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Discuss Temperature problems with large commercial hot water system in the Commercial Plumbing Forum area at UKPlumbersForums.co.uk.

  1. cr0ft

    cr0ft Trusted Plumber GSR

    Hi all. As most of you will know I've gone back on the cards now and am now fault-finding one of our commercial hot water systems in a 5,800sq.m building. It's a large building on a hospital site.

    The hot water system is an old system! Heating is run from a steam feed from the main hospital and that heats the water in 2 very large calorifiers (3000 litres each). The water is heated to 65C in these and is then pulled around the system with the standard 2x pumps (one on, one standby) on the hot water return.

    The hot water flow pipework is in 54mm and quickly splits off into 6x 35mm 'risers' feeding different parts of the building (6 branches really, all pipes run through between the 2 floors). The issue is that hot water return temperatures furthest from the boiler on some risers are down to 37-42C, and there is a legionella problem in the system. No shock there! When you open hot taps in the affected areas the temperature will then rise by up to 5C before dropping back as soon as the tap is closed. In the affected areas the water temperature doesn't even get up to 50C when running the hot tap.

    From having inspected two risers (they are above suspended ceilings) I see no balancing/gate valves on the main return pipework. What I do find odd though is that every individual tap has it's own tee'd off hot water return pipework. The end result is there are loads of loops for the water to follow to get back to the calorifiers on each riser.

    I suspect that the hot water is following the shortest routes back to the calorifiers and there are large stagnating areas where recirculation is not occurring. I don't see the point in the hot water return pipework to individual taps (they are all within 5m or so from the main pipework) and feel it complicates the system. My plan is to remove all of them and fit automatic balancing valves on the return leg of each riser in order to properly balance the system - something like the Honeywell Kombi 4.

    All the schematics I see online show taps etc connected to the hot water flow pipework only, I'm assuming connecting them all to flow and return is wrong and is going to cause this issue I have surely?

    Anyone else got any thoughts? Have spoken to our commercial plumber we contract in and he agrees with me but wanted to get other people's ideas before briefing management on Friday!

    Thanks a lot in advance for all advice.
     
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    So every tap is connected to the return loop ?

    It sounds like maybe someone's fitted a tmv wrong or its failed

    Even with it short circuiting to the first outlet you should be getting return temp higher than that 60 dc as it's doing a loop if it's insulted
     
  3. cr0ft

    cr0ft Trusted Plumber GSR

    Yes mate almost every hot tap!

    The 'primary return to the calorifiers is within 5 degrees of the 'primary flow from it which hid the problem.

    As you move along the 6 return loops the temperature drops the further you get from the plant room. Some loops drop more and some less but Working on the water being lazy and taking the easiest path it's taking the shortest routes back to the plant room.
     
  4. cr0ft

    cr0ft Trusted Plumber GSR

    Should say that all the tmvs get stripped and serviced regularly. Also, the hot and cold water pressure is balanced as both are fed from boosted cold water supply. Can't see how you would get flow from one side to the other in that case? None of the hot pipes on the TMVs I've felt so far feel cold at all either!

    Surely a system this size with this many return connections (circa 100 with all the taps) must need balancing?

    Also is this setup normal i.e. all taps connected to return directly??
     
  5. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Right I get you now

    Main hot 65
    Src 60

    Furthest outlet 40 (ish)

    Yep it's taking the easiest route

    Cap all tees into on the same run except one and fit a balancing valve to that with a lever valve also lever valve on before the first hot outlet
     
  6. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    You would be surprised how many hot single checks in tmv get stuck open or shut, common occurrence if there no hot water to give the check valve a poke and it opens
     
  7. cr0ft

    cr0ft Trusted Plumber GSR

    Brill. The way I see it is all the tap return connections are pointless and I will need to get rid of them all (bar the furthest one on each leg). Ultimately I will fit balancing valves to all 6 main return runs. Guessing i need a bypass of some sort too in case they all shut off??

    Whole job is complicated by it being a working hospital and no isolation valves on main flow and returns at all!!
     
  8. cr0ft

    cr0ft Trusted Plumber GSR

    Yeah I understand it's quite a common problem. I would look at that in more detail where it not for the fact that we have a regular service schedule for them all.
     
  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    If on a bank off the same line yes (remove the tees as someone will have a fit with the amount of dead legs :D

    And no as they will never shut just want to regulate flow and start putting lever valves in

    As soon as it tees off a main line lever valve after balancing valve lever valve (labelled and red handles normally)
     
  10. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Freezer kit is handy

    IMG_3045.JPG
     
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  11. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    First tap off each riser gets to correct temp ?
     
