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Low Flow Rate in Radiant Heating System

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Prevch
Prevch Member Posts: 106

Long Story short, I had a gas boiler powering my radiant heating system that leaked and made my family sick, so I swore off any and all gas appliances right then and there. Now I have the task of finding an electric system that can properly run my in-floor heat. I have 19 loops each 250-300 ft long. 7 loops in the basement, 7 loops on the main floor, and 5 loops upstairs.

To facilitate this, I purchased a Stiebel Eltron 24kWh Tankless water heater. I had seen a few setups similar on YouTube and others had some positive results so I thought I would give it a shot. I do also have forced air Electric Heat that at max runs at 28.8 kWh and that actually does a pretty good job of keeping the house warm until it gets REALLY cold outside, like -5 or less Fahrenheit. Trouble is, I live somewhere that it is not that unusual to have stretches of time colder than that. The in-floor heat kept up fine on the gas boiler. The logic I was operating off of was, well if 28.8kWh hot air can keep the house warm, then I bet 24kWh water will do even better. I still think I might be right, but I am running into a problem with the circulation.

I plumbed this “open-direct” style based on a conceptual map that I found at radiantec. Essentially, the cold water line (it’s confusing because it is plumbed in red it runs horizontal from the right pipe with the white valve on it and tees into the supply side of the heating loop. So, that pressurizes the loop, when the thermostat comes on, the pumps turn on and circulate the water to the cold inlet side of the hot water heater, which then sends the heated water out of the outlet back down into the supply side. No new cold water can enter the system because it is already pressurized. In the event that domestic hot water is used, some new cold enters into the loop and so on. By the way system is pressurized between 45-65 PSI because I am on a well pump and pressure tank. That being said, I am only getting 0.5 GPM in circulation. Both those pumps are rated to 16ish GPM or something like that. When I use domestic I can get over 2 gallons per minute heated up no problem, but when circulating, the tankless only registers flow at 0.5 and occasionally 0.6. I have tried isolating different valves for testing, but seems like no matter what I do, the best I can get are the aforementioned numbers.

Two questions:
1. Any ideas on why the GPM would be so slow?
2. How much flow is recommending in systems like these and why?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,301
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    1. Yup. That electric tankless hot water heater is a lovely gadget. With a huge head loss. Unless you changed the pump from the old gas boiler, there is no way the original pump is going to come even close to the same flow. You need a high head pump in that application -- not a heating system circulator at all.
    2. you need the same flow in the system that you had before with the gas boiler, whatever that was. Probably at least 8 gpm. I hear you say that your pump is rated at 16 gpm -- but at what head? Pump flows drop dramatically with increasing head.

    Now. All that said. In my humble opinion, open direct systems are an invitation to illness. I would never, ever use one. Second, water heaters -- such as you are using -- are meant to heat hot water for domestic use, not for use in heating systems. Wrong tool for the job.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Mad Dog_2GGrossbburd
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,929
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    No no no no...What do you mean gas "got you sick"?  Was there a Gas leak?  Carbon Monoxide issue? Mad Dog 🐕 
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,526
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    Tankless heater are not the right set up for radiant. They are made for domestic hot water.

    The tankless has a high pressure drop due to it's being designed for domestic water.

    You probably need bigger circulators.

    You need to know the flow (GPM you need on the total radiant loops. Then add the resistance of the longest radiant loop (or the loop with the most resistance) in feet of head at the gpm that one loop requires to the resistance of the tankless heater at the total flow for all radiant loops. Like This:

    1. Resistance of tankless heater in ft of head with the total flow of all radiant loops going through it
    2. Resistance in feet of head to the longest radiant loop (or loop with the most resistance) with that loops required flow going through it.
    3. Resistance of any common piping through the boiler feeding your manifolds.

    Once you have that come back and someone can help you size your circulators.

    You may want to pipe it primary secondary.

