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Problem getting my radiant heat to heat

I have a question about radiant floor heating. I am in the process of having radiant heat installed in an upstairs bathroom. The floor is installed and the system is connected. The only problem is that the floor has yet to put out any heat.

First a little background. I have a gas-fired hot water radiator system divided into three zones, pumped using one circulator pump. The circulator pump is a Taco model 007-F4, rated at 1/25 HP, 0.70 Amp. And the radiators in all three zones kick out significant heat. Two of the zones go to 3/4" headers for the first and second floor. The third zone is 1/2", for the basement baseboard heat (below the circulator). The new radiant zone now makes four zones. It is 1/2" PEX tubing. It runs about 70 feet with about 20 feet of rise. The path of least resistance does not appear to be through the new radiant zone.

My first impression was that the system was completely air bound. It was. The plumber bled it and said it was good to go. There was clear evidence of water flow. Still no heat in the floor though. So he came back and installed an air vent on the downstream of the mixing valve (which has been set to 160 degrees, with no improvement in heat output) on the riser side. Hours later, no heat. And here's what I found. The water on the downcomer is cold, and I clocked if flowing at about 0.22 GPM (that seems quite low to me). Also, there is not a dedicated circulator for the radiant zone.

Therefore, my questions are: how do you know if your circulator pump is big enough? If it isn't, how do you determine how big it needs to be? The thing is, I don't think a bigger pump will help

I think the answer is to install a dedicated pump on the radiant line (since the circulator seems to prefer sending water to the other, lower head loss zones). Is a 1/25 HP pump located on the return line the way to go for the radiant zone?

What are your thoughts?


  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    piping sounds


    You are saying you have three high temp zones, and the new radiant is the fourth (making it high temp!) off of one 007. Plumber came back and put in a mixing valve for the radiant. Oh, the horror.

    When mixing a radiant zone, the pump comes AFTER the mix, and it is a dedicated pump for that temperature zone. I don't see how your set up can work using the same pump for the high temp stuff, and the radiant, and where the heck was the mixer shoehorned in. And, the poor 007 trying to do it all.

    Sounds like it's back to Piping 101 :) Post a pic so we can see what you have. For morbid curiosity, tell us what kind of radiant tube attachmant is going on-is it supplement or total bath heat source. What kind of t-stat?
  • to answer your question

    I hope you can see the attached one-line drawing of the system. I don't think a photo would be much help. The numbers 1-3 indicate the zones for the radiators. 4 is the new radiant loop. Oh yeah, not pictured, but the zone valves are all located on the supply leg of the system.

    I don't see how it can work either. In fact, it doesn't. I'm kind of mad the plumber thought that the 007 would get water to the 1/2" radiant system while competing with 2 3/4" headers and a 1/2" header located below the circulator pump.

    The mixing valve was always there, he added an air ejector to get rid of trapped air.

    I'm sorry, I don't know the exact answer to your last two questions. The tubing is a generic-looking red PEX tube, and the thermostat is a basic Honeywell unit (the round kind).
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    asking a lot

    from that 007 to pull things through the mix from the return side, plus with all the other zone distractions. I would never pipe this way.

    But, you have something to start with. I would let the 007 deal with your high temp stuff (would be nice if it was on the supply side, pumping into the system for better air removal, but, oh well), and get thee a Grundfos 15/58 three speed onto the "mix" side of the mixing valve, pumping into the radiant loop. Forget about the idea of a circ on the return. You need to get a purge setup on the return side of the radiant loop too. Ditch the ZV that goes to the radiant, and return the radiant after the mix AFTER the existing 007, before the boiler.

    I was asking how is the tubing attached to the bath floor? Staples? Plates? In mud? Other? Slab sensor? Cuz, that'll be the next issue- how are you controlling the floor?

    I'm thinking your drawing may just be anatomically incorrect showing the supply coming out of the BOTTOM of the boiler:)
  • Mark Custis
    Mark Custis Member Posts: 539
    Bob is right

    Add a smaller the better injector pump from the "mixed" side of the three way to the radiant. Mixing valves for DHW will work with enough flow.
  • pumps

    I'm not really in the financial position to rework all the plumbing the guy just put in, though I do agree with your analysis and appreciate the advice. I will most likely ask the plumber to install the pump on the downstream side of the air ejector valve, unless you really recommend moving that stuff around. There just isn't room upstream.

    And yes, there is no particular reason why my drawing has the supply coming out of the bottom.

    Question: out of curiosity, why a variable speed pump?

    Thanks for the feedback! I'm not familiar with the abbreviation DHW though.

    How small a pump, because Bob recommended a 1/25 HP pump, though I've seen pumps rated at 1/40 HP?

    Thanks again to both of you.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083

    shouldn't be paying anything to correct this. The burden is on the plumber to make it right. Doesn't work now, won't work ever, the way it is.

    Pump HP is irrelevant here. The issue is flow rate and pump curves.
  • I agree

    Financial burden aside, by pump power, I meant different models and sizes of pump. If the shutoff head of the pump is less than the total head in the zone then the pump won't work. I need one powerful enough to get sufficient flow for the head in the system. I'm sure there's a way to determine system requirements before I purchase a pump.

    I'm still pretty new at this, and I've spend the last hour calculating friction and total system head, and have gotten results that don't make any sense. As I understand it, you can't get flowrate without knowing head, you need head to get flowrate (hence the pump curves), and around and around I go. Since it is a closed loop, the dynamic head should not apply, and all system head is due to friction. That is as far as I can get because friction head is based on velocity, among other things.

    The above ramblings can probably be classified as a gross concept error, but like I said, I'm new at this, and I want to provide proper oversight to a plumber who has so far given me an air free system of stagnant water. Thanks.

  • Did you design the system?

    Or did the Plumber design the system?

    Thanks, Bob Gagnon

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  • The plumber did. Bear in mind that he tied into an existing crappily laid out system. He didn't consider it a problem that the 1/2" new zone was competing with two 3/4" zones. That is why the new zone is basically stagnant. I actually just spoke with him and he is installing a pump tomorrow.
  • Mark Custis
    Mark Custis Member Posts: 539
    While he is at it

    have him consider swing or spring check valve to make sure the flow will go where the two of you want it to go without robbing the other zones.

    Your feet and the dog will thank you for your effort.
  • Rich L.
    Rich L. Member Posts: 414
    Hi Mike

    DHW stands for Domestic Hot Water. It's the stuff in the sink and shower as opposed to the stuff going to your radiators.

    Pump size can be determined by the total loop length of the radiant floor, or longest loop if mutiple loops, but it sounds like you only have one loop. You also need to know the required flow.

    Done properly your project should have started with a heat loss calculation for the room. Then variables can be determined such as floor output per sq foot, floor coverings, water temps, floor temps, tube spacing, total number of loops, and then pump size.
  • It works!

    Thanks everyone. The plumber came by and installed a 007-F5 pump on the supply side of the radiant loop. It works great now. Thanks for all the help.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083

    for taking time to update!
  • Mark Custis
    Mark Custis Member Posts: 539
    The supply side of the floor

    so the pump is pulling from the mixing valve?
This discussion has been closed.