  12. snowhead

    snowhead Well-Known Member

    I only work on Commercial buildings.

    The only time I see no secondary circ is where there is a local Electric hot water heater and the flow to the tap is a couple of meters max.

    All the return legs I see have individual balancing valves close to the tap.

    Be careful removing returns and leaving 100 x 5 mtr deadlegs;

    HSE guidance max 2mtrs
    http://www.hse.gov.uk/legionnaires/assets/docs/hot-cold-legionella-checks.pdf

    CIBSE Module 87: Ensuring safe and robust hot water flow and return pipework
    DHWS deadlegs should not exceed a volume of 0.5 litres – so, for example, a 20mm cross-linked polyethylene (PEX-c) pipe (internal 15.5mm) would have a maximum allowable deadleg length of 2.6m. This requirement is driven by the need to reduce legionella bacteria, but also reduces the risk of stagnation, ensuring better water quality.

    Faced with your issue I'd;

    Drain the system and fit Iso valves to both flow and return on the 6 risers, then you can work on each one after individually.

    Then I'd find the hottest tap returns , most likely the closest to the Cal /pump and fit balancing valves in those first and throttle them down.
    You'll get the fastest improvement in temperatures rather than having to do all 100 at once.

    Then work around the system finding the next hottest returns.
     
  13. cr0ft

    cr0ft Trusted Plumber GSR

    Thanks a lot. Good to get more experienced engineers opinions. I was hoping to avoid having to fit loads of automatic balancing valves on the individual tap returns due to the cost of the valves but if that's the best option then we will most likely have to go with it. System needs to meet HSE guidance and deliver water at 55C to TMVs within 1 minute.
     
  14. cr0ft

    cr0ft Trusted Plumber GSR

    Any ideas how long I could be looking at to drain and refill a system of this size?? Appreciate it's not an exact science but I'm thinking more than a day with fitting the isolation valves too!
     
  15. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Yea you could end up a day draining esp if you have to drain any cold water tanks

    If you can best to freeze main legs and fit lever valves
     
  16. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    How many groups of sinks you got ?
     
  17. cr0ft

    cr0ft Trusted Plumber GSR

    About 20. I'm very limited on what I can do with pipework to try and keep costs down. What are you thinking??

    I don't think mgmt will swallow buying 100 odd automatic control valves plus labour but will see what they say. May have to settle for pipe runs of up to 5m in 15 to taps to be honest. In healthcare the target is 55C at outlet or TMV within 1 minute which will be achievable with this if I can sort main flow/return temps.
     
  18. cr0ft

    cr0ft Trusted Plumber GSR

    Yes lovely and warm. They get progressively colder the further along each riser you go.
     
  19. cr0ft

    cr0ft Trusted Plumber GSR

    Out of curiosity how will I fit lever isolation valves to 54mm copper if there's no give in the pipe? I can get a pipe freezer no probs and leave hot water off for a day (and overnight to cool).

    As you can guess, this is a step up for me, it's like re-learning plumbing all over again tbh!!
     
  20. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Each group do you think they will all have hot water under 30 secs if yes you only need 20 balancing valves
     
  21. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Press fit or compression and slip coupling
     
  22. cr0ft

    cr0ft Trusted Plumber GSR

    Sorry see what you mean. Each group will be up to 55C in under a minute without the little returns to them.
     
  23. cr0ft

    cr0ft Trusted Plumber GSR

    Compression slip coupling will work a treat, plenty of pipe length to play with. Will hire a pipe freezer in and probably arrange to do it overnight on OT. Simplest way!
     
  24. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Go for that you can do one group at a time to spread the hit if they want

    Would do a test one first to check
     
  25. cr0ft

    cr0ft Trusted Plumber GSR

    Yup will recommend fitting isolation valves to all risers in one shift first then and may as well fit a balancing valve to returns on each at the same time.

    Then I can work on a section at a time which will make it a lot more realistic.
     
  26. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    And drain valves :D

    But on the src and hot risers you will need to remove all the tees for the return circuit bar the furthest one
     
  27. Dkdc71

    Dkdc71 Member

    Are the pumps sufficient from an original system with extension's and extra wings added to hospitals over the years might be undersized just a suggestion
     
  28. cr0ft

    cr0ft Trusted Plumber GSR

    Yes I'm going to recommend fitting some variable speed pumps in place of the existing ones too. Without balancing though won't they just push water round the easiest loops faster?
     
  29. Vee

    Vee Plumber GSR

    Somehow I think tackling the pumps first would be wise....
     
