Exterior Slab Radiant

Man with a question

I am often asked to install radiant heat systems in exterior patios. I have always been hesitant because there is always potential for unlimited heat loss and I have my doubts about how much effect wiil have on diners enjoying their meals on a cool night. I went ahead and took one, despite my aprehensions. Here's the set-up.
First of all a design day around here is 41°F . Typical evening in fall can be around 60°.

We have 1400 sq.ft. of 3" slab with 1" R-5 rigid insulation below and at perimeter.

Boiler is a Vitodens 200 8-32, low loss header. I have 12 250' pex circuits @ 6" OC.

Secondary pump is a Grundfos 15-58 set on lowwwwww. I'm looking for a Delta T of 30. 75Btu/sq.ft.

Water deliver temp is 144°F.

According to my calcs, I should be getting my 75btu/sq./ft. and my slab should be somewhere around 105°. It is not howver, at least not at the surface. The buried slab sensor gives me about 105 but one can feel that the surface is not.

My question is... how can I increase the performance of this slab? When I try to tighten the delta T (by raising the speed on the secondary pump) I get lower delivery temps across the LLH, probably because the Vitodens pump is not keeping up. I do believe I have raised the Vitodens on-board pump to 100% already.

Any thoughts? (Besides why the %!#**+! do they want to put a radiant slab OUTDOORS)

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Weezbo

you have installed insulation right?

do you idle the temp at 37 or so?

under the slab you have insulation and on the perimeter do you also have a 4' on the flat down about 2 '?

then the next idea that i have that might be somewhat different is what is the average wind speed and is there any wind break like hedges?.....

the next idea is some what similar , does this patio see direct sunlight and are there any pools or moving water ways near by...or large thermal mass deeper in the ground like a basement apartment built with poured concrete walls....

then another thought comes to mind on your pipe lay out, do you have loops that run parallel to oneanother or are the spacings made by single loops with lollipop loops at the ends? did you run 3 4 in parallel? if so try this take and run the return of every opposite loop as the supply and the supply as the return....

Gordy

Questions ?

What kind of outdoor temps are you dealing with at this point? Are the tables, and chairs in place which may help provide somewhat of a buffer zone for the slab. Plus the fact when the dining area starts to fill with people.

What kind of slab temps are you shooting for?

How long has the slab been up, and running? Is it even steady state yet if that is even possible.

A heated outdoor dining area could pose some health concerns. Once someone kicks their shoes off under the table, and feels the wamth of the slab it could get contagious ;-)

A slab of concrete or asphalt loses heat quickly once night falls in the outdoors. This could have some high operating costs once you get it dialed in.