Radient Floor Heat Boiler Config

Discussion in 'HVAC Heating & Cooling' started by devans175, Jun 29, 2009.

  1. devans175

    devans175 New Member

    I'm finally at the point I can start running tubing for radient floor heat. I plan to run the tubing myself and hire a contractor to make the final connections. Before I meet with the contractor, I hoped someone could give me a little info so I don't go into it cold.

    My house currently has 2 zones. I plan to switch the house of in 2 phases... zone A this year, then Zone B sometime in the future. I've attached a diagram of my current boiler configuration. Is it as simple as cutting disconnecting the old zone A copper system just past the zone valve and connecting into the new radient manifold, or is it a lot more complicated than that? View attachment Boiler Layout1.pdf
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    No, generally you can't just substitute a bunch of pex tubing for your existing radiators or baseboard heat.

    First, you need to know how much heat you need to supply, then that would help calculate how close together the loops need to be, how many loops you need, and the diameter of the tubing, then you need something to temper the water temperature, since baseboard or radiators typically use 180-degree water, which you can't put through your floor - talk about hot feet!

    You need the supply temp much lower, and deal with the max floor temp based on what covering is on the floor, the supply temp, and the flow rate. You want each loop to be approximately the same length, or you will need to have valves to balance them, or some will get more flow than others. It's not a bad idea to run the loop around the outside walls first, since that is typically the coolest, then come into the main part.

    There's a lot to designing a well-performing system.
  3. devans175

    devans175 New Member

    Thanks for the input Jadnashua. I've talked to a few suppliers and looked at a few web sites that explain the tubing layout and spacing very well. I have a loop plan for the tubing. I tried to attach it, but the file is too large. I'm installing the 1/2" tubing between existing 16" floor joists. The manufacturer recomends 8" spacing. All of my runs are about 250'. I plan to use a manifold that allows me to mix the return water in order to get the correct temperature in each of the loops.

    One of the suppliers indicated I'd need to completly re-pipe my boiler and that's what my real question is... why. Couldn't the plumber just run a line from one of the existing zone valves, to the manifold and a return from the manifold to the old return at the boiler. I attached my diagram of the current boiler configuration so you don't have to search for it. If I'm missing something, can you direct me to somewhere I can see a schematic of what the boiler piping should look like. I can't seem to find anything on the web. I just want to educate myself before the contractor shows up.

    Attached Files:

  4. Dana

    Dana In the trades

    (where to begin...)

    There's a huge amount of info to be gleaned about radiant heating on this forum- dig in!:


    Heating systems work better if they're designed. I have little faith in that the average online proto-design is optimized for your actual heat load. (Do you have a heat load calculation for the zone in question, Manual J or other?)

    The boiler piping layout is 10% of a radiant design, but very important if your return water from the radiant zone is going to be below 140F, if your boiler is standard-stock cast iron/steel non-condensing variety. This is very likely to be the case with a retrofit radiant system unless it's a fully-suspended (not staple-up, no-heat-spreader) system of limited loop length. With cooler return water temps the combustion gases will condense- first in the flue (ruining your chimney), but with return water below 125F it'll condense on the heat exchanger plates of the boiler itself, ruining the boiler in a single heating season or less.

    In order to know your return water temps, you need to be able to calculate them based on flow rates, tube-length, and heat-transfer efficiency (all unknowns in the equation at this point, but you can always fiddle with the mixers until you got enough heat, I s'pose. :) )

    A common boiler-protection scheme is to add a "boiler bypass", a pipe between the boiler's output & the input to the circulator on the return run to mix output water with return water raising the boiler input temp to non-condensing temps:


    With a valve on this branch the bypass water can be adjusted manually during operation (have at least a good 5-10+ minute burn going before adjusting) to get the return temp up to at least 140F (really 130F, with some margin.)

    But there are other boiler-protection schemes as well. A primer on the subject of boiler protection methods lives here:


    But whether this scheme is going to work well with both your high & low temp zones is another important design consideration. Study up on it before proceeding.

    If your radiation is capable of delivering design-day heat at 140F or less average temps it may pay to design it for high delta-T and use a condensing boiler with outdoor-reset controls, and use the cool return water for a huge efficiency gain. Otherwise, getting the return-water temp into the boiler as low as possible without running into flue or heat exchanger condensation problems boosts the overall efficiency of the system.

    A lot of old-skool hydronic systems were set up for 180F water and a 20 degree delta-T on radiation, with ~160F return water temps. But for every 10F you can drop the return water temp you get about a 3% drop in fuel use while delivering the same amount of heat. (Lowering the return water temp works by basically sending less heat up the flue, by since it lowers the flue-gas temp.) If the boiler is fired with natural gas or propane (not oil) and the flue has a stainless liner, setting the return water temp to 130F is generally "safe", but don't let it stray for long below 125F in normal operation. This can produce a ~10% boost in performance in many oversized older systems.
  5. devans175

    devans175 New Member

    Got it Dana. The bypass piping is what the supplier was trying to show me when he was talking about re-piping the boiler... he just couldn't explain it's purpose. The diagrams in the link were exactly what I was looking for. Now I'll have a little more knowledge when I meet with the radient heat contractor Friday.

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