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Thread: Balancing and delta T with a Modulating Pump

  1. #1
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    Default Balancing and delta T with a Modulating Pump

    Firstly apologies for asking a question similar to that which many have asked before.

    Trying to balance my system and achieve a reasonable delta T to get more condensing but just confusing myself at the moment despite lots of reading and research.

    I have an Ideal Vogue system boiler in a conventional S plan with the central heating 2 way valve latched open.

    The majority of the radiators (15 in total) are controlled via HR92s with the hall and landing still on traditional TRVs and the bathroom towel rad on two lockshields to act as a system by-pass when all the TRVs are closed. The rads are a mixture of sizes but are all of modern design.

    The problem I have is that in order to get a reasonable temperature drop on most of my rads (approx. 10C not even trying for 20C) the lockshields end up almost fully closed they have a full range of approx. 1and turns and most end up say 1/16 to 1/8th open.

    This would be fine but I then end up with quite a lot of radiator noise, especially as the house warms up and TRVs/HR92s start closing. The boiler has a modulating pump so I thought it might compensate as the TRVs close and therefore prevent the noise but it seems to run at 100% when it gets to this stage, not sure why, maybe needs the extra pressure in order to achieve a minimum flow rate via the open rads as the other TRVs close.

    It occurred to me that if I just open up all the lockshields to say 1 turn open then maybe the pump would modulate and lower the pressure whilst achieving the same flow rate.

    This has made the system virtually silent and Ive had no problems with rads being starved of heat (the system is made up of 28, 22 and 15mm pipe as things gradually tee off so probably provides a reasonable inherent balance regardless of lockshield settings).

    The pump however still seems to spend most of the time running at 100% and Im not getting good temperature drops across most of the radiators. I thought the pump might adjust to try and achieve a reasonable delta T and therefore opening up the lockshields would allow the pump to run at say 60% instead of 100% most of the time.

    The pump is largely automatic, I can change it from min to max modulation in the settings but this doesnt seem to make any difference.

    I therefore seem to be in a catch 22 situation either retain a reasonable delta T (and more condensing) at the expense of having system noise (and I suppose extra power used by pump) or vice versa.

    Is there something Im missing here? Maybe I need a 'proper' ABV rather than using the towel rad (along with the boiler inbuilt ABV) but I really don't fancy draining the system down and breaking into the pipework to do this.

    Any tips would be appreciated.

  2. #2
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    Are you sure the pump is in modulating mode ? It doesn't sound right that it is still at "100%" (how are you determining that ?) when most of the radiators have closed. You shouldn't get a lot of noise when most of the radiators have closed because a modulating pump should sense the restriction and reduce the flow - in fact most will reduce the pressure when they sense restriction, versus a fixed speed pump where pressure goes up as the system gets more restricted.

    Is the pump built into and supplied with the boiler or is it externally fitted ? What model of pump is it ? Most modern modulating pumps can be put either in a fixed speed mode or modulating mode, so I would check that it is not in fixed speed mode.

    I have a Grundfos UPS2 15-50/60, it has three fixed speeds and also three modulating speeds. I'm not able to use the modulating mode on my system as most of my radiators are microbore so it senses a high restriction even with all radiators open and slows right down to a flow that is insufficient. (My boiler needs a minimum flow anyway so I can't use modulating mode for that reason)

    There is some debate about whether you should balance radiators to give a specific temperature drop between supply and return pipe (typically 11-20 degrees) or whether you should balance them to cause all radiators to heat up at an equal rate with all radiators full on from a completely cold system start.

    Most information you'll find online seems to give the temperature drop method, but I'm squarely in the latter camp - balancing is to prevent radiators with "easy" flow paths stealing all the flow from those that are further away or have more restricted flow paths. A balanced system is one where all radiators heat up at about the same rate from a cold start with all radiators turned fully on. (On the TRV side only obviously) This can be easily checked by running around the house with an IR gun measuring the average panel temperature during that warmup phase. Keep going from radiator to radiator measuring the temperature as they warm up, looking for ones that warm up before all the others, or for stragglers that seem to be struggling.

    You can also measure the difference in temperature between the bottom left and bottom right corners of the panel to get a fairly good idea of the temperature drop across the radiator (not as accurate as a clamp on sensor on the pipes but good enough, and very quick) however what you will find is that if you have a mixture of radiator sizes and types that adjusting them all to get an equal temperature drop will not give you a balanced system and as you have found, sometimes its impossible to get the oft stated temperature drop without almost completely closing the valve. (Then it takes forever for this radiator to heat up as it is hardly flowing)

    This is particularly the case on a narrow radiator or one that is not a finned convector - a small non convector simply can't loose enough heat at normal flow rates to get an 11 degree differential across the radiator, unless you restrict the flow down to a ridiculously small flow. On the other hand a big 2 metre long 600mm high dual skin convector may have more than 11 degree drop even with the lock shield valve right open - as it is so good at loosing heat to the room.

    Personally I would aim to get the temperature drop you are looking for only for the biggest radiators you have and accept that smaller radiators will not be able to achieve this drop, and don't have to. Then verify that warm up rates from a cold start are about equal on all radiators. If there are any radiators that start getting hot before all the others, those need restricting. Typically these would be ones close to the boiler. If there are any that are lagging behind the rest of the house leave those fully open and restrict all the other radiators in the house a bit further. I have one room on the far corner from the boiler with a 1.6m convector and even with the lock-shield fully open it is the slowest radiator in the house, so not much I can do there. It does get nice and hot but just takes a bit longer than the others.

    If you can't get a sufficient temperature drop even across the large radiators without a ridiculously small lock-shield valve opening then your pump flow rate is too high - instead of trying to restrict all radiators to try to get around this its better to reduce the flow rate of the pump. If the largest radiators have too big a drop across them even with the lock-shield fully open you may need to increase the pump flow rate.

    I've only balanced a couple of systems but it can be a very tedious time consuming process if you want to get it right, especially when you have to let the whole system fully cool to re-test the warm up rate from cold. Also if you restrict a lock-shield valve you have to wait a long time for the radiator to cool before you can confirm what your change actually did to the drop across the radiator. So there is lots of waiting and going around and around the radiators repeatedly.

    If you don't have one, I'd recommend getting a cheap IR gun, this is the model I have:

    http://www.ebay.co.uk/itm/Digital-IR...YAAOSwhDdXD1Pr

    An IR gun will make the whole process much faster and easier.
    Last edited by DBMandrake; 11th November 2016 at 01:44 PM.

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    Quote Originally Posted by scut0 View Post
    Any tips would be appreciated.
    It sounds like you need to slow the pump down, which is your goal via the modulation of course. Does it have a manual setting you can use?

    Are you checking those drops when all the rads are open (eg TRV / Evo heads removed)? If only one rad is open it will get a lot more flow, which will reduce delta-T anyhow as the water speeds through.

    My general view is that a noisy system implies the pump is struggling to shift water through, and pump health is better to focus on than the other aspects at this time of year. Does the boiler not modulate to try and get the right return temperature, hence reducing the need for you to help it? If not then you'll probably struggle to find a setting that works well for all situations of heat demand. Is it oversized maybe, so can't modulate down far enough?

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    Than you for the replies. Generally understand and agree with you both. I'm now trying proportionally opening the lockshields based on my best guess of distance/resistance to boiler flow.

    Exceptions being that some of the very small radiators I'm keeping relatively closed down so they don't bypass a lot of heat/flow directly back to the boiler given they're not capable of disapaiting much heat.

    Wasn't able to monitor the system from cold this morning as it comes on before I'm out of bed.

    The house was therefore pretty much up to temperature before I could start monitoring.

    At this stage the boiler was cycling on for 1 to 2 minutes in each 10 minutes cycle.

    The pump is integrated into the system boiler and I can monitor it's speed on the LCD display. What's really useful on this boiler is that it can plot graphs of various inputs and outputs, including pump speed.

    The pump setting can only be changed from min modulation to max modulation. There are no fixed speed settings.

    I started with max modulation thinking this would allow the pump to reduce flow more but basically for the 1 to 2 minute burner on cycles it runs at 100 percent. When the burner turns off it then goes into a 3 minute pump overrun where it drops to 85 percent.I monitored this for 3 or 4 on/off cycles and it was identical each time. I didn't look at individual rad Temps but at the boiler, with a flow temp of 55 degrees, I was getting a retern of approx 42 to 45 degrees.

    I then tried changing the pump setting to min modulation to see what affect it has. Now the pump will initially run at 100 percent for approx 10s as the burner kicks in then drop back gradually to approx 60 percent for the remainder of the burn before going to 85 percent for the overrun period. The return temperature being a couple of degrees lower. During this phase of heating (tpi cycling after initial warm up) these setting seem pretty good with pump modulating, a descent delta T at the boiler and we'll into condensing return temperatures.

    My plan is now to leaves everything as it is and see what it does from cold with all the rads drawing heat. Fingers crossed for a similar result.

    Strange that 'max modulation' does not seem to modulate at all in these circumstances but 'min modulation' does. The manual states that min modulation is best for reliability whilst max is for well balanced systems and can save energy running the pump. That description doesn't seem to fit what I am seeing at all. Maybe there is something else going on with the boiler control logic which I'm missing.

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    Quote Originally Posted by scut0 View Post
    What's really useful on this boiler is that it can plot graphs of various inputs and outputs, including pump speed.

    My plan is now to leaves everything as it is and see what it does from cold with all the rads drawing heat. Fingers crossed for a similar result.
    Can you see heat modulation / gas flow? If that is not touching bottom of range then the boiler is perhaps able to do the right thing without any more changes.

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    You mention your heating valve is "latched open". Do you mean using the little lever? I think someone said that only half opens the valve, so isn't ideal.

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    Quote Originally Posted by paulockenden View Post
    You mention your heating valve is "latched open". Do you mean using the little lever? I think someone said that only half opens the valve, so isn't ideal.
    That was me. The standard Honeywell V4043 2 port zone valve only opens less than half way when on the filling latch, so if you want to bypass it my recommendation is to wire it to the power supplying the pump, if possible. This will make sure it fully opens whenever the pump is running but not leave it permanently on wasting power or seizing up.

    This is probably even more important with a variable speed pump configuration as the pump will "sense" the extra restriction and may be confused by this.

    Personally if you are using an S-Plan config on the Evohome with the Evohome also controlling the hot water, unless you use OpenTherm control for the boiler, I would leave the heating zone valve under control of the heating BDR91 even if you have HR92's on all radiators.

    I've found this gives better temperature control as the Evohome is better able to modulate the flow of heat from the boiler to radiators under light load conditions. It seems counter intuitive at first but the improvement shown in my temperature graphs after adding the zone valve is undeniable.

    I'm not entirely sure how this would interact with a variable speed pump but keep in mind that even if you wire the zone valve open when all the HR92's are almost closed you will get the same effect on flow as you would if the zone valve was closed.
    Last edited by DBMandrake; 12th November 2016 at 02:52 PM.

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    Nope. Can see graphs of most things but not gas flow/modulation. Only flame on or off. Can see fan speed so maybe that would be a guide. Ref my two way valve I've actually got that open with a permanent electrical (rather than the mechanical lever) supply so no issues there.

    Interestingly I turned on another zone this afternoon (office) and while that was getting up to temperature (40 minutes or so) the pump was running at 100 percent again and the delta T at boiler was very low.

    That radiator is quite small (approx 0.75kw) but i had left the lockshield quite wide open as it is one of the furthest from the boiler. I wonder if, while the hr92 was wide open on this rad and the rest of the open hr92s were quite closed (as their respective rooms were already at temperature) maybe the office circuit was flowing the majority of the water acting like a bypass and thus leading to a low delta T at the boiler.

    The thing that I can't work out is why the pump would run at 100 percent, I would of thought it would slow down to increase the delta T.

    Once the office got up to temperature (and presumably the hr92 started closing off) everything reverted to good pump modulation and good delta T.

    Makes my head hurt.

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    Quote Originally Posted by scut0 View Post
    Nope. Can see graphs of most things but not gas flow/modulation. Only flame on or off. Can see fan speed so maybe that would be a guide. Ref my two way valve I've actually got that open with a permanent electrical (rather than the mechanical lever) supply so no issues there.
    The fact that you can get graphs of any sort from your boiler is pretty neat though! Does it have an LCD screen that it displays them on or do you have to access them with a computer somehow ? What model of boiler is it ?
    Interestingly I turned on another zone this afternoon (office) and while that was getting up to temperature (40 minutes or so) the pump was running at 100 percent again and the delta T at boiler was very low.

    That radiator is quite small (approx 0.75kw) but i had left the lockshield quite wide open as it is one of the furthest from the boiler. I wonder if, while the hr92 was wide open on this rad and the rest of the open hr92s were quite closed (as their respective rooms were already at temperature) maybe the office circuit was flowing the majority of the water acting like a bypass and thus leading to a low delta T at the boiler.
    Yes that's probably correct. You won't get a large delta-T across a small radiator like that, so if that is the only radiator flowing significantly at that time and the others are mostly off I'd expect to see that.

    Do you know if there is any sort of automatic bypass valve in the boiler ? There is probably a safety valve even though you have a modulating pump, I wonder if your pump is modulating too high when only one radiator is open thus causing significant flow via the internal bypass ? That would also cause a very small delta-T.

    Is there an external automatic bypass valve on the pipework anywhere near the radiator ?
    The thing that I can't work out is why the pump would run at 100 percent, I would of thought it would slow down to increase the delta T.

    Once the office got up to temperature (and presumably the hr92 started closing off) everything reverted to good pump modulation and good delta T.

    Makes my head hurt.
    Mine too! Does the pump have a model number on it so we can look up spec sheets ?

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    Quote Originally Posted by DBMandrake View Post
    The fact that you can get graphs of any sort from your boiler is pretty neat though! Does it have an LCD screen that it displays them on or do you have to access them with a computer somehow ? What model of boiler is it ?

    Yes that's probably correct. You won't get a large delta-T across a small radiator like that, so if that is the only radiator flowing significantly at that time and the others are mostly off I'd expect to see that.

    Do you know if there is any sort of automatic bypass valve in the boiler ? There is probably a safety valve even though you have a modulating pump, I wonder if your pump is modulating too high when only one radiator is open thus causing significant flow via the internal bypass ? That would also cause a very small delta-T.

    Is there an external automatic bypass valve on the pipework anywhere near the radiator ?

    Mine too! Does the pump have a model number on it so we can look up spec sheets ?
    Yes it has an LCD Screen which displays the graphs. It is an Ideal Vogue System 32 boiler.

    It has an in-built by-pass. The manual does however say that at least one radiator with 10% of minimum boiler output must be left without TRVs or an external by-pass should be fitted. I have left the two bathroom rads without TRVs.

    I don't think the internal by-pass is opening as I'm pretty sure I can hear that when it happens. I initially had my system set up with the zone valve controling the boiler - when the heating demand went off the zone valve would close and the pump overrun would have nowhere to go so was just by-passing around the small loop within the boiler - this made a dsitinctive noise at the boiler which I no longer hear with the revised set up having the zone valve latched opn and at least three radiators always available to take flow (the two without TRVs and whichever rads are calling for heat).

    I'll pull the front of the boiler tonight and check the pump model number. Would like to get a better understanding of how this thing modulates.

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