Outdoor entertainment pad

Thaddeus

Hello everyone! I'm looking for ideas, and best suggestions.

I would like to build an outdoor concrete heated pad with my fireplace in the middle.

I live in Vermont and this would be for winter outdoor entertainment. 

I have an outdoor boiler, and 1000g thermal storage tank.  Water temp is 170-185 (low/high settings on boiler)

So I have a plan of a radiant pad around the fire pit. That can be turned on and off as needed. Ill be using ethylene glycol as the heat transfer fluid because I'd like the ability to turn it off and not worry about it.

So here is my question for the Post. All of you here who have had experience with Radiant Heating I am asking for your ideas and suggestions. I don't want anybody to get into a big debate or argument. I am simply asking for your thoughts on this subject. Like if you were to build it, how would you build it?

Thanks so much for anyone who participates! I appreciate anybody who takes the time to comment.

kcopp

Sound interesting. Outside in New England can be pretty cold. The slab could be heated for snowmelt. Typically for that it you figure 150 btu / ft2....that get the slab to just above freezing.
Radiant also takes a good amount of time to heat up...so turning it on and off randomly would take a lot of run up time and energy.
Not sure that is practical to heat the area.

Jamie Hall

Sounds like a nifty idea. The main addition I'd suggest would be a wind shield around at least half a circle -- probably glass on frames above benches. At least high enough for a tall man to be sheltered.

If I were doing it I'd probably use flat stones -- scrap slate? If you're down Rutland Castleton area? -- set on a concrete slab, and run the radiant in the slab near the top of the concrete. Insulate under and around the edges, of course.

Should work wonderfully. I'm a little jealous.

hot_rod

Are you trying to warm the slab, or just keep the snow off?  As Kevin mentioned a melt pad keeping the surface just above freezing will run 150,000 btu per square foot
what is the are you plan on heating? What is the boiler output?

heathead

I would do radiant gas heaters in the ceiling, it will make it feel warm, Unless you are just doing snow melting then the radiant in the concrete. I don't think you could get the slab warm enough to make you feel warm in the winter.

mattmia2

you should probably use a propalyne glycol hydronic antifreeze otherwise you need more strict separation between anything that is connected to potable water and the antifreeze loop.

hot_rod

heathead said:

I would do radiant gas heaters in the ceiling, it will make it feel warm, Unless you are just doing snow melting then the radiant in the concrete. I don't think you could get the slab warm enough to make you feel warm in the winter.

Bingo, some of those nice stainless mushroom infrareds

Thaddeus

Awesome! To answer a couple of questions or concerns to get a better understanding. 

1) boiler has a 272k BTU output. It is unpressurized open circuit. Set temps are 170 low 185 high
2) I am installing a 1000 gallon converted propane tank 30 feet from boiler location, it will be fully insulated and connected to boiler with 1.25" ThermoPex. Circ pump to be determined.... but I'll probably go as big as possible within calculation limits so I do not over draw on boiler capabilities. 
3) ethylene glycol circuit will be seperate and heated thru a heat exchanger. 
4) I would like more than snowmelt capabilities. [I LOVE THE IDEA OF RADIANT TUBING CLOSER TO SURFACE] It would be nice if I could possibly get slab to 60 or 70°F but not necessary if it would be too problematic. 
5) on/off feature... I understand completely it will take time to heat up. This is something I would plan days in advance, if the weather looks conclusive, it I would turn it on mid week and maybe keep blankets on it until it was time to enjoy it...
6) my heating plan as of now (subject to change with new information) is 1.25 ThermoPex ethylene glycol loop to the pad, and using 1.25 pex as radiant tubing in pad to get as much surface area contact as possible. And circulator pump to be determined based on actual installation parameters and BTU calcs at the time.
7) I do absolutely plan on having those radiant mushroom heaters! Thx!!

Zman

How big will the slab be?

1.25 tubing is really big. It is expensive, hard to work with and may create week spots in the slab. Not mention the pumping requirements.

A typical snowmelt slab would use 5/8" pex at 9" centers. Ideally the loops would be around 250'.

