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Thread: alpha pump flow rate not enough for some rads

  1. #21
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    Sep 2014


    Quote Originally Posted by MJNewton View Post
    It is usual practice to measure the pipes or valve tails, not the radiator itself, in order to ensure consistency. It is after all the flow and return temperatures you are trying to measure.
    "Usual practice" perhaps, but that assumes you are trying to adjust the balancing of the radiators to produce a certain temperature differential across each radiator. This is indeed the traditional approach but as I mentioned in my original post after having tried this method in the past I would argue strongly that this approach is fundamentally flawed regardless of being "usual practice", (it's trying to optimise for the wrong parameter) and I mention a couple of possible aggravating factors that contribute to this, such as short radiator vs long radiator where it's difficult to even achieve a large temperature drop on a short radiator without excessively restricting the flow.

    IMHO balancing via lockshield valves is about equalising the flow resistance to the different radiator branches so they all get a fair chance when all radiators a full on during initial warm up, to prevent any radiators from being left behind or completely starved. And for this purpose measuring the flow/return differential is meaningless, what you're interested in is the average temperature of each radiator (hence measuring half way along to get a quick single point measurement of the average of flow and return temperatures) and you want to be able to do it very quickly, as you need to measure all radiators quickly multiple times during a 10 minute warm up period to gauge the relative warm up rate of each radiator compared to the others.

    Clamp sensors are just too slow to get clamped onto pipes, take measurements then move onto the next radiator - while an IR gun isn't as precise its good enough and extremely quick, and if you follow the right measurement technique, very repeatable. Clamp sensors also don't work well if the pipes are painted as the paint will slow down the response of the sensor when the sensor is applied.

    At the end of this initial balancing you can then try to more accurately measure the differential across each radiator and if the differential is too small or too large on most radiators, increase or decrease the pump speed a bit, and in fact the overall temperature drop of the radiators should be set with the pump speed not the lockshield valves.
    Paint is almost certainly going to have variable emissivity depending on colour, pigment and compound. A piece of tape won't introduce any thermal lag whatsoever (it is too thin; indeed thinner than paint - but neither will make any difference anyway as the lag is already in the order of minutes due to the thermal mass of the metal component parts) but will significantly improve accuracy. Black PVC tape is very consistent in emissivity hence why it is the go-to surface for accurate IR temperature readings - most non-contact thermometer instructions will specify its use if they pass any comment at all.
    Unless the radiators are very different from one another (modern painted radiators vs old cast iron ones etc) then none of this is necessary. You don't need a super accurate absolute temperature reading, in the method I've described you're trying to get a comparison of how quickly the different radiators heat up - and for this relative readings are all you need. The emissivity of the IR gun doesn't even need to be calibrated as you don't care what the exact figures are, just how one radiator compares to another.

    If you have radiators with vastly different surface finish then sure, put a big square of black tape on the middle upper portion of each radiator, I have no problem with that. Just be sure that the entire surface area of the tape is pressed onto the metal and that half of it isn't sitting out in the air, and make sure it's big enough to fully cover the viewing cone of the sensor from the distance it is held at. You also have to be careful not to hold it too close for too long as heating up the plastic inside the IR gun sight can cause inaccuracy.
    Last edited by DBMandrake; 17th May 2021 at 09:23 AM.

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