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Discuss Air in CH system and hot water returning to FE tank via the cold feed in the Central Heating Forum area at UKPlumbersForums.co.uk.

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  1. doitmyself

    doitmyself Well-Known Member

    I see that you have 'solved' the problem by setting the pump to PP2. However this may be a compromise.

    When the water is heated it will expand so the water level in the FE tank will rise and feel warm (hot water rises).

    The level should be adjusted, by bending the ballcock valve stem, so the tank is only half full when the water is hot.

    If the level is too high water will escape via the overflow pipe. So when it cools down the level in the tank drops and has to be replenished via the ballcock valve. This introduces more air to the system meaning more bleeding.
     
    • Agree Agree x 1
  2. John.g

    John.g Member

    Good to hear that the pump over has stopped, I assume that you checked it with CH only & HW only. One concern I have is that you may have a partial blockage of the boiler heat exchanger as you ran for years with no pump over on a fixed speed setting, if there is a blockage and it gets progressively worse the the pump on PP control will just keep modulating down and the boiler deltaT will keep increasing for any given firing rate, I think some boilers cut out if the deltaT exceeds 25C or 30C, on the other hand you may have no blockage and/or you may have cleared it out with the chemical addition. Pump PP control is not recommended for boilers with a ABV (auto bypass valve) which opens on circ pump over run (if installed) for a short time on boiler shutdown to remove the residual heat from the exchanger, this is no problem with a fixed speed pump as the pump head will rise to its max (4M in your case) when all the zone valves shut so th ABV can be set at say ~ 3M so that it will open only on boiler shutdown, On PP control, the pump will reduce its speed to maintain its minimum head of ~ 1.2M but if you set the ABV down to say 0.8M then when the boiler comes back on the pump will ramp up to maybe 2M or so and there will be excessive bypass at this pressure. Again your boiler may not be fitted with this or it may use the post air purge to cool down the exchanger.
    Now for the good news....pump PP control.
    PP control was introduced on A rated circ pumps to save energy and in some cases to reduce noisy TRV,s when throttling down.
    I have one myself (Wilo) which consumes 21/23 watts with everything opened up and falls to 12/14 watts on CH only when some of the TRVs are throttled down.
    They work like this, depending on the PP setting you select the pump then sets a minimum head at zero flow and a maximum head at its full flow. For example assume that PP3 setting is selected on the UPS2, at that setting (see page 11 of the manual), the pump will modulate between 0 flow at 1.2M & 2.15M3/h (36 LPM) at a 3M head.
    Now assume that the pump is running on fixed speed1 and the system with everything opened up requires a 2.9M head at a f/rate of 0.9 M3/h (15 LPM)...you now change over to PP3 control, the pump will go minimum speed and then starts ramping up, for any given point on that "curve" the pump calculates the exact power needed and compares this with the actual absorbed pump power and because there is only one point on this (or any) curve where that calculated power and the absorbed power are exactly equal then the pump will stop ramping and remain at this setting until the system conditions change. I am able to calculate this in a spreadsheet (unfortunately that I am unable to post on here) so if we go back to our original requirement of a 2.9M head at 15 LPM and c/o to PP control then the pump head and f/rate will fall to 1.8M head & 11.6 LPM, as the system requirements fall then the pump will modulate still further. Again if you are back to fixed speed and you shut off "1/2" the rads you might need a 2M head for a f/rate of 7.5 LPM, change back to PP control and the head will fall to 1.52M & the f/rate to 6.7 LPM. If you "dead headed" the pump by say shutting the pump discharge isolating valve, the pump head will fall to 1.2 M at zero F/rate, if you fitted a controllable by pass around the pump and started opening it, the pump would ramp up to a 3 M head & a f/rate of 2.15 M3/h or 36 LPM.