It would be a good idea to mix the heating loops down to < 140 degrees to reduce the chances of thermal stress cracks.

At 150 btu/ft, you should be able to get the surface temp to 50 degrees or so. If you cover the slab, it will get a lot hotter. If you get the covered slab up to 80 degrees or so, it would probably stay that warm for a few hours after the cover is removed. With good wind protection, I think it would be very comfortable.

Don't forget to install a minimum of 2" ridged insulation underneath and at the edges.

So, when's the party?

Thaddeus

Hopefully the party will be next winter!!!

So right now, I invision the slab to be 20'x20' with a 5' diameter hole in middle for the fire pit. (One possible scerio is I make the slab round, but that requires rounding forms....) A 4' culvert will serve as the fire pit and the 6"radius between the concrete and pit will be filled with 3/8" stone for insulation to protect concrete from fire heat. 

I work for a concrete company, so I am going to take advantage of employment discounts and plan on a slab that is double rebarred 12"c with mesh grating and concrete fiber added. Im told by the concrete inspectors that we have ingredients that can be added to enable the concrete to withstand higher temps without stress cracking. The reason is for this overly strong slab is that the slabs physical location is on an area of a lot of ground water and thus frost heaving. I plan to dig down and install proper drainage, but I like to build things strong so I dont worry about them again. 

I am aware of the cost of pex. No worries there. But in my thought process, this loop, being a single purpose loop, I figured 1.25pex because why change sizes if I dont need too, thus increasing the amount of fittings and connections and I could locate the circ pump at the heat source instead of running wires to run smaller pumps near the slab. HOWEVER.... if 3/4 would do what I'm asking, which is more than snow melt, I can and will use that. To my knowledge ThermoPex is only available cost effectively in .75 and 1.25. Other variations are custom and expensive.  I have both. As this is not the only heating project planned. 

So with those answers.... do they change your thoughts at all??

Jamie Hall

Not really... except I would go to the trouble of making the thing round, or perhaps octagonal. Octagonal. with 8 foot sides. That would give you almost 20 feet across (19' 2" more or less) and you could use straight forms, and the windscreen could use 8 foot flat panels on the four sides upwind... but that's me. And I think I would arrange the piping not as a conventional grid, but as a spiral -- or actually two spirals, one starting at the edge and going in with the return coming back to the edge, and one coming with the feed in the centre and coming back out between the spirals going in. Taking advantage of the fact that PEX bends!

The question of pipe size isn't actually trivial. You will need around 55,000 BTUh to output 150 BTUh per square foot. If you want to limit that to a 20 degree delta T, that's 5 gpm. 3/4 inch will do that nicely. Inch and a half is going to be rather low velocity, so I'd be inclined to go with the 3/4 inch. Very roughly, i you set the spirals on 6 inch centres, you'd have 6 spirals, averaging 36 feet each on the spiral going in, and 6 spirals on the spiral going out. That works...

But as I've said before, that's just me!

Zman

The round concept would be cool. Don't fear the round forms, just swing the radius and paint it on the ground. Drive stakes on the line and bend the forms. Flexible material like masonite siding works great.

It is nearly impossible to design around the the thermal stress that dumping 180 degree heat into a frozen slab will cause. 3/4" tubing would work well in a round slab, you can just follow the curve. 3/4" tubing at 6"-9" centers would work well. Do 2 or 3 loops tied to a manifold. You could run the 1.25' to the manifold and locate the pump at the boiler. I would absolutely mix it down. You want to bring the slab up to temp slowly and evenly.

Don't blow off the insulation or you will need to defrost the ground in order to heat the surface. Radiant heat does not rise, it goes evenly in every direction.

Thaddeus

Wow! Thats very reassuring guys! In fact it now appears to me that going round would in fact make this simpler.  I didnt think about velocity so the 3/4 makes more sense. Especially if a spiral layout will work. 

I dont need wind guard as the area is well protected from trees and I get very little wind there.  But I'm thinking with this input I'm going to go round, with 4 or 6 inferred heaters added.  

