Radiant Slab Questions

[formula388]

I am planning on setting up my in slab radiant heat and have some questions prior to purchasing parts.

Is there an advantage of slab sensor vs thermostat air temperature regulating for my application?
What is an Outdoor Reset? From what I understand "IT" (not sure what it is) takes into account the outdoor temp? I've heard for small shops this might not be a big deal? My shop is very well insulated.
If the electric boiler is modulating, can I "oversize" it? I would like to stay on a reasonable budget but the cost difference from 3kw to 6kw seems minimal with most brands.
What size boiler do I need? I have spoken to several companies that sell prebuilt kits and have gotten estimates from 2kw to 5kw.

Background Info: Designed and used as a small personal auto shop. I have a two post lift and automotive tools in there. When I work in there I might open the door 4-5 times a day. Then I might go several days without opening the door at all. I poured my slab about 3.5 years ago with Oxygen Barrier Pex about 2" from the bottom of the 6" slab. Under and around the slab is 2" XPS foam. The building itself is about 500SF (21'x23.5') with 12ft ceiling. It is fully spray foamed with closed cell. Only window is a small one in the man door. The 9' x8' garage door is well insulated (Claims R19) . I have about 500 feet (1 loop of 240ft, another loop of about 270ft) of 1/2 Oxygen Barrier pex. I have a manifold already and the pex has been pressurized and holding pressure for the past 3.5 years.
My only reasonable fuel source for the boiler at this time is electric. I have more then sufficient room in the panel for this.

I have a mini split that can comfortably heat the space, but I was looking forward to a "warm floor."

I did not put a slab sensor in when I poured the slab. I didn't know much at the time. I don't mind drilling, once I can get some heat in the floor to verify where the tubes are using FLIR Thermal imaging camera.

I know this was a long winded post. I appreciate the insight.
Thank you,
Dennis

 

[jack stand]

I think with your insulation it should be very easy to heat and the outside reset isn't necessary. A slab sensor is a minor cost and installation job. It may be handy for dialing in the temp. I have a feeling that once you're into the "cold slab" temperatures you'll probably just run on the radiant and shut off the m/s. They'll be perfect up until you notice the cold slab.
I can't comment on the boiler.

 

[jlv03]

The outdoor reset will be what helps "modulate" the boiler. If it is warmish outside (say 40F) the boiler will drop down the temp, if it is really cold (0F) the boiler will put out as much heat as it can.

I have floor heat in my garage, but I use a thermostat (a Nest E) that only senses air temp. I usually run the garage around 60ish, so I'm okay with the heat "running away" on the warm days when the heat kicks on and the sun is also warming things up.

With a mini split you may want to experiment with different temp settings on both the floor and the minisplit so that they compliment each other but not fight. Maybe dialing back the minisplit at night and turning up the floor heat. Or vice versa.

 

[fitter30]

510' of pex 16,320 btus.
Btu per watt 3.41 = almost 5kw
Floor sensor keep the slab at a constant temp. Well design system water temp 5 - 10° over set point. Just remember radiant slab is slow to react if setback to 50° will take hours to bring it back up. Either thermostat will work. Outside reset can be used its will take some adjustments to dial it in. Most 6 kw heaters have two 3 kw elements could either disconnect one or use a two stage thermostat.

 

[PoorUB]

What size boiler do I need? I have spoken to several companies that sell prebuilt kits and have gotten estimates from 2kw to 5kw.

I did not put a slab sensor in when I poured the slab. I didn't know much at the time. I don't mind drilling, once I can get some heat in the floor to verify where the tubes are using FLIR Thermal imaging camera.

Like the other post, you will need around 5 KW. 510 feet of tube and depending on who you talk to about 30 BT per foot of tubing, so 15,300 BTU. 3.41 BTU per watt so just under 5 KW electric heater. No clue why anyone would recommend 2KW for a heater!

No slab sender,? I would not worry about it. Just go with a simple heating thermostat. You can drill in a floor sensor and buy a thermostat that uses both. the idea is you can set a limit on the floor temp and it will even out the highs and lows a bit. or you can just set a slab temp and run it there, but for comfort you will want to run of space temp.

As for getting an IR camera and finding the tubing, it is more miss that not. I have tried it on several occasions and was never able to pick up the tubing.

 

[86turbodsl]

I have a very well insulated house, spray foam and radiant. A couple of my rooms have high mass floors, like a slab.
I found i had real bad swings in temp until i put in outdoor reset. You're probably going to want it at some point. I didn't like
the temp swings in a living space. Where i live, we can be 20F in the overnight and 70F during the day. High mass just can't keep up.
Your mileage may vary.