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Low Water Return Temp

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CTHeating
CTHeating Member Posts: 15
edited October 2018 in THE MAIN WALL
I’m new to the group and have been trying to read as much as possible.

I have a 100K BTU Slant Fin cast iron natural gas non-condensing boiler (85% efficient). I’m located in CT and heating 2800 sq-ft. 2,000 sq-feet is heated with baseboard radiators. I recently added 800 sq ft and am heating that space with radiant heat (concrete slab).

My plumber hooked up the radiant system using a 3 way mixing valve. The return water (70 degrees) from the concrete slab is teed into the mixing valve and boiler return. I was concerned about return water temp but the plumber said most of the water is going back to the mixing valve. He also indicated the returning water isn’t enough to pull down the boiler temp. He seems to be correct, the lowest boiler temp I’ve seen is 170 degrees. Is this system okay? Is low water temp okay if it’s minimal and doesn’t decrease boiler temp?

Comments

  • hot_rod
    hot_rod Member Posts: 22,262
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    got a drawing or pic? Usually a small load load, low temperature zone like that should not pull return down too much to be safe.

    Watch with a temperature gun, or feel the return pipe to the boiler.

    Within 10- 15 minutes of start up that return should be above 120 for oil fired, above 130 for gas fired.

    Another thing to watch if in fact that small zone calls by itself, is how long the boiler runs, we like to see 10 minute or more run on a boiler. Short cycling, rapid on and off running, will be tough on components, maybe the fuel bill :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • CTHeating
    CTHeating Member Posts: 15
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    Thank you for the quick reply. The return temp into the boiler with only the radiant zone on is approx. 70 degrees after 10 min. The boiler does stay 170+ however.
    Boiler run time is only a couple minutes in the fall/spring, probably around 10 min during the winter.
    I'll have to take a picture tonight. Thanks again.
  • CTHeating
    CTHeating Member Posts: 15
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    Here are some pictures. From the radiant system the return goes to the cold side of the mixing valve and the boiler return. The boiler return is shared with 4 zones baseboard heat.
  • hot_rod
    hot_rod Member Posts: 22,262
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    You may only have a gpm or two running to the 800Sq. ft room? So that 70F return is blending with the 170 SWT to give you the desired mix temperature, the rest, whatever that zone circ is capable of providing, is going back to the boiler.

    Where are you measuring the return temperature? right at the boiler connection.

    If so, basically you are hitting the 170 boiler with a small gpm of 70F. That is IF only the radiant zone is running. If either of the fin tube zones are also running, their return is blending that 70F up, probably.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    edited October 2018
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    When you're mixing high heat and a low heat systems, a Honeywell mixing valve, only, just don't cut it. I would like to see drawing of how the low heat sys is connected to the high heat sys. Your pictures are a bit confusing to me.

    Short cycling mentioned by Hot Rod can be a problem. I might have used a buffer tank or maybe injection pumping. Is the 800' zoned separately with its own thermostat? Does that 800' run only by itself sometimes?

    The return water to the boiler need to be over 135 deg to prevent condensation in the flue and boiler and possible spillage depending on the length of the flue and outside temps.

    The important thing is the boiler return water is over 135 deg, the 800' addition is adequately heated (meets set point) and the boiler's not short cycling.

    The return water from in-slab heating can be 70 deg as the supply temp would be about 85 to 95 deg. Depend on the layout.
  • CTHeating
    CTHeating Member Posts: 15
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    Here is a rough sketch of the setup. The 800' of addition is on it's own zone and will run independently . The supply temp is set to 110 deg and the return is roughly 70 at the boiler connection. Should the supply temp be turned down to 85-90?

    I watched the boiler closely last night and it never got below 165 degrees. The low return temp from the radiant system just isn't enough to pull the temp down. Is condensation an issue if the boiler remains hot? I have cool return temps but the volume is very small.
    Thank you for all the input.


  • hot_rod
    hot_rod Member Posts: 22,262
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    Officially condensation happens when the metal surfaces fall below the dew point of the fuel. Dewpoint varies bases on a few factors and we typically measure return fluid temperature, not the actual surfaces.

    If there is not signs of fluid dripping inside, on the burner tubes when it is running, you may be okay. It is a very small load on that boiler. a large volume cast boiler probably blends that small flow quickly.

    All boilers condense for a short period of time when starting from room temperature, the goal is to dry them and the flue out as quickly as possible, 10 minutes is one rule of thumb.

