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delta T

Paul SPaul S Member Posts: 1,219
How does a higher delta T affect heating? I know the higher the delta t the lower Gpm at a certain amount of btus.... For example 100,000 btus 20 f Td = 10gpm and 100000 btus at 30f td is 6.6gpm....on condensing boilers installation manuals they always show a pump abs pipe sizing chart. ....I always use the 20f it usually results in bigger pipe and bigger and expensive pump. ... When should i use higher delta T? Thanks Paul S
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  • IronmanIronman Member Posts: 3,411
    edited February 2014
    Delta T


    It's gonna depend on the design delta T of the system and its type of emitters. If you have HWBB, then it most likely was designed for a 20* delta T and that's what you have to live with unless extra radiation is added.

    If, on the other hand, you have low temp emitters, then a wider delta T may be desirable.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • SWEISWEI Member Posts: 7,356
    Higher ∆T

    In the distribution loop reduces pumping costs, pipe size/costs, and valve size/costs by raising emitter cost   Two out of three win there.  Further analysis required in order to make a good decision, but high probability of success.

    In the boiler loop, it reduces pumping and piping costs while increasing boiler efficiency.  Three out of three win there.  As long as we keep everything within design parameters (no flue gas condensation, don't trip boiler safeties, don't risk boundary conditions where everything goes to hell) it's a no-brainer.
  • Mark EathertonMark Eatherton Member Posts: 5,582
    These conversations about Delta T amuse me….

    I guess there are two sides to every coin. Designing the system to operate at as low a temperature as possible is really important to the heating appliance and overall system efficiency.

    When people start playing games with flow trying to exert more differential, they are actually creating problems within the system. A big delta T is not always the optimum.

    Think about it. You are trying to get your emitters (what ever they are) as warm across the whole emitter as possible. Granted, there is a point of diminishing returns, but within reasonable circulator capacity and expectations, you are not wasting a WHOLE lot of parasitic energy to keep the differential in temperature low, and the AVERAGE emitter temperature high.

    As it pertains to the heat source, look at the opposite end of the scale. A lower average fluid temperature within the "converter", should equate to a higher average temperature differential between the flame and the working fluid, resulting in HIGHER net thermal (flame to water) efficiency.

    I have been recording and watching the delta T on my mountain property for quite some time now, and one thing I have found that is really interesting, is that if I let the fluid flow as fast as it can, even when I am coming out of my typical deep set back (40 degrees F) and headed for an occupied condition (70 degrees F), the delta T rarely exceeds 8 to 10 degrees between the supply and return. Even as the boiler goes through its ramping program, the delta T remains pretty much the same which kills the theory of higher water temperature putting out more heat. If I choke the flow to create a greater differential, I can physically sense the front loading of the heat emitters (radiant ceilings, walls and floors) and that doesn't equate to comfort in my book. Interestingly enough, I designed the whole shebang around a 20 degree differential, and in the 5 years its been running, have yet to see it actually happen

    The delta T is what the delta T is, and it is dictated by the emitter, surrounding ambient temperature, surrounding mean radiant temperature, color, convective potential, surface finish, Reynolds numbers, fluid type fluid temperatures and more.... Tubing at 12" OC in cold concrete is going to show (initially any way) a greater delta T than a light weight (enter methodology here) emitter.

    I also think people spend way to much time trying to guarantee that their condensing boiler is in the sweet spot of condensing. I think the real key to the overall efficiency of these new fangled machines is the fact that they MODulate their input and output to the real time load. Oh sure, the CONdensing capacity adds to the efficiency, but it has been proven time and time again that we spend so little time with high temperature emitters taking the appliance out of its sweet spot, that it is not worth worrying about…

    In my book, a LOW delta T makes more sense with a run of the mill system than does a HIGH delta T. Even though all baseboard is "designed" around a 20 degree delta T, it will rarely, if ever see those kind of numbers, and I don't see people with those systems complaining about system discomfort of system inefficiencies…

    I personally think (based on real world experience) people are making a big deal out of something that in reality, they have little to no control over, and when they try and influence those conditions in favor of the heat source, human comfort suffers.

    And then there's the conversations I've had with two very knowledgeable people who had hydronic systems that their heat sources were bouncing off of the high limits, with little to no DT, and little to no thermal output, and when they slowed the flow down, suddenly everything fell into line. Personally, I've never experienced that, and theoretically, it doesn't make ANY sense, but both of these people are very knowledgeable, experienced and have all of my respect.

    Just when you think you've seen it all and know what you are talking about, Mother Nature jumps up, slaps you alongside the head and humbles you…

    Trust, but verify, and if comfortable, don't mess with it :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • GordyGordy Member Posts: 7,111
    edited February 2014
    Super emitterated


    Did you ever keep track of those readings with just certain radiation surfaces in place?

    I know you have ceilings, floors,walls, and windows radiant surfaces. But they were,not all in place at once were they? If not did you data log with only certain surfaces in place?