    You would have been better off fixing the gas boiler, but that is spilt milk at this point
  • Prevch
    Prevch Member Posts: 106
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    Mad Dog_2 said:
    No no no no...What do you mean gas "got you sick"?  Was there a Gas leak?  Carbon Monoxide issue? Mad Dog 🐕 we had three separate gas leaks over three years.
  • Prevch
    Prevch Member Posts: 106
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    Prevch said:
    Mad Dog_2 said:
    No no no no...What do you mean gas "got you sick"?  Was there a Gas leak?  Carbon Monoxide issue? Mad Dog 🐕 we had three separate gas leaks over three years.
    Mad Dog_2 said:
    No no no no...What do you mean gas "got you sick"?  Was there a Gas leak?  Carbon Monoxide issue? Mad Dog 🐕 
    Yes gas leak
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,849
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    https://www.ecomfort.com/manuals/64565bad5a676631039f0c9fd0410b13.pdf

    Something like this, not necessarily directly  connected to DHW, would have let you use a pump essentially only sized for the loops, since it’s head loss is so low. 
  • GroundUp
    GroundUp Member Posts: 1,907
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    Wrong appliance for the job, and it's piped wrong anyway. Forget anything and everything you learned from Radiantec, an open system is NOT the answer. If you think a gas leak got your family sick, wait until you all have Legionnaire's Disease from this arrangement. You could very possibly make this work by adding a high head circulator in the primary position only flowing through the tankless, but the proper channel would be to replace the tankless with an actual boiler and go back to a closed loop system.
  • Jamie Hall
    Jamie Hall Member Posts: 23,301
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    three gas leaks... incompetent tech. people trying to "help" you. As to this as the solution. If you have three flat tires in a year, do you sell the car and buy a new car?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    KNPV_PSD
  • Prevch
    Prevch Member Posts: 106
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    three gas leaks... incompetent tech. people trying to "help" you. As to this as the solution. If you have three flat tires in a year, do you sell the car and buy a new car?

    No, but presumably one might have some warning of a tire deflating. The gas leaks were silent and our alarms did not go off for some reason. Either way, this is the decision I have made and I need to provide the best electric solution that I can. I appreciate your insights.
  • Prevch
    Prevch Member Posts: 106
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    https://www.ecomfort.com/manuals/64565bad5a676631039f0c9fd0410b13.pdf


    Something like this, not necessarily directly  connected to DHW, would have let you use a pump essentially only sized for the loops, since it’s head loss is so low. 
    Yeah I have been looking at those boilers. I might do a combination of one of those boilers for the heat and a tankless for the DHW. Thank you!
  • Prevch
    Prevch Member Posts: 106
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    GroundUp said:

    Wrong appliance for the job, and it's piped wrong anyway. Forget anything and everything you learned from Radiantec, an open system is NOT the answer. If you think a gas leak got your family sick, wait until you all have Legionnaire's Disease from this arrangement. You could very possibly make this work by adding a high head circulator in the primary position only flowing through the tankless, but the proper channel would be to replace the tankless with an actual boiler and go back to a closed loop system.

    Hi thank you for your response. Can you elaborate? How does one get sick in this set up? Also, where did I go wrong on the piping? I plan on going back to a closed loop, but just curious. Trying to learn.
  • Prevch
    Prevch Member Posts: 106
    edited September 2023
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    Tankless heater are not the right set up for radiant. They are made for domestic hot water.

    The tankless has a high pressure drop due to it's being designed for domestic water.

    You probably need bigger circulators.

    You need to know the flow (GPM you need on the total radiant loops. Then add the resistance of the longest radiant loop (or the loop with the most resistance) in feet of head at the gpm that one loop requires to the resistance of the tankless heater at the total flow for all radiant loops. Like This:

    1. Resistance of tankless heater in ft of head with the total flow of all radiant loops going through it
    2. Resistance in feet of head to the longest radiant loop (or loop with the most resistance) with that loops required flow going through it.
    3. Resistance of any common piping through the boiler feeding your manifolds.

    Once you have that come back and someone can help you size your circulators.

    You may want to pipe it primary secondary.

    You would have been better off fixing the gas boiler, but that is spilt milk at this point

    Thank you very much for this information, I will look into all of this. For the sake of completeness I have attached images of the circulating pumps. The red and green ones are still in place and were also in place when the gas boiler was hooked up. The silver one is the one that was inside of the old boiler.






  • Prevch
    Prevch Member Posts: 106
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    Tankless heater are not the right set up for radiant. They are made for domestic hot water.

    The tankless has a high pressure drop due to it's being designed for domestic water.

