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Thread: Is it possible to add heat to existing slab?

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    DIY Senior Member guy48065's Avatar
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    Default Is it possible to add heat to existing slab?

    In my cottage I'd like to get rid of my hot water baseboards & convert to radiant in-floor tubes but part of my cottage is on a slab. Is it possible (physically and financially) to do this to an existing slab--without adding to the top?
    Romeo and Atlanta, MI

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    Retired Defense Industry Engineer jadnashua's Avatar
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    On the slab area, you'd either need to do it with electric heating mats and probably tile (lowest buildup), or lots of cuts to bury pex in it, or pour a lot of additional cement over it, or use something like Bekotec from http://www.schluter.com/9_1_schluter_bekotec.aspx, then tile it. On a wooden subfloor, you can staple up plates and add pex. There are probably a few other ways, as well, but that's what comes to mind. Without chopping up the slab, I think the Schluter product may end up with the least buildup if you go hydronic on the slab. There are some ply/metal track systems, but I do not know if they are acceptable for use on a slab...I know they are designed for subfloors. Anchoring them in the slab is going to be a bit of a pain, but not too bad with the right tools (either shoot fasteners or a hammer drill and use screw anchors).

    If the slab does not have a moisture barrier and insulation underneath, you need to be much more careful of your selection - heating up the ground underneath can get expensive!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    If this is your north MI location, you'll want at least R10 between the tubing and the slab or you'll be spending quite a bit on heating up the sub-slab dirt. That would add at least 3" on the slab section.

    If you have crawlspace or basement under the other part with a joisted floor, you can still put radiant under that part, and use low-temp panel radiators or radiant ceiling on the slab area.

    In general the economics of retrofit radiant is kinda out-there, and there are trade-offs to be made.

    By contrast, replacing fin-tube baseboard with panel radiators is cheap, with a large uptick in creature comfort with more stable room temps and quicker response times than most radiant floor types.

    With the exception of just replacing functional fin tube with panel radiators of equivalent output, going with radiant for even part of it is a real hydronic design problem. Like any heating system design, the only way to zoom in on the "right" solution starts with a room-by-room and whole-house heat load calculation. If the fin-tube seems to have reasonable room to room temperature balance we can cheat a bit, and measure the fin tube length in each room, and use fuel-use against heating degree-days for a mid-winter bill to determine the whole house load, which will tell you how much of what type of radiant it would take to meet the 99% outside design temp load in your zip code.

    Without all of the necessary information it's kind of like asking, "Is it possible to get to town on my bike in under two hours?". It really depends on several things, like where you are, and where the town is, and whether you've been pumping up & doping in prep for the Tour de France. :-)

    Bottom line, it's always possible to add heat to the floor, but how much of the total design heat load it covers and how much it costs will vary by orders of magnitude.

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    DIY Senior Member guy48065's Avatar
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    I neglected to consider that the tubing needs to be insulated from the ground so that's a deal breaker. We'll just have to live with the inconvenient baseboards.
    Can radiant panels be run at 180F? When I start the inside renovations there may be one or two spots where a panel may work out better than baseboards.
    Romeo and Atlanta, MI

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Yes, the panels can run at higher temps, but their main beauty is that they provide comfortable heat at lower temperatures as well, they actually radiate, the baseboards rely mostly on convection, so to get the air moving, they need the higher temperature. There are radiant panels that can be added to either the walls or ceiling that might work out for you.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    I'd say almost anywhere you care about comfort there's an up-side to going with panel radiators.

    The single-number specified output (as with most heat emitters) is at an assumed 180F average water temp. Biasi's short-form brochure also gives a 150F output temp, and a "baseboard equivalent feet" number to keep you from having to do any math when doing swap-outs. Buderus panel radiators have trimmable flow valving on the radiator to allow you to adjust room-to-room balance easily ( but it's a cost adder.) Runtal is going for the clean-line Euro-modern look, but you pay extra for that. There are others.

    If you're breaking it up into zones or looking to optimize both comfort an efficiency the type & size of boiler would determine if the near-boiler plumbing needs to change (which it might.) Most cast iron boilers are grossly oversized for the actual heat loads of a house that would fit the "cottage" description, and would be made ever more-so if part of the renovations include better air tightness/insulation/better-performance windows, etc. Ideally you'd want to run the system at the lowest water temp that meets the load, which would keep the panel radiators at a nice warm temp 100% of the time with minimal temperature overshoots.

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