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I'm planning to install radiant heating lines in my new "studio" (essentially a detached garage). I want to save some money by doing parts of the project that I feel confident with. I think I can design and install the PEX tubing for hydronic radiant heat. I have read some already and can research more, but I wanted to put my plans out there for some constructive criticism if you have it.

I understand the concepts of

  • roughly equal loop lengths
  • doubling the tubing in your run so that you don't have the hottest water on one side of a zone and the coldest on the other
  • varying the distance between lines where there may be more demand for heat (e.g., near a window wall).

Please feel free to let me know anything I'm missing.

The lines will be embedded in an 8" slab.

I have 3 loops, but only two zones.

  • The "office" and bathroom (green) are 1 loop, 1 zone
  • the "garage" and "workshop" (blue) are 2 loops, 1 zone.

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  • That's a lot of slab thickness. 6" properly reinforced holds up my backhoe just fine. I can't imagine much need for more than 3 or 4 inches on the office-bathroom-workshop side at all. Insulate well underneath.
    – Ecnerwal
    Commented Jan 30, 2023 at 22:43
  • Just a little formatting goes a looong way...
    – FreeMan
    Commented Jan 31, 2023 at 14:28
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    Yeah, we aren't a consultancy. You need to revise to ask a single, specific question.
    – isherwood
    Commented Jan 31, 2023 at 15:13
  • I dunno, we often accept and answer "have I missed anything" questions... Might need a bit more in the way of specifics, but I think it's acceptable.
    – FreeMan
    Commented Jan 31, 2023 at 16:55

1 Answer 1

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Your question may be closed because it’s actually not a question. I would like to use this space to give you a few comments that won’t fit in comments. I have a hydronic radiant heat system, some in concrete, some under a wood frame floor.

Read up on the short life of cast iron pumps and accessories such as microbubble separators if you use regular plumbing PEX instead of oxygen barrier PEX. If you use regular PEX, you’ll need bronze or $tainle$$ pumps and brass fittings.

You haven’t said so, but I hope you plan to use adequate insulation under your 8-inch slab so you’re not warming the earth as much as your living space. Also consider insulating your foundation down to a depth of 4 feet to prevent cold soaking of the earth under your slab.

In my opinion, don’t make yourself crazy with close spacing of the PEX near windows and such. Radiant heat is diffuse, and I would challenge anyone to detect a difference in comfort near a window based on PEX spacing in this portion of slab vs that portion. Radiant keeps the entire room at the same temperature.

Forget clock thermostats. Forget smart thermostats. Forget IOT thermostats. You will set a dumb thermostat to your comfort level in the fall and you will leave it at that setting 24x7 until spring. An 8-inch slab will take at least a day to reach operating temperature and come into equilibrium with the room temperature. Setting back the thermostat at night will be pointless, since the room temperature won’t start dropping for several hours. If it manages to drop at all by morning, it will take all of the next day to get back up to your comfort level. So go into this knowing that your fuel bill will reflect 24x7 heating with no setback. (If this is not acceptable, skip the radiant and use hydronic baseboards instead.)

Will you ever want to drill into your concrete slab? Before you pour concrete, take photos from all angles and make a map of your PEX lines with accurate measurements.

And good luck!

EDIT: Some additional thoughts

Decide up front how you’re going to control the radiant water temperature, especially if your boiler is also making domestic hot water. Any boiler can produce boiling hot water, but you’ll want circulating water in the 125F range, plus or minus. Super hot water would crack your slab. Will you use a thermostatic mixing valve (prone to sticking) and mix boiler hot water with returning cool water? Use an indirect water heater (decent temperature control) to feed circulating water and serve as a heat reservoir? Use a buffer tank with its own aquastat (poor temperature control but an excellent air separator)? My boiler supplies baseboard heat, radiant heat and domestic hot water, so I use a buffer tank and an electric servo mixing valve for radiant water. Works great.

Your boiler ideally should be capable of modulating its output from 100% down to 10%. When your total system is running at equilibrium, it doesn’t take a lot of heat to raise the temperature of returning water at 110F back up to 125F. If the boiler only runs at full output, it will short-cycle.

Don’t oversize your boiler. That would also lead to short cycling. Suppose you have an oversized 150K BTU boiler that can modulate down to 10% output – 15K BTU -- but the building needs only 8K BTU to maintain comfort in mild weather? It will short-cycle. A buffer tank would lengthen the cycle but the boiler would still start and stop far more than it would if properly sized.

You may find that you need to raise the temperature of circulating water a few degrees during extreme cold weather. Boilers with outdoor temperature sensors can do that automatically, and so can electric servo mixing valves if so equipped. Thermostatic (wax capsule) mixing valves can’t sense outdoor temperature.

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  • And run a radon test before pouring a massive slab - way easier to put in radon remediation plumbing before, than after! Or just put the plumbing in as a "passive" vent and add a fan later if testing shows a need for it)
    – Ecnerwal
    Commented Jan 31, 2023 at 1:56
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    Not worth a whole answer but consider three zones so you can have three set temperatures according to usage/clothing: 1. bathroom (wet and naked) 2. office (sitting) 3. the rest (moving). Further to the point in this answer about the slab taking a long time to react, with these three zones you can also drop the temperature of the workshop or the office if you know you'll spend a long period not using one of them. This assumes the garage is more of a workshop extension than vehicle storage.
    – jay613
    Commented Jan 31, 2023 at 14:43
  • @jeremy foster I've added some additional info to my answer.
    – MTA
    Commented Jan 31, 2023 at 15:05
  • I'm under the opinion that I did formuulate a question. It was essentially, "Here's a thing. What do you think of it?" That's a question, right? @MTA, your thoughts are excellent are exactly what I would want and expect. So, if DIY on StackExchange is not the medium for a question like this mine, then were or how should it be asked. Help me understand. Commented Feb 2, 2023 at 17:36
  • @MTA, regarding your answer. Thank you so much. I don't care at this point about the system that will be used to run hot water through the tubes in the slab. I'm just going to leave the tubes stubbed out of the slab and consider a system at a later date. My only concern here is the layout of the tubes. Commented Feb 2, 2023 at 17:36

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