Best and acceptable media's for radiant heat for OUTDOOR applications.

Thaddeus · March 2017

Hi, my name is Thaddeus. I have recently purchased a new outdoor boiler and I am finding that I over sized it for my heat requirements. So with that said I am looking to use the boiler to its potential and heat a deck and walkway and maybe a small portion of my driveway in front of my garage.

So I have been cruising the internet and it seems as though there is to much information out there...

Now before you answer, please know that I am an educated person. I am a union Millwright, an FAA licensed A&P mechanic, a practicing electrician, and have solid basic plumbing skills.

My question is about heat sync's (media) in relation to application and climate, I live in Vermont and see significant snow fall in the winter months.

Because cement and asphalt are rather expensive I would like to know if sand, fine gravel, top soil, crushed hard pack, wood or any other readily available materials are suitable for radiant heating in outdoor applications?

If so, what kind of tube spacing and/or routing would be efficient and of equitable heat distribution thru a given media?

If I did a portion of my driveway should I use a flex additive in the cement to compensate for top load and frost heaving? Or should I attempt to make it a strong and inflexible as possible?

Are there chemical alternatives such as epoxy or hardening rubber that could be used as heat sync's?

Thank you for any help and/or advice you can offer.
Thaddeus

Jamie Hall · March 2017

Where in Vermont? Not that it matters, but I know the state pretty well.

The sort of radiant heating which you are looking at is a little unusual... but why not? If you have the heat.

The first question is: are you planning to pave this surface? Asphalt or concrete? It makes a difference. You don't need to, of course. I think if it were mine to worry about, I'd probably put down a layer of foam insulation board -- probably blue board (it's tough) and probably 2 inches thickness. Then I'd go over that with a foot to 18 inches of 3/8 to 3/4 inch stone. Then over that as a wearing surface, stone dust -- although you could leave that out. The insulation board should be sloped slightly to drain off water that infiltrates -- unless you are going to pave over the area. I'd use barrier PEX or PEX-Al-PEX for the tubing, as it can accommodate the inevitable flexing, and for the same reason I'd not lay it quite straight, but in a sort of gently snaking pattern. I'd lay it about mid-height in the stone.

I'd probably pave the whole thing with 2 inches of asphalt, to keep the infiltration down and make it easier to plow, but that's optional.

As to the amount of tubing -- that depends on how much heat you have to get rid of. I'd probably go with a 12 inch spacing, more or less.

Gordy · March 2017

Encasing the tubing in a rigid conductive media will give the best performance, and insure longevity of the tubing. I really don't care for the idea of tubing in abrasive medias that could let it fall victim to abrasive wear through movement. Movement from frost, vehicular traffic in times of thaw, expansion contraction during on, and off times. This could lead to tubing pumping its way to the surface.

I like concrete with a good sub base. You can pour sections as you can afford to. Asphalt your limited to the need of a paver, and roller, and asphalt is getting very expensive. you are a multi talented individual I'm sure you can finish concrete.

Concrete also has a lower permeability than asphalt. Asphalt really should be sealed periodically to give it longevity.

Any method needs a good sub base, insulation, etc. I'm sure you have seen in your search details for snow melt applications.

There are many different mix designs that give concrete added flexural strength. Air entrainment add mixtures protect from freeze thaw, fiber mesh adds flexural strength, along with certain plasticizers. Micro silica is the best, but pricey, and usually not batched only for high quantity orders.

There is also latex concrete which is very pricey, and limited to providers set up to do that type of concrete. It requires aggregates on site depending on how many yards you will pour. This method is usually limited to overlays, and not full depth applications. This method is also very time sensitive to application of wet curing method.

That covers vehicle traffic areas. Walk ways ect. You could play around with other surfaces. However the snow melt system still needs insulation to perform well.

Zman · March 2017

I agree with Gordy on this one. Good insulation and a concrete slab with rebar and tubing in the center.
The tubing in gravel adds a bunch of unpredictable elements. The change in thermal conductivity due to moisture, extremely slow response, excessive edge loss, and potential tubing abrasion are all on the list.

hot_rod · March 2017

An outdoor wood boiler by chance?

Thaddeus · March 2017

Thank you all for your advice. It greatly helps me in focusing my attention and research. Especially the foam board insulation.

To answer a few questions:
I live in Northfield Vermont.

I was hoping to avoid concrete or asphalt because they are just so expensive, but if I cannot then Ill have to save up...

The amount of heat I have... The boiler is rated at 240,000BTU (272k gross). I will be using about 1/3rd of that for automatic heating applications (house, domestic hot water, garage and hot tub)... As far as the (future) deck and drive way, I plan on a manual operation function. So that is to say... when I want to heat the deck or driveway I will load the appropriate amount of wood and activate the circulation manually, not by any automatic means...

to Gordy and Zman---- What about fine media like sand or even talc? can those be acceptable heat syncs? Or is there just to many variable's and unknowns to make it work?

And Yes an outdoor boiler... a Central Boiler Classic Edge 750. I choose it because of it internet connectivity option. Some other boilers had better customer ratings but I wanted to be able to monitor it frequently and from afar if needed. So far it has exceeded every single one of my goals and expectations.

hot_rod · March 2017

I doubt you will have much over 150,000 BTU/hr output from that boiler. They typically run around 40-45% efficiencies. That is full fired with dry hardwood

Keep you output expectations reasonable and about a dozen cords of wood on hand

Zman · March 2017

The problem I see with not having a hard surface is protection of the tubing. Between snow plows, vehicle wear and erosion, eventually the tubing will get damaged. There are materials other than concrete that will work. Concrete is by far the best.

