Vaillant Ecotec pro 24 high return temperature

Discussion in 'Plumbing and Central Heating' started by iany1954, 16 Dec 2010.

  1. iany1954

    iany1954

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    Hi, My new boiler seems to work well but I have noticed that the flow/return values reported by d40 and d41 are only 6 degrees different (76 and 70) after the system has reached operating temperature. However, the whole system has been carefully balanced (15 to 20 deg. across each radiator with one left fully open). When the actual temperatures of the flow/return pipes are measured externally they are 76 and 55 degrees! I really want to ensure that all is well and running efficiently. Is this caused by the bypass operating (radiators use microbore), and if so what can be done to ensure that the boiler is operating in condensing mode?

    Ian
     
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  3. YELLERBELLY

    YELLERBELLY

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    I wouldn't worry,it's efficient anyway.
     
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  4. ajrobb

    ajrobb

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    Possibly the automatic by-pass is opening. Anyway, a return temperature of 55°C will only cause a little condensation.
     
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  5. The831Bunny

    The831Bunny

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    I'd close the bypass nearly all the way to the stop on microbore anyway. Most 8mm and 10mm systems have a very high resistance.
     
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  6. Agile

    Agile

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    The last two posts seem to imply a different situation, the first an external auto bypass and the second the internal boiler bypass.

    The boiler is clearly receiving too much bypassed flow which elevates the return temperature to a value which is too high to ensure efficient condensing operation. In both situations the solution is to reduce the bypass flow but its essential to ensure there is still some if you have TRVs on all or most of the rads.

    Hopefully the installer mentioned that operation with a microbore system is not ideal but properly adjusted and balanced should still be good.

    Tony
     
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  7. ajrobb

    ajrobb

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    I meant the boiler's internal by-pass. There shouldn't be a need to install another (unless there is another pump).

    Anyway, even with a return temperature of 55°C, the boiler would only just begin to be condensing and the savings are three parts of naff all. Condensing improves all the way down to 30°C return. Maximum efficiency is stated with a flow of 40°C and a return of 30°C (UFH territory).

    The flow through the radiators needs to match the design flow through the boiler (even if the temperature drop across the radiators has to be reduced). That way, the by-pass can stay closed and significant condensing can occur at low return temperatures as the radiators warm up from cold. From the manual charts: With the by-pass at the factory setting of 0.25 bar, you'll need at least 1200 litres/hour flow through the radiators to keep it closed. This corresponds to a MAXIMUM achievable temperature rise across the heat exchanger of about 15°C.

    If you crank up the by-pass to the maximum 0.35 bar, you'll only need 900 litres/hour flow to keep it closed. Even 900 litres/hour at 20°C rise would be 20 kW. So, if your radiators total 10 kW, you should set the temperature drop to less than 10°C or the by-pass will open and condensing will stop. Basically for this boiler, work out the total radiator output in kW and set the temperature drop to the same number of °C and crank up the by-pass to maximum. Turn down the flow temperature in mild weather. Even after all that, when the TRVs close, the by-pass will open and condensing will stop (or at least reduce significantly).

    Unfortunately*, the boiler is conservatively designed to protect the heat exchanger against excessive temperature rises without any sophisticated controls, even with no external flow. Some boilers can modulate their pumps to maintain efficiency and others modulate output to protect their heat exchanger, but not this one.

    *This might make the boiler more reliable and reliability is much more important than efficiency.

    If the radiator output is much less than (19kW) boiler output (it usually is with combination boilers), you might want to de-rate the boiler - see manual "5.11 Adjusting the central heating output (range rating)". With all the TRVs open, I'd reduce the power so that the boiler takes at least 15 minutes to reach set temperature from cold. At least that way, the boiler will spend more time condensing as it warms up. As load reduces, the room thermostat might turn the heating off quickly before the boiler stops condensing.

    I predict that the next generation of high-efficiency boilers will use nearly stoichiometric gas/air mix and have a catalyst to control carbon monoxide. They would run with CO2 levels close to 15% and would start condensing at much higher temperatures. After that, they will have to start conserving entropy rather than just enthalpy and run with efficiencies of several hundred percent as gas powered heat pumps.
     
