pavilion radiant heat

utahDIY

Hello everyone,

I am planning a 12' x 16' pavilion in my backyard and looking into the feasibility and wisdom (or lack thereof) of putting in a radiant heat slab under it. I would enclose the sides with canvas or something of that nature in the winter time. I want the radiant heat to be powered via wood fire (wood stove, etc.). Could anyone tell me if this is possible to build? Also, would anyone know if radiant heat would be enough to heat this space to 60 + degrees F?

DCContrarian

I don't think it would work well for this application. You want something that can deliver a lot of heat in a short period of time, that's not really the forte of heated slabs.

The heat output of a floor is entirely determined by its surface temperature. In order to output enough heat to warm an uninsulated outdoor space you might find that the floor is too hot to walk on.

EdTheHeaterMan

Probably not enough to heat to 60° based on a load calculation where the side walls are canvas only. I helped a Wallie last year to get the proper radiator size to heat a glass room that was attached to her home. The room was all glass. It seemed that the plumber that installed the boiler figured that the room just needed a 12 ft section of baseboard radiator to heat a 13 x 25 ft glass room. It turned out that the radiator for the "Solarium" actually caused the ModCon boiler to fail because the return water was too hot. This was discovered after a factory tech support visited the house and found that the 3/4" loop was so short that the water was returning to the boiler return inlet too quickly and offering very little heat to the room.  Not enough temperature drop when only that zone operated.  Which was all the time.

I did the load calculation based on Hydronics Institute Form 1540WH.  I set up the walls as 9’ ’1” x 25’1” then used a window that was 9’ x 25’  the same thing for the two sidewalls that were 9’ x 13’ , and the ceiling was 13 x 25 glass. This made the load calculation of over 35,000 BTU, and that needed more than a 12’ section of baseboard.

You need to do the same thing.  Do a load calculation based on the construction of the roof and insert walls that have no insulation, and no thickness.  No siding, no sheetrock, no nothin.  And the infiltration will be above the charts.  Like maybe 10 or more air changes per hour.

With that load calculation you will find that the water temperature in the slab may need to make the floor 140°F to get the room to stay at 60° room temperature.  I believe that might be too hot to walk on.  

I have actually had experience with this. I built an awning to cover the HotTub on our back paver patio. We had vinyl/canvas side curtains made that rolled up in the summer and were deployed in the winter.  When we decided to use the HotTub on very cold days, that HotTub surface area of about 38 sq.ft. at 110°F warmed up the room by about 15° above the outdoor temperature.   Not very warm for a 17° day.  We also added an electric space heater on the other side of the enclosure (you don't want an electric space heater sitting on the edge of your hot tub) And that made the room go above freezing, but no way warm enough to sit outside and read a book.  

I believe that gas fired infrared heaters hanging from the ceiling may be your better option. 

Maxmadman

Very good feedback. I've been thinking about this topic for a while and glad to find this thread.

What if the goal is to get warm radiant up to keep the legs, maybe up to knee, from getting too cold? I don't care much about keeping a room warm, but I always complained about how firepit keeps one side warm but the other side still freezing. My thought is if I could generate enough heat to go up to knee high, then people can sit outside without freezing their bottom.

Can a hot water/heating liquid coil on composite deck be used in conjunction with wood burning stove to heat and circulate the hot water? I have endless supply of wood, so it is not a concern.

GGross

https://forum.heatinghelp.com/discussion/comment/1825085#Comment_1825085

I have some thoughts. first if the tubing is not too expensive, it won't hurt much to have it in there, though it will freeze if you aren't careful. I personally would not sign off on heating a space with canvas walls outside in the winter via a radiant floor. A draft house in a cold environment might require an 85 degree slab temp (thats on the really high side) to make the space warm. Imagine the slab temp required to heat an outdoor space in the winter, the slab will not like this, your feet probably won't like this, and you will need to keep it running pretty much all the time because to raise the temp in the coldest weather would take a long time. Instead, if you already have the wood heat available, and put the tubing in the slab, just run it like a snow melt. It won't make the space feel warm, but a slab that is 50 degrees or so over the outdoor temp might help somewhat, and in the shoulder seasons it might actually work for your needs. I wouldn't do this if I was paying for the fuel personally though!

If the goal is to feel warm while you are in the space, there will be nothing easier or more efficient than a free standing, or overhead style radiant heater. many electric options are available, and I believe gas models as well. If you ever go to a restaurant that has outdoor seating in the cold fall or winter weather you have seen these. You can turn them on and feel warm almost immediately after, then turn them off when you are not in the space. It would be my first choice if the goal is to just not feel cold outside.

