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Radiant Heating Design for 2800 sq-ft house. Feedback please :)

JonathanY Member Posts: 2
edited May 2019 in Radiant Heating
I am designing the radiant heating system to be installed in my own home and I've run into a couple of concerns. I would appreciate it if some of the experts on here could have a look at what I've got so far and see if it looks like I'm headed in the right direction or if I've lost the way.

I've read the Manual J and Siegenthaler's book, 'Modern Hydronic Heating' and done the calculations to determine heat loss, supply temperatures, flows, etc. It seems straight forward but the velocities that I ended up with in all my circuits are much lower than Siegenthaler recommends. He says to keep the velocities between 2-4 fps but my circuits range from 0.3 to 1.3 fps, flows are from 0.11 (bathrooms) to 0.93 gpm.

It turned out that my zones required supply water temperatures that fell into 3 groups clustered around temps of: 88°F, 102°F, and 114°F. So, I've gravitated toward a design that utilizes 3 zone circulators on supply circuits with max total flows of 3.00, 2.59 and 1.86 gpm for the 3 groups of zones. My cursory look so far at available circulators suggests that it may be hard to find circulators appropriate for such low flows as these.

I have not yet done flow resistance calculations to to determine the head losses, but I'm starting to think that I need to confer with experts first to see if I'm even on the right track.

1) Are these velocities acceptable for radiant floors?
2) Will I run into trouble trying to control/adjust such low flows?
3) Will I be able to find circulators in the flow ranges I've come up with?
4) Although the design is not finished, does it look like I'm taking a reasonable approach or am I headed for problems?

A brief list of the major design elements:
1) geothermal heat pump
2) hydronic radiant heat floors
3) house has 3 floors:
a) basement, concrete slab, 3 zones incl garage
b) main floor, gypcrete thin slab, 4 zones
c) loft, above floor tube & plate, 2 zones
4) Total heated floor space: 2800 sq-ft
5) Total house heat loss: 38,500 BTUH
6) Before I began work on the design, the house shell was erected by a GC who laid the basement radiant tubing and poured the concrete slab. It looks like one of the basement zones might have way too long of a circuit (perhaps 380'). Haven't yet considered how to deal with this.

I will attach a pdf of what I've designed so far. Thanks so much if anyone can have a look and provide a comment.


  • Ironman
    Ironman Member Posts: 7,225
    With a 1/2" loop at 250 -300', about .75 gpm is the best you're gonna get with a 15-58 circulator.

    Your mixing valves are gonna add more resistance, so you need to calculate their Cv value in. You probably don't need three of them as circuits that are within 15* of each other can of share the same SWT with some adjustments and tweaking.

    You shouldn't go over 330' with 1/2" pex ( one of your loops is 386').

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    delta TJonathanY
  • delta T
    delta T Member Posts: 883
    I would really suggest you do just two zones. One zone for the slab embedded tubing, and another for the main floor and the loft. Maybe change the common area to 6" OC and see what it does to the supply temp requirement if you really want to to get it down to the gnats you know what, but I doubt it will be a problem as is. Ironman is spot on, 15 degrees is not going to make a significant difference, and can be tuned out using flow setters on your manifolds. Zone valves and an ECM delta T or delta P pump would be my personal preference rather than three zone pumps on such a small load. Keep in mind that the btu requirements will far far less than design for 90% of the year. Close to half usually.

    I would also caution against stacking manifolds like you have, instead of supplying manifolds from other manifolds, supply each manifold with full port piping from the supply side of your pump (or zone valve ;) ). If you stack manifolds like that you will be pulling your hair out trying to get the balancing to work correctly. You can do all the math you want, but I guarantee that when you go to actually dial everything in, it will not react like you want it to. Ask me how I know.....

    That long loop in the basement rec room is going to be your biggest headache, (I gather that is already installed) though considering it is high mass, and in a basement, it may not be much of one. You will likely have a 'cold spot' somewhere near the end of the loop, but whether or not it ends up being noticeable outside of near design conditions is another matter.
    Paul PolletsJonathanYIronmanSTEVEusaPA
  • JonathanY
    JonathanY Member Posts: 2
    edited May 2019
    @delta T @Ironman Thanks so much for your comments. This is my first experience to post on a forum, so thanks for the response. My apologies if I mess up with any protocols.

    Your advice on stacked manifolds is noted- I will eliminate.

    I'm stuck with the basement circuits as-is. I plan to do some experiments to try and determine their exact length (I'm going to fill with water, blow out with air, measure the water to calculate volume and infer tube length, also circulate hot water from a bucket heater and shoot the slab with infrared thermometer, etc). So far I'm just estimating based on some photos that were taken just before the slab was poured. I don't even know which protruding tube ends go to which zones. From my reading, another problem I'm worried about with the slab is that the contractor never provided controlled contraction joints along the zone boundries. There are random cracks now and sure to be more in the future as different zones are brought to different temperatures from each other. So far I haven't found any info to help me anticipate possible problems I might have with this.

    I did the calculation for the effect of changing the tube spacing from 9" to 6" OC and it changes the total length from 893 feet to 1340. I would divide into 6 loops instead of 4. This would reduce the required supply water temp about 3-1/2 degrees to 111°F. I'm still thinking about this. Would you say your recommendation to do just two zones is more about the benefit of simplicity and reduced installation cost, or more about ease of control, or more about available pumps being oversized for such small loads? I want to understand your thinking on this.

    Again, thanks so much for your advice. Very helpful.
  • JohnNY
    JohnNY Member Posts: 3,226
    I wish we would all stop overthinking radiant floor heating. Then it might actually become a system that people are willing to install in their homes.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.