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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorgoodevans
    • CommentTimeAug 23rd 2020 edited
     
    Posted By: bhommelsAnother question regarding cooling using an ASHP - blissfully unaware of the setup you have:
    Most ASHPs have a buffer tank that also serves for DHW, for example to provide hot water for a shower in the morning. How would this be compatible with your overnight active cooling scheme? How to avoid spending money on temperature cycling a buffer tank?
    I did not see the point of a buffer tank - I have a DHW cylinder - 250 litre with a 3m2 coil that the ASHP pumps water through once a day. And the UFH circuit which it pumps water through for heating and occasional cooling. By the time the water has passed through the UFH pipes it is no more than 0.5 deg C above the slab temp (when heating).

    The ASHP suppliers wanted to put in either a) a low loss header or b) a buffer tank (which is in effect a big low loss header) or c) a heat exchanger to separate the water in the ASHP circuit from the water in the ufh/dhw circuit. I ignored the advice and the ASHP is directly coupled the the UFH and DHW coil.

    This means I have one pump, and one circuit (with zone valves) - I have no need for a separate buffer tank - the DHW cylinder is its own buffer tank and the UFH takes so long to heat up it is the equivalent to a buffer tank of approx 4m3 of water. The ASHP moduates it heat output if necessary - It can supply as much as 9kW of heating (15 l/min at delta T of 8.5 degrees c) down to as little as around 1.9kW (9l/min at delta T of 3 deg C). The pump pumps less through the UFH than through the DHW coil.

    The downside is that either alot of glycol is required (option c avoids this) - or you rely on the frost protection of the ASHP system (I did the latter). If there is a power cut during freezing conditions I will have to power the pump via a leisure inverter and a car battery to ensure the residual heat in the slab will keep the ASHP from bursting its internal heat exchanger.
  1.  
    Posted By: goodevansIf there is a power cut during freezing conditions I will have to power the pump via a leisure inverter and a car battery to ensure the residual heat in the slab will keep the ASHP from bursting its internal heat exchanger.

    Caution is needed using a 'leisure inverter' as many have a wave form that is not good for motors (too 'square' or chopped wave form) There are (more expensive) inverters that have a good sine wave form suitable for motors. Also car batteries are not designed for deep discharge and will be quickly wrecked when used for deep discharge cycles. Unfortunately lead acid batteries suitable for deep discharge are quite a bit more expensive than car batteries. Lithium batteries are an option but again cost is a factor.

    Are you intending to have a battery and inverter on the shelf as insurance or do a panic buy if the situation looks imminent?

    Will the battery back up be on a uninterruptible power supply or will it be manual change over? I guess that depends on the expected frequency of outages, duration and the length of time the ASHP can survive without power. Of course power outages will always happen when most inconvenient e.g. 02:00. Not too expensive to have a battery powered loss of power alarm to wake you up!

    What happens if you go away for a period?

    Is frost damage due to power failure an insurable event (without the insurance co. insisting on antifreeze)
  2.  
    Our Ashp had its own inverters, in the variable speed drives for the compressor and the circ pump, it would be neat if you could feed external DC into those! But unlikely to be easy or warrantied.

    As you only need a slow trickle circulation of the CH water through the ashp and the slab or DHW tank, maybe a tiny DC pump inside the house could do that? Or just a few W of DC heater, inserted into the ashp casing - a car lightbulb?

    Our ASHP died when the heat exchanger got a hairline crack and started leaking refrigerant. It wasn't caused by frost. The exchanger was mounted right down at the back of the case, and the labour cost was prohibitive to remove all the other components to access it.
    • CommentAuthorEd Davies
    • CommentTimeAug 23rd 2020
     
    Posted By: Peter_in_HungaryAlso car batteries are not designed for deep discharge and will be quickly wrecked when used for deep discharge cycles.
    Indeed, but if they're left in the car with the engine ticking over that needn't be a problem, assuming you have a car with an engine. Not the sort of thing you want to make a habit of but quite acceptable for a once-in-a-decade emergency.

    If you have a car with a big battery and a motor, it'll presumably have a 12V battery for lights, accessories, etc. Will it keep that charged off the big battery just sitting there?
    • CommentAuthorgoodevans
    • CommentTimeAug 23rd 2020
     
    we do not have many power cuts - and i'm not away from home that much - I have elected to take the risk of not being in at the wrong time.

    Given the infrequency of power cuts here I intend to have a manual cut over. Plug the pump into the inverter or ups when required. A mains off alarm is a good idea.

    The pump draws something like 27 watts an needs to run for say 5 minutes per hour. In the event of a power cut I can dial the pump down to minimum so it will probably draw less than 10w.

    My main fear is not maintaining good battery health - most systems require a discharge, charge cycle every 3 months or so which I don't trust myself to do - so I was thinking of using my actual car batteries - they always charged and known working and shouldn't take more than 10 minutes to disconnect. It will take hours to discharge the battery. because the power requirement is so low I may be able to use the cigarette lighter and not disconnect the battery.

    The pump has an EC motor - so I strongly suspect that unlike traditional AC motors so long as the inverter doesn't mangle the mains facing electronics the pump will be fine.

    I intend to buy the inverter and have it on the shelf (but have not done so yet - so a panic buy may happen this winter!).
  3.  
    I remember my granddad lashing up something similar from the alternator of his tractor, to run the milking machine.

    I'm sure you'll be careful, but for others reading in future, the supply from the car to the inverter to the pump must never be allowed to back-feed into the house and the mains supply...
    • CommentAuthorgoodevans
    • CommentTimeAug 23rd 2020
     
    Posted By: WillInAberdeenI'm sure you'll be careful, but for others reading in future, the supply from the car to the inverter to the pump must never be allowed to back-feed into the house and the mains supply...
    Yes - that why I have the pump on a 13amp plug - it will either be plugged into the inverter, or into the mains. Keeping the two systems separate.
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