Heating water for radiant slab with wood
I am Renovating a 40 x 30 Barn for my new shop. I have ripped the floor out of the back half ( 20 x 30, 600 sq. ft. ) for the machinery room. I plan to put in a radiant slab in itÕs place. I would like to heat the water with the wood stove. This way I can use the stove to heat the shop in the morning and at night when I leave the shop will stay warm (or at least above freezing) all night until morning.
I do not consider it an initially priority to design a system that will automatically maintain a certain temperature. ( i.e. an auxiliary electric water heater , however I would like to retain the ability to add that to the system later.)
I have received numerous suggestions from friends, they include:
1. Heat the water using copper tubing wrapped around a round barrel stove. He claims that the water will automatically siphon through the tubing to and from a reservoir tank. The heated water in the reservoir tank is then pumped through the slab and back into the reservoir.
This whole idea sounds great if it will work . My question on this methoed is : ShouldnÕt the hot water coming right out of the tubing around the stove go directly into the slab, intead of mixing back into the reservoir?
2. Simply place the reservoir above the stove and cycle the water in through the bottom and out through the top.
3. Another suggestion similar to #2 except the reservoir is built like a heat exchanger with the pumped water cycling through a coil inside the reservoir ( the reservoir itself being filled with water surrounding the coil)
4. Somone said why donÕt you just run a coil of some kind right in the fire box above the fire?
I am also courios about the size of the tubing I should use in the slab. Someone said that it may be beneficial to use larger than normal tubing to create a larger thermal mass of water in the slab as it cools overnight.
Any of these senarios create numerios questions. I would really appriciate any and all input. Hopfully someone out there has tried this.
Thank You , Eli
Edited 4/2/2002 9:08:36 PM ET by Eli Ellis
Replies
I hope you know what the heck you are doing or you are getting close to finding out if there really is an afterlife!
Sounds like you are beginings a phased project. You might want to run Pex tubing in the slab instead of copper, (less sensitive to corrosian). In the future you might want to continue the use of wood heat, but suppliment it with the use of oil or other sources of power. A multi fuel boiler could provide you this conveniece. The wood or coal can burn while you maintain it during the day, then as the fire burns low the oil burner kicks in to maintain a minimun temp. H.S. Tarm supplies multi fuel boiler and furnaces.
Good luck
Stephen Sullivan
I've done some work with in-slab heating. As suggested, PEX is a better choice over Copper. Also, check the web for radiant floor manufacturer sites. They offer some good design information.
Starting with the floor. 120degF. is the hottest that the water should be entering the slab. I can tell you that a continuouse feed of 120 degree water is not going to be an enjoyable experience. This brings us to the rest of the set-up. It's going to need to be a two loop system. Primary loop has a pump, boiler at the heat source, tubing to a decoupler (in this case a large container to hold lots of hot water). The secondary loop has a pump and an automatic 3 way mixing valve to to mix water returning from the slab with water being pumped from the decoupler. An adjustable thermostat located in the supply water line going to the slab adjusts the automatic valve to keep the supply water temperature below 120 deg. F. A second room thermostat shuts off the secondary pump when room temperature is satisfied. Here's the tricky part. A thermostat in the decoupler tank has to stop the heat from being added to the boiler (Big pot of water sitting on top of burning wood) If it were gas or oil fired, you'd just shut of the fuel. I don't know enough about wood burning appliances to go any further but I can tell you, if you can't get the heat off to the boiler, really bad things will happen.
Regarding proposal #1: (Heat the water using copper tubing wrapped around a round barrel stove) This will NOT thermoshipon fast enough on it on. Unless you use 3 or 4 inch pipe and have the reservior WAY above the stove. You've got to have a pump to move enough water through 1/2" or 3/4" tubing. I'd insulate around the tubing so more of the heat ends up in the water. And as noted, that water SHOULDN'T go directly to the slab. It needs to "tempered" or mixed with colder returning water to get down to 120F-ish.
Proposal #3: (Simply place the reservoir above the stove and cycle the water through it). Yeah, it would work. Except you don't have the control that Gorouser's scheme would give. That might be okay if the heat capacity of a load of wood is less what the floor can absorb. Here are the numbers: For a 30'x20'x4" slab, there about 90,000 BTU to absorbed in raising the slab temp by 20F. 90,000 BTUs are normally captrued from burning about 11 pounds of wood. But your efficiency in getting that heat into the water will be about 30%. So you could load up about 35 pounds of wood and walk away, knowing the slab will only go up 20 degrees. (Obviously, confirm these rough calcs during your initial testing).