    I know in my own home when I only had radiant ceilings the delta s in individual loops were 5-10 deltas with total system supply return delta of 15. It was always rock solid, and never varied with outdoor temps. Same delta always floats with supply temps.

    Now that I have added radiant floors to some of the rooms which now have radiant floors, and ceilings the delta is always a 9 for the floors, and total system delta dropped to 13. Still always the same though.

    What's interesting is my radiant is parallel piped. New loops were no longer than any existing loops. Circulator the same. So in theory my system flow rate gets divided up by additional loops added so flow rates in loops actually decreased.

    In your case, and mine designing for a 20 delta, and experiencing a 10 is it possible a certain square feet of radiation decreases design delta by a factor?
  • icesailoricesailor Member Posts: 7,265
    More Delta T's:

    "" I personally think (based on real world experience) people are making a big deal out of something that in reality, they have little to no control over, and when they try and influence those conditions in favor of the heat source, human comfort suffers. ""

    Sound advice.

    A former President said (Partly) in his inaugural address, "Some people see things and ask why?" If through out your career, you have been tasked to figure out why things aren't working why they are supposed to work, but do mostly, you develop a jaundiced view about problems. How important is it?

    Looking at the systems that all those dead Wetheads installed that work, you then find the small things that don't work and figure out why the rest of it works and the one thing doesn't.

    Where I spent my career, I think that most everyone thought that a "Delta T" was a brand or type of Tee Shirt sold in tee shirt shops open in the summer. I did all the IBR courses that I bought, read and did on my own. My location and scheduling never offered me the opportunity to attend a course. If I read about Delta T's, I probably though it was a Tee Shirt. I first came to understand that it had nothing to do with Tee Shirts at a Emerson-Swan seminar that Dan and Taco were the speakers on Heatway Radiant Heating. I started putting probes on any heating system I came across and noticed that most all had something around that Tee Shirt letter, "T". I also noted that as the burner cycled, it stayed within the differential of the High Limit. So, if the high limit was 200 degrees, it would over ride to as much as 210 degrees. And after the burner started at 190 degrees, the return temperature could drop to 180 degrees. That's a 40 degree Tee Shirt. But the difference was always 20 degrees, if that's what it started with.

    I always dealt with Baseboard. So, if you have rooms with 100' of copper (3/4") baseboard, and the high limit is 180 degrees and the end is 160 degrees (measured by the temperature going back into the boiler), what is the output of the last room getting 160 degree water? Not as much as the first room. My experience usually showed that the Tee Shirt was more like 10 degrees. But with the same Taco 007 or B&G Series 100, piping through 3/4" copper tube through the system, and pumping at a rated flow (whatever that is) and the emitter is emitting, (10 degree T Shirt), how many BTU's are given off in the given volume of water as compared to if the whole loop is piped with 1/2" baseboard and the Tee Shirt is now 20 or 30 degrees?

    ME talks about comfort. I did a bunch of houses for a designer that hated any sign of any utility that he had a personal dislike for. He lived on a boat so he hated bathrooms and made them as small and uninviting as he could. Every house was electric radiant Flex-Watt strips in ceilings. So you didn't see it. If I turned the water back on in a house like this in January, it would take three days to get the house up to temperature because the Flex-Watt was only good for 25 BTU's.

    Now, he uses all gas HAC.

    Its a wonderful thing to design things with perfection in mind. Nothing is perfect. With common sense in mind, life can be good.

    I don't consider myself to be some form of Luddite. The first time I saw a WILO ECM circulator at a Eat & Greet at the supply house, I understood immediately, their advantage to my personal home system with 6 zone valves. I bought it off the table. I installed quite a few.

    Anyone who wants to play with Tee Shirts should always use multi-speed circulators or ECM ones.

    ME, you have a well designed system. You should see some of the ones that can't be fixed because they are piped too small and can't be fixed. If owners would leave the thermostats alone, life would be good. "Experts" tell them that they can save money by turning the heat down. They don't explain that doing so is the same as manipulating the outdoor temperature and they just made the heating system think it is colder outside than it really is. And not big enough to do the job it was designed to do.
  • Mark EathertonMark Eatherton Member Posts: 5,582

    I installed all emitters at the time of construction, so no, I wasn't able to do a one by one evaluation. I could isolate certain components, but it would be difficult to do, and probably wouldn't really give me much in the way of useful information.

    In my PC based control system, I can monitor up to 64 individual points. I am only using about 1/4 of the systems capacity at present. And I can also switch some monitoring points around to "watch" what ever components I want to control or monitor.

    On my space heating/cooling system, I can see system supply, system return, room temperature, OSA temperature, and just to be able to monitor the possibility of solar gain, my solar collectors.