  30. cr0ft

    cr0ft Trusted Plumber GSR

    Been back to monitor system today and all return legs are getting up to 51-53 at the furthest points from the plant room.

    On further investigation the pumps haven't been swapped for ages as the maintenance team haven't been shown how to do it.

    Swapped the pumps, re-measured some temperatures and they are up by 4C.

    I think we will be replacing both pumps first and seeing how we get on from there.

    Out of curiosity, is it possible for a pump to intermittently pump at a lower speed?? I only ask as this system was sitting at 42C on furthest return pipework yesterday and the day before. Incredibly strange!!
     
  31. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    No as the only adjustment if any would be a selector on the side

    And sounds like it's on it's way out it's not bms connected is it ?
     
  32. cr0ft

    cr0ft Trusted Plumber GSR

    Hi, it is but it's an incredibly old BMS system. What are you thinking? Will I be able to get pressure drop through it or something useful?

    Just to be clear, here's an attachment which illustrates how the whole system is pipe (I've obviously only drawn a little bit of it). Had a chance to go through the whole building today tracing the main pipework. Turns out there are only 3 risers (duff information!).

    Given that switching from one pump to the other gives me a 4C difference in temperature drop from calorifier to end of return legs (57 vs 53 today at end of return legs) we need one new pump I reckon to replace the more worn one.

    I'm assuming the thermostatic balancing valves can stick like TBVs over time and so they are starting to play up. It's the only way I can explain temperatures of 42C and less at the end of the return legs earlier this week. Am going to test them more carefully tomorrow morning.

    What I don't understand is how just placing TBVs at the end of each riser makes much difference though. Surely the water is just going to be running through all of the individual tap branch flows and returns before it gets to the end.. I would have thought each tap return would need its own TBV as well in order for the system to run efficiently. I'd also have thought we would need variable speed pumps so they match this demand but as far as I can see they just run at one speed.

    I'm going with suggesting one new pump and fitting new TBVs atm unless anything else comes up tomorrow.
     

    Attached Files:

  33. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    I would if you could run the src on hand and not auto for a day

    And best to replace the main and back up as you don't want mis match pump sets

    The drawing to me looks like someone has teed into the src on the first 3 sinks (or x amount) after it was installed as the balancing valve is doing more harm that good where it is,

    Could understand if there were balancing valves on the first 3 sinks (or x amount) as this would stop short cycling

    As for pump spec wouldn't go ver speed more hassle than there worth esp in a hospital fixed speed less to go wrong bullet proof

    Best speak to grundfos or dab in relation to sizing if specs arnt on the pump
     
  34. cr0ft

    cr0ft Trusted Plumber GSR

    Hi again and thanks for all of the info. Basically nearly all sinks have their own dedicated return pipes. There are gate valves on both flows and returns to all sinks. My assumption is that originally these would have been used to balance the returns properly. Over time the contractors in the building have used them as isolation valves then just opened them fully...

    Currently there is a balancing valve at the end of each of the three return lines. There are no other automatic balancing valves on the system.

    I'd imagine grundfos will visit and specify us two pumps that will retrofit the existing ones?

    Our current setup is 2 separate pumps that are manually switched over each week. It's not an automatic process in this building hence my thoughts were change one pump first (they are bloody expensive as I'm sure you know), run it for a week or two and see the effect on temperatures. This is (I believe) the only building in our estate that has to have the pumps manually switched weekly. I also know for certain that our maintenance staff haven't been doing it so one has been running and the other not being used for ages. This is also the only building we have an issue with the hot water system in. Perhaps there's a clue there!

    If there were no flow and returns to individual sinks etc, the balancing valve would work a treat at the end of each main return branch. As it is, it will be shutting the return off and encouraging short cycling round the earlier taps right?

    I found out today there was a load of work done on the plumbing to the wards a few years back and we've had these problems with temperature ever since. I suspect the gate valves were used as isolation valves and now the whole system is out of balance. On some days it works great and on others it doesn't...

    Very hard to prove as today is was working a lot better but I can't see what else it could be.
     
  35. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Yep and they get messed with there's your problem how come there not on your drawings?

    They only want to be just cracked open just enough for a trickle ( about just less than a 1/4 open)

    Yep without working it all out your looking about 1lps (ish) flow rate max from each sink / pump manufacturer might work this out for you if you ask nicely :D hell if you can find the plans from when it was installed it will be listed on them

    Yes and no you want to have a site meeting with a rep

    And yes should be other than a flange change

    Yea spread the wear

    Yes you would have hot hot water on your main lines/ runs it would be the tees off to each sink that would not be recirculating so would chill
     
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