Virginia What is an acceptable backpressure on the return well line for open system?

Discussion in 'Maintenance and Troubleshooting' started by tommytx, Jul 21, 2018.

  1. tommytx

    tommytx Member


    The system is a Bosh 3 ton residential HVAC.. either its undersized or a little low on efficiency as it does well below 90% out side.. then wont hold the temp below 80... so question is what is an acceptable back pressure on the well return water line... and if its excessive can that cause lower return water and lower the efficiency...
    Also does anyone have any other ideas on how to measure water flow thru the system.. i have use the catch it in a 5 gal bucket and time it.. but of couse that will not be that accurate unless i know what the back pressure is and make sure the water into the bucket has to exceed the backpressure currently in place.. else the free flowing water with no backpressure will be more than the flow that would be passing when it working against backpressure...
    The only other method i have heard of is measuring the pressure drop across the input coil.. but that will be pretty inaccurate unless the gauges are super accurate since my guess is the difference will be only a very tiny difference..
    Thoughts, ideas, suggestions..
    1. For acceptable backpress range in return line..
    2. for good method to determine flow thru system w/o a fancy flow meter..
     
  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome!
    Flow rate can be easily measured with a pressure gauge through the PT ports on your unit. You can get the specs for your unit online. You measure the pressure difference between the in and out. I.E. the pressure drop. If inlet is 50 psi and outlet pressure is 45psi the pressure drop is 5 psi. Compare that to the chart for your unit. A given pressure drop equates to gallons per minute through the unit. Google PT ports to see what they look like.

    Acceptable back pressure is any pressure that does not decrease your flow rate below manufacturers recommendation.

    Hope this helps
    Eric
     
  3. tommytx

    tommytx Member

    Thanks for that... info.. the reason the question came up is my tech is suggesting i get the well return line redone to lower the back pressure since he hooked a hose to the well return line and dumped in the yard for a few minutes and showed me that the PT port temp difference increased to 20 degrees from maybe a diff of 15 to 16.. course the 15 to 16 is still in the mfg specs of being ok since i think the acceptable span is 15-20 degrees.. but his thoughts were that if the in out temp was rasied to 20 it may become more efficient and be able to bring the heat down lower when the peak temp hits...
    But when i asked him to measure the temp of the air coming out of the floor vents we could not detect any change from the 60 degee air.. so I am concerned if I spend a couple of thousand for a well me to put in a new pipe to the well it may not make any difference.. I don't mind paying for it if it will fix it but when it does not drop the air temp being generated.... will it really lower the temp any noticable amount over the long haul....
    To test it more thorougly i do have hte option of parallelling my return line to dump into another well that is not being used.. and hopefully between the two parallel returns it will bring hte back pressure lower.. and can run that way a few days to see if it really does improve efficiency.
    However I have another major concern.. and that is what you brought up... back pressure matters very little to none as long as it does not cause the required water flow to drop below the min.. and my concern is that the tech truly has no idea what the water flow is since he does not have the gages to measure the pressure drop at the PT ports which are installed.... he does have the pt port temp gages that plug in there and is using temp drop to tell if the flow is correct and we both know that you can't accurately do that. Howerver worse i have scoured the Bosh install manual from stem to stern and have not been able to find a pressure drop chart.... I can see the pressure at the inlet.. (bout 18 inches before inlet) due to gage installed.. and I can see the return line backpressure about 18 inches from the outlet... its apporx 40 in and 30 out.. and he is suggesting the backpressure should not exceed maybe 15 lbs.. My pump has the capability of 10 gpm max and for all i know he is forcing way more water thur the system than it can handle and that may be causing the increased backpressure... I can lower the flow to the inlet by SOV throttling and of course that will lower the back pressure but currently have not way of telling what the volume of flow is.. I can't call Bosh as I am sure they wont talk to me.. so maybe i can demand the tech contact Bosh and find out the proper methiod of determining flow... tech says he does it with temp in and out (the temp differential) I like the pressure drop (in and out diff) as that is the way its done with my Train up stairs as they give the pressure drop for you as you just measure the water time and find the section on the chart for the diff in input and output press and away go... however the train and the bosch as drastically different in flows required. For example the train calls for 3 gal per min per ton and i am 2 ton so ballpark is 6 gallons per minute... but the bosch is 3-5 gpm so 4 based on temp.... and here is the funny part. my tech has both units on same pump with ZERO restriction in the inlet or outlet to set the water flow... so the flow is just what ever it happens to be... and of couse would fluctuate when only one unit is runnng... as they run in parallel of the same pump.... Bottom line is i suspect the water flow is ok for both maybe one on the low end and hte other on the high end but never the less in the ok flow range... and the problem is it may have been incorrectly calculated and my be undersize for the hottest part of the day.. but tech may be pushing me to spend tons of money on the wells so he does not have to size it properly...
    As far as i know the sizing program has never been run on either of my installs... so can someone give me an idea what i might expect to pay for a good SIZER tech (designer) to come in and determine if I have been sized correctly? Thanks for any help..
     
