Radiant Heat in Exterior Spaces

Paul Pollets

Radiant in the slab (outdors) is not nearly as effective as infra-red heaters. The heaters may be an eyesore, but they heat the localized space even with wind.

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Man with a question

How many Btu's can the concrete accept?

One more time we have been asked to heat a semi-enclosed patio.

I have not had great success with this in the past and thought I might try again.

My question is this.

How much heat can I actually put in the slab?

The design would be for a 1600 sq ft insulated slab.

8 400'loops at 6" on center, 7" at the turns.

Design flow rate about 1 gl/loop for 8gpm total.

At a 20° Delta T this would give me 80,000 mbh or 50 btu/sq/ft.

Air temp on the patio would be a minimum of 50°.

My question is, can concrete accept more btu's than that if it is not so cold outside. I want a slab surface tmp around 105°.

Would using a larger boiler gain me anything under these circumstances? What if I double the flow rate?

I have designed Snow Melt systems with well over 100btu/sq/ft, but they have ICE sitting on th surface. This is a dining area of a restaurant.

Again, will a bigger boiler with more BTUs available gain me anything?

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Unknown

why not

consider schwank type radiant heaters from above? I haven't crunched any numbers but it seems to me you'll need an awful lot of heat to do what you're talking about. If I'm not mistaken on design days 100btuh/sq ft gives you in the vicinity of 40*F surface temp? I see you're in so cal though. Not as extreme as I'm thinking I'd say. Still. these might work nicely. Very quiet.

http://www.schwankgroup.com/en/prod-lum-eco.asp

hot_rod

the output of the slab

depends on the surface temperature and the ambient air temperature around it. For bare feet you want to stay around 82-85F max.

A rule of thumb is about 2 btu/ square foot per degree difference.

85F surface- 50F ambient = around 100 btu/ ft

Indoor slabs with 85F- 68F ambient = about 34 btu/ ft

With snowmelt you can run warmer surface temperatures (no bare feet) and usually start around 33F ambient, so that's how you get the high BTU/ ft output in snowmelt slabs.

So to get 100 btu/ ft out of a slab you need to be able to put that much into the slab.

The RPA Guidelines suggest .6 gpm max for 1/2 pex.

My RadPad only goes up to 50 btu/ ft outputs. At 50 btu/ ft output you would need about .7 gpm in 9" OC in a 200 foot loop.

1600 square feet at 9" OC would need 2080 feet of tube, 10- 200 footers would be close.

Any of the radiant design software programs should be able to calculate that application. Supply temperature, loop length, gpm, pressure drop, btu output etc.

Don't forget to factor in the glycol penalty.

Depending on your definition of a semi- enclosed space, even 100 btu/ ft radiant slab may not keep the space warm.

I agree overhead radiant heaters would be a much better choice.

I had dinner outside in Chicago last fall under infrared heaters, outdoor temp around 38F and I had to take my jacket off. Of course the top of my head was getting awful hot, and I started to sweat. Tall folks suffer under low infared heaters

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Josh_10

I was sitting outside last summer on a cool evening at my wifes relatives house. They have a patio that is stained an almost black color. The outside temperature was 54 degrees according to the thermometer. I noticed the slab was keeping us warm as we talked outside. Not warm enough for a tee shirt but the slab was definity adding comfort. My curiosity got the best of me and I happened to have driven my work truck to meet the family at the house. So I grabbed my little laser temp sensor and found the slab temp to be an average of 83 degrees.

With that in mind, I bet a slab at 100 degrees might actually do the trick. Let's say you design for a 50 degree delta between the outside temp. You will require 100 BTU/FT. You will need a 160,000 BTU supply. If you kept your loop lengths to 200 feet you would require 2 gallons per minute of flow per loop at a 10 degree delta for a supply temp of about 140 degrees. Assuming you will want a 30% glycol solution for freezer protection that yields a total head loss of about 28 feet. So total flow being 32 GPM.

I think the RPA recommends a flow of .6GPM for half inch to yeild a head of 10' or less. In our case, I don't think the standard applies, it's not a house. 27' of head at 32GPM is a pretty common pump size.

I will warn you about heating the outdoors though. Once the wind picks up, your system does nothing! We installed heat in the Mariners dugouts last year at Safeco Field and the radiators do great on calm days but are ineffective when the wind picks up, as any heating system would.