    Here is the weird thing though, Menards sells this unit under a different name as a boiler....just seems really weird to me -

    https://www.menards.com/main/plumbing/hydronic-radiant-heat/radiant-heat-boilers/stiebel-eltron-hydro-shark-reg-electric-microboiler/sh3-24/p-1444433730827-c-8519.htm?tid=43dba6c6-8bcc-44f1-b37c-550e7a37ff73&ipos=1&exp=false




  • GroundUp
    GroundUp Member Posts: 1,907
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    That internal circ that your gas boiler had is what's referred to as a "primary" or "boiler" circ. It separates the flow through the boiler from the flow through the system, so that the two loops can operate independently. By replacing the boiler with the tankless, you eliminated the primary/secondary piping and therefore the entire system is trying to flow through the high pressure drop tankless in series. In order for this to work at all, you need to add a primary circ again to separate the flows in a primary/secondary fashion. Menards sells those turds as boilers because HydroSmart found a loophole, but look at their "integrator panels" and you'll see that any system using that "electric microboiler" is piped primary/secondary. They used to sell them piped as yours is, and there were oodles of fires and lawsuits which resulted in a name change of the company. I have the 14.4 KW version in my shop that I installed 15+ years ago before I got into the trade with both feet and knew any better, and it only flows about 1.5 GPM total becuase it's piped like yours. I have an Electro Boiler awaiting install, but it's not high on my priority list because I don't keep it very warm in there and I also have a wood boiler that's used as the primary heat source.

    Regarding the Legionnaire's, that is caused by Legionella which is a bacteria that grows in stagnant water. You shut your radiant down for the summer and that water all sits in the lines, growing bacteria all summer, then when the thermostats begin circulating the water again in the fall, it introduces that bacteria into your water supply which can be ingested or even inhaled as it goes airborne (shower, for example). This is a common issue with open systems, and also the source of many lawsuits against Radiantec.
  • Prevch
    Prevch Member Posts: 106
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    GroundUp said:

    That internal circ that your gas boiler had is what's referred to as a "primary" or "boiler" circ. It separates the flow through the boiler from the flow through the system, so that the two loops can operate independently. By replacing the boiler with the tankless, you eliminated the primary/secondary piping and therefore the entire system is trying to flow through the high pressure drop tankless in series. In order for this to work at all, you need to add a primary circ again to separate the flows in a primary/secondary fashion. Menards sells those turds as boilers because HydroSmart found a loophole, but look at their "integrator panels" and you'll see that any system using that "electric microboiler" is piped primary/secondary. They used to sell them piped as yours is, and there were oodles of fires and lawsuits which resulted in a name change of the company. I have the 14.4 KW version in my shop that I installed 15+ years ago before I got into the trade with both feet and knew any better, and it only flows about 1.5 GPM total becuase it's piped like yours. I have an Electro Boiler awaiting install, but it's not high on my priority list because I don't keep it very warm in there and I also have a wood boiler that's used as the primary heat source.

    Regarding the Legionnaire's, that is caused by Legionella which is a bacteria that grows in stagnant water. You shut your radiant down for the summer and that water all sits in the lines, growing bacteria all summer, then when the thermostats begin circulating the water again in the fall, it introduces that bacteria into your water supply which can be ingested or even inhaled as it goes airborne (shower, for example). This is a common issue with open systems, and also the source of many lawsuits against Radiantec.

    Thank you for that information.

    So, based on your experience, if you were going to put in an electric boiler, you would go with the Electro Boiler on a closed loop?
  • Jamie Hall
    Jamie Hall Member Posts: 23,301
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    They're as good as any. For radiant heat, however, I'd use an air to water heat pump unless you are in a very cold climate. It will cost more to buy -- but a lot less to run (only half to a third as much). Unless your electric rates are low... that might be a consideration (around where I am a 28 KW electric boiler would cost me almost 10 dollars an hour to run -- and I don't have that kind of money to spare).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GroundUp
    GroundUp Member Posts: 1,907
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    Prevch said:

    GroundUp said:

    That internal circ that your gas boiler had is what's referred to as a "primary" or "boiler" circ. It separates the flow through the boiler from the flow through the system, so that the two loops can operate independently. By replacing the boiler with the tankless, you eliminated the primary/secondary piping and therefore the entire system is trying to flow through the high pressure drop tankless in series. In order for this to work at all, you need to add a primary circ again to separate the flows in a primary/secondary fashion. Menards sells those turds as boilers because HydroSmart found a loophole, but look at their "integrator panels" and you'll see that any system using that "electric microboiler" is piped primary/secondary. They used to sell them piped as yours is, and there were oodles of fires and lawsuits which resulted in a name change of the company. I have the 14.4 KW version in my shop that I installed 15+ years ago before I got into the trade with both feet and knew any better, and it only flows about 1.5 GPM total becuase it's piped like yours. I have an Electro Boiler awaiting install, but it's not high on my priority list because I don't keep it very warm in there and I also have a wood boiler that's used as the primary heat source.

