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How to provide pump load when all circuits are closed?

Dear fellows,

I hope you can tip me off to answer a puzzle for my floor heating installation, before I pour the screed. I have a new pellet boiler and want to feed 6 heating circuits in this "not so optimal but cheap way" as shown in the diagram:

My question is what is the best way to include a path for the fluid to run through when all the radiators are closed ?
If the pump in the boiler turns on while all the circuits are closed, it would burn up. I need to provide a way to load the pump, while not shorting out the rest of the system.

In other words, how do I provide a bypass path that has acceptable flow resistance but not lower than the total normal operational flow resistance?

I know I need to study more and eventually the answer will be clear as daylight, but the screed date is coming near and I am starting to panic, as I haven't figured it all out.

Thank you for your kind attention if you take the time to tip me off to the answer.

Best regards,
Branimir, Austria

PS. I should ask a local installer, but I am new to Austria and my German language skills are way too short to figure out the local dialect... and these guys talk fast.


  • Jamie Hall
    Jamie Hall Member Posts: 20,440
    Oh my. Well, the panel radiator and it's TRV in series with the radiant floor is not a good arrangement. If the TRV shuts off flow, then it will also be shut off to the radiant floor. Not good.

    So... the panel radiator and its TRV needs to be piped in parallel with the floor, not in series.

    Second, how do you propose to control the temperature of the water going to the radiant floor? As it is, the inlet temperature, at least, is going to be whatever the boiler puts out (assuming you make the modification just above, which you must). This is not good. The radiant floors need to be fed with the proper temperature water, and the usual way to do that is with a temperature controlled mixing valve and a circulating pump for the floor circuits. The mixing valve output is to the inlet of the pump, and the inlets are a T from the return from the floor and a T from the boiler primary loop.

    Now you also don't need just a bypass to keep boiler circulation running when the floors and raidators are off -- you also need a place to dump that heat. It does no good to circulate water through the boiler if the heat isn't being dumped somewhere -- the boiler will just get hotter and hotter, and the end result is not good. What do propose for a heat dump?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • branit
    branit Member Posts: 3
    edited September 2021
    Mr. Jamie Hall, thank you for the quick answer! The temperature of the water going into the radiant floor is controlled by the RTL - the return temperature limiter. I have seen this installed and operated here but I don't know what the drawbacks are.

    I admit this diagram is not by the book, but thought with the RTL it would work? The RTL would partially interfere with the flow through the panel radiator, but it should still work if I set the RTL to 35°C.

    Thank you for the suggestions -- I guess I have to tear it up and start again.

    But still, the original question, is there a piece of hardware that would allow the proper bypass? The controls would shut off the pellet burner when the return water temperature rises beyond a set point, but the pump needs to keep going.

    Kind regards :)
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 1,982
    @branit, I've merged your duplicate posts into one post here. Welcome to Heating Help!
  • Jamie Hall
    Jamie Hall Member Posts: 20,440
    OK -- let's go back to square one here.

    Your immediate problem, as I understand it, is that you have someone coming to pour and level the floor over the radiant tubing -- so let's address that first.

    Be sure that your radiant tubing layout is correct and suited to what you want to do -- the amount of heat your need from the floor. Also, be sure that it is laid out so that the loops are all roughly the same length, and that they are less than 200 feet (70 metres) long and that you have good access to both ends of all the loops.

    If you've got that, then we can go back and look at the rest of the piping and pumps and controls, which will -- I hope -- all be accessible.

    Deep breath.

    What the return temperature limiter is intended to do is to limit the return temperature -- as it says -- to a specific value. It does this by reducing the flow in the system, and is installed on the return. This does nothing for the temperature of the water being fed to the system. Installed on the feed -- as you have shown it -- it will reduce the flow into the floor loops in an effort to keep that temperature to a specific temperature -- but there is no way to reduce that temperature. If it were installed on the return, then reducing the flow to maintain a specific return temperature will work (the heat is lost and the temperature reduced in the loops) -- but the temperature of the water entering the loop is uncontrolled.

    You really have two problems -- other than getting anything controlled by a TRV in a parallel arrangement with independent feeds and returns.

    One is controlling your radiant loop temperature. That's best done with a circulating pump dedicated to the radiant floor and fed, as I mentioned, through the output of a thermostatic mixing valve, one input of which comes from the boiler (and possibly radiators) primary loop, and one input of which comes from the return flow from the floor (the remainder of the return flow, if any, going back to the boiler).

    The other is what seems to have prompted your initial post -- a situation where the pellet boiler is still merrily producing heat and the system doesn't want any. You have to have a place to dump that heat, as pellet stoves -- like all solid fuel stoves -- can't be turned off quickly. Sometimes this can be done with a big storage tank (sometimes called a buffer tank, but a buffer tank has a different purpose). Sometimes one needs an external heat dump.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,332
    I don't know what an RTL actually does inside, but if it's truly only an in and an out port, all it can do is limit flow—through the entire circuit. Probably not what you want.

    Unless you're intentionally trying to half-a$$ it in for €30 & a pack of cigarettes, start by reading the appropriate Idronics editions (https://idronics.caleffi.com/magazine-archive).

  • branit
    branit Member Posts: 3
    Mr Jamie Hall -- profusely thank you! Thank you for taking time to shine the light for me. Yes, I have completely botched this and will re-do it in the next days and post the new system.
    I did mistake the reduced flow through the RTL with reduced temperature -- clearly grave error.

    Your explanation is clear and it saves me a ton of money and time in monkeying around with the outcome of a wrong design. I will re-do this and learn the details around it.

    My radiant floor circuits are all 40m or less. I did a heat load calculation to start with and that is where it became clear that the floor heating will not be enough for the current state of this old house. As I add improvements like more "attic" insulation / ceiling insulation, much better windows and exterior insulation, I will eventually get there, but I still need panel radiators for the floor above the ground floor.

    If you ever come to Graz, Austria, I would be happy to treat you the local beer selection and/or find another way to thank you for your timely advice.

    To Mr. Ratio -- thank you too for pointing me to the archive where I can find relevant info. I am definitely serious about getting this right, because I am installing it in my own house where I intend to live in the coming years. Several reasons why I started to do this myself:
    -- 1. Very interesting subject and finally something I can do with my hands rather than on the computer screen.
    -- 2. The reputable companies don't give me the time of day for a 35sq.m. project. They are all busy with bigger projects.
    -- 3. Most of the installers talk in local dialect of German (almost no English) and that's a mistake waiting to happen too....
    -- I am originally from Bulgaria and I paid a company (which is reputable there) to advise me. Clearly their level is not up to par. As soon as I started looking through the books and looking at installations at my colleagues' homes, it became clear to me that my setup is flawed.

    Thank you HeatingHelp.com for providing this forum where fellow engineers are so helpful and quick!
  • heathead
    heathead Member Posts: 218
    I didn't see it, but I could have missed it. Please don't forget to insulate the bottom and sides of the pour. You want to get this right the first time.