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Boiler Protection


I have posted before about my heating system which is simply not performing. The installer does not fully understand the issues, so I am left to try to improve things. I am trying to base the modifications on an understanding of hydronic systems which I am trying to get my head around. I have two questions I would like to ask.

Firstly, I suspect that the boiler circulator is undersized. The boiler protection circuit is not correct as there is no hydraulic separation. I proposed to fix this by adding the pipe shown dotted. It will be 25mm copper and about 300mm long.

Now the boiler is 30kW. I have calculated (see attached sketch) that the flow required to remove this heat from the boiler is about 29l/min. I further believe that the TacoNova thermostatic mixing valve (TMV) will have a pressure drop across it of about 9m with this flow (see attached spec sheet). The current Wilo Star RS 26/6 circulator cannot produce this head. From this I conclude that the circulator will not do the job. Have I got this right?

Secondly, I have read in John Siegenthaler's book and elsewhere that the pressure vessel should be as close as possible to the inlet side of the circulator. In Fig5-78 (see attachment) the pressure vessel is separated from the inlet port by the TMV. There can be a significant pressure drop across this TMV. Also because of the low pressure drop across the boiler, the pressure vessel is close to the outlet port of the circulator, not the inlet port. This seems to go against the practice of placing the pressure vessel near the inlet port. What am I missing?

I hope someone out there can help me understand these issues. Thanks in advance.



  • SWEISWEI Member Posts: 7,356
    edited July 2013
    Mixing valve

    is not the correct choice for the application.  You want a motorized valve controlled by outdoor reset which includes boiler protection.  Not sure where you are located, but there should be several options available.

    The proposed "hydraulic separation" is flat out wrong.

    I'm going to leave the pump sizing to someone who works more regularly in metric units.
  • ChrisChris Member Posts: 3,040
    Why Not

    Just move the boiler pump to the return, put in 2 closely spaced tees for your separation, then add a 4-way mixing valve to provide boiler protection. Think you should read a little more.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • SantilalSantilal Member Posts: 40
    Thanks for the reply


    Thanks for the replies so far. You both (SWEI & Chris) advocate the use of a three way motorised valve for controlling the boiler inlet temperature. I have read the reference given and I am not against using the motorised valve, but I am trying to make the most of what I have inherited from my supplier (and I understand that this is not always possible or even the best option).

    I am trying to emulate Fig 6.3 in the reference Chris sent me. The boiler output temperature is about 70degrees, which is the same temperature that the system requires. Hence the TMV for the distribution system is not required. The TMV for boiler protection is required and I have shown this in my sketch. I have been told that the pipe shown dotted on my sketch will work like closely spaced tees as long as the pipe is short. I can re-pipe and use closely spaced tees, but I only want to do this if what I am proposing won't work. But either way I want to understand how it works.

    I like your quote about the bitter taste of a poor installation. In my case, the installer I selected was not the cheapest and had a good reputation. However, I felt my installation was not working properly and so I have embarked on research to try to find ways to improve it. In some cases I will be able to, in others, poor access to piping will prevent me doing much at all. If nothing else, I am starting to learn heaps, and feedback from this forum is a great help.


  • SWEISWEI Member Posts: 7,356
    edited July 2013
    Your mixing strategy

    Should do two things (assuming that is indeed a conventional, fixed-temperature boiler.)

    Supplying boiler water at a fixed temperature will not adapt to changing load requirements.  For the 95% of the season when this amount of heat is not needed, the boiler will cycle on and off, which will cost you both comfort and efficiency (roughly 15% fuel cost, perhaps a bit more.)

    Second, you need to protect the boiler heat exchanger from condensation caused by low return water temperatures.  You could buy a mechanical valve to do this (ESBE, Caleffi, LK Armatur.)

    The best answer is a motorized valve which will accomplish both tasks.  A Taco iSeries-R is the least expensive option here in the US, but we still don't know where you are located and what lines are available to you there.
  • SantilalSantilal Member Posts: 40
    Now I understand

    Hey, Thanks for that. Now I understand what you are suggesting and the benefits it provides. I thought that the motorized valve option and the TMV were just different ways to achieve the same result. Now I see the additional benefits that the motorised valve provides. I am in New Zealand by the way. I will now have another think about it and I am sure I will end up posting again. In the back of my mind I am wondering how this mixing strategy might impact with the rest of the distribution system - hopefully because of hydraulic separation, there will not be any interaction, but I am not sure. Once again, thanks. Your input is appreciated.

  • SWEISWEI Member Posts: 7,356
    Glad it's starting to make sense

    BTW, you can often use a single pump with a 4-way mixing valve.  See figure 6-11 on p.39 of the Idronics issue Chris linked to above.
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