    In a perfect world a return protection "device" would protect that low return by blending some output with return. another 3 way thermostatic is a simple method.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • CTHeating
    CTHeating Member Posts: 15
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    Thank you, I feel better based on your post. I will pull the cover and check for signs of water vapor dripping onto the burner tubes.
    In the winter I get decent cycle times but in the fall/spring they can be very short. I think part of the problem is all the loads are small. Some of the heating zones are only ~300 sq ft.
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
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    "Should the supply temp be turned down to 85-90?"

    Not necessarily, there are a lot of thing that go into the radiant design. Heat loss, tube spacing, location of the tube in the slab, back loss, etc. that determines supply temp. Rapid, exceptionally high supply temps, or uneven heating of the slab can lead to stress fractures. The lowest supply temp to satisfy the zone requirements is the best option.

    110 deg isn't out of the range and you have a good differential.
    What you want to be concerned with is the 70 deg water returning to the boiler. Is it mixing with hotter water from other zones. I don't see any circulators on your diagram, so how are you moving water thu the infloor sys.? How are the connection for the infloor made to the boiler?

    Of course, it all depends on whether the infloor zone can operate by itself, independent of the other hotter zones. The water returning to the boiler on a sustained basis must be 135 deg. or higher.

    But, the flow thru a infloor sys is in the range of 1 to 1.5 GPM, so I would think that low a flow would mix with the water in the boiler as Hot Rod suggested. I didn't see a circuit balancing valve on your diagram, so how are you regulating flow? A low CV mixing valve would certainly limit the flow, I would think. Flows above a certain velocity can lead to problems and flows that are low can lead to short cycling in a reservoir boiler. As resistances in your heating sys changes flows change. Balancing valves do what the name suggests--balancing out the flows in your sys.
  • CTHeating
    CTHeating Member Posts: 15
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    Here is an updated sketch. The radiant heat often does run independent of the other zones.

  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    edited October 2018
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    That makes more sense. There probably is short cycling in the boiler as the radiant circuit does run independently. At 70 deg boiler return temp, you have to figure what percentage of the boiler lite off that the boiler is running in condensing mode with the radiant circuit running independently. Then correct this if necessary.

    The way I do this is to use a Taco I-series mixing valve in a bypass from the outlet to the inlet of the boiler. The mixing valve has a sensor on the return to the boiler that allows the mixing valve to modulate to keep the return temperature to the boiler above 135 deg.

    If your plumber is correct about the flow thru the mixing valve and thru the boiler, than I don't see a problem. But be aware that 70 deg out of the radiant circuit must be 70 deg into the boiler, but the flow may be only a few gal/min and is mixing in the boiler or with the return from the other zones. Most of the condensing is in the first few min of a cold boiler lite off. But you may not have sufficient flow from the radiant circuit, running independently, to keep the boiler in the condensing mode because of the mixing.

    The problem then is short cycling, which is where a buffer tank come in to the picture. But maybe short cycling isn't as big a deal as it is made out to be.

  • Gordy
    Gordy Member Posts: 9,546
    edited October 2018
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    If your radiant supply is 110, and return 70 are you getting even floor temps with that large of a delta in temp? Typically radiant floors are around 10 for even heat across the panel. But then this also depends on the panel design, tube centers, room load, floor coverings, and flow rate.

    Seems the room temp is satisfied before the whole panel gets warm. You could back the supply temp down lower so you get a longer run time to reach setpoint, and a more even surface temp,

    What are the floor coverings?
  • Gordy
    Gordy Member Posts: 9,546
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    I should also add when are you measuring return temps? At the beginning, or near the end of the heat call?
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    edited October 2018
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    Gordy, he may not get the flow because of a low CV mixing valve regardless of the pump. If the flow thru the manifold balancing valves is adequate, then the large delta T may be because of the tube loop design. If the radiant panel keeps the area at set point, then I think it's ok.

    Reminds me of a contractor that laid a whole 1000' roll of 1/2" pex in concrete. The delta T was out of this world.

    He ran out of heat energy.
  • CTHeating
    CTHeating Member Posts: 15
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    The delta t does seem to drop as the system runs longer due to colder outside temps. I was getting short run times with the Tekmar thermostat with floor sensor (surprised me). The room is well insulated. Floor coverings are tile and vinyl plank flooring. Floor temps are even according to my temp gun, about 77 degrees. I am going to back off the supply temp to around 100-105 and see how that goes.
    The system has 3 loops, 300' each, 1/2" PEX tubing, all Uponor components.
  • hot_rod
    hot_rod Member Posts: 22,262
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    CTHeating said:

    The delta t does seem to drop as the system runs longer due to colder outside temps. I was getting short run times with the Tekmar thermostat with floor sensor (surprised me). The room is well insulated. Floor coverings are tile and vinyl plank flooring. Floor temps are even according to my temp gun, about 77 degrees. I am going to back off the supply temp to around 100-105 and see how that goes.
    The system has 3 loops, 300' each, 1/2" PEX tubing, all Uponor components.

