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Thread: How to control secondary pump from Evohome

  1. #41
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    Quote Originally Posted by fezster View Post
    I thought this may be the case. Obviously, with a LLH, the boiler has a pathway to expel it's heat, but I was thinking about the pump on the secondary side not having anywhere to pump to, perhaps leading to premature failure. I'm loathed to install an ABV as they often seem to pass water when not intended, so am thinking perhaps a modulating pump is the answer.
    If you can find a modulating pump that is happy to be deadheaded, please let me know. If you do go for a modulating pump, be aware that you can't then use an ABV at all as they end up fighting each other.

  2. #42
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    Quote Originally Posted by dty View Post
    If you can find a modulating pump that is happy to be deadheaded, please let me know. If you do go for a modulating pump, be aware that you can't then use an ABV at all as they end up fighting each other.
    @dty Could you tell me a little about your LLH setup? Do you have a fixed speed pump on your primary/boiler side? And if so, is it rated to overcome the boiler's hex resistance at max KW output?

    I wanted to try and understand what happens on the primary side of an LLH when the boiler modulates down? As the flow rate is fixed, the delta T at the boiler must surely reduce. Is this what you see in reality, though?

    The secondary side flow rate will vary according to heat demand (assuming a variable speed pump), but the primary side is fixed. Trying to get my head around why a LLH is often cited as the best way to separate primary and secondary flows - is it more about convenience than efficiency?

  3. #43
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    I have a system boiler, so it has a built-in pump that I don’t know a lot about other than it didn’t have enough remaining head to reach the extremities of my large system.

  4. #44
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    With a single zone calling for heat, have you ever examined the flow/return temps on the boiler? Are they still at a differential close to 20, even though the boiler is probably running at it's lowest modulation setting?

  5. #45
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    Checking the installer manual, it specifically says to set the pump to one of the fixed speed modes if using the boiler with a LLH. And, indeed, when I look at my modulation power and my dT, they are directly proportional to each other, as would be expected with a fixed speed pump. The secondary side, however, is running on variable speed.

  6. #46
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    Quote Originally Posted by dty View Post
    when I look at my modulation power and my dT, they are directly proportional to each other, as would be expected with a fixed speed pump.
    So as power goes down, so does dT?

    Flow rate=KW/(4185 * dT)

    Which is precisely my concern because it means when the boiler is modulating (most of its life), it is not running at the design 20 degree dT to be able to condense. This seems to contradict the manufacturers recommendations on a LLH being the most efficient way to run a boiler.

  7. #47
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    I've not thought about it too closely, to be honest. I have the LLH simply because the boiler's integrated pump doesn't have enough head after the losses in the heat exchanger to cope with my system.

    Given the manufacturers recommend this (and it seems to be standard practice in the US and Europe), I'm assuming it's still sane. Do you really need a dT of 20 to condense? I thought you'd get condensation with a reasonably low (< 50C or 55C) return?

  8. #48
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    The Vaillants are designed and speced for a 20C dT. Bit most run a 11C drop. Trying to achieve a 20C dT is quite hard, in practice especially with a large boiler.

  9. #49
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    Quote Originally Posted by dty View Post
    I've not thought about it too closely, to be honest. I have the LLH simply because the boiler's integrated pump doesn't have enough head after the losses in the heat exchanger to cope with my system.

    Given the manufacturers recommend this (and it seems to be standard practice in the US and Europe), I'm assuming it's still sane. Do you really need a dT of 20 to condense? I thought you'd get condensation with a reasonably low (< 50C or 55C) return?
    True - if your flow temps are low, then you'll get condensing. I guess it's only an issue once your flow temps are over 65. I, too, have the issue that the combined head loss of both the boiler and system are quite high, so am having to use a semi-commercial pump. I'm hoping the LLH will allow me to use 2 domestic pumps, and have the boiler operating more happily with constant flow rate through it (i.e. no more ABV, which I could never quite get right anyway).

    I have an added issue of having 2 heating zones (upstairs and downstairs), and if they are running at significant temperature differentials, the boiler gets a sudden rush of cold water meaning it hits S53. Sometimes this ends up in a perpetual cycle, meaning the boiler is in S53 mode for over an hour, and the heating running lukewarm only. I guess in your case, as you have EvoHome, you wouldn't have this problem, as hot water is circulating your entire system, even if it isn't actually entering all of the radiators.

  10. #50
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    Actually its worse with Evohome. Water doesnt actually flow unless its going through a radiator. So depending on how many are actually calling for heat and when you actually might have a situation where only one rad comes on first thing and so you get S53 because the hot water isn't leaving the boiler fast enough from the flow.

    Close Coupled Tees might be another option.

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