Proposal #3 (a heat exchanger reservoir with a (separate?) loop of water pumped through a coil inside the reservoir). Seems to add a bit of complexity without benifit. I do something like that for outdoor radiant sidewalks because I have gycol in one loop and water in the boiler. But if it is all water, I'd keep it simple.
Regarding proposal #4 (run a coil right in the fire box above the fire). I've installed about 30 of these. Used 1-inch stainless steel with about 2 to 4 feet inside the firebox. That's about all that fits in a normal wood stove and we were only doing it for domestic hot water needs, not for space heating.
Regarding the "the size of the tubing I should use in the slab". Either 1/2" or 3/4" PEX. End of story. All your heat capacity is in the huge mass of concrete. Unless you put in many times the length of much larger pipe, the water in the tubing won't make much difference.
And about Gorouser's concern about shutting off the heat: Since you can't shut off the heat easily, you need to be able to dump heat. A blow-off valve that activates on high temp is one way to avoid building up too much heat during, for instance, a power failure. But once the water has been blown off, the metal things will get overheated. Unless you allow for a refilling valve (a la toilet reservior) to keep adding cold water.
This seems a project for someone who loves to tinker, design, and babysit the resulting equipment. If not, bail now. There are lots of off-the-shelf systems that are easier to spec and safer to operate than any DIY boiler will ever be. And if you have to get this inspected, give it up.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Edited 4/5/2002 1:30:10 AM ET by David Thomas
Seems to me if you plan on not "stoking your fire" at night, the slab would really never heat up to keep the shop warm.
radiant heat isnt a heat on demand. And somehow im getting the impression you think your shop will just warm up when the fire starts.
Im also thinking the natural siphon effect wouldnt be enough force to heat your slab.
Also you really need zones and not one huge zone. IN the huge zone the only warm area youll really have is the area close to the fire because thats the area that will absorb all the heat and by the time its made its loop back to the fire its cold. And combined with limited use, im thinking youll have just once fancy exhibt rather then an effective heating system.
Once radiant gets te desired area its very effective, but that requires time. And wood fire systems require you to maintain the wood, whereas gas comes from a magical pipe in teh ground....
Edited 4/5/2002 10:17:08 AM ET by BILL_1010
Good point about the zones. A manifold is normally used. The PEX loops start off on the outside of the structure and loop back and forth toward eachother. Tube Length is important in these zones. You want enough length to give a good drop to the water temperature returning from the slab. If the square footage is high enough more zone loops should be added to the middle. Getting the tubing right the first time is important for obvious reasons.
Regarding using gravity to move hot water. Not only does the storage tank have to be higher than the heat source but you need to get the piping right. Hot water leaving the heater needs to be piped higher than the cooler water leaving the tank but not above the tank water line. You need good stratification in the tank without alot of turbulance. The water leaving the heat source needs to be at the highest point at the heater source. the pipes need to run as close to vertical as possible. Try not to cause a trap.
The slab will need to be pumped from the tank. If you don't pump it, you will more than likely heat the building with the storage tank than the slab.
Gorouser: We agree on the beauty of a $139 HWH as an insulated, pressure-rated, 40-gallon storage tank. You can't possibly do it cheaper and better on your own. They come with P&T valves and will pass inspection if you strap them to the walls and keep them 18" off the garage floor.
We also agree on "zones", although I think of a zone as having its own thermostat. I'd have multiple loops (four loops of 150 feet each for 600 sq ft) manifolded together but controlled by one thermostat if the 20'x30'area is all open (i.e. no separate rooms with different heat demands).
You are right to detail some of things that need to be done right to get a thermosyphon to work. Broadly, the pressure drop through the piping must be less than the density difference multiplied by the height of the run. The devil is in the details - entry and exit effects at a the tank adaptors, lossings in fittings, etc. I have done thermoshypon-heated hot tubs (both residential and backpacking versions) and it takes some sophiscation to visualize all the issues and do the calcs correctly. The previously proposed "wrap the barrel stove with tubing" will NOT work with any reasonable tubing size and heat rate. An alcohol-based percolator scheme is more versatile in terms of allowable heights and distances but is even trickier to get right.
