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Thread: What is the impact of running modern boilers at less than minimum rating

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    Default What is the impact of running modern boilers at less than minimum rating

    Trying to get my mind around understanding the design constraints of modern modulating boilers.

    I have a home brewed system where each room (apart from the toilet which has a thermostatic valve) has its own room stat and associated heating profile with specific temperatures at different times of the day depending on the day, season, outside temperature or unoccupied. There is no overall house thermostat. The radiators are fitted with 24v thermally controlled valves. Hot water is controlled just like a radiator.

    There are 12 radiators of which only 7* are typically ever in use and even then at totally different times during the day, there are many times when only one radiator is open and that radiator may well be below the minimum rating of the boiler 3.9kw in my case.
    Kitchen* - 1kw plus 2kw , Dining room* - 1.3kw, Lounge* - 2.8kw, Bed1* - 1.5kw, Bed2 - 1.1kw, Bed3* - 0.9kw, Bathroom* - 0.87kw, Toilet - 0.3kw, Study - 2kw, Workshop - 2kw, Sun Lounge - 1.5kw, Hot Water 3.2kw. As you can see the boiler will spend most of it's time running at less than the minimum rating.

    The Kitchen plinth heaters are currently on a permanently open connection with the power to the fans controlled when heat is required this provides a bypass function for the boiler which is not strictly needed as the boiler is never activated unless a valve is open and in the interests of economy will like to control as per the other radiators.

    With modern modulating boilers is there any reason why only the burner would be modulated say down to whatever heating is required even as low as 1 or 2 kilowatt and the pump not be modulated at all just manually adjusted to ensure adequate flow through the furthest and highest radiator say in a three story house.

    My current boiler an Ideal Vogue S18 (running at it's maximum temperature of 80 deg) does not seem to be coping with this setup at all well, 4 of the radiators are fan assisted and getting sufficient flow through them is proving problematic. My 35 year old cast iron sectional boiler worker perfectly, is this a case of modern is definately not better.

    What impact might this be having on the boiler.

    Is this the case that I have made my system too sophisticated for the boiler to handle.

  2. #2
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    Quote Originally Posted by GrahamW View Post
    With modern modulating boilers is there any reason why only the burner would be modulated say down to whatever heating is required even as low as 1 or 2 kilowatt and the pump not be modulated at all just manually adjusted to ensure adequate flow through the furthest and highest radiator say in a three story house.
    So are you saying that the pump is also modulating and that you can't set it to a fixed speed ? I'm not sure that I quite understand your question...

    Modulating pumps are all about saving electricity, and work on the theory that if the burner is running at a low level and producing a small amount of heat then you don't need the full water flow rate to transfer that heat to the radiators, thus the pump can slow down and save some electricity. (At a high burner output a slow water flow could not take heat away from the boiler fast enough to keep up so would limit total system output) But it doesn't HAVE to. You can perfectly well have a boiler where the burner modulates but the pump runs at a constant speed.
    My current boiler an Ideal Vogue S18 (running at it's maximum temperature of 80 deg) does not seem to be coping with this setup at all well, 4 of the radiators are fan assisted and getting sufficient flow through them is proving problematic. My 35 year old cast iron sectional boiler worker perfectly, is this a case of modern is definately not better.
    I presume you mean getting sufficient water flow through your fan assisted radiators is a problem, and that the modulating pump slows right down when only those radiators are open ? If so I am assuming that they have relatively high hydraulic resistance ? What pipe sizes and how long are the runs ?

    I can appreciate your problem - modulating pumps typically seem to be configured for systems with low hydraulic losses. I have a Grundfos UPS2 which has both fixed and modulating modes, however my radiators are mostly 8mm microbore - which has a lot more flow resistance than a modern system, that is all 15mm with 22mm headers everywhere, and some of the runs from the headers on my system are pretty long.

    The end result is that even if I fully open every radiator in the house (minimum flow resistance) the pump when in modulating mode still runs much slower than the lowest of the three fixed speeds, due to it sensing too much flow resistance and believing that most of the radiators are closed. Put simply the modulation slope the pump is calibrated for doesn't match my system - not by a country mile, so modulating mode is unusable for me. On fixed speed mode (either low or medium seem fine on my system) it works perfectly.

    It could be that the piping and radiators in your house just don't suit a modulating pump. I would be checking to see whether it is possible to switch it into a fixed speed mode, but if you do make sure you have an automatic bypass valve or a bypass of some sort.

    A quick look in the manual of your boiler seems to suggest a fixed speed mode is not an option and that only "minimum" or "maximum" modulation are options. I assume you have already tried the minimum setting ? It might be that there is a hidden installer menu option that could set it to fixed speed.

