New radiant in concrete Slab

Discussion in 'Boiler Forum' started by Cram108, Mar 4, 2013.

  1. Cram108

    Cram108 New Member

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    Bought my house 5 years ago, and converted to gas from oil. At the time I had a lot of renovations and $$ was tight so we used a non-condensing boiler (HB Smith) 100,000 btu. The boiler is set up with 4 zones, the 1st floor has 2 zones, both zones use cast iron radiators (sunrad type).

    My original intentions for the basement was to also use cast iron radiators on 2 zones, 1 zone for the basement room and 1 zone for the garage. 5 years later and a new slab of concrete in the basement room zone which now consists of 2 loops of radiant in the concrete floor. The garage zone will still be cast iron radiation.

    The boiler set-up is as follows, all zones are run off a Argo ARM-4P relay with a honeywell aquastat set at 180 degrees. All zones have there own circulator and thermostat. The piping comes out of the hot side into my spirovent and expansion tank and then into my header where each zone separates. At the header each zone has a flo-check and circulator pushing the hot side (top of the Boiler) to my rads and returning back from the loop into the cold side (bottom of the boiler). Thats the way the 3 cast iron rad zones are set up. The new radiant zone is as follows, from my header into hot side of a taco 5003 mixing valve, mixed side of mixing valve into radiant zone circulator , then into my supply side of radiant manifold at 120 degrees, return side of radiant manifold tee's off, one side to cold side of mixing valve, other side of tee goes into return on boiler. All returns enter a header seperatly, which then enters the return on the boiler.

    The zones are all working great my radiant in the concrete works wonderful as does my cast iron rads, where my problem is in the return on my radiant side, my temp out of the radiant is 80 degrees. I noticed since adding the radiant zone my boiler is firing much longer and normally it would have shut off at the aquastat's setting, now it seems the boiler runs past 200 degrees before the burner shuts down. When running only my cast iron rads it hovered at 185-190 before the burner went out. I added a bypass valve with a gate valve from a tee that come off the hot side and added an additional tee on the radiant return to get the temp to the proper degree the boiler likes (140). After adding the valve I really am stumped, when the radiant is running on it's own with the valve opened fully i can get a max of 130 degrees going into the radiant return, below the tee that goes up to the cold side of the mixing valve. As soon as the other Zones come on, the temp in the return from the radiant below the tee goes cold down to 80-90. And the pipe with the gate valve gets hot.

    Not sure where to go with this but I do know I want 140 going back into the boiler on the radiant return. Any help would be appreciated.

    Thanks
    Marc
  2. Dana

    Dana In the trades

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    Sending 80F water back to a cast iron boiler is severe abuse, and you can ruin it in one heating season or less(!). With 130F return temps a gas fired boiler is going to be OK, but with oil it could be an issue.

    I'm going to guess that rather than plumbed primary/secondary with a separate pump driving the boiler loop, when only the radiant is running you have one pump and a mixing valve. If that's the case the flow on the boiler loop will be ridiculously low, and it'll overshoot the high-limit aquastat's setpoint as soon as the pump stops (or even before.) With separate primary-loop pump to guarantee flow you'd be able to mix the return water to temp with the boiler bypass, and the boiler would cycle on/off during extended calls for heat from the radiant, but you would be able to tweak a minimum return water temp under all operating modes.

    See if you can sketch up and post a legible system diagram, showing every pump, manifold, and valve (including check-valves, even if they are incorporated into a pump, as is sometime the case.)

    BTW: I don't quite understand the rationale for moving from a cheaper cleaner-burning fuel to a more expensive dirtier fuel. Even 5 years ago the cost of heating with gas was substantially below that of oil. Although the Chicken Little/Henny-Penny analysts predicting $200/bbl oil when it crossed $140/bbl during the summer of 2008 should have been ignored, those who were calling the subsequent crash to $28/bbl "a return to normal" were also not credible sources, in view of the steady march of world demand exceeding growth that could ever be profitable at price points that low. (Even at $50/bbl the oil sands of Alberta would have to wait.) Neither $4 heating oil nor buck-a-therm gas will be around forever, but even $2/therm gas in an 80% burner is substantially cheaper heat than $3 oil in an 85% burner. The trend line on both is up, not down, but the back-story on regional gas supplies s much rosier than any argument you could make about heating oil. In most cases converting to gas pays for the conversion in under 2 heating seasons.
  3. Cram108

    Cram108 New Member

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    Location:
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    Hello Dana and thanks for the reply I'm gonna sketch something up, so maybe the understanding will be better. Also I think you misunderstood me on the oil and gas conversion. I did convert the system to gas. The previous owners had oil.
  4. Dana

    Dana In the trades

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    You started off with...

    "Bought my house 5 years ago, and converted to gas from oil."

    ..and I somehow mentally swapped the "from" and "to". (Mea culpa!)

    Most cast iron boilers would go pretty much forever with 130F return water with natural gas as the fuel, and most oil-boiler installations have stainless flue liners, which would tolerate gas exhaust flue condensate without a problem. Check with the manufacturer (and make sure they understand it has a gas burner now) but I'm pretty sure that would be the case here. All "cold start" oil boilers could handle 130F return water with a gas burner installed. The dew point of the exhaust product is a bit lower with a gas burner, and acidity of any condensate is MUCH lower with gas than with oil exhaust.
  5. Cram108

    Cram108 New Member

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    I checked my Manual and the recommended minimum temp is 140 degrees. I gotta get that diagram for you so you can see exactly what I'm talking about. I'm thinking I need to add a circulator to make a boiler loop.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    There's a limit on how hot you can supply a slab with radiant which is likely lower than the return water must be for your boiler! To keep the return water hot enough, you'll need to use a mixing valve from the output to warm the inlet from that branch to keep it above the 140-degrees. Or, you could probably use a buffer tank. You'd also need a method to drop the outlet supply to that loop. Then again, I may be all wet...the concrete slab nor your feet will appreciate excessively high water temps - the slab might crack.
  7. Dana

    Dana In the trades

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    Of course the manual for the boiler is for it's oil-fired incarnation, and thus 140F min is specified. But as I stated earlier, both the dew point & acidity of natural gas exhaust is lower than with oil, making lower return water temps just fine.

