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Why 20* TD?

bob_25 Member Posts: 97
Mitch, You haven't given us any details about your system so speculation is in order. Most probably your boiler is over sized and so is your radiation , the system has never been balanced and may not have provisions to do so. The flow rate through the system is probably too high. If it was mine and it wasn't noisy and heats O.K. I wouldn't worry about delta-T, it's seldom the system sees design conditions anyway. bob


  • mitch moore
    mitch moore Member Posts: 32
    supply and return

    Is this to insure that the water has given up all of it's btu's? Also, How can I determine the flow rate that's going through a particular connected load? Can you acctually decrease btu output by having to fast of a flow rate?
  • mitch moore
    mitch moore Member Posts: 32
    supply and return

    Is this to insure that the water has given up all of it's btu's? Also, How can I determine the flow rate that's going through a particular connected load? Can you acctually decrease btu output by having to fast of a flow rate?
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Which moves more energy...

    100 GPM @ a 1 degree rise, or 1 GPM at a 100 degree rise...

    I'll let you think about that for a while whilst I explain my take on the 20 degree delta T.

    Near as I can tell, the baseboard fin tube convector manufacturers came up with it to describe the flow through their convectors to guarantee a specific output per foot for board. It is the equivilent of 1 GPM delivering 10,000 btu H with a 20 degree delta T.

    What this all boils down to is this, once you kow what your btuH load factor is, you can size the GPM requirements of the pump based on 1 GPM @ 20 degree drop/rise delivering 10,000 btuH. In number words, if your load came out to say 78,000 btuH, then your pump would have to move 7.8 GPM. You haven't figured out the head requiremnts for the pump yet, but you nailed the GPMs necessary to "guarantee" a 20 degree delta T 'potential'.

    It's rather excessive in my humble opine, and will consume a lot less parasitic electricty if you choose a wider delta T. You just have to make sure the system is capable of working at those low temps and wide differentials. That equates to massive, large area heat emmitters, like floors, walls and ceiling. Ya need a good load to tye in to. The boiler manufacturers don't really care what your differential across thier boilers are (to a point) so long as you don't condense their appliance to death. This means that if your heat source were designed to deliver 180 degree water, it wouldn't hurt it to see a 140 degree F return temp.

    However, if you design the "system" to work around a lower temperature and a large delta T, say like 100 to 140 deg F, then you could keep high efficiency condensing equipment right in it's "sweet" zone as it pertains to thermal efficiency. 95% to 98% thermal efficiency is a heack of a lot better than 80 to 85% thermal efficiency...

    Shoot a litle solar and waste heat recovery in there and you've got yourself some real energy savings!

    So, in a nut shell, your delta T may vary depending on your driving habits:-)

    Did you figure out the answer to the question on the first line yet???

    They deliver the same, about 50,000 btuH:-)

    Happy Holiday Hydronicing!!


  • bob_25
    bob_25 Member Posts: 97

    Mitch, the answer to your first question is NO. Second question, a flow meter or a calibrated orfice or circuit setter and a differential pressure gauge(expensive). Third question, some posters on the wall think you can reduce heat transfered by increasing flow rate. I'm skeptical on that one. I've never seen it happen. The physics doesn't wash but anything is possible. bob
  • JimGPE_3
    JimGPE_3 Member Posts: 240
    We engineers are lazy....

    With a 20F dT the math is easy! Load in BTUH divided by 10,000 = GPM!

    Also, with series loads, like long runs of baseboard, the water gets colder as you move along the heater. With a 20F dT along the zone, the drop in temperature isn't particularly significant in terms of loss of heating capacity (BTUH/Ft).

    With a 40F dT, the loss in capacity can be significant. Its not a problem, you just need to increase the amount of active fin near the end of the run to compensate, but you do need to understand this will happen, particularly if the last few feet of finned element is in another room.

    Also, high dT's means low flow, and if the finned element is a large tube, you can have laminar (non-turbulent) flow which can have a bad effect on heat transfer. Most finned element catalogs have a minimum flow per pipe size, so don't go below that flow.

    Lastly, most boiler catalogs list a minimum return water temperature, and you cannot go below that or you will cause the flue gasses to condense, which will rust out your boiler/burner pretty quickly. Watch this especially if you plan to do outdoor temperature reset (the higher the outdoor temperature the lower the supply water temperature).

    Probably more than you wanted to know, but I had some time on my hands and felt like writing!
  • JimGPE_3
    JimGPE_3 Member Posts: 240
    Too fast?

    More than 10 fps and you can get erosion of the tubing, but generally the faster the flow the MORE heat you get. Here's why.