I need to get my drafting table out and start drawing this. When I do ill post the picture to this thread! When I build it, you'll all be invited for a winter night bonfire where you can wear sneakers and t shirts!! 

Zman

I am thinking flip-flops for me....

mattmia2

The ~300 ft of 1.25" tubing may also be difficult to get enough velocity in to effectively purge.

GroundUp

I have done a number of these types of heated patios, outdoor kitchens, hot tub areas, etc and a couple even using OWB like yours. My first question, is what is the model of the OWB you will be using and what is the thought process behind the 1000 gallons of storage? Second, please reconsider the ethylene glycol thing and spend the extra $1.79 to use propylene. Third, ThermoPex is the insulated underground lineset from the OWB to the load, not something you'd embed in the slab. Assuming you embedded 3/4" pex in the patio slab, you'd likely want 3 loops for maximum output which would require a manifold location somewhere as well as a location for the mixing valve, expansion tank, air eliminator, pump, and heat exchanger. Is there any sort of cabinet for all of this equipment to hide?

PerryHolzman

I'd put a minimum of 4" of 25 psi foamboard under the concrete, and around the edges. There is no reason to heat the ground.

I also think you are better off with 3/4" pex (with O2 barrier) than larger.

Vertical infrared heaters are a must as well - as that is what the people will notice (not just the warm concrete). Those are unlikely to be usable with the water temperatures that the slab requires. You may have to install those separate.

I see no reason for your large storage tank (just build a normal slab heating system). It will save you a lot of $$$ and future grief. Also, you would have to build a spill containment system (it will leak someday). Glycol - of any kind is nasty in large quantities.

Have fun and be sure to invite us all to the party...

Perry

Thaddeus

Hi. So thanks for the new input. To answer a few of the questions.

Central Boiler Classic Edge 750. 272,000btu

The 1000g tank has other purposes. My garage will be heated with hot water Modines. The current plan is 1.25" piping for the 3 Modines heating the garage. The tank is serving as a heat storage/surge tank as the modines will draw to heavy on the boiler. 

Ethylene vs propylene? It was my knowledge that propylene can and will freeze. As I do not wish to run this component all of the time, it was my conclusion to use a plate heat exchanger and ethylene. 

To address the other components you list, I had hoped to make this as simple as possible... 1 or 2 loops, 1 circ pump 1 heat exchanger.

hot_rod

PG is a non toxic glycol, EG is considered a higher toxicity, not the best fluid for your application. The freeze point of either is based on the % you mix

Thaddeus

Yes, I've used pg in marine winterizing. It does infact freeze in pure form. Its indearing quality is it does expand. I used it in every aspect of winterizing marine vessels including running it thru ice makers and making pink ice cubes with it. 

I want the ability to shut this component off for upto or more than a month. I might only use it once a year, or 30 times a year. The point is, I want to be able to leave it unattended for prolonged periods if time. EG, unless there is something better that I can afford, is the best choice for me.

I do thank you for your input and if it were anyone else asking this question they might not know the difference. But I do. So I am open to other suggestions. But knowing my environment and temps, PG is not an option for me.

hot_rod

Dow is one of many brands of glycols, they offer a DowFrost a PG and DowTherm an EG.

DowFrost PG or HD which is heavy duty, a bit higher operating temperature for solar, is the most common glycol used in residential hydronics and protects to -50F, do you see colder temperatures than that? DowFrost or mother brands of PG is used in snowmelts across the US and Canada.

DowTherm EG is still used in larger commercial applications, less $$ and a bit better heat transfer. But it should not be used if potable water is generated with the system. Unless you use double walled heat exchangers.

bill_brooks

move to florida.
i know. i know. there's one in every crowd.

GroundUp

Marine/RV antifreeze is a propylene based fluid, but it is not the same as propylene glycol used in heating systems. I know you say you know the difference, but you don't. EG is toxic and should not be used for this purpose. PG is the proper choice (NOT RV type), and when mixed to the proper solution can remain fluid down to whatever temp is required. Forget about the EG. Something like Hercules Cryo-Tek can be purchased in a premixed 45% concentration and is perfect for snowmelt applications like yours, and will still pump when it's -40F outside. I have thousands of gallons of it in service doing exactly what you're looking to do.