    What impact might this be having on the boiler.
    The only impact of the heat demand being below the modulation range of the boiler's burner would be that it has to cycle on and off rather than be able to do a constant low burn. This is not quite as efficient but in itself is not anything to worry about as the boiler will still be cycling on and off if you use TPI modulation anyway, so they are built to be able to handle this.

    If you can't get sufficient flow through your radiators due to the pump modulating down too low all the time that is a bit of a problem though - you may find that the system struggles to put out enough heat overall if the pump won't ramp up its speed sufficiently, and the burner may short cycle or even go into lockout. Have you contacted Ideal for any advice on the pump settings ?
    Last edited by DBMandrake; 8th February 2017 at 01:22 PM.

  3. #3
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    Quote Originally Posted by DBMandrake View Post
    So are you saying that the pump is also modulating and that you can't set it to a fixed speed ? I'm not sure that I quite understand your question...
    [B][GrahamYes that is correct.

    Modulating pumps are all about saving electricity, and work on the theory that if the burner is running at a low level and producing a small amount of heat then you don't need the full water flow rate to transfer that heat to the radiators, thus the pump can slow down and save some electricity. (At a high burner output a slow water flow could not take heat away from the boiler fast enough to keep up so would limit total system output) But it doesn't HAVE to. You can perfectly well have a boiler where the burner modulates but the pump runs at a constant speed.

    I presume you mean getting sufficient water flow through your fan assisted radiators is a problem, and that the modulating pump slows right down when only those radiators are open ? If so I am assuming that they have relatively high hydraulic resistance ? What pipe sizes and how long are the runs ?
    GrahamPipe sizes from the boiler, 6m of 28mm, splits into 2 x 5m of 22mm, one of which then splits into 3 x 15mm, one of which 5m goes to the fan radiators which splits into 2 x 15mm feeding each radiator. The 15mm feed to the fan rad connects internally to a header with 4 parallel estimated 10mm pipes through the core. I do assume that it must have a higher hydraulic resistance than a normal rad, hydraulic resistance quoted as 1046mm hg at 455 l/h down to 95 mm hg at 113 l/h for what it is worth
    I can appreciate your problem - modulating pumps typically seem to be configured for systems with low hydraulic losses. I have a Grundfos UPS2 which has both fixed and modulating modes, however my radiators are mostly 8mm microbore - which has a lot more flow resistance than a modern system, that is all 15mm with 22mm headers everywhere, and some of the runs from the headers on my system are pretty long.

    The end result is that even if I fully open every radiator in the house (minimum flow resistance) the pump when in modulating mode still runs much slower than the lowest of the three fixed speeds, due to it sensing too much flow resistance and believing that most of the radiators are closed. Put simply the modulation slope the pump is calibrated for doesn't match my system - not by a country mile, so modulating mode is unusable for me. On fixed speed mode (either low or medium seem fine on my system) it works perfectly.

    It could be that the piping and radiators in your house just don't suit a modulating pump. I would be checking to see whether it is possible to switch it into a fixed speed mode, but if you do make sure you have an automatic bypass valve or a bypass of some sort.


    A quick look in the manual of your boiler seems to suggest a fixed speed mode is not an option and that only "minimum" or "maximum" modulation are options. I assume you have already tried the minimum setting ? It might be that there is a hidden installer menu option that could set it to fixed speed.
    GrahamIt is not possible to switch the boiler into a fixed speed mode, that modulation option you have found is not available on my boiler, apparently it is a very early model and that option has been added later. the boiler was only installed 18 months ago!.

    The only impact of the heat demand being below the modulation range of the boiler's burner would be that it has to cycle on and off rather than be able to do a constant low burn. This is not quite as efficient but in itself is not anything to worry about as the boiler will still be cycling on and off if you use TPI modulation anyway, so they are built to be able to handle this.

    If you can't get sufficient flow through your radiators due to the pump modulating down too low all the time that is a bit of a problem though - you may find that the system struggles to put out enough heat overall if the pump won't ramp up its speed sufficiently, and the burner may short cycle or even go into lockout. Have you contacted Ideal for any advice on the pump settings ?
    Graham i have been in touch with Ideal but they are trying to blind me with science and misinformation to avoid updating the boiler.

    Thanks for the response that is the conclusion I had come to, persuading Ideal is another matter.
    Last edited by GrahamW; 8th February 2017 at 09:34 PM.

  4. #4
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    I too had exactly this problem with my Alpha2 pump. The Auto Adapt mode would just make my boiler shut down all the time. So in the end I had to use one of the proportional speeds, which does a pretty good job. Ofcourse I could use the fixed speeds too, but when proportional works fine and at a lower power consumption, I left it.

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