    Regarding the Legionnaire's, that is caused by Legionella which is a bacteria that grows in stagnant water. You shut your radiant down for the summer and that water all sits in the lines, growing bacteria all summer, then when the thermostats begin circulating the water again in the fall, it introduces that bacteria into your water supply which can be ingested or even inhaled as it goes airborne (shower, for example). This is a common issue with open systems, and also the source of many lawsuits against Radiantec.

    Thank you for that information.

    So, based on your experience, if you were going to put in an electric boiler, you would go with the Electro Boiler on a closed loop?
    I've got extensive experience with most, if not all electric boiler manufacturers. Electro is without a doubt, the best there is.
  • Prevch
    Prevch Member Posts: 106
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    They're as good as any. For radiant heat, however, I'd use an air to water heat pump unless you are in a very cold climate. It will cost more to buy -- but a lot less to run (only half to a third as much). Unless your electric rates are low... that might be a consideration (around where I am a 28 KW electric boiler would cost me almost 10 dollars an hour to run -- and I don't have that kind of money to spare).

    Got a link to a system you might like?
  • Prevch
    Prevch Member Posts: 106
    edited September 2023
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    GroundUp said:

    Prevch said:

    GroundUp said:

    That internal circ that your gas boiler had is what's referred to as a "primary" or "boiler" circ. It separates the flow through the boiler from the flow through the system, so that the two loops can operate independently. By replacing the boiler with the tankless, you eliminated the primary/secondary piping and therefore the entire system is trying to flow through the high pressure drop tankless in series. In order for this to work at all, you need to add a primary circ again to separate the flows in a primary/secondary fashion. Menards sells those turds as boilers because HydroSmart found a loophole, but look at their "integrator panels" and you'll see that any system using that "electric microboiler" is piped primary/secondary. They used to sell them piped as yours is, and there were oodles of fires and lawsuits which resulted in a name change of the company. I have the 14.4 KW version in my shop that I installed 15+ years ago before I got into the trade with both feet and knew any better, and it only flows about 1.5 GPM total becuase it's piped like yours. I have an Electro Boiler awaiting install, but it's not high on my priority list because I don't keep it very warm in there and I also have a wood boiler that's used as the primary heat source.

    Regarding the Legionnaire's, that is caused by Legionella which is a bacteria that grows in stagnant water. You shut your radiant down for the summer and that water all sits in the lines, growing bacteria all summer, then when the thermostats begin circulating the water again in the fall, it introduces that bacteria into your water supply which can be ingested or even inhaled as it goes airborne (shower, for example). This is a common issue with open systems, and also the source of many lawsuits against Radiantec.

    Thank you for that information.

    So, based on your experience, if you were going to put in an electric boiler, you would go with the Electro Boiler on a closed loop?
    I've got extensive experience with most, if not all electric boiler manufacturers. Electro is without a doubt, the best there is.
    Ok great thank you....are they expensive to run? Do you know if they come with a primary circulator like you mentioned?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,849
    edited September 2023
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    Ok great thank you....are they expensive to run? Do you know if they come with a primary circulator like you mentioned?
    They’ll be as expensive as your existing electric boiler, electric resistance is what it is. You’ll save some by having less pumping losses, but minor. You currently have a large boiler, if you replace it, replace it with something sized to your heat load (if you’re able to use a storage water heater or separate tankless for DHW). 
  • Prevch
    Prevch Member Posts: 106
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    Ok great thank you....are they expensive to run? Do you know if they come with a primary circulator like you mentioned?
    They’ll be as expensive as your existing electric boiler, electric resistance is what it is. You’ll save some by having less pumping losses, but minor. You currently have a large boiler, if you replace it, replace it with something sized to your heat load (if you’re able to use a storage water heater or separate tankless for DHW). 
    Thank you for all of that information
  • GroundUp
    GroundUp Member Posts: 1,907
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    Electro boilers do not come with a primary circ, because they don't need one in most cases. There is almost zero pressure drop through them- they're nothing like the tankless you have.
  • Prevch
    Prevch Member Posts: 106
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    GroundUp said:

    Electro boilers do not come with a primary circ, because they don't need one in most cases. There is almost zero pressure drop through them- they're nothing like the tankless you have.