I've kicked this scheme around for myself since there is an airplane hanger in my future and my 13 acres has lots of wood on it. I envision a big tank (1000-2000 gallons) for 2-4 days of heat storage. But the killer is always the comparison between my time and the low cost of natural gas. Would I rather fell, buck, split, haul, and burn 80 pounds of wood? That takes what, about 40 minutes? Or pay $2.00 to the gas company? (i.e. work in the office another 3 minutes) Hence my perspective that these endeavors are better as a hobby than as a cost-benifit decision.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave, this thread rally caught my eye, as I recently built a new 40 x 60 building that will eventually house my woodshop. I put in an insulated floor slab with pex tubing run 18" o.c. running off a mnifold. I've been thinking about possibly having a boiler system that is dual fuel, probably wood/propane. But on the other hand, with radiant floors slabs it's such a slow reacting heat, I've about decided to stick with propane as the main source for the floor system, install a used woodstove to be the backup heat for when we are in the shop all day working, and I've got a used hanging, propane fired, ceiling mount, forced air unit for quick heat up if needed. I would keep the radiant set at a lower temp. to keep floors comfortable and maintain say 55 degree temp in winter. Scraps can be burned up in the woodstove along with wood from our 40 acre woodlot.
Bish
Bish: That seems a reasonable apprach. Use the radiant to maintain a minimum heat (e.g. 55F) and a fast-acting alternative to acheive a quick warm-up. However, consider a staged approach. Run any plumbing/wiring/thermstat wire needed for your hanging, propane forced-air unit. But wait a winter to install it. I find that my radiant garage responds fast enough for me. I keep it at 45F for my wife's car (or 55F if there's a lot of snowmelt off the car to evaporate). I reset the thermostat to 70F when I am going to work out there and it comes to temp in 60-90 minutes.
The extent to which radiant seems to react slowly is in part the large mass to heat up. But in many houses, it is also the carpet, wood, etc flooring that insulates and isolates the heat source (slab) from the heat's destination (room air). In a garage or slab, you have bare concrete - the best possible flooring for RFH. So consider giving RFH-only a try and add your forced air unit later if needed.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave,
Thanks for the info. I may do just as you suggest as the shop will be well insulated. The forced air unit came from a remodel we did, turning warehouse space into offices, so it is basically free. On the other hand, no sense in the additional work if not needed.
Bish
You state that you are set on using propane. Have you considered using a high efficiency oil boiler like a Bock? Depending on the cost of the fuel in your local area oil is actually cheaper and with the new systems it is even more environment friendly. Just food for thought.
Jason
All this sounds a bit extreme. I have heated a many workshops with just a wood stove.
I e-mailed dave winkelman to stop in here and contribute, he has written articles on the subject that are thoughtful and knowledgeable. http://www.winkelman.com
RJT
Good points Dave. I was thinking about what you said about inspections. Here's a $150.00 idea. If the storage tank (decoupler) was a domestic hot water heater, you could have it fitted with a propane orafice and leave the wood burner out for future connection.
Home made boilers make me feel uneasy when they are unattended.
Hi David,
A friend is planning his house and has come upon a site that caters to DIY. http://www.radiantcompany.com They recommend 7/8 tubing @ 16" and no oxygen diffusion under 145 degrees, and in emails he's shared with me, consider any preference for 1/2" as part of a "Wirsbo disinformation campaign." My question is, what are the numbers that support the pref for your recommendation? And that support the last sentence of the quote. He can't understand how with larger tube you'd need much more pipe to make a difference. I've reached the end of my ability to explain these nuances. Any deeper questions and I'll just say, "Well 1/2" @ 12" oc is what I put in my house, so pffffffftttttttttt." So what's the math that leads to the conclusions on optimal tube size and spacing?
Thanks. Jim
Edited 4/11/2002 9:30:52 PM ET by Cloud Hidden
Edited 4/11/2002 9:32:55 PM ET by Cloud Hidden
Jim: To slightly retract my terse sizing thought: You may have a benefit from larger tubing in that more even temperatures can be acheived throughout the slab and less pumping energy is required in bigger tubing. But I was answering about boosting the slab's heat capacity. Let look at two cases:
First: 30'x40'x4" slab with 1/2 tubing on 12" centers (1200 linear feet of tubing). Heat capacity = 400 cu. ft. of concrete x 144 pounds/cu ft density x 0.20 BTU/pound/F heat capacity of concrete = 11520 BTU/F. e.g. if you raise the slab 1 degree F, it stores 11,520 BTU/F. Factoring in the 12 gallons of water in the tubing adds only 101 BTU/F and subtracts 46 BTU/F for the missing concrete for a net gain of 55 for a new total of 11,575 BTU/hour. An increase of 0.5% in heat capacity for installing the 1/2" tubing.