  4. geoxne

    geoxne Active Member Forum Leader

    Your question does not have an answer based on the information given.

    Bosch engineering submittals are here-
    https://www.bosch-climate.us/suppor...r/downloads/engineering-submittal-sheets.html
    Choose your model family then download. If yours is not listed, you can look at "obsolete products" to the left. Once you download the correct document you can lookup the exact model size in the performance charts to find pressure drops based on different flow rates and water temperatures.

    I will pass on a few tips about open loop as follows-

    You do not want pressure to build up in the return well. It should be vented to atmosphere. If the vent overflows your return well is not adequate for your flow rate. Perhaps it can be redeveloped.

    A source pump can pump more water at lower head pressure. Raising supply pressure to overcome return pressure is inefficient, makes more noise and reduces the life of a pumping system.

    Open loop systems with entering water above 50F should flow 1.5 gpm/ ton. Any more is a waste of pumping power and will do very little to increase capacity. Consider fixed flow controls such as Hays or Dole in the gpm required to ensure proper flow.
     
  5. tommytx

    tommytx Member

    Thanks for that ever little bit help but i have the actual Bosch ESO35 install manual with all mfg specs given and no where do they show any water pressure and temp charts converting differential pressure and temp to flow as does Trane.. they are very clear.. But i did search thru sevearal of the PDFs you included even though i did not find on on the ESO35 i looked at a few to see if i could find a chart.. and then correlate that chart to my model and see if i could find it in my PDF for my exact model..
    I am wondering if they (Bosch) even correlate differential water pressure to flow..

    Anyway the do say the flow needs to be between 3.5 to 5.o for my model so that will be fine once i figure out how to measure it.. I used the 5 gal bucket method and came close to 6 gpm...Trane sets it base on tons but Bosch gives you the flow based on your particular model.. since i am a 3 ton that would equate to in the ball park of what you suggested.. 1.5 gpm per ton..

    Thanks again..
     
  6. tommytx

    tommytx Member

    Just noticed your comment about overflow the vent... LOL.. I have no idea if i have a vent or where it is... i suppose there is no way to find it without a wellman with a wizzles stick or whatever that douser thingy is.. HaHa.. my back pressure may all be about trying to shove more gpm down a 3/4 inch pipe than the law allows.. the reason is say this is while experminting i connected a full normal water hose (i think most are 5/8 id or close) to the well return line and let water flow straight to the street for a few minutes.. the back presssure at the entry of the hose was 30 psi.. so its clear there that i am pumping way more water than can travel down a 5/8 pipe which also suggests too dang much water for a 3/4 pipe which is what i am using as supply and return... so i may be my own worst enemy.. so what i plan now is to try to measure the actual flow via the old 5 gal bucket method and then throttle the water in to no more than the 3-5 gal the system needs.. and hopefully the 3/4 pipe will carry that away without the high backpressure..... don't think the hi backpressure will hurt me as long as the proper flow is there.. other then maybe reduce the life of the pumping system, making it do unnecessary work to overcome the useless backpressure... as it adds nothing in goodies..
     
  7. geoxne

    geoxne Active Member Forum Leader

  8. tommytx

    tommytx Member

    Thanks that gives me some more information to study.. but not sure it will help as i don't have any gages accurate enough to tell me much on that.. About 18 inches from each PT port i do have a gage but its the plain old water pressure gage and not very accurate.. I am seeing about a 15 psig difference in the inlet and outlet pressure and that would equate to maybe 15 gpm and i am sure my pump probably maxes out at around 10 so that cant be correct... also not sure what the differences would be since that is for the old Florida Heat Pump ES036 and i have the ESO35... since Bosch Took it over... the series now goes like this
    ESO25 = 2 ton ESO35 = 3 ton ESO49 = 4 ton ESO61 = 5 ton ESO71 = 6 ton and mine is a 3 ton... and i have the hard copy of the PDF but NO pressure drop..
    you would think they would even explain how to do the pressure drop.. but you need very accurate gages to read a difference of .86 difference...
    Will scan that ESO36 to see is they describe the press diff check... but thanks for the additional info.. i may pick up some more info there...
     
  9. geoxne

    geoxne Active Member Forum Leader

    Thanks for the correction of model #. Try this page 22 for pressure drop table and for your specific model pages 25-26.
    https://www.bosch-climate.us/files/Bosch_FHP_ES_Catalog_US.pdf

    You are correct. It is difficult to measure a small pressure drop normally found in open loop applications. However you can quickly discover if you are flowing at all or too much.
     