    Edit: Should have asked, what PP setting are you using??, thanks.
    Have just seen that you are on PP2, I will adjust the numbers above shortly to reflect this, I would suggest PP3 if still no pump over.
    I have left the above PP3 and shown the PP2 settings below, very little difference, one of the draw backs of UPS2 PP control.

    PP2 SETTING
    assume that the pump is running on fixed speed1 and the system with everything opened up requires a 2.9M head at a f/rate of 0.9 M3/h (15 LPM)...you now change over to PP2 control, the pump will go minimum speed and then starts ramping up, for any given point on that "curve" the pump calculates the exact power needed and compares this with the actual absorbed pump power and because there is only one point on this (or any) curve where that calculated power and the absorbed power are exactly equal then the pump will stop ramping and remain at this setting until the system conditions change. I am able to calculate this in a spreadsheet (unfortunately that I am unable to post on here) so if we go back to our original requirement of a 2.9M head at 15 LPM and c/o to PP2 control then the pump head and f/rate will fall to 1.58M head & 11.1 LPM, as the system requirements fall then the pump will modulate still further. Again if you are back to fixed speed and you shut off "1/2" the rads you might need a 2M head for a f/rate of 7.5 LPM, change back to PP2 control and the head will fall to 1.35 M & the f/rate to 6.2 LPM. If you "dead headed" the pump by say shutting the pump discharge isolating valve, the pump head will fall to 1.1 M at zero F/rate, if you fitted a controllable by pass around the pump and started opening it, the pump would ramp up to a 2.7 M head & a f/rate of 2.34 M3/h or 39 LPM.
     
    Last edited: Jan 17, 2019
  3. doitmyself

    doitmyself Well-Known Member

    You can post it if you zip it first. I would be interested in seeing the spreadsheet.
     
  4. Ric2013

    Ric2013 Plumber Top Contributor!!

    Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.

    Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.

    Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.

    On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.
     
    • Optimistic Optimistic x 1
  5. John.g

    John.g Member

    There you go, works exactly as the spreadsheet shows.
     

    Attached Files:

  6. John.g

    John.g Member

    I would like to ask that grundfos engineer why in a 6M pump that they only allow a max 3M PP head, even the "cheap" pumps allow a 5 M PP head. I think that even the UPS3 only has a max PP setting of 3.5M.
    If you take the above example that I used and assume a perfectly normal CH system. Ideally to set up the PP control, one should open up all the zone valves including HW and open all or any TRVs fully to get the maximum flow, you can then calculate the head and flow rate, I did mine by measuring the boiler deltaT and because I knew its output I was able to calculate the flow rate, I then looked at the (fixed) speed pump curve and was able to read off the head.
    So if the head & flow required are 2.9M @ 15 LPM, you should then be able to set the PP control high enough to give you this but because the UPS2 can only be set to a max of 3M the head and flow will fall to 1.8M & 11.6 LPM so you are getting 77% of the max flow. IF you could increase that PP setting to 4M (3.8M to be exact) then you would have the required head & flow of 2.9M & 15 LPM on change over to PP mode. I know this is exactly how it works because I can set my Wilo pump any where between 0.5 M & 5.5 M PP head in increments of 0.1 M so if I set it to the 3.8 M above I will get that 2.9M head & 15 LPM, it will never go any higher but will ramp down as I described on reducing heat demand.
     
  7. John.g

    John.g Member

    Here is very interesting article on Grundfos AuotAdapt.
     

    Attached Files:

  8. Tiggy1955

    Tiggy1955 New Member

    So in really simple non-technical terms:

    A pump set to a Fixed Speed will apply a constant speed/pressure regardless of what’s ahead of it in the system. The result being that it could be too much pressure (particularly when a change takes place such as some rads being closed off) and it is also inefficient.

    A pump set with Proportional Pressure will apply a variable speed/pressure depending on what is ahead of it (when a rad is closed/opened the pump compensates accordingly). The result being a more appropriate pressure is applied and it is more efficient.

    Is that about right?
     
