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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlcforbes
    • CommentTimeAug 12th 2016
     
    An inverter GSHP to service UFH and DHW in a Passive House - has anyone any experience of this type of GSHP that they are willing to share?
    • CommentAuthorSprocket
    • CommentTimeAug 12th 2016
     
    I remain unconvinced of the improved efficiency claims made for inverter drive for GSHP.
    If the output is buffered, either by a water tank or a big slab of concrete then surely you want full power or none so simple on/off is pretty efficient? Surely the performance limit of most GSHPs is down to temperature difference and the actual efficiency due to the mechanical construction of the compressor?

    Am I missing something? Is there some new application method for GSHP here to make good use of a modulated output? Is this just manufacturers striving to differentiate their products?
    • CommentAuthorgyrogear
    • CommentTimeAug 12th 2016
     
    better reliability and efficiency...

    http://www.airconditioning-systems.com/dc-inverter.html

    gg
    • CommentAuthorJonG
    • CommentTimeAug 14th 2016
     
    Inverter GSHP's in test perform no better than fixed speed. I saw some data sets recently from a manufacturer who have both.The inverter has a sweet spot but it is generally in the higher output temps and still drops off even on a low temp to give scops just lower than fixed.

    We have fitted inverter ground source in the past but now reserve the specification of them in situations where the heat load is higher than hot water load and the inverter can then ramp down and allow for a smaller cylinder.

    Can't see the need in a ph though where loads will vary less than in most other scenarios.
    • CommentAuthorringi
    • CommentTimeAug 14th 2016
     
    JonG,

    Are you always installing a buffer tank for heating?
    • CommentAuthorJonG
    • CommentTimeAug 15th 2016
     
    Not always it depends on the system and its design but we do use them a lot even on gas or oil. With gas if the lowest modulation of the boiler is above min load, it gets a buffer and on oil we use them for high temp from the boiler and weather comp to the system.

    On heat pumps if we have enough open circuit volume to match buffered volumes AND we can guarantee flow rate we do sometimes omit them.

    Mostly they are fitted in parallel but sometimes series is fine too because primary and secondary flow rates are usually the same.
    • CommentAuthorringi
    • CommentTimeAug 15th 2016
     
    I was just thinking that a inverter may reduce the need for buffer tanks
    • CommentAuthorJonG
    • CommentTimeAug 15th 2016
     
    They do ringi but even with an inverter turn down ratios can be poor in comparison with say gas, but sizing is based on the min ramp down when an inverter is present.
    • CommentAuthorlcforbes
    • CommentTimeAug 18th 2016
     
    Thanks for all the feedback - seems the jury's out for a ph with low loads
    • CommentAuthorwoodgnome
    • CommentTimeJun 21st 2017
     
    JonG
    What's your opinion on the Nibe inverter driven pumps compared to the fixed speed?
    Are they worth the extra cost?
    I'll have a low heat load and will use it for DHW as well.
    • CommentAuthorJonG
    • CommentTimeJun 26th 2017
     
    In our opinion and experience the inverter units are mainly useful in scenarios where the DNO have restrictions on connection to the network, where the lower starting current can allow connections/installations to go ahead.

    Running costs are not significantly different to fixed speed and in some cases higher due to the way that an inverter works, generally their sweet spot is at a higher temperature than that of a fixed speed.

    Arguably too the higher levels of complexity in an inverter may result in higher life-cycle costs, but this is conjecture.

    They do allow for a smaller buffer if space is an issue.

    We have 2 costed but have only specified them for the DNO issues mentioned above.
    • CommentAuthorwoodgnome
    • CommentTimeJun 29th 2017
     
    Thanks.
    • CommentAuthorwoodgnome
    • CommentTimeSep 12th 2017
     
    JonG ...Sorry...slightly off topic.
    Just had my Nibe 1145 non inverter heat pump installed by an MSC installer.
    They advised Nibe recommend a buffer in all instances. I wanted to avoid one as i have approx 90ltrs of water in my UFH Pipework, which is all open zone.
    I'm struggling with the way it's set up, the buffer is heated constantly to temp and as the stat calls for heat, a circulatig pump draws from the buffer to feed the pipework.
    Would it have been better to omit the buffer and just heat directly to the floor slab when required. It seems counterproductive to keep the buffer (100ltr) up to temp with heat being lost, as opposed to heat being lost in the actual slab only.
    Few other questions to ask...which may be better whispered. Thanks
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