    Thank you! If I need one, do I ask for a primary circulator at the supply store? The reason I asked is because I googled it and I am not able to differentiate on the google search results between a primary circulator and the red and green one that I have
  • Jamie Hall
    Jamie Hall Member Posts: 23,301
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    Prevch said:

    GroundUp said:

    Electro boilers do not come with a primary circ, because they don't need one in most cases. There is almost zero pressure drop through them- they're nothing like the tankless you have.

    Thank you! If I need one, do I ask for a primary circulator at the supply store? The reason I asked is because I googled it and I am not able to differentiate on the google search results between a primary circulator and the red and green one that I have
    You can't differentiate them because there isn't a difference... a pump is a pump. If it's on the primary loop, it's a primary circulator. If it's on a secondary loop, it's a secondary circulator or zone pump.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    GroundUp
  • Prevch
    Prevch Member Posts: 106
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    Prevch said:

    GroundUp said:

    Electro boilers do not come with a primary circ, because they don't need one in most cases. There is almost zero pressure drop through them- they're nothing like the tankless you have.

    Thank you! If I need one, do I ask for a primary circulator at the supply store? The reason I asked is because I googled it and I am not able to differentiate on the google search results between a primary circulator and the red and green one that I have
    You can't differentiate them because there isn't a difference... a pump is a pump. If it's on the primary loop, it's a primary circulator. If it's on a secondary loop, it's a secondary circulator or zone pump.
    ok got it. Thank you!
  • hot_rod
    hot_rod Member Posts: 22,144
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    That Grundfos is probably a bit large for a typical electric bioiler. It is a nice stainless pump!
    Built the 13th week of 1996.

    1/2" pex loops 250- 300' length generally run .5- .65 gpm per loop
    You have 7 loops? So 3-4 gpm is all you need.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Prevch
    Prevch Member Posts: 106
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    hot_rod said:

    That Grundfos is probably a bit large for a typical electric bioiler. It is a nice stainless pump!
    Built the 13th week of 1996.

    1/2" pex loops 250- 300' length generally run .5- .65 gpm per loop
    You have 7 loops? So 3-4 gpm is all you need.

    That's great information thank you. Any idea why I was only get .5gpm on that electric tankless hot water heater? Is that just from head loss ya think? Seems odd to me.
  • Jamie Hall
    Jamie Hall Member Posts: 23,301
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    Prevch said:

    hot_rod said:

    That Grundfos is probably a bit large for a typical electric bioiler. It is a nice stainless pump!
    Built the 13th week of 1996.

    1/2" pex loops 250- 300' length generally run .5- .65 gpm per loop
    You have 7 loops? So 3-4 gpm is all you need.

    That's great information thank you. Any idea why I was only get .5gpm on that electric tankless hot water heater? Is that just from head loss ya think? Seems odd to me.
    An electric boiler and an electric tankless water heater are two completely different critters. The electric boiler has a very low head loss -- almost negligible. The tankless water heater, on the other hand, likely has a head loss of 30 to 50 feet at your desired flow rate. It's a wonder you get any flow at all...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Prevch
    Prevch Member Posts: 106
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    Prevch said:

    hot_rod said:

    That Grundfos is probably a bit large for a typical electric bioiler. It is a nice stainless pump!
    Built the 13th week of 1996.

    1/2" pex loops 250- 300' length generally run .5- .65 gpm per loop
    You have 7 loops? So 3-4 gpm is all you need.

    That's great information thank you. Any idea why I was only get .5gpm on that electric tankless hot water heater? Is that just from head loss ya think? Seems odd to me.
    An electric boiler and an electric tankless water heater are two completely different critters. The electric boiler has a very low head loss -- almost negligible. The tankless water heater, on the other hand, likely has a head loss of 30 to 50 feet at your desired flow rate. It's a wonder you get any flow at all...
    Good to know.