If, on the other hand, you wanted to increase the heat capacity of the slab by 50% (not double it, just half more): Your tubing would have t o hold 104 times as much water. Therefore it must be 10x the diameter. And it's mighty hard to fit 5" tubing in a 4" slab. So let's say 2" pipe - that takes up half the slab's depth and seriously weakens it, but maybe that's okay for light use. You need 3,070 liner feet of 2" pipe, filled with water, to increase the slab's heat capacity by only 50%. But that implies spacing the tubing 4.7" on center. So not only is half the height consumed by tubing but 42% of the plan view is as well. And no way can you bend 2" tubing on a 2.3" radius.
I didn't present calcs the first time because it was so far from being practical I thought I could kill by shooting from the hip without aiming.
So have we put it to bed? Buying pipe to get heat capacity ain't the way to go. Buying more concrete would definitely increase heat capacity and it's the cheapest uilding material around. But than you'd have an even slower response.
But is bigger tubing (within reason) ever valid? Sure. Absolutely. Except for weakening the slab, bigger is better. Less pumping energy to pump X gpm if the tubing is bigger. And the option to deliver a given BTU/hour with a lower water temperature (avoid damage to vinyl flooring and excess expansion/contraction).
If your question is about pumping costs and heat delivery (and not heat capacity) that's another discussion. And a worthy one, I think.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
If I understand, the summary of your answer is that a water floor would hold more heat than a concrete floor. Of course, the rugs would get wet, but... :)
>If your question is about pumping costs and heat delivery (and not heat capacity) that's another discussion. And a worthy one, I think.
I'm game if you are, be/c I get tired of hearing the left hand say X and the right hand say Y and there should be definitive answers on something like this. So, to frame the question: For a 4" slab w/ 2" rigid insulation, and the room 1' above the slab is 67° and you want to raise it to 70°, what is the performance difference be/t 1/2" tube @ 12" and 7/8" tube at 16"? Make any otther assumptions you need as long as you state them. Put your pencils down after 30 min (oops, SAT flashback).
Bonus: what is the practical effect of striping with the wider spacing? I know that there must be temp dropoff the further you get from the heat source, but I don't know the rate.
Really, Dave, thanks for exploring this. Jim
If the slab is 3F hotter then the air will approach 3F hotter over time (I'm looking first at the long-term steady-state solution and ignoring the harder mathematics of the transient effects). The mental box I'm drawing to say this is the first foot of air and the flooring. That stuff has some R-value. A constant delta T across that R-value results in a given amount of BTU/hour whether the slab is, for instance, 87F and the air is 67F or the slab is 90F and the air is 70F. A delta of 20F either way gives the same BTU/hour and presumably the house needs the same BTU/hour is the outside temps haven't changed. (Actually, it needs a bit more at 70F than at 67F air temperature, but that is a small effect when it is 20F outside.
Assume minimal losses through the 2" foam. So how many BTU's to raise the slab (1000 sq ft) by 3F? 1000 x 0.33 x 144 x 0.2 x 3F = 28,800 BTUs. Now we can do some interesting comparisons:
3 loops of 1/2" tubing, 300 feet each versus 6 loops of 150 feet. Constant pumping pressure. Constant outgoing water temp of 90F, return temp = 2F about slab (average = 70.5F for this heating period). So BTUs delivered are the per gallon in either scenerio at 162 BTU/gallon. The simplistic appraoch would be to say that 1/2" pipe takes 3 gpm which is a good rough estimate, but of course the longer tubing loop takes less than the short loops. At 5 psi, the 300' loops each take 1.92 gpm (x 3 loops = 5.8 gpm total). And the 150' loops each take 2.72 gpm (x 6 loops = 16.3 gpm total). So the scheme with 6 loops instead of 3 longer loops comes to temperature almost 3 times faster!
And to fine-tune from our assumptions above. Return temp will be a but lower in the longer loop (5-8% benefit). And pumps are not truly constant pressure output, especially as you approach their maximum volumetric flow. So let's credit the longer loops with another 10-15% and acknowledge that you may be buying a bigger pump (e.g. 1/15 hp instead of 1/25 hp) to take advantage of the greater number of shorter loops. Still the shorter loops heat up the slab 2.3 times faster! And you only have a slightly bigger manifold to make, NO additional tubing required and a slightly larger pump.