    The delta T has more to do with the temperature of the space and the temperature of the heat emitters. Unless you have your heat emitters on the outside of your home :), then the outside temperature would be the driver.

    Here is how the heat transfer works and how and why the ∆T changes and moves as the system runs. If you have a wide ∆ on first start up, even beyond you design ∆ it indicated higher amount of heat transfer, delta will shrink as the space warms and obviously go to 0 when the load is satisfied.

    The heat emitters are ALWAYS in charge of the boiler operating conditions, not vice versa. of course safety devices are added to prevent the boiler from running at unsafe conditions.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HomerJSmith
    HomerJSmith Member Posts: 2,468
    edited October 2018
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    This is my take in support of Hot Rod's post.

    All heating and cooling runs on a differential. Heat energy seeks equilibrium. Nature abhors a vacuum and will try and equalize the state of disharmony. It's natural law. Mankind tries to create disharmony in forces to get nature to create work. That's what a boiler is, a machine to create disharmony in heat energy.

    That being said, when a fluid containing heat energy move thru a pex tube, it tries to equalize the temperature (temperature is an indication that heat energy exists) between the fluid carrying heat energy and the colder slab. Likewise, the slab carrying heat energy tries to equalize the temperature between the slab and the colder room and the room tries to equalize the temperature between the room and the colder outdoors. All buildings leak heat energy.

    The rate of transfer of heat energy is dependent on the disequilibrium between temperature states. If the fluid in a pex tube is 90 deg and the room is 90 deg, there will be no heat energy transfer from the fluid to the room and the delta T will be zero. A widening delta T is an indication that heat energy is being transferred. There is nothing sacred about a 20 deg delta T or a 5 deg delta T or even a 70 deg delta T. The only thing that matters is, does your design reach setpoint on the coldest day in winter. All the other considerations like economy of operation, etc. are incidental to that specific goal.

    Delta T is not fixed and will change as the heat energy transfer changes. Yes, CTHeating, the delta T will widen as heat energy move outdoors. But that's not a concern. What matters is will the design reach setpoint. Nature is concerned about one thing, the equalization of temperature differences. It doesn't give a hoot about how the RWT effects your equipment or what the delta T is.

    That's all I have to say.
  • Gordy
    Gordy Member Posts: 9,546
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    I was simply extracting information about the system with out getting to deep into the heat transfer side of it. The wide delta had me thinking about “when”readings were taken. Which should be near the end of the cycle in my opinion after “thermal equilibrium” is reached.


    Plus we are in a bit of a shoulder season not full blown heating season.

    Isn’t outdoor temperature really in charge of how the system reacts? It is the reason why we are heating, or cooling. Yes the emitters are in charge of the heat transfer as far as the system, but they react to the induced load.
  • hot_rod
    hot_rod Member Posts: 22,262
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    I would say the temperature outside vs inside, and the insulation value certainly effects the energy load, heating or cooling. But many, most heating and cooling systems are responding to the t-stats inside. It's a limit switch, whenever energy is being added thermal equilibrium drives the heat transfer course.

    ODR attempts to anticipate a changing load, but it too will perform much better with indoor feedback in the logic. Just because outdoor temperature drops, doesn't necessarily mean the building needs more heat energy dumped into the space. High mass in the building and or heat emitters are energy storing devices. Passive gains also change the need to add heat energy.


    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_23.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,546
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    It’s interesting to note in how envelopes have evolved to become more efficient The reaction becomes more delayed from the induced load to the interior structure.

    When homes were poorly sealed, and insulated the resulting load penetrated the structure faster especially with wind. ODR becomes Less of an asset because the interior thermostat sees the load faster.

    As envelopes became more efficient ODR helped anticipate the induced delay for better envelopes . Now I think as you say with even tighter high rvalue envelopes ODR coupled with indoor feed back becomes an even more efficient system in anticipating that load at the right time.

    I picture a well insulated envelope as having fly wheel effect much as a radiant slab if not designed, and controlled correctly.
  • hot_rod
    hot_rod Member Posts: 22,262
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    Crazy as it sounds, I have been in "engineered" mega homes in Colorado with heating and AC running at the same time. Massive south glass walls, and near windowless north side, about 50F when I visited. So how does outside conditions relate to that home?

    At least it had ground source GEO for efficiency :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2018
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    Well solar gain does have its advantages if “engineered” right. Sounds like some auto suntracking window treatments would be a good investment for all that glazing :) I wonder what happens with all that energy when the sun sets.