    Gas-fired modulating-condensing boiler designers STRUGGLE to get the dew point of the exhaust to break 125F, but usually don't 122-123F is pretty common. (I read a 127F spec regarding one model recently, but I'll have to see the test data to believe it.) At the levels of excess combustion air inherent with retrofit gas burners for cast iron boilers you won't have exhaust. condensation inside the boiler with 130F return water- if it's so starved for excess combustion air that it's condensing at 130F it's probably WAY out of adjustment and leaving soot deposits from incomplete combustion! If a mod-con designer can't get it there, it simply won't happen in a cast iron boiler! With extended operation with 120F return water you'd be at real risk though.

    It's possible to run heating water at arbitrarily low temps without endangering the boiler, but it's a real design issue facing dual-temp hydronic designers all the time, but to do it with radiant slabs it generally requires a primary/secondary pumping configuration, not just a boiler-bypass or system-bypass. You need the boiler loop to have it's own pump which guarantees the amount of boiler output flow through the boiler, and a separate pump on the mixer loop for the low temp side, and unless you designed the pumps PERFECTLY for the loops, there's sometimes a ball-valve to tweak the maximum amount of return-flow from the low-temp radiation mixing into the boiler loop. You can find some discussion of the relevant issues here.

    There are different ways to skin this cat, but it's done every day.
  8. Cram108

    Cram108 New Member

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  9. Cram108

    Cram108 New Member

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    So if I'm gonna add a primary loop to this design what circulator should I use and what control would you recommend i Add to my argo ARM4p, I also picked up an argo outdoor reset. All the pumps right now are taco 007, I was told to add A grundfos alpha 15/55, but I don't know if thats for the radiant loop or the boiler primary loop. Questions, questions questions?????
  10. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    "At the levels of excess combustion air inherent with retrofit gas burners for cast iron boilers you won't have exhaust. condensation inside the boiler with 130F return water- if it's so starved for excess combustion air that it's condensing at 130F it's probably WAY out of adjustment and leaving soot deposits from incomplete combustion! If a mod-con designer can't get it there, it simply won't happen in a cast iron boiler! With extended operation with 120F return water you'd be at real risk though."

    This is correct per our field tests on all sorts of boilers, condensing and atmospheric on mixed cast iron and radiant slab installations.

    Your drawing and controls do nothing to protect the boiler or counteract "thermal depression", when the radiant kicks in lowering return water temperature to the boiler.
  11. Cram108

    Cram108 New Member

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    What would you recommend for proper protection?
  12. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    When we have to install a low-efficiency boiler or are called out to trouble-shoot a radiant floor connected to one, we start with a motorized three-way with outdoor reset and boiler protection built-in. Yours is a common and challenging control issue. We try to design hydronic systems--even those with disparate radiation--with similar design water temperatures. Using an cast iron boiler just makes it more complicated. Look to Tekmar and Danfoss for some common solutions. We don't think the extra effort is worth it, but if you have an existing low-efficency cast iron boiler (all cast iron is low-efficiency) then you must keep the stack hot enough to avoid sustained flue gas condensation. We retrofit boiler controls everyday and make the best of it since the worse hydronic system beats the best scorched air system every time...hehheehee
  13. Dana

    Dana In the trades

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    A (usually code-required for oil-burners) stainless steel flue liner will tolerate copious natural gas exhaust condensation from lower stack temps, as long as you set up a means of management & disposal. If the stack is setup where condensate could drip directly into the boiler it could spell disaster, so a condensate trap on a horizontal section of stack pipe is in order (even with 140F return water temps.)

    This is a real hydronic design problem- you (or someone you hire) will have to do the math, not just hack it into kinda-working.
  14. Cram108

    Cram108 New Member

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    Location:
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    If I add a boiler loop and make it my priority zone and the other four zones secondary. Wouldn't that help with my cooler return water so it would mix with my boiler water as it goes thru the primary loop as opposed to it returning the cooler water direct. Would it also help with the flow through the boiler and help the other loops get better flow by taking the strain of them pulling through the boiler?
  15. Dana

    Dana In the trades

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    It's "primary" not "priority" loop, which could make for some confusion when discussing this with pros. ("Priority zone" is a different term, with a different meaning.)

    Plumbing it primary/secondary is a likely-necessary first step, but it's just the beginning, not the end. You'll either have to read up on this quite a bit to get to a reasonable DIY design, or (probably the better step) hire someone who already has the hydronic design tools and experience to get it rigth the first time. Dual-temp multi-zoned systems heated with fixed-output boilers can be made to run at reasonable efficiency, but it's not amenable to a "design-by web-forum" easy-fix solution. (I'd trust someone BadgerBoilerMN's experience and credentials to take what you have as a starting point and come up with a reasonable solution, but it won't be & shouldn't be free- it takes time to do the full analysis.)

    But if you want to hack on it, setting it up primary/secondary with at least a ball-valve-tweakable (or thermostatic) boiler-bypass loop to mix-in sufficient boiler output with the return from your low-temp radiation might get you to 130F minimum return water temp at the boiler, which will keep it from being ruined in short order (which is measured in months, not years, with 80F return water.)
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