Note that faster heating will only occur if your heat source can keep up. At 16.3 gpm, 19.5F delta T, you'd need 158,300 BTU/hour. But you would only need that heat rate for 11 minutes (which my HWH can do) versus the lower heat rate of 69,000 BTU/hour for a longer time of 25 minutes for the long-loop scheme.
Conclusion: Use more, shorter loops for better performance.
Next (and last) comparison: 1/2" versus 3/4" tubing. Assume constant pumping pressure, three 300' loops. Other stuff the same. 1/2" tubing takes 1.92 gpm per loop x 3 loops = 5.8 gpm total. 3/4" tubing takes 7.3 gpm at 5 psi x 3 loops = 21.9 gpm total. An improvement of 3.77 times! Again, you need a bigger pump to see that advantage. Leaving the pump the same would give you some benefit, but not that large. So with a constant delta T, the floor comes to temperature in 7 minutes instead of 25 minutes.
OR, reduce the delta T from 19.5F to 10F if you use 3/4 tubing. It still heats faster than 1/2" at a 19.5F delta (13 minutes versus 25 minutes) AND you get much more even temperatures. None of the floor has gotten to 90F, only to 80F and since a smaller fraction of the heat was removed in the first 50 feet of tubing, the end of the loop see more heat in the higher-flow, lower delta system.
Conclusion: Significant benefits in larger tubing (if you install a larger pump). You can get much faster heat delivery AND more even distribution at the same time. All for the cost of 3/4 versus 1/2" PEX. At my supplier, w/o my discount that would $667 for 900' of 3/4" versus $421 for 900' of 1/2" PEX. A $246 difference. And $30 for bigger fittings in the manifold and $50 for a larger pump. So $300 for a significantly better and more flexible set-up.
So, was that helpful, or were there too many numbers?
David Thomas Overlooking Cook Inlet in Kenai, Alaska
>So, was that helpful, or were there too many numbers?
Fine on the numbers. Might not pass a test unless it was open book, but followed along.
Lastly (I _think_ lastly anyway), increase the spacing on the 3/4 by 33%, be/c the company that's getting all hot and bothered in this is recommending 7/8 @ 16" and making it sound like any other recommendation comes from Satan himself. That would result in 25% less tubing which would almost equalize the price. I'm assuming that it would cut the btu transfer by 25%, too, usless you lowered all tube lengths equally to benefit from your first conclusion. But this still gives big advantages over 1/2" at no cost penalty, yes? Of course, you also pointed out the need for a larger pump and a larger heat source to avail yourself of the greater capacity for heat transfer, and this also comes with a pricetag (for example, I'd think it would be easier for a gas boiler to meet the steep btu/h requirements than a ground source heat pump--a kind of hare v tortise comparison). Nonetheless, the numbers seem to favor the larger tube at wider spacing.
Which begs the questions: what's the effective upper limit on tube/size and spacing. 1" @ 18"? something else? And, following your numbers, why would anyone use 1/2", unless as in my case here, they were limited on heat capacity (our well depth limited the GSHP to 4.5 tons, for example) and thus couldn't take advantage of the higher possible transfer rates afforded by larger tube, be/c they'd turn out to be only theoretical for us?
Thanks. This is fun.
Jim: Right on all counts. Going to 16" spacing would eliminate the cost premium but still offer significantly better heat delivery and the evenness of heat distribution. BTU delivery would be 25% less than in my expample. Any pump will pump somewhat more through 3/4" than 1/2" (or 6 loops instead of 3). But I can imagine some people are installing oversized pumps without knowing it. Or undersized. Just buy whatever circulator pump is cheap or available or kind of worked last time.
You'd need a bigger burner (or a storage tank) to get a better heat delivery rate, but again, some systems may be constrained currently not by the heater source but by the tubing. And you'd get the more even heat distribution even if your heat source couldn't keep up. A higher flow of cooler water delivers the same heat (i.e. everything the heater can put out) but whole slab gets heated instead of the area nearest the heater getting so much hotter sooner than the rest of the floor.
Everything I've said about uniformity of heat distribution is about one large area of floor versus another. Not about "stripping" - ever wet down a bare RFH slab? makes it easy to see where the heat is. Rather than do the theoretical calcs, I might go crank the garage slab and measure it with the infrared thermometer. (The house is already out of the heating season because with 35F sunny days and 15F nights, we leave the windows cracked all afternoon and evening.) SO I'll get back to you on the 12", 16", 18" o.c. question.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks. Looking forward to the answer.
