Balancing radiators

Discussion in 'Plumbers' Talk' started by Scadamike, Feb 26, 2021.

  1. Scadamike

    Scadamike New Member

    Out of curiosity why when balancing radiators do you close them all down and then gradually open them until all radiators are getting warm, why don't you open them all fully and gradually close them down until all radiators get warm?
    The second method would allow more flow and less chance of sludging up! or am i missing the point here?
  2. koolpc

    koolpc Screwfix Select

    To be honest, i have trouble lifting one rad without trying to balance 2!
  3. andy48

    andy48 Screwfix Select

    Because you are trying to get the temperature drop across each radiator to be the same, 11 degrees for a non-condensing system, 20 degrees if condensing (assuming properly sized system). If they were all open, the drop across the one you are working on would be affected by the flow through the others. Ideally, you start with the index radiator (the one hydraulically furthest from the boiler) and work you way towards the boiler.
  4. Scadamike

    Scadamike New Member

    Thanks for the reply.
    Makes more sense now, but once you have set the furthest away aren’t you going to have a similar issue when you set the second radiator I.e. setting the second will affect the flow in the first one you set?
    I didn’t realise the temperature drop was different for a condensing boiler and quite a lot as well.
    You mention a properly sized system, if the boiler is over sized is there anything that can be done or is there only a problem if it’s undersized.
    Sorry about all the questions but I like to know how things work and why they are ser the way they are.
  5. Mike83

    Mike83 Screwfix Select

    In today’s terms an oversized boiler ain’t good.
    You ideally want a boiler with a decent modulation ratio. This is turn means a lower output boiler would usually have a lower minimum output.
    Say you turn on all radiators and they need 5kw to heat up correctly. If you have a 30kw boiler it will start to modulate as the temperature increases. If the boiler can only cut down to 7kw the boiler will start to cycle quite quickly.
    However if the boiler can cut down to 4kw the boiler can sustain its minimum output for far longer thus its in condensing mode more often.
    People’s definition of a properly sized system can vary.
    On a system with 10 radiators a 15kw heat only boiler will be more efficient than a 30kw combi as it would be able to modulate lower and more often.
    On some boilers the maximum output can be reduced but the minimum is fixed.
  6. andy48

    andy48 Screwfix Select

    1. To an extent setting the second radiator, and subsequent ones, will have an effect on the preceding ones. However, provided the flow and degree of heat is sufficient, the effect aren't significant.
    2. Traditionally, the drop across each radiator was 11 degrees. This worked with a flow temperature of around 76, and a return temperature of around 65. However, with the advent of condensing boilers, a return temperature (at the boiler) of 55 degrees or less is needed to force the boiler into condensation mode. To ensure the system heats up in a reasonably quick manner, a higher flow temperature (and thus increased temperature drop) is needed, and so generally needs to be 75 degrees.
    3. The average temperature of a radiator in a traditional system is (((76 - 65) /2 ) + 65) = 70.5. In a system designed for a condensing boiler the average temperature is (((75 - 55) / 2) + 55) = 65.
    4. Using a desired ambient air temperature of 20, gives (approximately):
    4a. A difference between air and radiator temperature of 70.5 - 20 = 50 degrees for a traditional system.
    4b. A difference between air and radiator temperature of 65 - 20 = 45 degrees for a condensing system.
    5. As the transfer of heat from radiator to air is a function of temperature difference, a condensing system needs slightly larger radiators.
    6. If a traditional system has its boiler replaced by a condensing boiler, it will take longer to reach the desired temperature unless the radiators are changed at the same time. This is rarely of any significance.
    7. Mike83 has covered the issue of oversizing boilers in post #5.
  7. sam spade

    sam spade Active Member

    If the system was originally designed for 50 degree differential with 76/65 temperatures, but the new boiler runs with a 45 degree differentia with 75/55 temperatures, there will be a reduction in radiator output of about 15%. This may not matter if the radiators have already been oversized, but it stiil needs to be taken into account.

    As for balancing temperatures, 11 (now 10) for non-condensing boilers and 20 for condensing are only guidelines -it's normally the maximum permitted temperature drop across the boiler when running at maximum output. Provided this is not exceeded, a lower temperature is acceptable.

    Balancing each radiator for the same diffrential assumes all radiators have been correctly sized for the calculated heat loss in each room, rather than for the width of the available space, e.g. window opening.
  8. Scadamike

    Scadamike New Member

    Thanks for your replies that more than satisfies my curiosity what I thought was a relatively simple subject is quite complex, on the downside i’m Going to have to work out if my current system is working to it’s optimum!!

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