GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Hydronic radiant design questions

Bdgray | Posted in Mechanicals on

Hello GBA,

I posted a pretty naive question last week regarding selecting the proper heat source. I’m not an HVAC expert by any stretch but as an owner of multiple degrees and a home improvement nut, I don’t want to back away from this just because it is quite a bit more intricate than I realized.

I’m in the process of educating myself on hydronic systems. I’ve purchased one text book and downloaded a design manual from Uponor (light reading). I know phase one is just to get an understanding of how the various components come together to form the system. That said, I couldn’t help myself and I’ve also downloaded a number of software packages (all free via trial) to begin the design process. I realize this is putting the cart before the horse to some extent.

With that said, I have some general questions as I begin educating myself.
1) Is this the best forum to ask questions? I’ve received excellent advice thus far but I suspect there are websites dedicated to hydronic design. I post here because I subscribe to GBA and FHB.
2) Cost not being the primary concern (we’re a modest home surrounded by million dollar mansions), in a between joist retrofit is it correct to assume that heavy extruded aluminum plates will deliver the best results and lowest operating temperatures? I’ve seen a number of options including thinner (and cheaper) aluminum sheeting, new graphite plates, and fins (where the pex doesn’t touch the subfloor). No need for a lengthy explanation. It seems to make sense that a thick aluminum plate with solid contact to the tubing would transfer the most heat from the pex. Would just like validation.
3) I have tubing layout software (free for 30 days) that I plan to use to design the pex layout. When designing between joists, I’m curious if I can/should design the circuits by zone rather by room? In other words, do I need to concern myself with the walls above me? It seems like designing a single circuit for two small rooms would offer a more efficient layout than two separate circuits. If it is bad practice to heat directly beneath walls, couldn’t I just pull the tubing 2-3″ off the subfloor when I cross a wall?
4) Are there other best practices I should consider when designing the circuit layout (manifold location, entry location for rooms, perimeter banding, keeping water temp of circuits at/below certain temps, keeping all circuits at the same temp, ideal PEX type for between joist)? FYI, I have a steel I-joist running across the center of our basement. My thought is to run the manifold connections in a soffit I’ll build adjacent to this I-Joist. Hence all circuit access/egress will come along the center of the building’s floor plan.
5) Something tells me this is a bad idea but I have to ask as some DIY Radiant companies mention this (albeit for open systems which I don’t want). Going radiant means I will not have an air conditioning system for the summer months (we’re in Chicago so there is ~month every year when we really could use some cooling & conditioning). We’re also on a well (crazy since we’re only 10 miles from downtown) which means I do have access to (limitless?) ~50 degree water. I already have several dehumidifiers that we run in the summer to tackle humidity. So, the question is whether I could safely/efficiently run that well water through the circuits without burning out our well pump or creating a mold farm from condensation? If this is a terrible idea I don’t need a lengthy explanation. Just tell me its crazy and I’ll drop it. I already suspect I need to install a small minisplit system for summer cooling/conditioning.
6) Lastly, (maybe this should have been firstly) would any expert on this forum be willing to review my draft circuit layout, mechanicals selection (likely centered around the HTP Hydro), material list, and occasional installation advice? I would of course compensate for this activity. I’m not ready for this yet but I also know it would be asinine to spend $20K on a system without knowing it will work efficiently and effectively.

Thank you,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    For under-the subfloor systems, yes, extruded heat transfer plates will deliver more heat at any given water temp than sheet metal plates or other approaches. But above-the subfloor systems can go even lower still. The thickness & materials of both the subfloor & finish floor matter. It's fine to run the PEX under walls, but under cabinets where food might be stored spoilage rates will increase, even if you opt not to run the heat transfer plates under those areas.

    Laying out the PEX by zone rather than by the room is the right approach, unless you are considering leaving the option of micro-zoning it on a room-by-room basis later.

    The PEX needs to have an oxygen barrier or you will go through pumps pretty fast (unless you use bronze impeller pumps & oxygen tolerant boilers, and avoid use of hydronic components that would have iron in contact with the water.)

    Running fresh water through your system causes corrosion (see comment about PEX type.) With 50F well water you wouldn't get a whole lot sensible cooling out of a radiant floor, but it is cold enough that you could run into condensation issues on the distribution plumbing. With a lot of pipe insulation and an isolating heat exchanger you might get some, but for the amount hardware cost and pumping power involved you'd probably be better off buying some mini-splits and solar panels with that money. With radiant cooling you get higher heat transfer rates if the radiant is on a wall or ceiling rather than floors.

  2. kevin_in_denver | | #2

    Hands down the best website is

    Dan Holohan is the Martin Holladay of hydronics. (Common sense, excellent and clear writing, myth busting, etc.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |