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Green Building Bible, Fourth Edition
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Heating and cooling: Sizing a heat pump hot water storage tank

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    • CommentAuthorShevek
    • CommentTimeApr 13th 2022 edited
     
    - 2 bedroom, 4 person apartment, 68 m2
    - 1 bathroom with bath/shower, 1x washing machine, 1x dishwasher (we'll occasionally have all running at once in the evening, including 2x showers)
    - currently just the 2 of us, but plan to have visitors often and it may well get rented to 4 person family
    - Considering the Aquapura Monobloc: https://www.energie.pt/en/products/aquapura-monobloc

    The question is 114 lire (maybe too small) vs 195 litre (maybe too big)? (120 model vs 200ix model)

    Is it true to say that we'll mostly only pay for the water that we use rather than what we store, so therefore it's better to slightly oversize than to slightly undersize, so we don't risk running out?

    The 120 also has a COP of 2.8 while the 200ix has a COP of 3.72. What does this mean in the context of sizing?

    (some additional context: had a quote to get the 200i installed for €2600, and we should be able to get 85% of this back through EU funded programme here in Portugal for energy efficiency upgrades)
    •  
      CommentAuthordjh
    • CommentTimeApr 13th 2022
     
    Posted By: Shevekcurrently just the 2 of us, but plan to have visitors often and it may well get rented to 4 person family ...
    The question is 114 lire (maybe too small) vs 195 litre (maybe too big)? (120 model vs 200ix model)
    Is it true to say that we'll mostly only pay for the water that we use rather than what we store

    I think the 200ix will be a better fit. Are you getting the ix because you have solar thermal or what?
    You will mostly pay for the water you use but you will also pay for heat that is lost from the tank (standing losses) so you might want to calculate the difference. I haven't looked up the figures, sorry. It sounds like the 200ix is also more efficient, so might actually work out cheaper to run.
  3.  
    You can store the same amount of heat in 200l heated to 40degC or 100l heated to 70degC, and so get the same useful amount of water when you blend it with cold down to bath/shower temperature. But the heat pump will likely work much more efficiently at 40 than 70degC, and the cylinder will lose less heat.

    Big cylinders were traditionally for people to heat a whole day's supply of hot water on a cheap rate overnight (or maybe with solar). Do either of those apply to you? If you are willing to reheat several times during the day, you might need less storage, if you can split the usage into several chunks over the day. Our family of 5 have a 150l cylinder at 50degC which is plenty as some people shower in the morning and others in the evening and we reheat during the day.

    Be aware of legionella.

    In the UK, washing machines and dishwashers don't use hot water from a cylinder, they have cold feed only and heat it themselves. Different with you?

    Does that cylinder suck warm air from the house (EAHP) at a rate that is acceptable to you?
    • CommentAuthorbhommels
    • CommentTimeApr 14th 2022
     
    Posted By: WillInAberdeen
    In the UK, washing machines and dishwashers don't use hot water from a cylinder, they have cold feed only and heat it themselves. Different with you?

    I would not recommend a hot feed for washing machines as washing woollens at 40 C would give interesting results. For dishwashers it is a very good solution though. Most modern dishwashers can readily accept water at any temperature between 10 and 60 deg C - but do check the specifications. If there is enough head, even a gravity fed hot water cylinder might be OK. We have ours plumbed in with our solar HW system, it reduced the electricity consumption from 1.5 to 0.5 kWh for an ECO cycle.

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