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  8. iany1954

    iany1954

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    Thanks for the replies. I've set the bypass to maximum and adjusted all radiators for 10-12 dedgrees drop (approximately the radiator total output). This has increased the differential to 8 degrees. I've also just reduced the boiler output to 14 Kw and the flow temp to 70 degrees. Now I need to let it all cool down and see what happens! Will report ASAP.

    Thanks
    Ian
     
  9. ajrobb

    ajrobb

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    With your original figure of 6°C rise across the heat exchanger with the by-pass at 0.25bar (1200 litres/hour), the boiler load was roughly 8.2 kW (1200/3600*4125*6 W).
    • 1200 litres/hour
      3600 sec/hour
      4125 J/litre/°C
      6 °C
    With your new figure of 8°C with the by-pass at 0.35bar (900 litres/hour), the boiler load is about 8.2 kW again (900/3600*4125*8 ). At 900 litre/hour, it is handy that you can estimate the boiler output directly from the temperature rise across the heat exchanger - 8°C => 8 kW and 4°C => 4 kW etc.

    If that is with all the TRVs open, then 8°C should be your target temperature drop across each radiator and I'd try dropping the boiler rating to 10 kW. Dropping the temperature drop from 12°C to 8°C will increase the output of your radiators slightly; 44°C mean temperature difference up to 46°C or about 5% extra.

    Actually, a temperature drop of 70°C to 62°C probably corresponds to the design flow rate of the radiators. They may have been rated at 85°C to 74°C (185°F to 165°F or similar based on the old BS). As the flow temperature drops, so does the radiator output so does the difference between flow and return temperatures. So at 70°C flow, the total output drops from 11 kW to 8 kW and the difference across the radiator drops to from 11°C to 8°C.

    Based on your figures, then at 10 kW, you should eventually be able to reach 80°C flow and 70°C return. If that is not enough to keep you warm in the coming cold snap, you could up the power to about 11 kW and raise the flow temperature to the maximum 85°C and return of 74°C. Bear in mind that you have stated a temperature rise across the heat exchanger as 8°C to the nearest degree, this could be up to 1°C out or 12.5%. All other figures would be affected accordingly.
     
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  11. iany1954

    iany1954

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    Thanks ajrob. The best I can get is about 10 degrees accross the furthest radiators and so have gone for that all round. Still have 8 degrees flow return difference and the boiler set at 12 Kw at present. I'm going to let it all cool down now, reset the trv's and then see what happens! If heat up is still fairly fast I will reduce to 10 Kw.

    Ian
     
  12. D_Hailsham

    D_Hailsham

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    Since when was Joules constant 4125?

    Why are you assuming that the OP has pre 1997 radiators?

    But the rad output will drop even if the difference is the same!

    A 1000 rad (75/65/20) will give out 870W at 70/60/20 and 740W at 65/55/20.

    How do you work that out?
     
  13. iany1954

    iany1954

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    The system is pre 1997 with 8 mm microbore but new Vaillant ecotec pro 24 (was rigorously powerflushed). It does seem that I cannot do better than 8 degree flow/return drop at the boiler because of the effect of the bypass. The minimum 10 degree furthest radiator drop happens because of the microbore resistance.

    Ian
     
  14. ajrobb

    ajrobb

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    You seemed to have missed the point - constant target flow rate of 900 litre/hour.

    OK 4187 might be closer than 4125 - but given the temperature rise across the boiler is 8 +/- 1 it won't change the conclusions.
     
  15. ajrobb

    ajrobb

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    You can safely go to 8°C on nearer radiators. You'll mainly be using flow that would have gone through the by-pass. It won't make a huge difference, mostly they will heat up a bit quicker and slow down the boiler flow temperature rise slightly.
     
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  16. The831Bunny

    The831Bunny

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    Also I would increase the pump over-run to 7-8 minutes on D.1, this will stop the boiler entering S.8 or S.0, and even out the comfort levels in regard to room temperatures.

    Check the flow through the radiator where your roomstat is sited too. If it's close circuit wise to the boiler, you may have too much bypassing there.

    I wouldn't say raising the flow temperature above the preset maximum of 75C would give you much benefit. In reality the boiler runs better and is less stressed at sub 75C flow temperatures.

    Have you considered weather compensation? Once set up it will ensure the lowest flow temperature is used to your required room temperature.

    Cheers
     
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  17. D_Hailsham

    D_Hailsham

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    OK, got you now.

    You haven't answered my last question.
     
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