Can't get delta temp correct.

Discussion in 'Boiler Forum' started by tom3holer, Jan 15, 2018.

  1. tom3holer

    tom3holer Member

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    I have an Burnham Alpine boiler installed in our home. As I understand it a zone should be set up to have a delta T of about 20deg to be most efficient basically.
    We installed a Taco Bumblebee circulator in the main loop for the downstairs in my home. It is approx 100' of standard baseboard. With this cold wx the boiler has been running 180 deg out. The Taco pump is set up in the DT mode at a diff of 20deg. The problem is that it is running about as slow as it can at 11 watts but the DT is only around 4-6 deg. I switched it to constant flow at the higher setting and it doesn't seem to change much still no where near the 20 deg diff I wanted. I am checking the temps with a seperate meter with clamps that have the sensors built in. They both check accurate when compared to the dial temp gauge on the boiler output. One at the output at the boiler and the other at the input of the BumbleBee which feeds the return manifold.
    It just seems to me after 100' of baseboard the water should have cooled a lot more than 6 deg with the pump running slow.
    Tom
     
  2. NY_Rob

    NY_Rob In the Trades

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    20deg DT is a design standard... it's not often seen in everyday use.

    From a cold start you may see a DT of 20deg or higher- but once closer to equilibrium (your rads are up to temp and putting out as much as they can) your DT will narrow. You can also ensure max baseboard output by cleaning the fins with a vacuum, make sure they're not blocked by carpet and drapes, etc. Lower CH pump speeds will increase DT but too little flow through your boiler's heat exchanger will fry it so be careful when tinkering with flow. The Install manual will specify minimum flow rates through the heat exchanger.

    You have just one zone with 100' of fin-tube (actual fin-tube element not total length of fin-tube enclosure)?
    You should probably not be running the CH pump in DT mode as it will be fighting the boiler's internal temp control- picture a car driver and passenger both having hands on the steering wheel at the same time.
    Run it in a fixed speed mode for better results especially if you have just one big zone without zone valves. If you have multiple zones (via zone valves) a DP pump like the Grundfoss Alpha2 would serve you better.
     
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  4. Dana

    Dana In the trades

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    Most systems (like this one) are over-pumped on the radiation end, since most installers don't even bother to calculate the pumping head.

    If the boiler is plumbed primary/secondary the delta-T at the boiler will always be lower than the delta-T on the radiation. If it's pumped direct (no primary/secondary) one still has to be mindful of the minimum flow requirements on the boiler.

    At lower output temperatures the delta-T shrinks too since the baseboard is emitting less heat. Even on systems set up for a true 20F delta-T across the radiation at a boiler output of 180F controlled by outdoor reset can see delta-Ts well under 10F when the boiler output is 120F.
     
  5. tom3holer

    tom3holer Member

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    I did discover I was measuring things incorrectly but still had a DT of less than 10F.
    This installation was done 4 years ago and had to be changed after I did my homework and with Dana's help. I arranged a meeting with the plumber and the tech rep from Burnham. The tech rep asked how the plumber determined what size boiler to get and he said "I just look whats there and always add 20%." The rep politely let him know that he had put in too large a boiler for my home. He did replace it with an AL80 which is ok till it get into the very low teens or single digits. After correcting the location of the temp sensors for the circulator I noticed something very interesting. The boiler is plumbed pri/sec. I measured the temp just out of the boiler before the "T" of which one side leads to the return lines and the boiler loop. The other side leads to the intake manafold and the zone pumps. The boiler temp was 180. Just afer the "T" on the zone pump side the temp was only 168. That was only inches away. The intake to the running zone pump was 168 also. So although the boiler was putting out 180 the zone pumps were only seeing 168 or even less as time went on. I closed the 2 iso valves for the other two zone returns which were not running for several seconds then opened them and noticed the temp rising on the zone valve manafold. It came up to the boiler temp as it should be. Therorizing here but wondering if the setup is such that when the zone valve came on it creates a situation where it can draw from the return lines as well as the boiler thus the lower temp.

    Tom
     
  6. Dana

    Dana In the trades

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    In a primary/secondary the return water from radiation is going into the boiler at return water temp, but some of the return water is also being mixed with the boiler output at the hydraulic separator. You never get the full boiler output temp going into the system radiation, and that's just the way it is.

    If you throttle back the flow through the hydraulic separator the primary and secondary flows will interact.
     