Maxmadman

https://forum.heatinghelp.com/discussion/comment/1825087#Comment_1825087

I was thinking more of wood decking instead of cement slab. Basically heat up the tubing under the decking hot enough that it will radiate the heat hot enough to make warm enough to walk outside while wearing jacket, or of lying down.

hot_rod

put a large enough wood stove and heat with just that, the radiation of a 300 degree wood stove will transfer more than an 85 degree floor

Jamie Hall

This^^^ plus one other consideration: keep the draught off of floor level. This means the patio or deck is surrounded by solid down to the floor railings. This arrangement works astonishingly well — I know of several outdoor swimming pools at ski resorts (the first one was at Mt. Snow in Vermont) which, while the water is warmed, have no other heat than the sun — and solid railings. And which are wildly popular.

Maxmadman

https://forum.heatinghelp.com/discussion/comment/1825116#Comment_1825116

The idea is to spread the heat source around. As I mentioned, I can have a big fire pit but it will heat one side that's facing the for but not the opposite site. I'm wondering if I could spread the heat around a little by using heating pipe under the"floor".

GGross

I don't think what you are describing will work well at all no. and you would really need to get that fire going a few hours before you went out there to feel anything on the floor, and even then I don't think it will get what you want, maybe in mild weather. i would mount an electric infrared heater above the cold area, and turn it on and off as needed. second choice would be a little propane infrared heater. They also make those little electric snowmelting mats that just plug in (various sizes, think welcome mat size pad), one of those right under your feet might be a cheap solution to try out but I'm not sure exactly how hot those get they are just meant to melt a little snow.

Maxmadman

https://forum.heatinghelp.com/discussion/comment/1825132#Comment_1825132

Thanks for the feedback!

Maxmadman

https://forum.heatinghelp.com/discussion/comment/1825118#Comment_1825118

Great idea. I should look into incorporating this design as well along with big fire pit in the middle.

Jamie Hall

If you don't block the draught at floor level up to chair level, nothing is going to work.

Maxmadman

Thanks again.

My initial idea is, from top to bottom of an outdoor deck:

1. Wood/composite deck
2. Deck joist with hot water radiant heating coil, heated by wood burning stove with circulator
3. Metal roofing galvanized metal under the joist to capture water and heat from the hot water radiant heating coil
4. Some kind of insulation, most likely spray foam against the bottom of the galvanized metal

And for the railing, glass/plexiglass railing that can start all the way from the floor to prevent draft.

In my mind, this design would work to keep the area under the deck dry and while at it, by using galvanized metal, it will trap the heat that can be used to dry the joists after rainy day and maybe in the winter, enough to trap and reflect the heat upward to warm up the deck.

Maybe if I incorporated this design with large stove or firepit in the middle and no-gap railing, I might be able to warm up the deck a little more evenly than just single firepit in the middle?

GGross

I've already been a nay-sayer on this quite a bit and my opinion hasn't really changed. But if you are determined let me make a few comments. The tubing transfers heat through surface area contact, so if you were to use a metal underlay that touched the entire deck, and groove it so the tubing had more surface contact with the metal it may transfer heat better, the reflective properties are never going to work beyond day 1 as the metal will become tarnished or dirty enough it will not reflect heat like you expect. The tubing will expand and contract, so you need to account for that. The water will need to drain somewhere, if you just leave an end open to drain, heat will move to cold and you will lose all your heat out the open end, remember heat does NOT rise (thats hot air), it moves to cold, you need to fully insulate around any direction you do not want heat directed.

hot_rod

when people talk about heating the great outdoors with radiant floors, you need to think more like snowmelt load numbers

some football fields have hydronic bench warmers. Better to heat objects by conduction than the great outdoors.
That’s what the radiant tube and mushroom heaters you see outdoors are doing. Radiating to anything in their line of sight. Turn your back on one and your front get chilled quickly

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1825847#Comment_1825847

I'd be thinking more like the heated seats in luxury automobiles. Heat the points where people are making contact, the floor, seatback and seat bottom. Don't try to warm the environment. I'd also try to insulate and wind-proof them.

I think with your back and feet kept reasonably warm and your front facing the fire you'd be comfortable. I have no idea how to engineer that though.

Maxmadman

Thanks everyone. I appreciate everyone's feedback and expertise. I just didn't want to leave anything on the table and left wondering but you all had convinced me. I won't waste my time testing that theory.