Bluntly, is Wirsbo blowing smoke with their 1/2 @ 12 recommendation? Is 7/8 @ 16 definitively superior? Is 1 @ 18 even better? Plain or peanut? Ha ha ha
Did the ol' water on the slab trick. Best for me is barefoot.
Jim: From where I sit (in a RFH house in the Arctic), 300' loops of 1/2" is bogus. 150' loops of 1/2" I could accept. 150' OR 300' loops of 3/4" would also be fine, arguably better. 1" would be past the point of diminishing returns even ignoring the strutural aspects regarding the concrete.
Spacing I see as a function of heat load per square foot. A well-insulated house or one in a mild climate doesn't need to have ALL the floor at max temp. And since it doesn't need a lot of heat, many of these concerns are minimized. But a house with poor energy efficiency and in a cold climate needs all the heat it can get (short loops, big pipes, closely spaced, big heat source).
Spacing is also a function of flooring. Under carpet, the slab is pretty uniform in temp, even with 18" spacing. The spacing isn't the limiting factor, the flooring is. Only on tile or vinyl, or bare concrete is the stripping perspectible. Because it is a more effective heat exchanger, the slab loses more of its heat near the tubing before the heat spreads to the areas between the tubing.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
It is marketing pure and simple. In order to market to a DIY market like they do you have to discredit the pros. One good way to do that is by discrediting one of the biggest supplier to the pros.
Anyone who has been to Wirsbo's plant and seen the amount of research they do would not take their recommendations lightly. I may not agree with everything but when someone starts mentioning their dis-information campaign it is obvious to me there is a whole lot more at work here than just a differing opinion.
Hope you now have a better understanding of WHY they say this stuff aside from the technical issues. I am not just spouting opinions without having done my homework on this issue. They make thier living by bashing the pros. And by selling inferior design systems which are illegal in many jurisdictions. They tell you they will fight the inspector for you. What hogwash.
When I see marketing like that, I'm always suspicious. Trying to get to the facts is sometimes difficult.
If you had to decide for your own house, and wanted RFH, what size/spacing of tubing would you look at first, assuming that's a fair question?
With Wirsbo, I'm curious about why they speced 300' loops of 1/2" rather than 3/4. They could have had just as much money from me...
Jim: I promised to measure the temperature variations in my bare garage slab. while keeping the thermostat at 45F, towards the end of a heating cycle, I measured several paths perpendicular to the tubing layout with a non-contact infrared thermometer. Of course, the "period" of the pattern was 12" because that was the tubing spacing. Typically, the temperature peaks were 3-4 degrees above the valleys (e.g. 52F over the tubing and 48-49F between the tubing). A touch less at the far corner. More temperature difference only very near where the two loops start. There, high = 55-56F and low = 49-50F. So with 1/2" tubing on 12 centers, it seems that there is not a lot of temperature variation between the tubing. And this is bare concrete. With any flooring, the slab will be MORE uniform in temperature. Details: 4" slab. 1/2" Wirsbo tied onto the 6"x6" #10 WWF. Tubing is probably about 3" down because dobbies or saddles weren't used to elevate the WWF and tubing.
Now it's got me wondering about 18" spacing. That would need 44% as much tubing. Which would not only for 3/4" tubing (versus 1/2") but would also more than buy the larger manifold to use 150' instead of 300' loops.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Can you rebuild the floor to test that, please. For me.
Ha ha ha! Thanks for the numbers.
Sure, We'll be pouring the slab for the new in-law place* in a few weeks, once the nights get close to freezing (currently 10-15F). I definitely using off-brand $0.20/foot 3/4" PE tubing this time. And two loops of 3/4". Let me kick around 12" versus 18" spacing a bit but I might experiment with that too.
*It's all those danged summer house guests. Rather than scheduling the guest room and losing our privacy, we'll just build a 300 sq ft detached effiency.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Could you try to be a little less accommodating? And are you really gonna make me wait till next winter to catch the results? Geez, some people...
Will you make sure the tube has O2 diffusion barrier? The company I've referred to that recommends 7/8 @ 16 also says the barrier is only necessary if > 145 degree water and there are steel components in the pumps or fittings. What the hell do I know? What do you think?