    On the solar system, I can see solar storage tank top, bottom, line going from and to solar array, and of course the solar collector absorber temperature. The solar storage tank is 120 gallon SuperSor. Aux. is an 80 gallon SuperStor. I installed circ lines and a small circulator between the solar preheat and the aux. so that if the solar tank gets real hot (summer) I can transfer heat from the PHS to the aux. tank before I show up on the weekends. If both the PHS and the AUX. tanks are maxed out at 180 degrees, I have a 15' deep well I drilled and set a 2" ID copper heat exchanger (concentric piping from the top) that I can use as a heat dump. It is drilled into blue shale and is grouted tightly in, so it works very effectively as a heat sump. I could also use it as a cooling source for my radiant ceiling heating/cooling system.

    I have sensors at the top, middle and bottom of the thermal storage/dump well.

    I am also monitoring the run time, heated window temperature, and one unheated window temperature (for reference).

    Hopefully, this summer, I will get my woody biomass boiler and 240 gallon storage tank system on line, and will be able to monitor everything going on with that chunk of real estate.

    Eventually, when I am done remodeling my little cabin next door, I will be able to monitor all of its critical vitals and control the LP modcon I will be setting up over there.

    Did I mention that I can control 32 different points in the system?

    To save chewing up too much memory , I only store the data for a maximum of one year, but can take a digital picture of any "event" that I want and evaluate it later.

    I can also divert the solar energy from teh DHW system directly to my house radiant heating system and extend the boilers OFF cycle before it starts using that expensive LP again. Based on my most recent LP bill, I am thinking its time to start looking at installing an electric boiler. LP is now around $5/therm, electricity is at $2/therm.

    I can manipulate ALL of this system remotely using any PC, cell phone or iPad. Hence, I can turn my operating temperature from the 40 degree F setting I use when I am not there, to the higher 68 degree temperature when I am there, and by doing it remotely, the house is toasty warm when we arrive there. It gives me the opportunity to watch the house during recovery and acceleration, as well as normal running conditions.

    I also have a remote PTZ camera that I can turn on at any time, and see what the wethers doing, day or night. Funny to see the wild life enjoying my property when I am not there. I've seen elk herds, deer and foxes hanging around the lot.

    In regards to your last question, increasing surface should increase delta T, IF the flow rate through the emitter remains the same (doubtful unless you upgraded the pump) but the hydraulics may have changed such that it appears to be a decrease in output, which could be a function of the Reynolds numbers being affected by lower flow rate and less velocity. Laminar flow dontcha know…

    Got questions? :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Mark EathertonMark Eatherton Member Posts: 5,582
    And here all along I thought...

    Delta Tea was a special hot drink from Louisiana. :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • GordyGordy Member Posts: 7,111

    Serious data logging, and controlling goin on there Mark!

    Well being that I can't monitor 64 data points, and control 32 actions I'm only making some ass-umptions.

    Initially my particular system had 21 loops all parallel piped. With the circ I have in place, and the calculated head I'm running 15 gpm @ 11' of head so IF the loops are equal length which they are most likely not the gpm to each loop would be .71 gpm. 15gpm/21=.71 gpm

    Now I have added 5 more loops to the system so now I have 26 loops/15gpm=.58gpm....approximately.

    Now what I have in place is thermometers at the main supply, and return at the boiler after mixing, and at the other end of the main supply, and return at the farthest point from the the boiler which supply the radiant ceilings. I also have thermometers on the main supply, and return of the floor loops I added which are parallel piped also. All other temperature monitoring is done with an IR thermometer at various points.

    The main supply return temps for the radiant ceilings has always been a 15 delta before, and after. Adding floor loops. The delta should have increased because of the lower flow rates in the loops. The added floor loops delta is a 9.

    What's stuck,in my head is the main supply/return delta should have increased due to a theoretically lowered flow rate due to the addition of extra loops to the parallel piping system. It has not changed, so what I'm thinking is now that I have 4 rooms with opposing radiant surfaces that those surfaces are giving up less heat due to double the radiant surface area. There for even though the flow rates decreased the delta t remained the same....pretty much by accident, unintended,and now wondering why.

    For your amusement Mark as if you already don't have enough to think about.
  • icesailoricesailor Member Posts: 7,265
    Delta Tea:

    Is that like "Texas Tea"?

    Unrefined or refined crude from oil leaks?
  • Mark EathertonMark Eatherton Member Posts: 5,582
    I forgot to mention a key factor in thermal loading for radiant panels...

    It's a factor called AUST. Average Unheated Surface Temperature. By having a radiant combo sandwich, you actually reduced the load for both the floor and the ceiling. You are also dealing with two different conductivity factors. Copper versus plastic.

    Both of those factors will play off of each other and will affect your temperature differential.

    I have one of those radiant sandwiches in my mountain home. Radiant walls and radiant windows. :-) And I rarely run the windows above 74 degrees F. Snug as a bug in a rug.

    Now, if I can just figure out a way to make the glass cool, I'd be a millionaire. Maybe if I just switch the polarity of the windows… Nahhh. Too easy.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • SWEISWEI Member Posts: 7,356
    Reversing windows

    A transparent PV panel excited by low level infrared?  Many, many millions.
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