  7. tom3holer

    tom3holer Member

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    I didn't realize that Dana, thanks. When you say throttle back the flow through the hydraulic separator can you explain?
    I did slow down the variable speed boiler boiler pump but that didn't seem to do much.
    Here is a pic of the setup with the vertical line on the left the boiler out and the one on the right the boiler loop.
    I just checked the temps and the boiler out just before the "T" is 180 and just after the "T" to the left its 158 that is with all 3 zones running.

    upload_2018-1-16_7-24-25.jpeg
     
    Last edited: Jan 16, 2018
  8. Dana

    Dana In the trades

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    I was envisioning a ball valve between the closely spaced tee hydraulic separation point, a valve which you don't have:

    [​IMG]

    With the flow through the stub constricted the radiation pump (secondary loop) would create a pressure difference between the ingoing and outgoing side of the boiler loop at the tees, driving more water through the boiler. Similarly the boiler pump (primary loop) would be directly drive more water to the radiation. The point of hydraulic separation is to be able to use different flow rates on the loops.

    To raise the temperature of the water going out to the radiation you can either increase the flow on the primary loop, or throttle back the flow on the secondary loop, but the radiation will never see the full boiler output temperature due to the mixing that happens in the hydraulic separator.
     
  9. tom3holer

    tom3holer Member

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    When I increased the flow rate on the primary side I did see a gain on secondary temp with 2 zones running of 5 deg below boiler out. With all 3 zones running about 10-12deg. I gather this heat loss really is not totally wasted but need to review my book as its a bit foggy. Is one actually loosing efficiency with a dual loop system? How does one determine the optimum point? I can go up one speed on the boiler pump but would adding a ball valve be better?
     
  10. Dana

    Dana In the trades

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    The heat from that temperature drop at the hydraulic separator isn't lost, but if you need more heat to be emitted from the zone radiation it either needs to be fed hotter water, or there needs to be more zone radiation.

    Yes, there is a (usually but not always small) efficiency hit due to the higher entering water temperatures at the boiler when plumbed primary/secondary. If there is enough radiation that it operates in the condensing zone almost 100% of the time the hit is pretty slight, and if there is so little radiation that it pretty much NEVER operates in the condensing zone the hit is nearly zero, but in-between it can make measurable difference, a few percent in as-used AFUE, but not more than 5% if the outdoor reset curve is set up pretty close. In a well designed system it's worth saving the power use of the second pump and buying back that few percent in average combustion efficiency, but manufacturers don't like it, due to the higher number of boiler failures from less-well-designed systems. (An installer who sizes the boiler by looking at the DOE output of the beast it's replacing and adding 20% is not the person to let design a direct-pumped system.)

    Pumping direct rather than primary/secondary can often work out well and will deliver higher temp water to the radiation. But it's best if you (or a competent hydronic designer) does the math first, to ensure that under all operating conditions the boiler flows and delta-Ts remain within the manufacturer's specs.
     
  11. tom3holer

    tom3holer Member

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    Dana, thank you for the explanation. The problem is finding someone that is competent.
    Increasing the primary flow has increased the secondary temps and helped but as outside temps have moderated all is well.
     
  12. tom3holer

    tom3holer Member

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    One more question if I may. Is the DT of 20df also a good number for my indirect HW tank?
     
  13. Dana

    Dana In the trades

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    For the heat exchanger in the indirect to have adequate heat transfer efficiency usually requires about a 20F delta-T between the boiler output temperature and the storage temperature. But the delta-T of the boiler out to boiler return during a call for heat from the indirect could be higher than that at the beginning of a cycle, and would be substantially lower at the end of a cycle.
     
  14. tom3holer

    tom3holer Member

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    So would a delta T pump be of any benefit as I have one around I am not using.
     
  15. Dana

    Dana In the trades

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    Delta-T pumps are all high-efficiency ECM drive, but they are designed to slow down if the delta-T is too low, speed up if the delta-T is too high, which isn't necessarily the right thing to do when serving an indirect. At the end of a call for heat from an indirect the limitations are the narrowing delta-T between the storage temp and boiler's maximum temp, and slowing down the pump doesn't increase that delta-T.

    But if it's replacing a low-efficiency pump with a similar pump curve it's worth it- just program the thing for a high delta-T and let it rip- it'll still use less than half the pumping energy in most cases.
     
  16. tom3holer

    tom3holer Member

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    Thats kinda what I thought, thanks again Dana.
     
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