Still wondering where you plan to place the manifolds, circ, mixing valve, HX, expansion tank, controls, etc to operate this system and why you feel the need for 1000G of storage with a boiler that's designed to run with onboard storage

Thaddeus

Okay, I stand corrected on the antifreeze. I hadn't done any research on the matter and just figured that ethylene would do the job.

To answer your question for the thousand gallon tank, I am a person who does everything in excess. I believe in being 150% prepared not 100% prepared. I plan on hitting my garage with some large Modine heaters. Three of them 150000 BTU. Knowing that those heaters might run together oh, I want the extra capacity in my heating system so as not to starve the other components outside my garage such as snow melt pad, hot tub, house and hot water.

GroundUp

That's..... Not really how storage works here. Yes, it will lessen the blow on the system for a few minutes but then you're back in the same predicament trying to recharge the storage before the next heat call. You'll lose more from the tank and associated piping than you'll gain by adding it. These units are designed to run perfectly with the exact amount of onboard storage that they already have. If your UH are rated at 150k, their actual output with OWB temp and flow rates is derated to roughly half on the top end- which is already maxing out the OWB with or without the storage. This doesn't need to be complicated. Where are you planning to house the manifolds, circ, mixing valve, HX, controls, etc?

Thaddeus

The current plan is to store the tank in an underground vault. I work for a concrete company and am having a vault built to house the tank. After I have plumbed the tank for permanency it will then be wrapped in milar insulation and then incased foam insulation and the vault sealed. All plumbing connections will be welded and the system pressure tested prior to incasement.

There are four plumbing connections. There is a second vault upon which I will be moving my boiler to and it will be with in 6 feet of the tank vault. The boiler has 3 water circuits each 1.25". The plumbing connections on the tank will be each 3". The boiler will use 3 taco 9 series circ pumps and feed the the tank at one end. The opposite end will return to the boiler.  The sides of the tank in the middle will also have the same 3" ports, thos two ports will be the hot water supply via 3" piping where a manifold consisting of 12 output and 12 return ports up to 1.25" each. That 3" line will have a singular circ pump. (I have not yet worked out the rate and volume of that pump, however it will be more than needed. 

From the 3" manifold in the second vault below the boiler is where all cric pumps and mixing valves (if necessary) will be placed. 

I understand the BTU rating of the modines. Which is why each modine will be on its own independent circuit. And why the 3" circ pump will be able to exceed total demand in a worst case scenario. However I do not anticipate this scenario as nothing I heat except the house and domestic hot water is in any way automatic (possibly the hot tub kept at low temp until I make a plan to heat it for a weekend) Every heated component this system will have will be of manual control. Therefore I can load my boiler appropriately and plan accordingly. 

The post of the tank is simply to expand my capacity even by a small amount. I have the tank. I have the 1.25 and .75 thermopex. And I will have the vaults. 

The boiler being 272k BTU and yhe planned heating components as follows...

House hot air exchanger 80 kBTU
Side Arm heat exchanger (domestic hot water) 5 kBTU
Hot tub plate exchanger 40 kBTU
3 Modines ~150k using ~85 kBTU 
PLUS.....
1 snow melt pad approximately 16X28'
1 20' diameter heated pad described in this post
1 radiant floor for my kiln, 20x32. (This is to dry my wood for the boiler.) 

So the fact is I have higher total BTU requirements than my source can handle. Therefore I am adding the tank as a buffer/surge protection and added total capacity. And also... because I can.


Now back to my original post. The heated pad. If the temp is too hot for the concrete and mixing is indeed needed then ill have to devise a plan for that. However my coworkers who are quality control specialists for concrete tell me that my plan of the slab being both thick and round, plus the added rebar and meshing and fiber should be able to handle the heat from a cold starting point. As long as the slab heat evenly and slowly. 

Thaddeus

Joking about what?

GroundUp

Okie dokie, good luck.