goes into the pavers, sand, and snow; but the longer it takes to happen, the more you are heating the great outdoors. Higher fluid temperatures (good HX and not overly long loops) will help achieve faster heat transfer but still not as well as a radiant slab.I know the above paragraph is true from a theoretical basis. It WILL take longer to melt snow through pavers. But I haven't tried it with pavers and maybe the effects I describe would still result in acceptable performance. Try to get a manufacturer (such as Wirsbo) or installer to report on some paver applications. But if they recommend against it, believe them. You could consider a dyed concrete slab imprinted with a brick pattern. They lay down plastic and press a pattern in with a large roller. Looks pretty good when well done. -David
*Thanks, David -You've spent enough of your time guiding me through this.This is more of a tinkering project for me and it would be hard to justify laying a concrete driveway to more efficiently accomodate a PEX loop. I may scale down the thought and come up with something that will melt a "track" for the wheels to ride on, which would help with our 25 degree sloped driveway that's only about 100' long.It would be maybe 4-5 times a year when I wished for a heated path instead of throwing sand and gravel for traction.Since we are looking at paving the driveway I thought it would be a good idea to consider the PEX loop ahead of time.The brick patterened dyed concrete is a good idea.Our town ripped out the brick paved crosswalks with them after the bricks started settling from heavy traffic.Take care and thanks for the technical info,Alan
*Alan....lay pavers in Stone dust that has been well tamped...and you will have much less problems with settling pavers. And remember to sweep fill the joints well...We leave it for a few days...then clean up with one last sweep/fill.near the stream,aj
*Thanks, AJ -I just did a 12x30 paver brick patio with 4-tons of crusher run base and 2-tons of sand under the bricks. Used a gas compactor on the soil base and crusher run. In hindsight, I wish I had used stone dust instead of sand, but it should be ok for a patio.Do you use stone dust also for sweeping in between the bricks ?I still have about a ton of sand left either for top sweeping or a nice sandbox for the kids.I definitely will use stone dust for the driveway and thanks for the reminder. (This, ofcourse, with a crusher run sub-base.)Alan
*Stonedust underneath is the most important...try the sweeping of top either way yourself and decide.near the rockhard stonedust paved stream,aj
*Thanks, AJ.My neighbor jumped on the bandwagon after he saw several cubes of paver bricks in my yard.He went with stonedust for his patio "sand" base and it packed down very hard and flat. I was impressed at how well it held togetherIt could be a paver surface in itself.Might be a little tougher then sand for screeding and keeping things level flat, though.But I see what you mean.It will be stone dust, for sure, for the driveway or parking spots.Alan
*Alan...screeds easy too.... you screed with loose on top of packed mostly...think layers...foundation...rough screed...lay more dust, finish screed...the closer you get to finish...the less material used...so it goes quick.near the stream,aj
*Yes, think layers...think layers....Just one more thing.Especially for a driveway, do you recommend using a power compactor on stone dust or does it hold together well just by screeding.Reason I ask is there will be a compacted crusher run as the base and the brick, ofcourse, will be compacted after sweeping on the dust.(Textile, crusher run, textile, stone dust, then brick.)Thanks for the tip, AJ
*Your first layer of stone dust needs to be compacted and any significant added layers. A hand tamper nearby.near the stream,ajTwo geo layers is overkill but fine.
*You're right about the overkill.My friend owns a contractor supply biz so I figure what the heck.Then there's the matter of our area being kinda heavy in clay.Spreading out an extra layer on the bottom shouldn't hurt.Thanks so much for the advice you've given me.Alan
*Alan, Most certainly compact the stonedust to be used in a driveway. A plate compactor is worth it's weight when doing this.I generally place a layer, plate compact it, then go over it with a 36" landscaping rake, raking back and forth. The tines take down the high spots. Flip the rake over and use the flat side to redistribute the loosened stone dust. Compact again.I don't mind the workout of using a hand-tamper on smaller jobs, but on large ones a plate compactor does a better and more consistent job.