It has happened here, Ireland, that use of non oxygen barrier pipe caused steel oilburning boilers to rust and leak in 5 years from new.
Dermott
Last time I tried to breath through the walls of off-brand PE tubing, I got really blue. Just as deoxygenated as when I tried to breath through 1/16" thickness of fancy PE available only in plumbing supplies stores to licensed plumbers. Copper pipes in the manifold, bronze pumps - that stuff sees oxygen all the time in potable piping. No cast-iron boiler, so I'm going to go for it - no guts, no glory.
Seriously, I agree with that recommendation. Steel or iron or high temperatures make oxygen more of an issue. 110F in copper and bronze will last forever - that is what goes thru everyone's regular pumping all the time.
The other thing I'm going to do on the new building is a thermostatically controlled vent fan that pulls air from the high point of the cathedral ceiling. I debated about but didn't do it in the main house and now have to open two windows whenever it is above 45 outside (30 if sunny). Would be nice if that were automatic. Then a very secure cover for the heating season.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for all the great info.
I have a few questions if your willing:
- How critical is the temperature of the water entering the zones?? I understand that 120 degrees is prefered, but I'm not after heat on demand. It can take all day to bring the temperature of the slab up. In the middle of the winter here in Maine I should think that even if the water was 70 or 80 degrees it would effect the slab over the coarse of 10 hours.....????......
- I recently aquired some 1 inch PEX tubing for free at the dump. They built an ice skating rink nearby and they used this stuff for the coolant I'm told. There was a huge ammont of it , and all in brand new spools!!!! I took all that my truck would hold. Anyhow my question is obvious : Is this tubing suitable to put in a slab?? Does size matter in my situation, especially when it is free tubing????
Thank you very much,
Eli EllisBrookfield Woodworking
Cushing, Maine
Eli: Larger tubing lets you use lower temerature water to deliver the same heat. And you get more even heat distribution as well! But it is a classic capital cost versus operating cost balance. And the people who build most houses actually install 14 gauge romex instead of 12 gauge. So of course the no brainer is to use the cheapest components that can be made to work because, after all, you not going to live there, right?
But if you ARE going to live there? Then spend an extra $20 to use 12 gauge wire throughout. Likewise, spend an extra $150 and use 3/4" PEX where others get by with 1/2".
120 degrees is not preferred. 120 degrees OR LESS is preferred. Higher temps can discolor vinyl flooring. And a 120F floor is noticeable hot to one's bare feet. If you can deliver the necessary heat with a lower temperature water, great. Do it. And having larger tubing makes that possible. And getting the tubing for free is fantastic. Check with your concrete gal/guy if the 1.25" inches of missing concrete is any structural issue. And if you are using gycol for an outdoor loop (like my snow-melt sidewalk) you could be buying a lot of anti-freeze. But I like plain old water for an indoor loop and the 1" free tubing gives you options most people don't have. Note that you'll need higher volume circulator pumps to take full advantage of that tubing. NOT higher pressure, your pumping losses will be very low.
Where's the spool you grabbed for me? How does it go in grade school, "If you didn't bring enough for everyone. . . ."?
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for the info David
- Eli EllisBrookfield Woodworking
Cushing, Maine
Boy am I late to this party, but if you see any more of that pex, pick some up for me, too.....you'd have thought the ease of unloading surplus on e-bay would have made dumping stuff like that SO 10 minutes ago.....
Eli Ellis,
Unlike David, I would not be so inclined to emphasize the difficulty of what you are attempting. It is not too hard to put a stainless-steel coil inside the firebox of a steel wood stove. As long as your storage tank is situated higher than the stove, thermosiphoning should work fine. Be sure to have at least two PT relief valves and somewhere to dump the extra hot water safely, and be sure your loop is pressurized with a cold-water feed to make up for the blow-off water. This is not a boiler, it is a wood-fired hot water system, and people do it all the time. While I have never run a radiant loop off such a system, people run radiant loops off gas-fired domestic water heaters all the time -- so what's the difference? Check out the JLC article from a few years back on using a hot water heater to supply a radiant floor.
My first thought would be to go with 2 separate systems - A dedicated boiler for the in-floor heat, and keep the wood stove separate.
That way it wouldn't take so much babysitting and tinkering to keep the thing goping, and you wouldn't have the potential for any major troubles the other guys have mentioned. You could fire up the wood stove when you were in there, and just use the slab heat to keep it barely warm or to heat it when you were gone.