Funny fact: wet sand transfers heat 5x better than dry sand.
Try running the numbers on a sand bed system.....

Gordy · March 2017

You can try what ever you like, so long as you are okay with well that did not work very well.

Conduction is king

Gordy · March 2017

I do not care for Jamie's example detail. To much granular material between the insulation, and the tubing. Hot goes to cold so you want the insulation as close to the bottom of the actual tubing as possible while keeping it imbedded.

One needs to think about off season usage of the driveway also. Being a mill wright / mechanic I assume you have a larger weighted down vehicle. You know how spring thaws go, and why roads get posted.

Thaddeus · March 2017

So as to the boiler, it is an EPA Phase II down fire boiler. It's effeciency is well beyond 50%. Using BTU calcs I estimated I would burn 7-8 cord this winter. I have only burned 4. And I have not filled my 5gal ash bucket yet. I readjusted my calculations given the known running experience and I estimate the boiler is running in the 90%+ range. So I am not in any way worried about being under on BTU's.

So the walkway idea (that I have been thinking of) I think may work given the comments here. Around me we have a crushed shale we call "hard pack" it is great for making driveways because it resists water and can handle heavy loads. I was thinking of this for the walkway.

After reading your comments I am certain that I will have to use concrete for the driveway. I guess there is no getting around that given I do have dump trucks entering my garage.

Thaddeus · March 2017

So let me ask this pertaining to the walkway. Say I use railroad ties as the border (already my plan) lay down 2" blue board, lay 3/4" PEX AL PEX in 12" wavy spacing on the blue board, then cover with hardpack and then go over the hardpack with 5/4 deck boards to make a deck-like walkway. Would a scenario like this work? (Bare in mind that the hardpack is very dense, almost goat-like consistency)

Leonard · March 2017

Styrofoam under tubing......I've heard ants and termites like to burrow in it. Maybe lay down some insecticide first, but being in VT I assume your on well water so that may be a problem.

Overall I wonder about just sleaving the driveway tubing, so if it leaks in future you can just dig up the edges and pull in new tubing. Will have a cost of lower conductivity from water to soil though, air gap.

I bigger question is why look for a place to dump the excess heat, just figure out a way to throttle down the boiler. Dumping excess heat will just cost you fuel and labor feeding the boiler.

Thaddeus · March 2017

There is a couple of reasons besides the convenience of clearing snow. I am looking for a summer time heat dump because I can throttle down the boiler but not to a low enough level to maintain the 2 things I actually want to heat with it, which is my domestic hot water and my hot tub... Both of these things together run me about $120-$150 a month in the summer. The boiler is a much much much cheaper means of heating then the very high electric bill. With that said, I have a regular trade with a local logger and get my wood for zero money, so my source is abundantly cheap because it is just a little of my time in trade for all the wood I need... As for the labor of loading the boiler.... Doesn't bother me a bit considering what it saves me in money

Zman · March 2017

Thaddeus said:
So let me ask this pertaining to the walkway. Say I use railroad ties as the border (already my plan) lay down 2" blue board, lay 3/4" PEX AL PEX in 12" wavy spacing on the blue board, then cover with hardpack and then go over the hardpack with 5/4 deck boards to make a deck-like walkway. Would a scenario like this work? (Bare in mind that the hardpack is very dense, almost goat-like consistency)

The only reservation I would have would be the lack of contact between the boards and the hard pack. I am envisioning the heat melting the snow very well between the boards and not as well on the middle. How do you feel about pavers?

Thaddeus · March 2017

Well, I am not against pavers, just trying to be cheap and within my skill level so I do not have to hire someone else... the last paver job I did looked horrible (bad enough I knew I probably would not attempt it again). Now that said, I was not planning on an air gap, my "idea" was that the pressure treated 5/4 planking would rest directly on the hard pack. Then replace the planking in 5-6 years as it rots with composite deckboards and never worry about it again... think that might work?

Gordy · March 2017

Thaddeus, What type of timber does your logging buddy deal with?

Thaddeus · March 2017

only hard wood. He has a fire wood operation in which he sells about 500 cords a year. Its good wood. Also something else I am doing with the boiler for a summer time heat dump is I have acquired a 20 freight container. I plan on flipping it over, plumbing the underside of the floor with pex and filling it in with epoxy just enough to cover the tubing and bond it to the container floor making the entire floor a heat sync. then installing a fan and a dehumidifier and presto, my very own high temp kiln... Also I have a saw mill and a kiln has been a dream of mine for a while so theres that too ;-)

Gordy · March 2017

Darn thought maybe cedar could have been bartered

What species hardwoods?

Thaddeus · March 2017

Primarily Beach, Ash, Maple, Birch (although my boiler dealer tells me to stay away from Birch because it is acidic), some Oak.

Gordy · March 2017

Thinking some live edge slabs of various wood species for that walk way may look interesting. Something darn near free, replace as needed. Of course a wood more resistant to moisture, and insects would be nice,but I see you dabble in the woods.

Best and acceptable media's for radiant heat for OUTDOOR applications.

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