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Thread: Evohome upgrade from CM927, worth the investment?

  1. #31
    Site Sponsor The EVOHOME Shop's Avatar
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    Quote Originally Posted by DBMandrake View Post
    I'm no expert on these matters so I am happy to be corrected, but I don't think you would normally run the pump in variable speed mode on a system with an external automatic bypass valve.

    I have a Grundfos UPS2 15-50/60 which I fitted new a few months ago to replace the very old (original 20 years ??) Grundfos pump that had got very noisy and started to fail... unlike the old pump that had I, II and III it can also be switched between three fixed speeds and 3 variable speeds, probably like yours. (in fact it's possible we have the exact same pump, it seems to be a pretty common model)

    I too experimented briefly with the variable speed modes but they were a complete no go for my system with the pump running far far too slow on any of them. I think you'll find that you can only use a variable speed mode with a boiler that is specifically designed for a variable speed pump.

    Variable speed mode slows the pump down as the flow becomes more restricted (eg as radiators close) and speeds it up as the flow becomes less restricted. If you look at the load lines for the pump you'll see that they actually trend in the opposite direction to fixed speed mode. Eg in fixed speed mode as radiators close and start restricting flow the pressure will increase, (to a dangerous point if you didn't have some kind of bypass) but in variable speed mode pressure will actually decrease as flow reduces due to electronically limiting the torque of the motor...

    This inverse relationship does not react well with an automatic bypass valve that is mounted after the pump as ABV's usually are, and will prevent the ABV from regulating the pressure properly. But as I say I don't think you're supposed to use a variable speed pump with an ABV anyway, it is an alternative to an ABV, but it obviously can't be used with any boiler that has a minimum flow rate requirement and does not have any internal bypass of its own or other design feature to allow it to work with a variable speed pump.

    So if your ABV is mounted after the flow output of the pump and/or your boiler isn't designed for a variable speed pump you can't use variable speed mode.
    The information above is correct. If you have an external ABV and external modulating pump it should remain in constant pressure mode, not variable as you can risk overheating the boiler.

  2. #32
    Automated Home Jr Member
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    Thanks you for the guidance all. Ive setup the pump for CP1.

    What balancing method are people using? get all the rads to get warm at the same time or a delta T of 12 deg across the rad?

  3. #33
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    Anyone had any suggestions where i can find the minimum flowrate for my boiler? (Worcester Bosch Greenstar Ri 24) There is nothing in the tech specs in the manual i have in front of me nor on the WB website.

  4. #34
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    The WB installer manuals are particularly bad for technical information Leigh and you will have to call them for particular flow rates of their appliances.

  5. #35
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    Thank you Richard

  6. #36
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    Hi I`ve found this thread which is exactly what have been looking for information on. Basically I have an old 1950s house with 13 radiators in total. I spend roughly 1200 PA on gas heating. I have looked at evohome as an option for a new smart heating mechanism but to cover the new wifi controller and subsequent trvs it woud be almost 1000. Really all I need is my house split into upstairs/downstairs as my biggest loss is keeping the heating on around 19.5/20 overnight via our wireless cm927 honeywell controller kept in our bedroom (we have 2 small children so need consistent heat level for them). This means the downstairs warms to this level also, bar some minor manual permanent tweaks on indivdual radiators such as dining room. Can your products offer a better option if i want to minimise zones and still have control, even over 2 different levels? Also is this difficult to install or could I do it myself and what happens my combi boiler and water heating if I do this? Many thanks

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