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outdoor reset for large slab?

ratio Member Posts: 3,173
Ok, here's a question for the thinkers out there. I've got a large slab, roughly 75' x 150'. Poured in the 50's, the supply & return pipes are 2-1/2", with a PD 35S circulator. I'm guessing the slab is between 4 & 6" thick. Church sanctuary, carpeted. Current control is a stat with a remote sensor drilled as far up into the slab as I dared (from a tunnel below) that starts the pump. Tempering is via a manual valve on a 2" boiler bypass. The problem with this setup is that I have to set the stat up & down as the temperature drops & climbs, & poor control of the water temperature. What I'm thinking (purely because I have no idea) is to rework the pump to DT & add an injection pump, at that point I should be able to dump in x number of BTUh depending on the OAT.

Is this the stupidest idea you've ever heard? Doomed to unreliability? Nifty idea that outta just work but way harder than it needs to be? I realize that there's a lot of work that needs to be done before this can be answered with hard numbers, but if it's a non-starter I'd like to know that before I invest a lot of time.

And, here's a few pics showing the piping:

The radiant loop is to the right, pumping towards the boiler, the pipe rising up immediately to the left of the pump is the bypass.

Here, you can see the bypass coming horizontal & then rising up into the radiant supply (with the round thermostat, on the very left). The lower valve is the one I use to temper the water, although it's now wide open & I've actually closed the large gate valve on the discharge of the pump, to try and get the supply temps down & keep the pump on.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,687
    Without knowing all the particulars, I think you are on the right track. Continuous pumping, injection loop, outdoor reset. You may not need the delta T pump if continuous & injecting. As you are aware, if it's set up right, it will save you a lot of time.
    Depending on your climate, you may have trouble. If you're in a climate where it could be 80 during the day and drop down to below freezing at night, and quickly heats up the next day, you will have obvious lag time with the slab-both under heating and over heating.
    The trick is figuring out the right size for the injection loop, piped correctly and the controls.
    Is this all one loop? How long?
  • ratio
    ratio Member Posts: 3,173
    Ok, good. I just wasn't sure if I was making it too complicated.

    Central Ohio, so 80's to freezing isn't too likely, although a 40* swing isn't unheard of. What I want to do is slowly track the outdoor temperature. My gut feeling is that I'm going to be four or five days behind, just because of the mass involved. But as long as I'm increasing when it's colder and decreasing when it's warmer I think we'll be good.

    I think the maths will give me a good size of the injection loop, just a matter of crunching numbers.

    As for the loop, I have no idea. It goes in and out as 2-1/2", it's pumped with a 35S, & it can keep that 75' x 150' hunk of concrete comfy is all I can tell you. I can't even tell you if it's reverse return! I'm planning on adding a water meter, at that point I'll be able to cypher up the water content of the loop, but after that I may never know any more details.

  • Firecontrol933
    Firecontrol933 Member Posts: 73
    edited May 2016
    I agree that you are on the right track. Go with a standard circulator on the slab running 24/7 during heating season. I like this strategy on unknown slabs because it helps to level out the dispersion of heat across the whole slab even if you somehow put too much energy in (helps to even out the surface temp across the entire slab). Do the math and take some notes on things as they are now, like temperature difference across the slab inlet and return.

    The injection pump could be controlled by a Tekmar 356 which gives you everything you are looking for along with doing it automatically plus it gives you boiler protection if needed so you don't slam it with cold water.
    IronmanPaul S_3
  • Ironman
    Ironman Member Posts: 6,868
    Go to Tekmar's site and read their essays on variable speed injection mixing. That will also provide you with the info needed to size the injection bridge and circ.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Paul S_3
  • ratio
    ratio Member Posts: 3,173
    @Ironman, I was going to tell you that I couldn't find the essays, but I just did: http://tekmarcontrols.com/solutions/educational-literature.html if any one else is looking for this information.

    I'm reading through them now.

    I've got the boiler running with ODR, via a Johnson Series 350 control with reset, (and I plan to add two stage control to the boiler this off-season), but I think I'm going to have to abandon the boiler reset, because I suspect that my convector circuit needs 180* water most of the time.

    There's a lot of good reading here, I'm sure I'll have more questions after I think about it. Thanks!

  • STEVEusaPA
    STEVEusaPA Member Posts: 5,687
    ratio said:

    I think I'm going to have to abandon the boiler reset, because I suspect that my convector circuit needs 180* water most of the time.

    What convector circuit are you talking about?

  • ratio
    ratio Member Posts: 3,173
    Ah, this is a church from the 50s. I've got, in addition to the radiant loop, a circuit of convectors piped direct return in e.g. the sacristy & entrances, and two air handlers with diverting valves that feeds the sanctuary. The pumps are visible in the first pic, pumping towards the boiler. I'm not sure yet what controlled the convectors, the air handlers had series 90 controls on the discharge air. One of the diverting valves had been replaced with a straight pipe sometime in the last decade. :/ The boiler is a five section Peerless 211A.

    My plan, such as it is, is to keep the slab slightly warm via reset all season, & use the air handlers to pick up the slack during occupied times. However, I'm starting to suspect that if the slab is warm I'll be overshooting the heat loss.

  • STEVEusaPA
    STEVEusaPA Member Posts: 5,687
    edited May 2016
    By slab is warm, I mean you dont let the slab get cold in the winter. If you do, it will take hours to heat up, probably wont heat up by the time everyone goes home. Or use a thermostat that you can set (or control via wifi), to turn the temperature up long before you need it.

    If the convectors are on their own loop, you can leave it all up to the boiler control.
    For example you can set the max temp for the convector loop to 180, and the max for the radiant loop to 130. Protect the boiler from low water return temps either with piped by-pass, 4 way mixing valve, etc., and let the reset control do it's thing. If both zones are calling, you'll have the right reset temp for the convector loop thru the boiler, and the radiant loop will only inject was is needed (via the sensors) to maintain it's reset temperature.

    Take a look, specifically page 26, but the whole book, heck the whole series is a good read:
  • ratio
    ratio Member Posts: 3,173
    The slab takes days to heat up, not hours! We started tripping the high limit, and I wouldn't notice for a week, that's how warm the slab stayed. (Pretty sure the boiler inspector "simulated" a high limit shutdown by turning the limit down and didn't get it set back quite high enough. It had never occurred before his visit, temperatures weren't out of spec, & it went away after I touched it a hair higher.)

    I've had no-heat complaints about the convectors, they can't seem to keep the space temp up in the areas they are located when it's really cold (they're the only heat source in the areas they're located in), which I currently attribute to low water temp since I'm resetting, 1:1 curve, 180 SWT @ 30 OAT, low limit set to 140 (cast iron sectional) I believe is what it's set at now. Is it possible that a general lack of cleaning could be hampering the heat transfer that significantly?

    I've got most/all of the Idronics issues that could be downloaded. I'm in the middle of the Tekmar docs, but I'll read that next. Thanks!