  10. tommytx

    tommytx Member

    Thanks.. that has the exact tables i was looking for..
     
  11. arkie6

    arkie6 Member Forum Leader

    The Fluid Pressure Drop table shown above and on the linked document is shown in PSI and FOH (Feet Of Head or Ft of H2O). For a 3 ton unit with open loop @ 1.5 GPM/ton you are looking at around 4.5 GPM of flow needed. The lowest flow shown for the Bosch ES035 is 6 GPM and that equates to 4.30 ft of H2O. If you want to be accurate in measuring these low pressure differentials, you can build a manometer to measure ft of H2O. You would need a couple of football/basketball inflation needles and about 25' of clear vinyl tubing that will fit tightly over the threaded part of the inflation needles. You will need an air tight seal between the tubing and the needles, so you might want to apply some RTV silicone to the threads before attaching the tubing and clamp the tubing to the needles with the smallest hose clamp you can find or even a couple of nylon zip ties cinched down tight. You would make a U of this tubing from floor to ceiling with ~4' of water (maybe add some food color to make it easier to see) in the bottom of the U. Insert the needles in the PT ports, then start the system up and watch the difference in height of the two water columns. If you are looking for a differential of ~4 ft of H2O (just below 6 GPM), you would see the supply side of the U tube drop down ~2' and the discharge side of the U would rise up ~2'.
     
  12. tommytx

    tommytx Member

    Can you give me a clarification.. you said 3 ton equates to 4.5 gpm for my model.. but then you said Bosch ESO35 is shown as 6 gpm.. so since the lowest they show is 6 gpm should i have at the 75 degree range 4.5gpm or 6 gpm... not sure what you meant... but thanks that helps me understand it all a little better.. but building that contraption may be a little more than i want to bit off...

    i have a trane in the upstairs that is a two ton and they do say at my temp would normally run 2gpm per ton so that needs maybe 4 gpm... and using no throttling and all same size pipes paralleling both with 3.4 inch pvc lines thru out from source to coil then to the well... so of course there is some flux of the volume when they both kick in.. and i am pretty sure my pump is capable of a max of 10 gpm.. as i get about a 40 to 50 psi when they run... but the back pressure is maybe 30 psi so i am sure that is slowing the flow thru both...the downstairs bosch 3 ton is fine at the pressures as it runs maybe 10-15 min per hour and keeps it cool and moisture free but the upstairs split train is a 2 ton and runs damn near 24x7 and cant keep up when the temp hits 85-90.... I am pretty sure the upstairs is undersize and should have been a 3 ton.
    I am about to hire a pro to come in and tell me what the specs call for and demand an upsize if the analyzation says it needs it.. can't let the existing tech do the analysis since probably would be biased.. and why would he not just pull out the numbers that he did before the install.... (hard to do when no numbers were run.. LOL)
     
  13. arkie6

    arkie6 Member Forum Leader

    1.5 gpm/ton is just a rule-of-thumb flow rate for open loop systems (3 ton x 1.5 gpm/ton = 4.5 gpm). Nothing specific to your unit. This is similar to 3.0 gpm/ton is the rule-of-thumb flow rate for closed loop systems. These are not absolute values, but general guidelines and a good target minimum flow rate. 6 gpm was just the lowest value shown in the manual posted above for determining the pressure drop across the coil on your unit. Nothing wrong with running 6 gpm or 9 gpm through your 3 ton unit other than possible wasted energy moving more water than is necessary. But with you being in a cooling dominated climate and having relatively warm open loop water, 6 to 9 gpm flow rate through your 3 ton unit probably isn't such a bad idea.
     
  14. tommytx

    tommytx Member

    Thanks.. all this detail helps me understand the overall hang of things soo much better and help protect me from that ocasional unscrupulous HVAC guy thankfully those are few and far between.. but i have had more than my share of them this week... tried to get me to run a new well line due to excessive backpressure causing the compressor to lock out at overpressure.... when in fact it was condensate lockout due to water backup in the condensate line due to double trap.. i am surprised that a double trap would not let water pass.. i would have guessed yolu could have as many traps as you wanted as long as the slope continued downward.. i have now learned that a double trap in a condensate line is the kiss of death.... and the worst thing of it all is it was a Tech's tech telling me this crap... What is a tech's tech its when a air to air company has a go getter salesman and convinces someone to buy a water to air from an air to air company where the air to air guys are dumb as rocks in regard to water to air and the air to air install the water to air and have no idea what they are doing so the go to a supposed water to air company forhelp and the water to air guy is dumber than the air to air techs.... LOL... like blind leading the blind.
     

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