    Heat flow for a given configuration is proportional to the temperature difference between the air and the AVERAGE water temperature. The higher the flow, the lower the dT, so the higher the average water temperature and the higher the heat flow.

    Also, the more turbulent the flow, the better the heat transfer, so more is better.

    The effect of flow on hot water heating appliances exists, but is not particularly significant when you are talking about 190F average water temperature. The dT between the 190 and the 70F air temperature is very high. A 20F change in a.w.t. is only a 17% change in a.w.t., so a 17% change in heat flow. The change in GPM is very significant.

    Look at the finned tube catalogs. The difference in heat flow at 1 GPM and 3 GPM, a three-fold increase in GPM, is insignificant.

    So yes, flow does have an effect, but on hot water heating appliances in the 200F range, the effect is not significant.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    20° delta-t in water systems seems to have been used for a LONG time as it is frequently referred to as the "standard" regarding gravity systems. Some texts say though that in operation the delta-t was typically higher in the 30°-40° range.

    Lacking a flow meter you have to "reverse engineer" to find the approximate value. Compute the highest pressure drop in the system and check against the curve of the particular circulator used.

    Would be really hard to decrease output by increasing flow in any practical application as it would seem to require extremely high velocity. That said, strange things can happen in converted gravity systems--particularly short (lengthwise) rads close to the boiler when restrictors were used in upper floors.

    With convective/radiative emission devices higher flow=lower delta-t=higher output. With extremely conductive devices like copper in heavy aluminum plates it [seems] that increased flow=higher delta-t=higher output however... This is a strange situation that seems to vary with the actual mean radiant load on the panel to which such are attached.

  • Rob T
    Rob T Member Posts: 64
    So what about a system

    that has a 7-10º DeltaT?

    That's what I am getting now that I have zoned my system. My limit is set to 185º and my return (On the one I have a thermo on, the rest have to wait until the new pump arrives.) temp is 175º-178º consistantly.

  • Tom Anderson
    Tom Anderson Member Posts: 40
    Modern Design Practice

    As stated, the old 20F TD design practice is a holdover from the days when all of these calcs were done by hand, and 10,000 Btu per gallon made things easy and simple. There is absolutly no technical or economic basis for 20F TD; in fact, quite the contrary.

    Many equipment manufacturers, as well as ASHRAE, suggest designers apply higher TD's... in the 30F to 40F. The reasoning has been stated: less flow. Less flow gets more heat per gallon pumped. Pumps become smaller, piping sizes are reduced, and reduced electrical requirements result. Furthermore, central air handler heating coils are easier to control using higher TD's. Most importantly, parasitic losses from pump energy are minimized.

    The same thing is happening for chilled water design. The standard 10F TD is rapidly becoming obsolete, as is the 3 gpm per ton condenser flow rate.

    It is usually in the clients best interest.... from the perspective of first cost, as well as operating cost, to apply hgher than standard TD's when apprpriate. And there is really no excuse these days with all the computer sizing programs out there.

    Let's face it, this industry is, in general, very slow to change (and many exceptions to this are apparant by reading this board), which is why it is still common to see some still use obsolete sizing parameters (in my opinion).
  • mitch moore
    mitch moore Member Posts: 32
    low TD's

    low TD's might indicate oversized boiler or running supply temps higher than needed for the heat loss?
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Not suprising...

    I never saw anything more than a 7 degree D.T. on my current system with a 40 gallon water heater. And that was at design condition.

    If I cut the flow to 1/4 of it's original, it would approach 20 degress, but it felt Eurocave-ish.

    Your delta T is whatever YOUR delta T is, for the given current conditions. Add some wind and EVERYTHING changes..


  • mitch moore
    mitch moore Member Posts: 32

    I not refering to any particular boiler. I'm just tring to get the theory down.wouldn't the TD be normal to high in a situation where the boiler matched the heat loss at design conditions and the edr was oversized?
  • Rob T
    Rob T Member Posts: 64

    Mark, for some reason I thought I had to achieve a 20º Delta to have a correctly working system.

    It heats just fine, last couple of days we had temps in the low 30s with 30-40 mph winds and everything was good inside.

    Picking up the new pump and outdoor re-set today, it'll be interesting to see what effects they have on the system.

  • Bryan_16
    Bryan_16 Member Posts: 262
    how little

    water can you flow and still maintain space heating demand? I just checked a job yesterday where TD was 2 degrees. I installed a smaller pump, will I get in trouble doing this? Seemed like 2 degrees was inefficient way to run the boiler. Any thoughts?
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