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Radiator not projecting any heat

Discussion in 'Plumbing and Central Heating' started by k4renm4c, 9 Aug 2011.

  1. D_Hailsham

    D_Hailsham

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    I should have said that it depends on the size of rad. A 2kW rad will need the LS valve open further than a 500W one as the flow rate is four times faster.

    A sticking TRV would show different symptoms to a closed LS valve. The obvious difference is that the rad surface will be cold with a sticking TRV but warm with a closed LS valve.
     
  2. gasafengineer

    gasafengineer

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    Never the case in real world LPHW hydronic heating systems.............

    Always a topic for confusion too :mrgreen: , to get 80% of the design heat output (80f/70r) we need only about 25% of the design flow rate (non mixing) , as you can see control response is highly non-linear , if the temperature difference between design flow & return is increased the response becomes closer to linear ie: 80f/60r would require around 48% of design flow rate for 80% of design heat output , this implies that a larger design temperature difference between flow & returns is preferred as control of emitter heat output would be improved............ :idea: , take a look @ the drayton TRV , orifice entry size is around 8mm :eek: , 'technically' speaking these would be no good fitted to any convector larger than around 1.5 kw (velocity not exceeding 1m/s) , but they work & system balancing is a breeze , you can point me in the direction of the manufactures literature which on many times you have (drayton) ;) but i've been using Drayton TRVs for years & never have i had to faff around with balancing issues when commisioning (domestic) , modulation of water flow rate can be very forgiving. :mrgreen: .................WC can have it's uses too :rolleyes: with regards to 'flow temperature modulation' , response is very nearly linear so good overall control can be achieved unlike the above (MWFR).
     
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  4. D_Hailsham

    D_Hailsham

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    Obviously the flow rate through a 2kW rad will only be four times that of a 500W rad if both are giving of the rated output at the specified flow and return temperatures.

    That will depend on how you reduce the rad output to 80% of the quoted output.

    If you maintain the same flow temperature and reduce the return temperature, by closing down the LS valve, then the flow rate will not be proportional as you will now have a higher temperature differential. But if you reduce the flow temperature and maintain the same differential, then the flow rate will be 80% of the full output flow rate.

    Why do you use non-standard temperatures?

    A 1kW rad has a flow rate of 0.024 litres/sec at standard temperatures (75/55/20). If you want to reduce the output to 800W, there are several, related, ways you can do this: reduce the flow temperature and /or reduce the return temperature.

    If you maintain the same flow temperature (75C), the return temperature will have to reduce to 51C and the flow rate will then 0.009l/sec, which is 37.5% of the flow at 1kw output. But if you reduce the flow temp to approx 67C and maintain the 10C differential, the flow rate will reduce to 80% of 0.024 l/sec.

    I don't follow your logic.

    Drayton say that you shouldn't exceed 200mB differential pressure across the valve, which works out at 3kW. That would give a velocity of 0.45m/s in 15mm pipe.

    Have you ever looked at the size of the opening in a TRV4 body if you alter it using the special key (the setting numbers are on the black ring)?
    This shows the opening when the setting is on 1 or 2 (can't remember which).


    Setting 1 is suitable for a 500W radiator and 2 for a 750W.
     
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  5. Onetap

    Onetap

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    Check there is an unobstructed air gap below and behind the radiator; check that it is not, for example, blocked by the skirting board.

    There is a minimum clearance required below the radiator, you'd need to check the installation instructions for the manufacturer's recommendations, I think it is usually about 100mm. If you reduce this, then you won't get the rated heat output.



    It is usual to have the radiator below the window because the air adjacent to the glass gets cold and you get a cold draught dropping to the floor.

    If the radiator is under the window, the hot upflow of air cancels out and overwhelms the cold draught from the window.

    If the radiator is on the opposite wall, the cold draught from the window flows across the floor to the opposite wall, where it is warmed, rises to the ceiling, flows to the window, etc.. You get a powerful 'conveyor belt' draught in the room. The upwards warm convection from the rad reinforces the downwards cold convection from the glass. The coldest draught is at ankle level, where people are very sensitive to cold air movements.

    It is most pronounced with singe glazing.
     
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