  9. Tiggy1955

    Tiggy1955 New Member

    I have one final question on this topic:

    I replaced the old Grundfoss UPS pump (because it was old) with the Grundnfos UPS2. I got the UPS2 because that seemed to be the direct replacement for the UPS.

    Until John G mentioned PP I had never heard of it. What’s more, I did not even know that the UPS2 had PP.


    My question is: If I had not replaced the pump I would have been left with a pump that only had Fixed Speeds. In which case, how could I have resolved the pump over problem if the slowest speed of the pump was still too much?
     
  10. John.g

    John.g Member

    With your system like Ric2013 has shown above, cold feed before the boiler and vent after it I am a bit surprised that you wern't get some pump over "all the tine" depending on the number of rads in service. The cure: ideally I would ensure that the boiler heat exchanger has no (or partial) blockage but of course you need a RGI to do this. If you are happy not to inspect it then if you want to retain the gravity system you could shift the cold feed from before the boiler to after it adjacent to the vent but not more than 150mm away from it OR fit a fully pressurized system with a expansion vessel.

    If the PP control is "curing" the problem then why worry too much?.

    Are all your rads etc heating up to your satisfaction?.

    I don't know if you have a boiler flow temperature indication but (if so) sometime you might watch it when the boiler shuts down and see what the temperature rise is over the next few minutes.
    All this has been and is very very interesting, thanks for the feed back.
     
  11. Chuck

    Chuck Top Contributor!!

    In an ideal world, the pressure drop across a given radiator would always be the same. The flow through this radiator is then limited by the lockshield valve and controlled by the local TRV.

    A so-called 'constant pressure' pump, which adjusts its displacement rate to achieve a fixed pressure difference across its ports, will give a constant pressure drop across the radiators provided there is no pressure drop along the connecting pipework.

    In practice, there will will be a flow-dependent pressure drop due to the connecting pipework. The 'proportional pressure' behaviour of a smart pump is an attempt to cancel this out.

    Constant pressure operation is definitely, in my opinion, an improvement over a traditional constant speed pump, although it makes setting up the external by-pass correctly a rather delicate operation. Whether the additional complication introduced by PP is actually worthwhile in practice I'm not so sure about; the theory is based on an assumption about the pipework that is at best a rough approximation.
     
    • Agree Agree x 2
  12. Tiggy1955

    Tiggy1955 New Member

    I'm draining the system down right now to remove the X400 and flushing through. I will refill and add the X100. All rads working really well (better that they have for a long time) and HW piping hot so I'm going to stick with the PP control.
     
    • Agree Agree x 1
  13. John.g

    John.g Member

    More or less, a fixed speed pump head falls as the flow rate increases and increases with flow rate decrease (Trv's or zone valves closing) where as PP control works in the opposite sense, as the flow rate increases the pump head increases (pump ramps up the speed) and visa versa. Most of these pumps also have CP (constant pressure) control where the head is kept constant whatever the flow rate is.
     
  14. John.g

    John.g Member

    I picked up this description of PP control somewhere along the line and it bears out what you are saying.
    I would have to say that PP control works very well for me.

    PP Control
    "It starts with the assumption that about half of your pressure loss in the system will be in the distribution pipe while the other half is lost in the radiators. Consequently, the pump is controlled such that it will respond to a decrease in flow with a reduction of its head and that at zero flow, when all valves are closed, it will provide half the head pressure it has at maximum flow."
     
  15. doitmyself

    doitmyself Well-Known Member

    Unfortunately the spreadsheet is difficult to understand as you don't explain where the data comes from or what each column contains.

    For example:
    How did you obtain the Know Head and Known Flow?
    Where does the UPS2 data at the top come from? It doesn't agree with the Grundfos published data.
    What does "rem 7.9 lpm" mean (apart from the obvious litres per min)?
    What does series 1 and series 2 refer to?
    etc etc.
     
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